Division 03 — Isolating Transformer Troubleshooting

01 · Electrical & Winding Continuity

9 of 9 topics published

IT-121 Transformer Open Circuit

Symptom — A fixture on the series circuit is completely dark and initial testing shows no continuity through the isolating transformer as a whole — neither winding is confirmed conducting yet, and it is not yet established whether the fault lies in the primary or the secondary.

Equipment / System

Series-circuit isolating transformer (encapsulated/epoxy-potted, single-lamp type), typically 1:1 ratio, 6.6A primary, various VA ratings (30/45/65/100W), installed in an L-867 light base or below-grade pit.

Possible Causes

HighPrimary winding open-circuited internally

HighSecondary winding open-circuited internally

MediumInternal lead/connection broken at the potting compound interface, common after thermal cycling or a manufacturing defect

MediumPrimary or secondary L-823 connector failed open — corroded pins, a cracked connector body, or a pulled-apart crimp

LowerLightning or switching surge damage causing an internal open fault

Required Tools

True-RMS clamp meter (loop-rated)Isolating transformer bench test setInsulation resistance tester (5kV)MultimeterL-823 connector test leadsFixture wiring diagram

Safety Precautions

Never open or disconnect an isolating transformer while the CCR is running — isolate the CCR and apply LOTO first, since the series loop can present hazardous induced voltage at any break point.
Follow airside work permit and NOTAM procedures before accessing runway/taxiway light bases.
Assume the pit/light base may contain standing water — inspect before kneeling or placing tools inside.

Diagnostic Procedure

ObservationConfirm the CCR is delivering rated loop current at the commanded step (per CCR-021/CCR-031 in Division 01) before concluding the fault is local to this transformer rather than a CCR-side issue.
MeasurementWith the CCR isolated and LOTO applied, disconnect the transformer's primary and secondary L-823 connectors and bench-test the unit using an isolating transformer test set.
MeasurementCheck primary winding continuity/resistance against the manufacturer's nameplate value.
MeasurementCheck secondary winding continuity/resistance against the manufacturer's nameplate value.
DecisionPrimary winding open → proceed to IT-122 for primary-specific diagnosis, noting the fail-safe shorting device implication for loop continuity. Secondary winding open → proceed to IT-123. Both windings show continuity on the bench but the fixture was still dark in the field → suspect a failed L-823 connector rather than the transformer itself.
Fault LocationIsolate to the primary winding, the secondary winding, or an external connector at the transformer's plug interface.
Root CauseConfirm the specific open point through the winding-specific diagnostic procedure (IT-122 or IT-123).

Test Points

Primary winding resistance (bench test)

Secondary winding resistance (bench test)

Primary L-823 connector pins

Secondary L-823 connector pins

Root Cause Verification

After replacement or repair, bench-test the transformer to confirm both windings show correct resistance and turns ratio, then re-install and confirm the fixture illuminates correctly at every commanded intensity step.

Corrective Action

Replace the transformer if either winding is confirmed open.
Repair or replace a failed L-823 connector if the windings themselves test healthy.
Re-terminate a broken internal lead only if the manufacturer's repair procedure permits field repair of a potted unit — otherwise replace.

Preventive Action

Bench-test spare isolating transformers before storage so failures are caught before installation, not after.
Include a periodic sample bench-test programme for transformers pulled during other pit works.
Track transformer replacement history by circuit to identify recurring failure locations.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785, FAA AC 150/5345-10

Diagram: Series Circuit Single Line Diagram, Isolating Transformer Wiring Diagram

Case Study: —

Rev 0 — Initial issue

IT-122 Transformer Primary Open Circuit

Symptom — The isolating transformer's primary winding is confirmed open. Because the primary sits in series within the loop, the field presentation depends entirely on whether the transformer's internal fail-safe shorting device operated: if it did, only this one fixture goes dark while the rest of the circuit runs normally; if it did not, the entire series circuit loses continuity and the CCR raises an open-circuit/no-load alarm.

Equipment / System

Isolating transformer primary winding and its internal fail-safe shorting device (where fitted) — a component specifically designed to short the primary on winding failure so a single transformer fault does not take down the whole series circuit.

Possible Causes

HighPrimary winding failed open due to age, thermal stress, or an internal manufacturing defect, with the fail-safe shorting device operating correctly — only this fixture affected

HighPrimary winding failed open and the fail-safe shorting device also failed to operate — the entire series circuit loop is open, escalate immediately per CCR-021/CCR-030 in Division 01

MediumPrimary-side L-823 connector corroded or pulled apart, presenting as an open primary without an actual winding fault

LowerSurge/lightning damage to the primary winding and, in some cases, to the fail-safe device itself

Required Tools

True-RMS clamp meterIsolating transformer bench test setMultimeterInsulation resistance tester (5kV)L-823 connector test leads

Safety Precautions

If the entire circuit is open (fail-safe did not operate), treat the loop as carrying an elevated open-circuit voltage at the break point — apply LOTO on the CCR before touching any part of the loop.
Do not assume a single dark fixture with the rest of the circuit healthy is automatically safe to handle — verify zero energy at the specific transformer before disconnecting it.

Diagnostic Procedure

ObservationDetermine which scenario applies: is only this one fixture dark with the rest of the circuit operating normally, or has the CCR raised an open-circuit/no-load alarm for the whole loop?
DecisionWhole-loop open-circuit alarm present → the fail-safe device did not operate; this is now a full-circuit fault, proceed per CCR-021/CCR-030 (Division 01) to locate the break, which will be at this transformer's primary.
MeasurementIf only the single fixture is affected, with the CCR isolated and LOTO applied, disconnect the transformer and bench-test the primary winding and fail-safe device continuity.
MeasurementCheck the primary-side L-823 connector for corrosion, looseness, or physical damage as an alternative to a true winding fault.
DecisionPrimary winding confirmed open with the fail-safe correctly shorted → replace the transformer as a routine single-fixture repair. Connector fault found with the winding itself healthy → repair/replace the connector only.
Fault LocationIsolate to the primary winding and fail-safe device, or to the primary-side connector.
Root CauseConfirm the specific failure by bench test before returning the transformer to service or scrapping it.

Test Points

Primary winding resistance

Fail-safe shorting device continuity/operation

Primary-side L-823 connector

Root Cause Verification

After replacement, confirm loop continuity and rated current at the CCR, then re-energize and confirm the repaired fixture illuminates correctly alongside the rest of the circuit.

Corrective Action

Replace the transformer if the primary winding is confirmed open.
Repair or replace the primary-side L-823 connector if the winding tests healthy.
Escalate to a full loop continuity investigation per CCR-021/CCR-030 if the fail-safe device did not operate.

Preventive Action

Verify fail-safe device function as part of the bench-test acceptance criteria for spare/replacement transformers.
Track which circuits have experienced a fail-safe non-operation event, since it indicates the device population on that circuit may be ageing together.
Include primary winding resistance trending in periodic sample testing.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing), SOP-CCR-06 (Series Circuit Loop Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Series Circuit Single Line Diagram, Isolating Transformer Wiring Diagram

Case Study: Division 25 — Field Case Study Troubleshooting (cross-reference where applicable)

Rev 0 — Initial issue

IT-123 Transformer Secondary Open Circuit

Symptom — The fixture served by this isolating transformer is dark, but the CCR shows normal loop current and no open-circuit alarm — the primary loop is intact, and the fault is isolated to the secondary winding or the secondary-side connection to the lamp/LED driver.

Equipment / System

Isolating transformer secondary winding and its output connector/lead to the fixture.

Possible Causes

HighSecondary winding open-circuited internally

HighSecondary-side L-823 connector corroded, loose, or pulled apart at the lamp base connection

MediumSecondary lead broken at the potting compound interface

LowerSecondary-side surge protection device (where fitted) failed in an open-circuit mode

Required Tools

True-RMS clamp meterIsolating transformer bench test setMultimeterL-823 connector test leadsFixture wiring diagram

Safety Precautions

Apply LOTO on the CCR before disconnecting the transformer, even though the primary loop is confirmed healthy, since the secondary can still carry a hazardous voltage while the loop is energized.

Diagnostic Procedure

ObservationConfirm the CCR shows normal loop current and no open-circuit alarm, isolating the fault to this transformer's secondary side or the fixture itself rather than the loop.
MeasurementWith the CCR isolated and LOTO applied, disconnect the transformer's secondary connector and bench-test secondary winding continuity.
MeasurementInspect the secondary-side L-823 connector and lamp base connection for corrosion or looseness.
DecisionSecondary winding confirmed open on the bench → replace the transformer. Winding tests healthy but connector/lead is damaged → repair or replace the connector/lead only, redirect to CCR-031 (Division 01) if the fixture itself (lamp/LED driver) is then found to be the actual fault.
Fault LocationIsolate to the secondary winding, the secondary connector, or the fixture's own wiring.
Root CauseConfirm the specific open point by bench test and connector inspection.

Test Points

Secondary winding resistance

Secondary L-823 connector pins

Secondary lead at potting interface

Root Cause Verification

After replacement or repair, bench-test the transformer to confirm secondary winding continuity, then re-install and confirm the fixture illuminates correctly at every commanded intensity step.

Corrective Action

Replace the transformer if the secondary winding is confirmed open.
Repair or replace the secondary-side connector or lead.
Replace a failed secondary surge protection device if fitted and confirmed as the open point.

Preventive Action

Include secondary winding resistance in the bench-test acceptance criteria for spares.
Inspect secondary connectors for corrosion whenever a fixture is opened for lamp/LED driver replacement.
Track fixture-level dark-lamp reports by transformer age to spot a batch nearing end of life.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Isolating Transformer Wiring Diagram, Fixture Wiring Diagram

Case Study: —

Rev 0 — Initial issue

IT-124 Low Secondary Voltage

Symptom — With rated primary current confirmed flowing through the transformer, the secondary output voltage measured at the fixture connector reads below the expected value for the unit's rated transformation ratio — the lamp or LED fixture runs dim.

Equipment / System

Isolating transformer winding and core, secondary connector/lead to the fixture.

Possible Causes

HighInternal winding degradation increasing effective winding resistance or reducing effective coupling, reducing secondary output for a given primary current

HighCore degradation (lamination damage, partial saturation) reducing magnetic coupling efficiency between windings

MediumSecondary connector or lead has a high-resistance connection, dropping voltage before it reaches the fixture even though the transformer itself is healthy

MediumAn incorrect transformer (wrong VA rating or ratio) was installed at a previous replacement, undersized for the fixture's actual load

LowerBench test measurement error — voltage measured under a load current that does not match the actual field primary current

Required Tools

Isolating transformer bench test set (with turns-ratio function)True-RMS clamp meterMultimeterFixture wiring diagram / transformer schedule

Safety Precautions

Apply LOTO on the CCR before disconnecting the transformer for bench testing.

Diagnostic Procedure

MeasurementBench-test the transformer at its rated primary current and measure secondary output voltage against the nameplate value.
MeasurementPerform a turns-ratio test to compare the actual primary:secondary ratio against the nameplate ratio.
ObservationVerify the installed transformer's part number/rating against the design transformer schedule for this fixture type.
MeasurementWith the transformer confirmed healthy, check the secondary connector and lead for a high-resistance connection dropping voltage under load.
DecisionRatio test confirms a genuine deviation from nameplate → internal winding/core degradation, replace the transformer. Ratio and bench voltage both correct but field voltage at the fixture is still low → connector/lead fault, not the transformer. Wrong-rating transformer confirmed installed → correct with the design-specified unit.
Fault LocationIsolate to the transformer's internal winding/core, the secondary connector/lead, or an incorrect unit having been installed.
Root CauseConfirm the specific cause through bench ratio testing and physical rating verification.

Test Points

Secondary output voltage at rated primary current (bench)

Turns ratio (bench)

Secondary connector/lead voltage drop under load

Installed unit's nameplate rating

Root Cause Verification

After correction, bench-test the transformer (or its replacement) to confirm secondary voltage and turns ratio match nameplate values, then re-install and confirm correct fixture brightness in the field.

Corrective Action

Replace the transformer if winding or core degradation is confirmed.
Repair or replace the secondary connector/lead.
Install the design-specified transformer rating if a wrong unit is found.

Preventive Action

Verify transformer part number against the design transformer schedule whenever a unit is replaced.
Include a turns-ratio test in periodic sample testing, not just a resistance check.
Maintain an accurate, current transformer schedule by fixture type and circuit.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Isolating Transformer Wiring Diagram, Transformer Schedule

Case Study: —

Rev 0 — Initial issue

IT-125 High Secondary Voltage

Symptom — With rated primary current confirmed flowing through the transformer, the secondary output voltage reads above the expected value for the unit's rated transformation ratio, risking over-driving the lamp filament or LED driver and causing premature fixture failure.

Equipment / System

Isolating transformer winding and core, secondary load condition.

Possible Causes

HighAn incorrect transformer (wrong ratio, intended for a different fixture type or wattage) was installed at a previous replacement

HighInternal winding fault (partial inter-turn short on the primary side) effectively reducing primary turns and raising the ratio, elevating secondary voltage for the same primary current

MediumSecondary circuit lightly loaded or partially disconnected, with the reading taken close to an unloaded/no-load condition rather than true rated load — a measurement artefact rather than a genuine fault

LowerBench test performed at an incorrect primary current, not matching the actual field loop current

Required Tools

Isolating transformer bench test set (with turns-ratio function)True-RMS clamp meterMultimeterFixture wiring diagram / transformer schedule

Safety Precautions

Apply LOTO on the CCR before disconnecting the transformer for bench testing.
Do not leave a confirmed over-voltage condition connected to an LED driver or lamp longer than necessary — it accelerates fixture degradation.

Diagnostic Procedure

MeasurementBench-test the transformer at its correct rated primary current, with the secondary correctly loaded to the fixture's rated load, and measure secondary voltage against nameplate.
MeasurementPerform a turns-ratio test to compare the actual ratio against nameplate.
ObservationVerify the installed transformer's part number/rating against the design transformer schedule.
DecisionRatio confirmed deviated from nameplate under correctly loaded test conditions → internal winding fault, replace the transformer. Ratio and bench voltage correct once tested under proper load → the original field reading was a measurement artefact from an unloaded/lightly loaded condition, not a genuine fault. Wrong-rating transformer confirmed installed → correct with the design-specified unit.
Fault LocationIsolate to the transformer's internal winding, an incorrect installed unit, or a measurement condition issue.
Root CauseConfirm the specific cause through a correctly loaded bench ratio test.

Test Points

Secondary output voltage under correct rated load (bench)

Turns ratio (bench)

Installed unit's nameplate rating

Root Cause Verification

After correction, bench-test under correct rated load to confirm secondary voltage and turns ratio match nameplate values, then re-install and monitor the fixture for correct brightness with no signs of over-drive.

Corrective Action

Replace the transformer if a genuine winding fault is confirmed under a correctly loaded test.
Install the design-specified transformer rating if a wrong unit is found.
Correct the bench test procedure/loading if the original reading is confirmed to be a measurement artefact.

Preventive Action

Always bench-test isolating transformers under correct rated load, not open-circuit, to avoid misleading voltage readings.
Verify transformer part number against the design schedule whenever a unit is replaced.
Include turns-ratio testing in periodic sample testing.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Isolating Transformer Wiring Diagram, Transformer Schedule

Case Study: —

Rev 0 — Initial issue

IT-126 Incorrect Transformation Ratio

Symptom — A turns-ratio test on the isolating transformer returns a value that does not match its nameplate-rated primary:secondary ratio, identified during bench testing, acceptance testing of a new/repaired unit, or while investigating a voltage complaint (IT-124/IT-125).

Equipment / System

Isolating transformer winding assembly — turns-ratio relationship between primary and secondary.

Possible Causes

HighInternal winding fault — a partial inter-turn short on either winding altering the effective number of active turns

HighWrong transformer model/part number was supplied or installed, with a genuinely different rated ratio than intended for this fixture type

MediumBench test set calibration error or an incorrect test procedure (wrong excitation voltage/current) producing a false ratio reading

LowerManufacturing defect present since new, not caught at incoming inspection

Required Tools

Isolating transformer bench test set (with turns-ratio function)Calibrated reference transformer or known-good unit for cross-checkMultimeterTransformer schedule / nameplate reference

Safety Precautions

Apply LOTO on the CCR before removing any transformer for bench testing.

Diagnostic Procedure

MeasurementRepeat the turns-ratio test carefully following the bench test set's specified procedure, to rule out a test error.
MeasurementCross-check the test set itself against a known-good reference transformer with a verified correct ratio.
ObservationConfirm the transformer's nameplate part number and rated ratio against the design transformer schedule for this fixture type.
DecisionTest set confirmed accurate against the reference unit and the deviation persists → genuine winding fault or wrong unit supplied. Test set itself found inaccurate → recalibrate/repair the test set and re-test the transformer before drawing further conclusions.
Fault LocationIsolate to the transformer's winding, an incorrect part supplied, or the test equipment.
Root CauseConfirm the specific cause and reject or accept the unit accordingly.

Test Points

Turns ratio (bench)

Reference transformer ratio (for test set cross-check)

Transformer nameplate part number

Root Cause Verification

Confirm the turns-ratio test result matches nameplate on a verified-accurate test set before accepting the transformer into service, or confirm the correct-rated replacement unit tests within tolerance.

Corrective Action

Reject and replace a transformer confirmed to have a genuine winding fault.
Correct a wrong-part-number installation with the design-specified unit.
Recalibrate or repair the bench test set if it is confirmed to be the source of the error.

Preventive Action

Include turns-ratio testing as a mandatory incoming-inspection step for new and repaired transformers, not just resistance checks.
Maintain a calibration schedule for the bench test set itself.
Keep the transformer schedule current and clearly cross-referenced to fixture type.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Transformer Schedule

Case Study: —

Rev 0 — Initial issue

IT-127 High Primary Current

Symptom — During a rated-voltage bench excitation test (with the transformer off the series loop and energized from a dedicated AC test source), the primary draws current noticeably above the normal value for the applied test voltage — indicating the unit is absorbing more power than a healthy transformer would at that excitation level.

Equipment / System

Isolating transformer primary winding and core, as evaluated during off-loop bench excitation testing (not a loop current measurement, since loop current is fixed by the CCR's constant-current regulation).

Possible Causes

HighPartial winding short (inter-turn fault) on the primary or secondary, increasing current draw for a given test voltage

HighCore lamination damage or degraded core steel increasing eddy current/hysteresis losses

MediumMoisture or insulation degradation within the winding creating a partial leakage path that adds to normal excitation current

LowerTest procedure error — excitation voltage applied above the correct bench test value for this transformer's rating

Required Tools

Isolating transformer bench test setTrue-RMS clamp meterInsulation resistance tester (5kV)Manufacturer bench test procedure/reference values

Safety Precautions

Apply LOTO on the CCR before removing the transformer for bench testing.
Do not exceed the manufacturer's specified bench test excitation voltage — an incorrect over-voltage test can itself cause or worsen a winding fault, confusing the diagnosis.

Diagnostic Procedure

ObservationConfirm the exact bench test excitation voltage applied matches the manufacturer's specified value for this transformer's rating.
MeasurementRe-run the excitation test at the correct voltage and record primary current draw.
MeasurementMegger the winding insulation resistance to check for a moisture-related leakage contribution.
MeasurementCross-check the reading against a known-good reference unit of the same rating under identical bench conditions.
DecisionExcitation voltage confirmed correct and current draw remains elevated versus the reference unit → genuine winding/core fault, reject the transformer. Test voltage found incorrect → repeat the test correctly before drawing conclusions. Insulation resistance also found low → treat as a combined winding/insulation fault (see IT-130).
Fault LocationIsolate to a genuine winding/core fault, an insulation-related leakage contribution, or a test procedure error.
Root CauseConfirm the specific cause through correctly-conducted, reference-compared bench testing.

Test Points

Primary current at rated bench excitation voltage

Winding insulation resistance

Reference (known-good) unit comparison reading

Root Cause Verification

Confirm any replacement unit draws primary current within the manufacturer's normal range at the correct rated bench excitation voltage before returning it to service.

Corrective Action

Reject and replace a transformer confirmed to have a genuine winding or core fault.
Correct the bench test procedure/voltage if a test error was the actual cause.
Address insulation degradation per IT-130/IT-131 if that is found to be the contributing factor.

Preventive Action

Standardise and document the correct bench excitation voltage for every transformer rating used on site.
Keep a known-good reference unit of each common rating available for comparison testing.
Include insulation resistance testing alongside every excitation current test as standard practice.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785, IEC 60076

Diagram: Bench Test Procedure Reference Sheet

Case Study: —

Rev 0 — Initial issue

IT-128 Abnormal No-Load Current

Symptom — With the secondary open-circuited (no lamp/LED load connected) and rated voltage applied to the primary during a bench test, the measured magnetizing (no-load) current falls outside the manufacturer's specified acceptance range — either significantly above or, less commonly, implausibly low.

Equipment / System

Isolating transformer core and primary winding, evaluated specifically under the standard no-load magnetizing current acceptance test.

Possible Causes

HighCore lamination damage or degraded core material increasing magnetizing current beyond the normal range

HighPartial winding short reducing effective turns, which increases no-load current for the same applied voltage

MediumTest connection error — secondary not genuinely open-circuited (a partial load or a shorted secondary during the test), invalidating the no-load reading

LowerManufacturer's published acceptance range applied incorrectly for this specific transformer rating

Required Tools

Isolating transformer bench test setTrue-RMS clamp meterManufacturer no-load current acceptance criteria (by rating)

Safety Precautions

Apply LOTO on the CCR before removing the transformer for bench testing.
Confirm the secondary is genuinely open before applying primary voltage — an inadvertently shorted secondary during a no-load test can damage the transformer or the test set.

Diagnostic Procedure

ObservationConfirm the secondary is genuinely open-circuited (verified with a meter, not just visually) before starting the test.
MeasurementApply rated primary voltage and record the no-load current.
ObservationCompare the result against the manufacturer's published no-load current acceptance range for this specific rating, not a generic figure.
DecisionSecondary confirmed genuinely open and the reading still falls outside the acceptance range → genuine core/winding issue, reject the transformer. Secondary found not fully open during the original test → repeat the test correctly before concluding a fault.
Fault LocationIsolate to a genuine core/winding fault or a test setup error.
Root CauseConfirm the specific cause through a correctly conducted, rating-specific no-load test.

Test Points

No-load (magnetizing) current at rated primary voltage

Secondary open-circuit verification

Root Cause Verification

Confirm any replacement unit's no-load current falls within the manufacturer's published acceptance range for its rating before returning it to service.

Corrective Action

Reject and replace a transformer confirmed to have a genuine core or winding fault.
Repeat the test correctly if a setup error (secondary not truly open) was the actual cause.
Update the site's acceptance-criteria reference sheet if the wrong range was being applied.

Preventive Action

Maintain a rating-specific no-load current acceptance table for every transformer type used on site.
Verify secondary open-circuit condition with a meter, not visual inspection alone, before every no-load test.
Include no-load current testing as a standard incoming-inspection step for new and repaired units.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IEC 60076

Diagram: Bench Test Procedure Reference Sheet

Case Study: —

Rev 0 — Initial issue

IT-129 Excessive Load Current

Symptom — During a bench test with the secondary connected to its rated fixture load (or an equivalent test load), the secondary current draw exceeds the fixture's rated current for the applied primary excitation — indicating the transformer, fixture, or connected load is drawing more current than designed.

Equipment / System

Isolating transformer secondary circuit and its connected lamp/LED fixture load.

Possible Causes

HighAn incorrect lamp/LED fixture (higher wattage than the circuit and transformer are rated for) has been fitted

HighA partial short within the fixture wiring or lamp base is drawing additional current, reflected back through the transformer

MediumThe transformer's own winding fault is presenting as excess loading rather than a true fixture-side cause — cross-check against IT-126/IT-127

LowerTest load used on the bench does not match the actual field fixture's rated current, giving a misleading result

Required Tools

Isolating transformer bench test setTrue-RMS clamp meterMultimeterFixture/lamp rating referenceTransformer schedule

Safety Precautions

Apply LOTO on the CCR before removing the transformer or fixture for bench testing.
Sustained excessive secondary current can overheat the transformer — do not leave a confirmed over-current condition connected for extended bench testing.

Diagnostic Procedure

ObservationConfirm the exact test load used on the bench matches the fixture's rated current specification.
MeasurementWith the correct test load connected, measure secondary current at rated primary excitation and compare against the fixture's rated value.
ObservationInspect the actual field-installed lamp/LED fixture for correct wattage/rating against the design specification.
MeasurementIf the fixture rating is confirmed correct, bench-test the transformer alone (per IT-126/IT-127) to rule out an internal transformer fault presenting as excess loading.
DecisionWrong-rated fixture confirmed installed → correct with the design-specified fixture. Fixture rating correct but current still excessive → inspect fixture wiring for a partial short. Transformer itself found faulty in isolation testing → replace the transformer.
Fault LocationIsolate to an incorrect fixture, a fixture-side wiring fault, or the transformer itself.
Root CauseConfirm the specific cause by testing the transformer and fixture independently.

Test Points

Secondary current under rated test load

Field-installed fixture wattage/rating

Fixture wiring insulation and continuity

Root Cause Verification

After correction, confirm secondary current at rated primary excitation matches the fixture's design current, both on the bench and after re-installation in the field.

Corrective Action

Replace an incorrectly rated fixture with the design-specified unit.
Repair fixture wiring or lamp base if a partial short is confirmed.
Replace the transformer if it is confirmed faulty independent of the fixture.

Preventive Action

Verify fixture wattage/rating against the design specification whenever a lamp or LED driver is replaced.
Bench-test transformer and fixture separately when investigating an over-current complaint, to isolate which component is responsible.
Maintain a current fixture and transformer rating schedule by circuit.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Transformer Schedule, Fixture Wiring Diagram

Case Study: —

Rev 0 — Initial issue

02 · Insulation & Earth Faults

6 of 6 topics published

IT-130 Low Insulation Resistance

Symptom — A megger test of the isolating transformer (winding-to-winding or winding-to-earth) returns a value below the acceptable threshold (commonly a minimum of 100 MΩ at 500V/1000V per site standard), without yet indicating an outright breakdown — an early-warning finding rather than a confirmed failure.

Equipment / System

Isolating transformer winding insulation system — potting compound, winding varnish/insulation, and the winding-to-core/earth barrier.

Possible Causes

HighMoisture ingress into the potting compound or winding insulation, gradually reducing resistance (see IT-134/IT-135)

HighAge-related insulation degradation from long-term thermal cycling and service life

MediumContamination on the transformer case/terminals providing a surface leakage path, mistaken for internal insulation degradation

MediumTest performed on a wet/damp unit immediately after removal from a flooded pit, giving an artificially low reading that may partially recover once dried

LowerTest equipment/lead fault giving a false low reading

Required Tools

Insulation resistance tester (5kV, or per site standard voltage)Contact cleaner and lint-free clothIsolating transformer bench test setSite insulation resistance acceptance criteria

Safety Precautions

Apply LOTO on the CCR before removing the transformer for testing.
Handle a unit with confirmed low insulation resistance as a potential shock hazard until it is either repaired/replaced or confirmed safe — do not return it to service pending investigation.

Diagnostic Procedure

ObservationConfirm the test is being performed at the correct test voltage per the site's insulation testing standard, and record the exact reading.
MeasurementClean the case and terminals and re-test to rule out surface contamination as the cause.
ObservationNote whether the unit was recently removed from a wet or flooded location, which could explain a temporarily depressed reading.
MeasurementIf moisture is suspected, dry the unit per manufacturer guidance (where field drying is permitted for this transformer type) and re-test.
DecisionReading recovers to an acceptable value after cleaning/drying → moisture or surface contamination was the cause, monitor and consider trending. Reading remains low after cleaning and drying → genuine insulation degradation, reject the unit and proceed toward IT-131 assessment.
Fault LocationIsolate to surface contamination, temporary moisture, or genuine internal insulation degradation.
Root CauseConfirm the specific cause through a repeat test after cleaning and, where applicable, drying.

Test Points

Winding-to-winding insulation resistance

Winding-to-earth/case insulation resistance

Case and terminal surface condition

Root Cause Verification

Confirm the insulation resistance reading is stable and within the site's acceptance threshold on a repeat test before returning the unit to service.

Corrective Action

Clean the case and terminals and re-test if surface contamination was the cause.
Dry the unit per manufacturer guidance and re-test if temporary moisture was the cause.
Reject and replace the transformer if low insulation resistance persists after cleaning and drying.

Preventive Action

Trend insulation resistance results over successive tests for each transformer, rather than treating each test as a pass/fail in isolation.
Include insulation resistance testing in every routine bench test, not only when a fault is suspected.
Investigate and correct the pit/light base drainage condition if repeated moisture-related low readings occur on the same circuit.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785, IEC 60076

Diagram: Bench Test Procedure Reference Sheet

Case Study: —

Rev 0 — Initial issue

IT-131 Insulation Breakdown

Symptom — A confirmed dielectric failure within the transformer — a direct short or flashover between windings, or between a winding and earth/case — distinct from a degraded-but-intact insulation resistance reading (IT-130). Typically presents as a tripped earth fault protection, a failed HV withstand test, or a visibly burnt/tracked insulation path on inspection.

Equipment / System

Isolating transformer winding insulation system, at the point of confirmed dielectric failure.

Possible Causes

HighProgressive insulation degradation (from IT-130) reaching the point of complete breakdown under normal operating or test voltage

HighSustained water ingress causing a direct conductive path across the insulation (see IT-134)

MediumA surge/lightning event causing sudden dielectric puncture

MediumManufacturing defect (void or contamination in the potting compound) creating a weak point that eventually breaks down

LowerMechanical damage to the case exposing internal insulation to the environment

Required Tools

Insulation resistance tester (5kV)HV withstand (Hipot) test setIsolating transformer bench test setInspection torchPPE for HV testing

Safety Precautions

Treat any unit with a confirmed insulation breakdown as scrap — do not attempt to return it to service after a dielectric failure, even if it appears to test acceptably afterward.
Apply LOTO on the CCR before removing the failed transformer, and handle it as a potential earth fault source until confirmed isolated.

Diagnostic Procedure

ObservationNote how the breakdown was identified — a protection trip in the field, a failed HV test, or visible physical evidence — as this shapes the immediate priority.
MeasurementPerform an insulation resistance test to confirm the breakdown and characterise its severity (a near-zero reading versus a partial reduction).
ObservationVisually inspect the case and accessible internals (per manufacturer guidance) for tracking, burning, or a visible puncture point.
MeasurementIf the unit came from a wet location, confirm whether water ingress is present, since this is a common direct cause (see IT-134).
DecisionBreakdown confirmed by test and/or visual evidence → reject and replace the unit; do not attempt repair on a potted transformer with a confirmed dielectric failure. Investigate whether the cause was isolated (age/defect) or systemic (a circuit-wide water ingress issue) to determine whether other units on the same circuit need inspection.
Fault LocationConfirm the breakdown is contained to this single transformer, or escalate to a circuit-wide inspection if a systemic cause (e.g., flooding) is suspected.
Root CauseDocument the confirmed cause for the maintenance record and to inform preventive action.

Test Points

Winding-to-winding insulation resistance

Winding-to-earth/case insulation resistance

Visual condition of case and accessible internals

Root Cause Verification

Confirm the failed unit is removed from service and replaced with a bench-tested spare that passes insulation resistance and HV withstand testing before re-installation.

Corrective Action

Replace the transformer — a confirmed dielectric breakdown on a potted unit is not field-repairable.
Investigate and correct the systemic cause (e.g., pit drainage, connector sealing) if the breakdown is linked to water ingress affecting the wider circuit.
Report the failure for the maintenance/failure history record.

Preventive Action

Trend insulation resistance results to catch degradation before it reaches breakdown (see IT-130 preventive actions).
Investigate any earth fault trip event to determine the specific failed transformer rather than simply resetting protection and moving on.
Review pit/light base sealing and drainage on circuits with a history of insulation breakdown.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785, IEC 60076

Diagram: Bench Test Procedure Reference Sheet

Case Study: Division 25 — Field Case Study Troubleshooting (cross-reference where applicable)

Rev 0 — Initial issue

IT-132 HV Test Failure

Symptom — An isolating transformer fails the dielectric withstand (Hipot/HV) test performed during acceptance testing of a new or repaired unit, or during periodic testing — the unit either flashes over, breaks down, or draws leakage current above the test set's trip threshold at the specified test voltage.

Equipment / System

Isolating transformer insulation system, evaluated specifically under the standard HV withstand test (commonly an AC test voltage per IEC/IS acceptance criteria for the transformer's rated voltage class, applied for a specified duration).

Possible Causes

HighGenuine insulation weakness or a developing defect that survives normal operating voltage but cannot withstand the elevated test voltage

HighMoisture present in the unit at the time of test, temporarily reducing dielectric strength (see IT-135)

MediumTest procedure error — incorrect test voltage, ramp rate, or duration applied for this transformer's rating

MediumTest set fault or miscalibration causing a false trip

LowerA genuinely marginal but otherwise serviceable unit failing right at the threshold due to normal manufacturing tolerance, warranting a retest per the site's documented retest procedure

Required Tools

HV withstand (Hipot) test setInsulation resistance tester (5kV)Manufacturer/standard HV test procedure and acceptance criteriaPPE for HV testing

Safety Precautions

HV withstand testing must only be performed by personnel trained and authorised for HV test procedures, following the test set manufacturer's safety instructions and site HV test permit process.
Establish a clear test area with barriers and warning signage — HV test voltages are hazardous even for a brief test duration.
Do not touch the unit under test or its leads while voltage is applied; wait for the test set's full discharge cycle to complete before handling.

Diagnostic Procedure

ObservationConfirm the test was performed at the correct voltage, ramp rate, and duration for this transformer's specific rating per the manufacturer or applicable standard.
MeasurementPerform an insulation resistance test on the same unit to check for a correlating low reading, which would support a genuine insulation issue rather than a test artefact.
MeasurementIf moisture is suspected, dry the unit per manufacturer guidance (where field drying is permitted) and retest.
MeasurementVerify the HV test set's calibration status and, where practical, cross-check against a known-good reference unit.
DecisionTest parameters confirmed correct and insulation resistance also low → genuine failure, reject the unit. Moisture confirmed and resistance recovers after drying → retest per site procedure. Test set found miscalibrated or test parameters were wrong → correct and retest before drawing conclusions.
Fault LocationIsolate to a genuine insulation failure, a moisture-related temporary condition, or a test procedure/equipment error.
Root CauseConfirm the specific cause and record the final test outcome for the acceptance record.

Test Points

HV withstand test result at rated test voltage

Insulation resistance (correlating check)

HV test set calibration status

Root Cause Verification

A unit that passes HV withstand testing on a correctly parameterised, calibrated test set is accepted into service; a unit failing under correct test conditions is rejected and replaced.

Corrective Action

Reject and replace a transformer confirmed to have failed HV testing under correct conditions.
Dry and retest a unit where moisture is confirmed as the cause, per the site's documented retest allowance.
Correct the test procedure or recalibrate the test set if either was the actual cause of a false failure.

Preventive Action

Maintain a documented, rating-specific HV test procedure and acceptance criteria for every transformer type used on site.
Keep the HV test set on a current calibration schedule.
Include HV withstand testing as a mandatory acceptance step for new and repaired transformers before installation.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IEC 60076, IS 7785

Diagram: HV Test Procedure Reference Sheet

Case Study: —

Rev 0 — Initial issue

IT-133 Earth Leakage

Symptom — The CCR's earth fault protection detects and trips on leakage current traced to a specific isolating transformer on the series circuit — either a hard trip during normal operation or a leakage current above threshold identified during a dedicated earth fault location procedure.

Equipment / System

Isolating transformer winding-to-earth/case insulation barrier, and the CCR's earth fault protection loop (see Division 01, Subsection 04 — Protection & Alarms for CCR-side earth fault diagnostics).

Possible Causes

HighWinding-to-earth insulation breakdown within this specific transformer (see IT-131)

HighWater ingress into the transformer or its enclosure providing a conductive leakage path to earth (see IT-134)

MediumDamaged case or cracked potting compound allowing a direct or moisture-assisted leakage path

MediumA degraded connector or lead with exposed conductor contacting an earthed surface within the light base/pit

LowerThe leakage is not actually at this transformer but elsewhere on the circuit, and this unit was misidentified during location testing

Required Tools

Earth fault location test set (per site's CCR earth fault procedure)Insulation resistance tester (5kV)Isolating transformer bench test setInspection torch

Safety Precautions

Apply LOTO on the CCR before working on any part of the loop during earth fault location.
Treat every suspect transformer as potentially live to earth until confirmed isolated and tested dead.

Diagnostic Procedure

ObservationConfirm the CCR's earth fault protection has actually trended/tripped for this circuit, and review the segment-location result (per the CCR earth fault location procedure) that pointed to this specific transformer.
MeasurementWith the CCR isolated, disconnect this transformer and measure winding-to-earth/case insulation resistance independently.
ObservationInspect the transformer case, potting compound, and connectors for visible damage or water ingress.
DecisionInsulation resistance confirmed low on this specific unit → this transformer is the earth fault source, proceed per IT-130/IT-131. Insulation resistance on this unit tests normal → the location procedure may have pointed to the wrong segment; re-run earth fault location and check adjacent transformers/cable sections.
Fault LocationIsolate to this specific transformer, or redirect to an adjacent segment if this unit tests healthy.
Root CauseConfirm the specific transformer or cable section responsible for the leakage.

Test Points

Winding-to-earth/case insulation resistance

Transformer case and potting compound condition

Adjacent cable/connector sections (if this unit tests healthy)

Root Cause Verification

After replacing the confirmed faulty transformer, re-energize the circuit and confirm the CCR's earth fault protection remains clear over a sustained run period with no repeat trip.

Corrective Action

Replace the transformer confirmed to be the earth leakage source.
Repair or reseal a damaged case/connector if the leakage path is external to the winding itself.
Continue earth fault location on the circuit if this unit tests healthy and the fault lies elsewhere.

Preventive Action

Include insulation resistance testing of transformers as part of the response to any earth fault trip, not just a reset-and-monitor approach.
Maintain accurate pit/light base records to speed up earth fault location on recurring-trip circuits.
Review case sealing and connector integrity on circuits with a history of earth leakage.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing), SOP-CCR-10 (Earth Fault Location Procedure)

Standards: ICAO Annex 14 Vol I, IS 7785, IEC 61820-3-2 (supersedes IEC 61822)

Diagram: Series Circuit Single Line Diagram

Case Study: Division 25 — Field Case Study Troubleshooting (cross-reference where applicable)

Rev 0 — Initial issue

IT-134 Water Ingress

Symptom — Physical evidence of water inside the transformer enclosure or the surrounding light base/pit is found during inspection — standing water visible on opening, or water draining from the unit when disconnected — typically discovered while investigating a dark fixture, an earth leakage trip, or during routine pit maintenance.

Equipment / System

Isolating transformer case/enclosure, its sealing/gasket system, and the light base/pit drainage condition.

Possible Causes

HighFailed or degraded case seal/gasket allowing water entry over time

HighLight base/pit drainage blocked or non-functional, allowing water to pool and submerge the transformer beyond its rated conditions

MediumCracked or physically damaged transformer case, from mechanical impact or freeze-thaw cycling

MediumConnector boot/seal at the L-823 connector failed, allowing water to track in along the cable

LowerUnit installed below its rated submersion depth/duration for the specific site's flooding conditions

Required Tools

Inspection torchInsulation resistance tester (5kV)Isolating transformer bench test setPit dewatering equipment (pump/absorbent) as neededReplacement seals/gaskets per manufacturer spec

Safety Precautions

Treat any transformer recovered from standing water as a shock hazard until confirmed isolated and tested — do not handle live.
Follow confined-space and electrical safety precautions when working in a flooded or partially flooded light base/pit.
Do not re-energize a circuit with a water-affected transformer still connected until it has been tested, dried where applicable, or replaced.

Diagnostic Procedure

ObservationConfirm the extent of water ingress — standing water inside the transformer case itself versus water only in the surrounding pit with the transformer case intact.
MeasurementWith the CCR isolated, disconnect the affected transformer and perform an insulation resistance test (see IT-130) to assess whether the ingress has already compromised the winding insulation.
ObservationInspect the case seal/gasket and connector boots for the specific point of entry.
ObservationAssess the pit/light base drainage condition to determine whether this is an isolated case failure or a site drainage issue likely to affect other transformers on the same circuit.
DecisionInsulation resistance confirmed low or breakdown found → reject and replace the transformer (see IT-130/IT-131), do not attempt to dry and reuse a unit with confirmed internal water ingress and failed insulation. Insulation resistance still acceptable and the ingress is recent/limited → the unit may be dried and retested per manufacturer guidance if the transformer type permits field drying, otherwise replace as a precaution. Drainage issue identified → address the pit/light base drainage independent of the transformer itself.
Fault LocationIsolate to a failed case seal, a failed connector boot, or a systemic pit drainage problem.
Root CauseConfirm whether this is an isolated unit failure or a drainage issue likely to recur or affect adjacent fixtures.

Test Points

Winding-to-earth/case insulation resistance after water exposure

Case seal/gasket condition

Connector boot/seal condition

Pit/light base drainage condition

Root Cause Verification

Confirm the replacement or dried-and-retested transformer passes insulation resistance and, where required by site procedure, HV withstand testing before re-installation, and confirm the pit drains correctly on a follow-up inspection.

Corrective Action

Replace the transformer if insulation resistance is confirmed degraded by water ingress.
Dry and retest per manufacturer guidance only where the transformer type explicitly permits field drying and insulation resistance remains acceptable.
Replace failed case seals, gaskets, or connector boots.
Clear or restore pit/light base drainage.

Preventive Action

Include case seal and connector boot condition in routine pit inspections.
Address drainage issues at the light base/pit level proactively, not only after a transformer failure.
Track water-ingress-related failures by circuit/pit to identify locations needing a drainage upgrade.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing), SOP-AGL-07 (Light Base & Pit Maintenance)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Light Base/Pit Installation Drawing

Case Study: Division 25 — Field Case Study Troubleshooting (cross-reference where applicable)

Rev 0 — Initial issue

IT-135 Moisture Inside Transformer

Symptom — No standing water is visibly present, but a transformer shows a gradually declining insulation resistance trend, condensation observed on internal surfaces during inspection, or an unexplained intermittent earth leakage — consistent with internal humidity/condensation rather than direct water ingress (IT-134).

Equipment / System

Isolating transformer case and potting compound integrity — specifically its ability to prevent humid air exchange (breathing) rather than only bulk water entry.

Possible Causes

HighA small pinhole or micro-crack in the case/potting allows humid air to be drawn in during thermal cycling (the unit 'breathes' as it heats and cools), with moisture condensing internally without any bulk water ever entering

MediumA degraded but not fully failed case seal allowing slow humidity ingress over time, a precursor stage to full water ingress (IT-134)

MediumHigh ambient humidity at the site combined with significant day-night thermal cycling of the light base/pit

LowerMoisture trapped internally from the original manufacturing/potting process, not fully cured or sealed

Required Tools

Insulation resistance tester (5kV, with trending capability if available)Isolating transformer bench test setInspection torchManufacturer field-drying guidance (where applicable)

Safety Precautions

Apply LOTO on the CCR before removing the transformer for inspection or testing.
Treat a unit with a declining insulation trend as a developing hazard — do not wait for a full breakdown before acting.

Diagnostic Procedure

ObservationReview the insulation resistance test history for this transformer, if available, to confirm a genuine declining trend rather than a single low reading.
MeasurementPerform a current insulation resistance test and compare against the trend.
ObservationInspect the case exterior for any visible pinhole, crack, or seal degradation, even in the absence of standing water.
ObservationConsider the site's ambient humidity and thermal cycling conditions as a contributing environmental factor, particularly for circuits with a pattern of similar findings across multiple units.
DecisionDeclining trend confirmed with no other insulation defect found → treat as an early-stage moisture issue, reject the unit before it progresses to breakdown. Isolated low reading with no trend history → treat per IT-130 general low insulation resistance procedure. Pattern found across multiple units on the same circuit → investigate a systemic environmental or batch-related cause.
Fault LocationIsolate to this specific transformer's case integrity, or to a broader environmental/batch pattern across the circuit.
Root CauseConfirm whether the cause is unit-specific or systemic, to guide the scope of corrective action.

Test Points

Winding-to-earth/case insulation resistance (trended over time)

Case exterior condition for pinholes/cracks

Pattern across other transformers on the same circuit

Root Cause Verification

Confirm the replacement unit shows a stable insulation resistance reading with no declining trend over subsequent periodic tests.

Corrective Action

Replace a transformer showing a confirmed declining insulation trend consistent with internal moisture, rather than waiting for full breakdown.
Investigate and address a systemic environmental or batch-related pattern if multiple units on the same circuit are affected.
Escalate a suspected manufacturing/potting defect to the supplier if a batch pattern is identified.

Preventive Action

Maintain and review insulation resistance trend data per transformer, not just pass/fail results, to catch a slow decline early.
Consider proactive sampling of transformers on circuits in high-humidity or high-thermal-cycling locations.
Feed confirmed batch-related findings back into future procurement/acceptance criteria.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Bench Test Procedure Reference Sheet

Case Study: —

Rev 0 — Initial issue

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03 · Thermal & Mechanical

11 of 11 topics published

IT-136 Overheating

Symptom — During field inspection or thermal survey, the isolating transformer case or the pit/duct surrounding it runs noticeably hotter than adjacent, identically loaded units on the same circuit, with no CCR alarm raised, since a series circuit's constant current does not by itself flag a thermal problem.

Equipment / System

Isolating transformer case, potting compound, and surrounding pit/duct enclosure.

Possible Causes

HighInternal winding or core fault (partial short, degraded insulation) generating excess internal losses without yet tripping any electrical alarm

HighTransformer undersized or mismatched VA rating for the connected fixture, running continuously above its thermal design point

MediumPoor heat dissipation due to a flooded, silted, or poorly ventilated pit restricting normal case cooling

MediumAmbient conditions (high pavement or duct temperature, direct sun on an elevated fixture housing) compounding an otherwise normal loss level

LowerThermal imaging technique or emissivity setting error producing a falsely elevated reading

Required Tools

Infrared thermal cameraTrue-RMS clamp meterIsolating transformer bench test setTransformer schedule (VA rating by fixture type)

Safety Precautions

Apply LOTO on the CCR before opening a pit or duct to physically inspect a transformer flagged as hot.
Do not touch a suspected overheated case directly — confirm temperature with a non-contact thermal instrument first.

Diagnostic Procedure

ObservationThermally survey the transformer against adjacent, similarly loaded units on the same circuit to confirm a genuine relative temperature rise rather than a normal ambient effect.
ObservationCross-check the pit or duct for water, silt, or debris restricting normal heat dissipation.
MeasurementWith LOTO applied, remove and bench-test the transformer for winding resistance, insulation resistance, and no-load current against nameplate values.
ObservationVerify the installed transformer's VA rating against the design transformer schedule for this fixture type.
DecisionBench test reveals a genuine winding/core fault → replace the transformer. Bench test normal but unit is undersized for the fixture → correct with the design-specified rating. Bench test normal and rating correct → treat as an environmental/pit drainage issue.
Fault LocationIsolate to an internal transformer fault, an incorrect VA rating, or restricted pit heat dissipation.
Root CauseConfirm the specific cause through bench testing, rating verification, and pit condition inspection.

Test Points

Relative case temperature versus adjacent units (thermal image)

Winding resistance and insulation resistance (bench)

No-load current (bench)

Pit/duct drainage and debris condition

Root Cause Verification

After correction, re-survey the transformer under normal load and confirm its temperature has returned in line with adjacent units on the same circuit.

Corrective Action

Replace a transformer confirmed to have an internal winding or core fault causing excess heating.
Install the design-specified VA rating if an undersized unit is found.
Clear and restore drainage on a pit found to be restricting heat dissipation.

Preventive Action

Include a periodic thermal survey of transformer pits in the preventive maintenance programme.
Verify transformer VA rating against the design schedule at every replacement.
Maintain pit drainage as part of routine circuit inspection.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Bench Test Procedure Reference Sheet

Case Study: —

Rev 0 — Initial issue

IT-137 Burnt Winding

Symptom — Upon removing a failed isolating transformer for bench testing, visible scorching, discolouration, or a burnt odour is found on the winding insulation or potting compound, confirming the unit has failed thermally rather than simply by a clean open or short.

Equipment / System

Isolating transformer primary/secondary windings and potting compound.

Possible Causes

HighProlonged internal short or inter-turn fault generating localised heating until the winding insulation charred

HighSustained overload beyond the unit's rated VA, driving continuous excess I²R heating in the winding

MediumPrior insulation breakdown or moisture ingress (see IT-130/IT-134) that progressed undetected to a full thermal failure

MediumManufacturing defect in winding insulation or potting reducing thermal margin from new

LowerExternal heat source (adjacent equipment, fire, or pit event) rather than an internal electrical cause

Required Tools

Isolating transformer bench test setInsulation resistance tester (5kV)Visual/borescope inspection kitTrue-RMS clamp meter

Safety Precautions

Apply LOTO on the CCR before removing a failed transformer.
Handle a burnt unit as a potential contaminant — burnt potting compound and insulation can release fumes; avoid prolonged unprotected handling and dispose of per site environmental procedure.
Do not attempt to re-energise a unit showing visible burn damage, even briefly, for diagnostic purposes.

Diagnostic Procedure

ObservationVisually document the location and extent of scorching or discolouration on the winding and potting compound before further handling.
MeasurementBench-test winding resistance and insulation resistance to confirm the extent of internal damage, without re-energising the unit at full voltage.
ObservationReview the transformer's service and test history, if available, for a prior insulation or short-circuit warning that preceded this failure.
ObservationCheck the fixture and circuit for evidence of a sustained overload condition (incorrect fixture wattage, wrong transformer rating) that could explain continuous excess heating.
DecisionBurn damage confirmed with prior insulation/short history → treat as a progressed, previously-undetected fault; reject and replace. No prior warning found and rating/load confirmed correct → treat as an isolated winding or manufacturing defect. Overload condition confirmed on the circuit → correct the underlying loading issue in addition to replacing the unit.
Fault LocationIsolate to a progressed internal fault, a sustained overload condition, or an isolated manufacturing defect.
Root CauseConfirm the originating cause through bench testing and a review of service history and circuit loading.

Test Points

Winding resistance (bench)

Insulation resistance (bench)

Visual extent of burn/scorch damage

Fixture wattage and circuit loading versus transformer rating

Root Cause Verification

Confirm the replacement transformer's winding and insulation resistance are within acceptance criteria at commissioning, and re-check circuit loading if an overload was identified as a contributing cause.

Corrective Action

Replace any transformer showing confirmed burn or scorch damage — do not attempt to return it to service.
Correct fixture wattage or transformer rating if a sustained overload is identified as the originating cause.
Dispose of the burnt unit per site environmental/hazardous-material procedure.

Preventive Action

Track units with a prior insulation-resistance or short-circuit flag for early replacement rather than waiting for thermal failure.
Verify fixture wattage against transformer VA rating at every fixture or transformer change.
Include burn/scorch inspection as a standard step when any failed transformer is removed.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Bench Test Procedure Reference Sheet

Case Study: —

Rev 0 — Initial issue

IT-138 Winding Short Circuit

Symptom — Bench testing of a removed transformer shows an abnormally low winding resistance compared with the nameplate/reference value, or the unit draws grossly excessive current at rated bench voltage, indicating turns within a winding are shorted together rather than the winding being simply degraded.

Equipment / System

Isolating transformer primary and/or secondary winding.

Possible Causes

HighInsulation breakdown between adjacent turns allowing current to bypass part of the winding (see also IT-139 Inter-Turn Fault for the partial case)

HighMoisture ingress or contamination within the winding creating a conductive path between turns

MediumMechanical damage to winding insulation during a prior repair, handling, or transport event

MediumThermal degradation from a prior sustained overload or overheating event compromising turn-to-turn insulation

LowerBench test lead or connection fault producing a falsely low resistance reading rather than a genuine winding short

Required Tools

Isolating transformer bench test setMicro-ohmmeter or low-resistance winding testerInsulation resistance tester (5kV)Manufacturer reference winding resistance values

Safety Precautions

Apply LOTO on the CCR before removing the transformer for bench testing.
Do not apply full rated excitation voltage to a winding already suspected of being shorted — use a low-voltage resistance test first to avoid further damage or a test-set fault.

Diagnostic Procedure

ObservationVisually inspect accessible winding areas and potting compound for discolouration, moisture, or physical damage.
MeasurementMeasure winding resistance at low test voltage and compare against the manufacturer's reference value for this rating; a resistance significantly below reference indicates shorted turns.
MeasurementPerform an insulation resistance test to check for a moisture or contamination path contributing to the short.
ObservationVerify the bench test leads and connections are sound before concluding the winding itself is at fault.
DecisionWinding resistance confirmed low against reference with sound test connections → genuine winding short, reject the transformer. Test connections found faulty → correct and repeat before concluding a fault. Insulation resistance also low → treat as a combined short/insulation fault, cross-reference IT-130.
Fault LocationIsolate to the primary winding, the secondary winding, or a test setup error.
Root CauseConfirm the specific short location and contributing cause (moisture, mechanical, thermal) through bench testing.

Test Points

Winding resistance vs. manufacturer reference (bench)

Insulation resistance (bench)

Bench test lead/connection integrity

Visual condition of winding and potting

Root Cause Verification

Confirm the replacement transformer's winding resistance matches the manufacturer's reference value within tolerance before returning it to service.

Corrective Action

Reject and replace a transformer confirmed to have a shorted winding — a shorted winding is not field-repairable.
Address any moisture or contamination source identified (see IT-134/IT-135) if it contributed to the failure.
Correct bench test procedure/connections if a test error was found.

Preventive Action

Maintain a manufacturer reference winding resistance table for every transformer rating used on site.
Include low-voltage winding resistance testing as a standard bench acceptance step, not just an insulation check.
Investigate and correct any moisture source affecting transformers in the same pit or circuit.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Bench Test Procedure Reference Sheet

Case Study: —

Rev 0 — Initial issue

IT-139 Inter-Turn Fault

Symptom — The transformer passes a basic winding continuity check but performs abnormally under load or on a turns-ratio test — for example, a slightly low secondary voltage, an elevated no-load current, or mild but persistent heating — consistent with a small number of turns shorted together rather than a full winding short.

Equipment / System

Isolating transformer winding, evaluated specifically under turns-ratio and no-load current testing rather than simple continuity.

Possible Causes

HighA small, localised insulation breakdown between adjacent turns, shorting out a portion of the winding without fully open- or short-circuiting it

MediumEarly-stage moisture ingress affecting only a section of the winding rather than the whole coil

MediumLocalised mechanical or thermal damage from a prior event, insufficient to cause a full winding short but enough to compromise a few turns

LowerTurns-ratio test performed at the wrong excitation voltage or with an incorrect reference ratio, giving a misleadingly abnormal result

Required Tools

Isolating transformer bench test set with turns-ratio functionTrue-RMS clamp meterInsulation resistance tester (5kV)Manufacturer nameplate turns ratio and no-load current criteria

Safety Precautions

Apply LOTO on the CCR before removing the transformer for bench testing.
Treat a suspected inter-turn fault as a developing failure — do not return the unit to service pending confirmation, even though basic continuity passed.

Diagnostic Procedure

ObservationConfirm basic winding continuity passes, which by itself does not rule out an inter-turn fault.
MeasurementPerform a turns-ratio test at the correct rated excitation voltage and compare the actual ratio against the nameplate value.
MeasurementPerform a no-load current test and compare against the manufacturer's acceptance range for this rating.
MeasurementPerform an insulation resistance test to check for a moisture contribution to the fault.
DecisionTurns ratio deviates from nameplate and/or no-load current is elevated at correct test voltage → genuine inter-turn fault, reject the transformer. Test voltage or reference ratio found incorrect → repeat the test correctly before concluding a fault. Insulation resistance also low → treat moisture as a contributing factor.
Fault LocationIsolate to a localised inter-turn fault within the winding, or to a test procedure error.
Root CauseConfirm the specific cause through correctly-conducted turns-ratio and no-load current testing.

Test Points

Turns ratio vs. nameplate (bench)

No-load current vs. acceptance range (bench)

Insulation resistance (bench)

Basic winding continuity (for reference only, not conclusive)

Root Cause Verification

Confirm the replacement transformer's turns ratio and no-load current both fall within the manufacturer's acceptance range before returning it to service.

Corrective Action

Reject and replace a transformer confirmed to have an inter-turn fault — continuity passing alone is not sufficient acceptance criteria.
Address a moisture contribution if identified, per IT-134/IT-135.
Correct the bench test procedure if an incorrect voltage or reference ratio caused a false result.

Preventive Action

Include turns-ratio and no-load current testing as standard acceptance steps, not continuity testing alone.
Maintain accurate, rating-specific nameplate ratio and no-load current reference data for every transformer type on site.
Treat any transformer with a marginal or borderline ratio/no-load reading as a candidate for closer trending rather than an automatic pass.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Bench Test Procedure Reference Sheet

Case Study: —

Rev 0 — Initial issue

IT-140 Core Damage

Symptom — Bench testing shows an elevated no-load (magnetizing) current and/or reduced secondary output that cannot be explained by winding resistance or insulation results alone, and physical inspection of the accessible core (where the design permits) reveals lamination damage, corrosion, or displacement.

Equipment / System

Isolating transformer magnetic core and lamination stack.

Possible Causes

HighLamination damage or corrosion increasing core losses and magnetizing current, reducing effective coupling to the secondary

MediumCore displacement or looseness within the winding assembly, altering the magnetic circuit from its designed geometry

MediumMoisture ingress causing corrosion between laminations, degrading their intended insulation from one another and increasing eddy-current losses

LowerA prior severe electrical fault (e.g., an internal short) that damaged the core as a secondary consequence rather than the core being the originating fault

Required Tools

Isolating transformer bench test setTrue-RMS clamp meterVisual/borescope inspection kitManufacturer no-load current acceptance criteria

Safety Precautions

Apply LOTO on the CCR before removing the transformer for inspection or testing.
Do not attempt to disassemble a sealed/potted transformer in the field to access the core — send to a qualified repair facility or replace, per manufacturer guidance.

Diagnostic Procedure

ObservationConfirm winding resistance and insulation resistance are within acceptable range, to rule out those as the explanation for reduced performance.
MeasurementPerform a no-load current test and compare against the manufacturer's acceptance range; an elevated reading with healthy windings points toward the core.
ObservationWhere the transformer design and access permit, visually or borescopically inspect the core for lamination damage, corrosion, or displacement.
ObservationReview whether the unit has a history of moisture ingress or a prior severe electrical fault that could explain core damage as a consequence.
DecisionElevated no-load current confirmed with healthy windings and visible/suspected core damage → core fault, reject the transformer. Core found displaced but otherwise sound and accessible for repair → refer to manufacturer for repair feasibility. No core damage found and no-load current is within range → re-evaluate other causes (see IT-124, IT-127, IT-128).
Fault LocationIsolate to the core (lamination damage, corrosion, or displacement) as distinct from a winding fault.
Root CauseConfirm whether core damage originated independently (moisture, mechanical) or as a consequence of a separate electrical fault.

Test Points

No-load (magnetizing) current vs. acceptance range (bench)

Winding resistance and insulation resistance (bench, for elimination)

Visual/borescope core condition where accessible

Service history for prior moisture or severe fault events

Root Cause Verification

Confirm the replacement transformer's no-load current falls within the manufacturer's acceptance range and remains stable on a follow-up bench check.

Corrective Action

Reject and replace a transformer with confirmed core damage — cores in sealed/potted units are not field-repairable.
Send a unit with accessible, repairable core displacement to the manufacturer or a qualified repair facility for assessment.
Address the originating moisture or electrical fault if core damage is found to be a consequence rather than the primary cause.

Preventive Action

Include no-load current trending as part of periodic sample bench testing to catch developing core issues early.
Investigate any transformer with a prior severe fault for possible core involvement before returning it to service.
Protect stored and installed transformers from moisture ingress as a core-protection measure, not just a winding-protection measure.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Bench Test Procedure Reference Sheet

Case Study: —

Rev 0 — Initial issue

IT-141 Core Heating

Symptom — Thermal survey or bench testing shows the transformer core running hotter than expected for its no-load current level — heating that tracks with the applied voltage/frequency rather than with load current — distinct from winding-driven heating that tracks with load.

Equipment / System

Isolating transformer magnetic core and lamination stack.

Possible Causes

HighElevated eddy-current or hysteresis losses from lamination insulation breakdown or corrosion between laminations

MediumCore operating closer to saturation than designed, due to an incorrect supply voltage/frequency or a design/rating mismatch

MediumCore displacement increasing localised flux concentration and associated losses

LowerHeating misattributed to the core when it is actually winding-driven — requires cross-check against load current to distinguish

Required Tools

Infrared thermal cameraIsolating transformer bench test setTrue-RMS clamp meterManufacturer core loss/no-load current reference data

Safety Precautions

Apply LOTO on the CCR before removing the transformer for bench testing.
Allow a hot core adequate time to cool before handling, and confirm temperature with a non-contact instrument first.

Diagnostic Procedure

ObservationThermally survey the unit at no load and at rated load to determine whether heating tracks with applied voltage (core-related) or with load current (winding-related).
MeasurementBench-test no-load current and compare against the manufacturer's reference; elevated no-load current with core heating points to lamination or core losses.
ObservationConfirm the primary supply voltage and frequency to the transformer match its rated values, ruling out an over-excitation condition as the cause.
ObservationWhere accessible, inspect the core for displacement or visible lamination degradation.
DecisionHeating confirmed to track with no-load/voltage rather than load current, with elevated no-load current → genuine core loss issue, reject the transformer. Supply voltage/frequency found incorrect → correct the supply condition and re-test before concluding a transformer fault. Heating found to track with load current instead → treat as winding-related, cross-reference IT-136/IT-137.
Fault LocationIsolate to core losses, an incorrect supply condition, or a winding-driven heating source.
Root CauseConfirm the specific cause through voltage/frequency verification and no-load versus loaded thermal comparison.

Test Points

No-load current vs. manufacturer reference (bench)

Case/core temperature at no load vs. at rated load (thermal)

Supply voltage and frequency to the transformer

Visual core condition where accessible

Root Cause Verification

Confirm the replacement transformer's no-load current and no-load thermal behaviour are within the manufacturer's normal range at commissioning.

Corrective Action

Reject and replace a transformer confirmed to have excessive core losses.
Correct the supply voltage or frequency if found outside the transformer's rated range.
Refer a unit with accessible, repairable core displacement to the manufacturer for assessment.

Preventive Action

Verify CCR output voltage/frequency is within specification as part of routine circuit checks.
Include a no-load thermal check in periodic sample testing to catch developing core loss issues.
Track units with elevated no-load current for early replacement before core heating becomes severe.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Bench Test Procedure Reference Sheet

Case Study: —

Rev 0 — Initial issue

IT-142 Abnormal Transformer Noise

Symptom — An audible hum, buzz, crackle, or intermittent noise is reported from a transformer pit or duct during energised operation, beyond the normal low-level hum expected of a healthy unit at its rated frequency.

Equipment / System

Isolating transformer core and winding assembly, and its mounting/enclosure within the pit or duct.

Possible Causes

HighCore lamination looseness or displacement causing magnetostriction-driven vibration and audible hum/buzz

MediumAn internal partial discharge or intermittent arcing condition associated with developing insulation breakdown, producing a crackling or hissing noise

MediumLoose mounting hardware or a loose connector within the pit resonating with normal operating vibration

LowerNoise actually originating from an adjacent component (CCR, cable, or another fixture) and only perceived as coming from this transformer's pit

Required Tools

True-RMS clamp meterInsulation resistance tester (5kV)Isolating transformer bench test setBasic hand tools for mounting hardware inspection

Safety Precautions

Apply LOTO before opening a pit to investigate a noise source at close range.
Treat a crackling or hissing noise as a possible sign of internal arcing — prioritise isolation and testing over prolonged close-range listening while energised.

Diagnostic Procedure

ObservationWith the circuit re-energised under controlled conditions (or using a listening/vibration tool), confirm the noise source is this specific transformer's pit rather than an adjacent one.
ObservationCharacterise the noise type — steady hum/buzz suggests a mechanical/core cause, while crackling or hissing suggests a possible electrical discharge.
MeasurementWith LOTO applied, inspect and tighten mounting hardware and connectors within the pit.
MeasurementBench-test the transformer's insulation resistance and no-load current if an electrical cause is suspected.
DecisionNoise traced to loose hardware/connectors, resolved on tightening → mechanical cause, closed. Noise persists with hardware sound and bench tests show degraded insulation or abnormal no-load current → treat as a developing internal fault, replace the transformer. Noise traced to an adjacent component → redirect investigation accordingly.
Fault LocationIsolate to core-driven mechanical noise, an internal electrical discharge condition, loose hardware, or a misattributed external source.
Root CauseConfirm the specific source through hardware inspection, noise characterisation, and bench testing where an electrical cause is suspected.

Test Points

Mounting hardware and connector tightness

Insulation resistance (bench, if electrical cause suspected)

No-load current (bench, if electrical cause suspected)

Noise character and pit-of-origin confirmation

Root Cause Verification

After correction, confirm the abnormal noise no longer recurs on a follow-up site visit under normal energised operation.

Corrective Action

Tighten or replace loose mounting hardware or connectors found to be the source.
Replace a transformer confirmed to have a developing internal fault associated with the noise.
Correctly identify and redirect investigation if the noise traced to an adjacent component instead.

Preventive Action

Include a listening check for abnormal noise as part of routine pit inspections.
Torque-check mounting hardware and connectors at scheduled maintenance intervals.
Treat any crackling or hissing noise as a priority investigation item rather than a routine hum complaint.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Isolating Transformer Wiring Diagram

Case Study: —

Rev 0 — Initial issue

IT-143 Transformer Vibration

Symptom — Physical vibration is felt or observed at the transformer case or its mounting during energised operation, beyond the minor magnetostriction-related vibration normal to a healthy unit, and may be accompanied by abnormal noise (see IT-142).

Equipment / System

Isolating transformer case, core assembly, and its mounting arrangement within the pit or duct.

Possible Causes

HighCore lamination looseness allowing greater-than-normal magnetostriction movement within the case

MediumLoose or degraded mounting hardware/brackets failing to adequately restrain the unit within the pit

MediumExternal source — pavement or structural vibration from nearby operations transmitted into the pit and case

LowerPerceived vibration actually originating from an adjacent piece of equipment or ducting, not this transformer

Required Tools

Basic hand tools for mounting hardware inspectionTrue-RMS clamp meterIsolating transformer bench test setVibration/contact-level assessment (by feel or simple instrument)

Safety Precautions

Apply LOTO before opening a pit to physically inspect a vibrating unit at close range.
Confirm de-energised status before handling a transformer showing vibration, since vibration alone does not indicate the unit is safe to touch while live.

Diagnostic Procedure

ObservationConfirm the vibration source is this specific transformer and mounting, not an adjacent component or the surrounding duct/pavement.
ObservationWith LOTO applied, inspect and tighten mounting hardware and brackets securing the transformer within the pit.
MeasurementBench-test the transformer's no-load current if core-related vibration is suspected, since elevated no-load current can accompany lamination looseness.
ObservationCheck for an external vibration source (nearby paving, ground, or airside operations) that may be transmitted into the pit independent of the transformer itself.
DecisionVibration resolved on tightening mounting hardware → mechanical mounting cause, closed. Mounting sound but vibration persists with elevated no-load current → treat as core lamination looseness, replace the transformer. External source identified → note as environmental, no transformer action required.
Fault LocationIsolate to loose mounting, internal core looseness, or an external transmitted source.
Root CauseConfirm the specific cause through mounting inspection and, where indicated, bench testing.

Test Points

Mounting hardware and bracket security

No-load current (bench, if core cause suspected)

Vibration source confirmation (this unit vs. adjacent/external)

Root Cause Verification

After correction, confirm vibration is no longer present or has returned to a normal minor level on a follow-up site visit under normal energised operation.

Corrective Action

Tighten or replace mounting hardware/brackets found to be loose or degraded.
Replace a transformer confirmed to have internal core-related vibration.
Document an external vibration source and monitor rather than replacing the transformer if it is confirmed to be the cause.

Preventive Action

Torque-check transformer mounting hardware at scheduled maintenance intervals.
Include a vibration check alongside the noise check (IT-142) during routine pit inspections.
Track units with elevated no-load current for early replacement before vibration becomes severe.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Isolating Transformer Wiring Diagram

Case Study: —

Rev 0 — Initial issue

IT-144 Incorrect Wattage

Symptom — The transformer installed at a fixture is found to have a VA/wattage rating that does not match the design specification for that fixture type — either under- or over-rated — discovered during inspection, a rating audit, or while investigating a related symptom such as low secondary voltage or overheating.

Equipment / System

Isolating transformer nameplate rating versus the design-specified rating for the fixture type at this location.

Possible Causes

HighAn incorrect-rating transformer was installed at a previous replacement without checking the design transformer schedule

MediumThe fixture itself was changed (e.g., lamp to LED conversion) without updating the transformer rating to match the new fixture's requirements

MediumStock/spares of multiple ratings are not clearly segregated, leading to an inadvertent mix-up at installation

LowerThe design transformer schedule itself is out of date or contains an error for this fixture type

Required Tools

Transformer schedule (VA rating by fixture type)Isolating transformer bench test setFixture wiring diagramNameplate/rating label reference

Safety Precautions

Apply LOTO on the CCR before removing a transformer to check or correct its rating.

Diagnostic Procedure

ObservationRecord the installed transformer's nameplate VA/wattage rating and compare it against the current design transformer schedule for this fixture type.
ObservationConfirm the fixture itself (lamp/LED driver) matches its own design specification, in case a fixture change is the actual source of the mismatch.
MeasurementWhere a related symptom (low/high secondary voltage, overheating) prompted the investigation, bench-test the transformer to confirm it is otherwise healthy at its actual installed rating.
DecisionNameplate rating confirmed to differ from the design schedule and the transformer is otherwise healthy → replace with the correct design-specified rating. Fixture found to have been changed without a matching transformer update → correct both to a consistent, design-verified pairing. Design schedule itself found to be in error → correct the schedule and verify other installations of this fixture type.
Fault LocationIsolate to an incorrect unit installed at replacement, a fixture change not matched by a rating update, or a schedule error.
Root CauseConfirm the specific cause through nameplate, fixture, and schedule cross-checking.

Test Points

Installed transformer nameplate VA/wattage rating

Design transformer schedule rating for this fixture type

Fixture (lamp/LED driver) rating

Root Cause Verification

After correction, confirm the newly installed transformer's nameplate rating matches the design schedule, and confirm normal fixture brightness/current in the field.

Corrective Action

Install the design-specified transformer rating where a mismatch is confirmed.
Correct the design transformer schedule if it is found to be in error.
Segregate spares stock by rating clearly to prevent future mix-ups.

Preventive Action

Verify transformer part number/rating against the design schedule at every replacement, not just at commissioning.
Update the transformer schedule whenever a fixture type is changed (e.g., lamp to LED conversion).
Label spares stock clearly by rating and store separately.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Transformer Schedule

Case Study: —

Rev 0 — Initial issue

IT-145 Transformer Overloaded

Symptom — The transformer is confirmed to be operating with a secondary load current exceeding its rated VA output on a sustained basis — distinguished from a single high-reading test by a consistent pattern across multiple checks or by physical evidence of sustained thermal stress.

Equipment / System

Isolating transformer secondary circuit and its connected fixture load, evaluated for sustained loading against nameplate VA rating.

Possible Causes

HighFixture wattage exceeds the transformer's rated VA output, either from an incorrect fixture installation or an undersized transformer (see IT-144)

MediumMultiple sustained high-current excursions from CCR step changes are compounding transformer thermal stress beyond what an occasional peak would cause

MediumA partial fault within the fixture (e.g., a degrading LED driver) drawing more current than its rated design value

LowerA single elevated reading mistaken for sustained overload rather than a normal transient

Required Tools

True-RMS clamp meterIsolating transformer bench test setTransformer schedule (VA rating by fixture type)Fixture wiring diagram

Safety Precautions

Apply LOTO on the CCR before removing the transformer or fixture for testing.
Treat a confirmed sustained overload as an active thermal risk — do not defer correction, since continued operation can progress toward IT-136/IT-137 (overheating/burnt winding).

Diagnostic Procedure

ObservationReview current readings for this fixture across multiple site visits or CCR logs, if available, to confirm a sustained pattern rather than a single transient reading.
MeasurementMeasure secondary current under normal operating conditions and compare against the transformer's rated VA output.
ObservationVerify the fixture's actual wattage/rating against the design specification for this transformer.
MeasurementIf the fixture rating is correct, bench-test the fixture (lamp/LED driver) in isolation to check for an internal fault drawing excess current.
DecisionSustained overload confirmed with a mismatched fixture/transformer pairing → correct with the design-specified transformer rating (see IT-144). Fixture rating confirmed correct but drawing excess current → treat as a fixture-side fault, redirect to Division 01 as applicable. Overload was in fact a single transient, not sustained → no correction needed, continue routine monitoring.
Fault LocationIsolate to a mismatched transformer/fixture pairing, a fixture-side fault, or a misread transient.
Root CauseConfirm sustained overload and its specific cause through repeated measurement and rating verification.

Test Points

Secondary current under normal load vs. rated VA

Fixture wattage/rating vs. transformer rating

Trend of current readings across multiple visits

Root Cause Verification

After correction, confirm secondary current under normal operation is within the transformer's rated VA output on a follow-up check.

Corrective Action

Install the design-specified transformer VA rating if an undersized unit is confirmed.
Address a fixture-side fault causing excess current draw independently of the transformer.
Continue monitoring if the original reading is confirmed to have been a transient rather than sustained overload.

Preventive Action

Trend secondary current readings over multiple visits rather than relying on a single measurement to judge overload.
Verify transformer VA rating against fixture wattage at every fixture or transformer change.
Prioritise correction of any confirmed sustained overload to prevent progression to thermal failure.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Transformer Schedule

Case Study: —

Rev 0 — Initial issue

IT-146 Transformer Underloaded

Symptom — The transformer is confirmed to be operating with a secondary load current well below its rated VA output on a sustained basis — not itself a failure mode, but flagged during a rating audit or while investigating an unrelated symptom, since a significantly oversized transformer can mask a developing fixture-side fault or represent an avoidable inefficiency.

Equipment / System

Isolating transformer secondary circuit and its connected fixture load, evaluated for sustained loading against nameplate VA rating.

Possible Causes

HighA significantly oversized transformer was installed relative to the fixture's actual wattage, either from a previous replacement error or a design specification set higher than needed

MediumThe fixture was changed (e.g., halogen to LED conversion, which typically draws less power) without the transformer rating being reviewed and downsized accordingly

MediumA developing fixture-side fault (partial LED driver failure, dimming) is reducing actual current draw below the fixture's own rated value, masked by the oversized transformer margin

LowerA single low reading mistaken for sustained underload rather than a normal low-intensity CCR step

Required Tools

True-RMS clamp meterIsolating transformer bench test setTransformer schedule (VA rating by fixture type)Fixture wiring diagram

Safety Precautions

Apply LOTO on the CCR before removing the transformer or fixture for testing.

Diagnostic Procedure

ObservationReview current readings for this fixture across multiple site visits or CCR logs to confirm a sustained pattern rather than a single low-intensity-step reading.
MeasurementMeasure secondary current under normal rated-intensity operation and compare against both the fixture's design current and the transformer's rated VA output.
ObservationVerify the fixture's actual wattage/rating against the design specification and check whether the fixture type has changed since the transformer was last matched.
MeasurementIf the fixture rating and transformer both appear correctly matched on paper but current is still low, bench-test the fixture (lamp/LED driver) for a developing fault reducing its current draw.
DecisionOversized transformer confirmed relative to current fixture wattage → downsize to the design-specified rating at next replacement. Fixture found to draw less current than its own rated value → treat as a developing fixture-side fault, redirect to Division 01 as applicable. Reading was a single low-intensity-step transient, not sustained → no correction needed.
Fault LocationIsolate to a mismatched (oversized) transformer/fixture pairing, a developing fixture-side fault, or a misread transient.
Root CauseConfirm sustained underload and its specific cause through repeated measurement and rating verification.

Test Points

Secondary current under normal rated-intensity load vs. rated VA and fixture design current

Fixture wattage/rating vs. transformer rating

Trend of current readings across multiple visits

Root Cause Verification

After correction, confirm secondary current under normal operation is appropriately matched to a correctly sized transformer, and that any underlying fixture fault has been resolved.

Corrective Action

Downsize the transformer to the design-specified VA rating at next scheduled replacement, where a significant sustained mismatch is confirmed.
Investigate and correct a fixture-side fault if reduced current draw is traced to the fixture rather than the transformer pairing.
Update the design transformer schedule if a fixture type change was not previously reflected.

Preventive Action

Review and update the transformer schedule whenever a fixture type is changed to a lower-wattage design (e.g., LED conversion).
Trend secondary current readings to catch a developing fixture-side fault that a generously sized transformer might otherwise mask.
Avoid specifying transformers oversized relative to actual fixture wattage beyond the manufacturer's recommended margin.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Transformer Schedule

Case Study: —

Rev 0 — Initial issue

04 · Connections & Physical Integrity

9 of 9 topics published

IT-147 Wrong Polarity

Symptom — For a polarity-sensitive fixture or circuit arrangement (e.g., certain LED fixtures with a polarised secondary connection), the fixture fails to operate correctly, operates at reduced output, or a protective device trips, despite the transformer testing electrically healthy and normal loop current being confirmed at the CCR.

Equipment / System

Isolating transformer secondary output connection to the fixture, and the fixture's polarity-sensitive input.

Possible Causes

HighSecondary leads reversed at the transformer's connector or at the fixture's connector during a previous installation or repair

MediumA replacement transformer or fixture installed with a different lead colour/marking convention than the original, leading to an inadvertent reversal

MediumThe L-823 connector was reassembled with reversed pin connections after a repair

LowerThe fixture is not actually polarity-sensitive and the symptom has a different cause — verify against the fixture's own specification before concluding a polarity fault

Required Tools

Multimeter (DC polarity check where applicable)Fixture wiring diagram with polarity markingsL-823 connector test leadsIsolating transformer bench test set

Safety Precautions

Apply LOTO on the CCR before disconnecting and reconnecting any secondary connection.
Confirm the circuit is fully de-energised before handling connectors, even though the fault is polarity-related rather than an open/short.

Diagnostic Procedure

ObservationConfirm from the fixture's specification whether it is genuinely polarity-sensitive, to avoid pursuing a polarity fault on a fixture where it does not apply.
MeasurementWith the circuit de-energised, verify the secondary lead connections at the transformer and at the fixture connector against the wiring diagram's polarity markings.
ObservationInspect the L-823 connector for correct pin assembly if it was recently reassembled or replaced.
DecisionA reversed connection is found at the transformer, the connector, or the fixture → correct the connection to match the wiring diagram. No reversal found and the fixture is confirmed polarity-sensitive → re-investigate for a different cause (fixture fault, transformer output issue). Fixture confirmed not polarity-sensitive → close as not a polarity fault, pursue Division 01 fixture troubleshooting instead.
Fault LocationIsolate to a reversed connection at the transformer secondary, the L-823 connector, or the fixture input.
Root CauseConfirm the specific reversal point through wiring diagram verification.

Test Points

Secondary lead polarity at the transformer connector

L-823 connector pin assignment

Fixture input polarity marking

Root Cause Verification

After correction, confirm the fixture operates correctly at all commanded intensity steps and that any tripped protective device resets normally.

Corrective Action

Correct the reversed connection at whichever point (transformer, connector, or fixture) is confirmed.

Preventive Action

Standardise and clearly mark lead colour/polarity conventions across transformer and fixture replacement stock.
Verify polarity against the wiring diagram whenever a polarity-sensitive fixture's connector is reassembled after any repair.
Include a polarity check step in the commissioning procedure for polarity-sensitive fixture types.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Isolating Transformer Wiring Diagram

Case Study: —

Rev 0 — Initial issue

IT-148 Incorrect Primary Connection

Symptom — After installation or maintenance, the transformer's primary-side connection into the series loop is found to be incorrect — for example, connected in a way that bypasses the loop, uses the wrong terminal pairing, or was made with an incompatible connector — discovered through an abnormal CCR reading, a failed insulation test, or a physical inspection.

Equipment / System

Isolating transformer primary terminals and their connection into the series circuit loop cable.

Possible Causes

HighPrimary leads connected to the wrong terminal pairing on the transformer, inconsistent with the manufacturer's wiring diagram

HighLoop cable connected in a way that bypasses the transformer primary rather than routing through it, effectively removing the isolation function

MediumAn incompatible or incorrectly rated primary-side L-823 connector used at installation

MediumPrimary connection made with reversed loop-in/loop-out cable ends, affecting downstream circuit continuity or fault-finding on the loop

Required Tools

Fixture/transformer wiring diagramMultimeterL-823 connector test leadsIsolating transformer bench test set

Safety Precautions

Apply LOTO on the CCR before inspecting or correcting any primary connection.
Confirm the loop is fully de-energised and isolated before opening any primary-side connector, since the loop carries the full series circuit current when energised.

Diagnostic Procedure

ObservationCompare the as-found primary connection against the manufacturer's wiring diagram and the design loop routing for this circuit.
ObservationConfirm whether the loop cable is correctly routed through the transformer primary rather than bypassing it.
MeasurementVerify the primary-side L-823 connector type and rating match the design specification.
DecisionConnection confirmed to deviate from the wiring diagram (wrong terminals, bypass, wrong connector) → correct to match the diagram. Connection confirmed correct → re-investigate the original symptom for a different cause.
Fault LocationIsolate to the specific point of incorrect connection — terminal pairing, loop routing, or connector type.
Root CauseConfirm the connection error occurred during original installation or a subsequent maintenance/repair event, to guide corrective scope.

Test Points

Primary terminal connections vs. wiring diagram

Loop cable routing through the transformer primary

Primary-side L-823 connector type and rating

Root Cause Verification

After correction, confirm the transformer's primary connection matches the wiring diagram and that the loop shows normal current and no open-circuit indication at the CCR.

Corrective Action

Correct the primary connection to match the manufacturer's wiring diagram and design loop routing.
Replace an incorrect or incompatible primary-side connector with the correct type.

Preventive Action

Verify primary connection against the wiring diagram as a standard step whenever a transformer is installed or replaced.
Include loop routing continuity as part of post-installation commissioning checks.
Standardise primary-side connector types across the site to reduce the risk of an incompatible connector being used.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Isolating Transformer Wiring Diagram

Case Study: —

Rev 0 — Initial issue

IT-149 Incorrect Secondary Connection

Symptom — After installation or maintenance, the transformer's secondary-side connection to the fixture is found to be incorrect — for example, connected to the wrong secondary tap on a multi-tap unit, using an incompatible connector, or wired to the wrong fixture on a multi-fixture arrangement — discovered through abnormal fixture brightness, a wrong-fixture-lit condition, or physical inspection.

Equipment / System

Isolating transformer secondary terminals/taps and their connection to the fixture.

Possible Causes

HighSecondary leads connected to the wrong tap on a multi-tap (multi-wattage) transformer, delivering an incorrect voltage to the fixture

MediumSecondary connected to the wrong fixture where the transformer serves, or is adjacent to, more than one possible connection point

MediumAn incompatible or incorrectly rated secondary-side L-823 connector used at installation or repair

LowerSecondary connection is correct and the brightness symptom actually originates from the fixture or the transformer's own rating — verify before concluding a connection error

Required Tools

Fixture/transformer wiring diagramMultimeterL-823 connector test leadsIsolating transformer bench test set with turns-ratio function

Safety Precautions

Apply LOTO on the CCR before disconnecting and reconnecting any secondary connection.
Confirm the circuit is fully de-energised before handling secondary connectors.

Diagnostic Procedure

ObservationCompare the as-found secondary connection against the manufacturer's wiring diagram, noting the specific tap used if the transformer is a multi-tap unit.
MeasurementWhere a multi-tap unit is suspected of being connected to the wrong tap, perform a turns-ratio/output voltage bench test on each tap to confirm which is actually in use.
ObservationOn a multi-fixture arrangement, confirm the secondary is physically connected to the intended fixture, not an adjacent one.
DecisionWrong tap or wrong fixture connection confirmed → correct to match the wiring diagram and intended fixture. Connection confirmed correct → re-investigate the original brightness/wrong-fixture symptom for a different cause (see IT-124/IT-125).
Fault LocationIsolate to an incorrect tap selection, an incorrect fixture connection, or an incompatible connector.
Root CauseConfirm the connection error occurred during original installation or a subsequent maintenance/repair event.

Test Points

Secondary tap selection vs. wiring diagram (multi-tap units)

Secondary connector routing to the intended fixture

Secondary-side L-823 connector type and rating

Root Cause Verification

After correction, confirm the fixture operates at correct brightness and that any adjacent fixture previously mis-lit now shows the expected condition.

Corrective Action

Correct the secondary tap selection or fixture routing to match the wiring diagram.
Replace an incorrect or incompatible secondary-side connector with the correct type.

Preventive Action

Clearly label tap selection on multi-tap transformers at installation.
Verify secondary routing against the wiring diagram as a standard step on multi-fixture arrangements.
Include secondary connection verification as part of post-installation commissioning checks.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Isolating Transformer Wiring Diagram

Case Study: —

Rev 0 — Initial issue

IT-150 Terminal Damage

Symptom — Physical inspection of a transformer's primary or secondary terminals reveals damage — corrosion, pitting, a cracked or melted terminal block, or a stripped/broken terminal — that may or may not yet be causing an electrical symptom, discovered during routine inspection or while investigating a connection-related fault.

Equipment / System

Isolating transformer primary and secondary terminal blocks and their immediate connection points.

Possible Causes

HighCorrosion from moisture ingress at the terminal block, consistent with a broader moisture issue (see IT-134/IT-135)

MediumThermal damage (melting, discolouration) from a prior high-resistance connection or overload event at that terminal

MediumMechanical damage from a previous repair, over-tightening, or handling during a prior transformer or connector replacement

LowerCosmetic surface discolouration or minor wear not yet affecting electrical integrity

Required Tools

Visual/borescope inspection kitMultimeterInsulation resistance tester (5kV)Isolating transformer bench test set

Safety Precautions

Apply LOTO on the CCR before inspecting or handling any transformer terminal.
Treat a corroded or thermally damaged terminal as a potential high-resistance point capable of localised heating even at normal loop current — do not assume it is safe simply because the circuit currently passes current.

Diagnostic Procedure

ObservationVisually inspect the primary and secondary terminals for corrosion, pitting, thermal discolouration, or mechanical damage.
MeasurementWhere a terminal shows visible damage, measure the connection's resistance or insulation resistance at that point to assess whether it is electrically compromised.
ObservationDetermine whether the damage pattern is consistent with moisture ingress, a prior thermal event, or mechanical handling, to guide the corrective scope.
DecisionTerminal confirmed electrically compromised (elevated resistance, failed insulation test, or structurally unsound) → replace the transformer or repair the terminal per manufacturer allowance. Damage found to be cosmetic only, with sound electrical readings → note for monitoring, no immediate replacement required. Moisture ingress confirmed as the cause → address per IT-134/IT-135 in addition to terminal correction.
Fault LocationIsolate to the specific damaged terminal (primary or secondary) and its contributing cause.
Root CauseConfirm whether the damage originated from moisture, a prior thermal event, or mechanical handling.

Test Points

Visual terminal condition (primary and secondary)

Terminal connection resistance

Insulation resistance at the affected terminal

Root Cause Verification

After correction, confirm the repaired or replaced terminal shows sound connection resistance and normal insulation resistance on a follow-up check.

Corrective Action

Replace the transformer if a terminal is confirmed electrically compromised beyond manufacturer-allowed repair.
Repair or replace the terminal block per manufacturer allowance where the transformer itself is otherwise healthy.
Address any underlying moisture source contributing to terminal corrosion.

Preventive Action

Include terminal condition as a standard checklist item during routine pit inspections.
Avoid over-tightening terminals during installation or repair, per manufacturer torque specification.
Investigate and correct moisture sources affecting terminals across a pit or circuit, not just the single unit found.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Isolating Transformer Wiring Diagram

Case Study: —

Rev 0 — Initial issue

IT-151 Cable Lead Failure

Symptom — The flexible primary or secondary lead between the transformer's potted body and its L-823 connector is found damaged, chafed through, or broken, discovered during a continuity fault investigation, a physical inspection, or while handling the unit for another reason.

Equipment / System

Isolating transformer primary and secondary flexible leads, from the potting compound exit point to the L-823 connector.

Possible Causes

HighChafing or abrasion where the lead exits the potting compound, from repeated flexing or a sharp edge in the pit/duct

HighThe lead was pulled, kinked, or crushed during a previous installation, cable-pulling, or pit-works activity

MediumInsulation degradation on the lead from prolonged exposure to standing water or contaminants in the pit

MediumA poor factory or field splice at the potting interface progressively working loose or fracturing under thermal cycling

LowerRodent or wildlife damage to the exposed lead within the pit or duct

Required Tools

Visual/borescope inspection kitMultimeterIsolating transformer bench test setL-823 connector test leads

Safety Precautions

Apply LOTO on the CCR before handling or testing a transformer with a suspected lead fault.
Treat a lead with damaged insulation as a potential shock hazard even at loop current levels — do not handle exposed conductor with bare hands while any possibility of the circuit being energised remains.

Diagnostic Procedure

ObservationVisually inspect the full exposed length of the primary and secondary leads from the potting exit to the connector for chafing, cuts, kinks, or crushing.
MeasurementBench-test continuity and insulation resistance of the affected lead to confirm whether the damage has caused an open circuit or a compromised insulation path.
ObservationInspect the potting compound interface where the lead exits for signs of the fault originating at that joint rather than along the lead's length.
DecisionLead confirmed open, shorted, or with failed insulation → repair per manufacturer-permitted field repair, or replace the transformer if the fault is at the non-repairable potting interface. Lead intact but insulation marginal → note for monitoring and re-test at next scheduled visit.
Fault LocationIsolate the damage to a specific point along the lead — mid-length, at the connector, or at the potting interface.
Root CauseConfirm whether the cause was mechanical (chafing, pulling, crushing), environmental (water/contaminant exposure), or a workmanship defect at the potting joint.

Test Points

Lead continuity (bench)

Lead insulation resistance (bench)

Visual condition along the full exposed lead length

Potting compound exit point condition

Root Cause Verification

After repair or replacement, confirm the lead shows sound continuity and insulation resistance on the bench, then re-install and confirm normal fixture operation in the field.

Corrective Action

Repair the lead only where the manufacturer's procedure permits field repair; otherwise replace the transformer.
Route and protect the corrected lead away from sharp edges or pinch points within the pit.

Preventive Action

Route leads clear of sharp edges, connector bodies, and other hardware within the pit during installation.
Inspect exposed lead condition whenever a pit is opened for other works.
Avoid pulling or handling transformer leads directly when moving or positioning the unit.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Isolating Transformer Wiring Diagram

Case Study: —

Rev 0 — Initial issue

IT-152 Rubber Moulding Damage

Symptom — The rubber/elastomeric moulding around a transformer's connector boot, cable entry, or case seal area is found split, perished, or torn during inspection — a condition that may not yet have caused an electrical fault but compromises the barrier intended to keep moisture out.

Equipment / System

Rubber/elastomeric moulding at connector boots, cable entries, and case seal interfaces.

Possible Causes

HighAge-related perishing (hardening, cracking) of the rubber compound from prolonged heat and UV/ozone exposure over its service life

MediumMechanical damage from a previous connector mating/unmating done without proper technique, or from a tool slipping during a repair

MediumChemical degradation from contact with fuel, oil, or de-icing fluid residue in the pit

LowerCosmetic surface crazing not yet penetrating through the moulding's full thickness

Required Tools

Visual/borescope inspection kitInsulation resistance tester (5kV)Manufacturer replacement boot/moulding kit (if field-serviceable)

Safety Precautions

Apply LOTO on the CCR before handling a connector or moulding for close inspection or repair.
Treat any moulding damage found in a wet pit as a probable active moisture ingress path — prioritise the associated insulation resistance check.

Diagnostic Procedure

ObservationVisually inspect connector boots, cable entries, and case seal moulding for splits, cracking, perishing, or tears.
MeasurementWhere moulding damage is found, perform an insulation resistance test on the transformer to check whether moisture has already entered.
ObservationAssess whether the damage is superficial surface crazing or has penetrated through the moulding's full thickness.
DecisionMoulding damaged and insulation resistance confirmed normal → replace the moulding/boot per manufacturer procedure where field-serviceable, monitor insulation trend afterward. Moulding damaged and insulation resistance already degraded → treat as a combined moisture ingress fault, cross-reference IT-134/IT-135/IT-130. Damage found to be superficial only → note for monitoring, no immediate action required.
Fault LocationIsolate the damaged moulding to a specific location — connector boot, cable entry, or case seal.
Root CauseConfirm whether the cause is age-related perishing, mechanical damage, or chemical exposure.

Test Points

Visual condition of connector boots and moulded seals

Insulation resistance (to check for consequential moisture ingress)

Extent of damage — superficial vs. through-thickness

Root Cause Verification

After repair, confirm the replaced moulding forms a sound seal and that insulation resistance remains stable on a follow-up test.

Corrective Action

Replace damaged moulding/boots per manufacturer-supplied field repair kit where the design permits.
Replace the transformer if moulding damage is at a non-repairable location or has already led to confirmed moisture ingress.
Clean any fuel, oil, or de-icing fluid residue found contacting moulding in the pit.

Preventive Action

Inspect rubber moulding condition as a standard item during routine pit inspections.
Use correct connector mating/unmating technique to avoid mechanical damage to boots.
Track moulding age/condition against known perish life for the compound used, where data is available.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Isolating Transformer Wiring Diagram

Case Study: —

Rev 0 — Initial issue

IT-153 Case Damage

Symptom — The transformer's outer case is found cracked, dented, punctured, or otherwise physically damaged during inspection — potentially compromising the moisture barrier and, in more severe cases, the internal winding or core assembly itself.

Equipment / System

Isolating transformer outer case and potting compound.

Possible Causes

HighImpact damage from a tool, vehicle, or dropped object during pit works or fixture maintenance

HighA crack propagating from age-related material fatigue or repeated thermal cycling stress on the case material

MediumDamage from being over-compressed or improperly seated within an undersized or poorly maintained light base/pit

LowerSuperficial scuffing or scratching not penetrating to the potting compound or internal components

Required Tools

Visual/borescope inspection kitInsulation resistance tester (5kV)Isolating transformer bench test set

Safety Precautions

Apply LOTO on the CCR before removing a damaged transformer for inspection or testing.
Handle a cracked or punctured case carefully — a compromised case may have already allowed water ingress and could present an unexpected shock or contamination hazard.

Diagnostic Procedure

ObservationVisually document the extent, location, and depth of the case damage, noting whether it penetrates to the potting compound or appears to be surface-only.
MeasurementPerform an insulation resistance test to check whether the case damage has already allowed moisture to affect the winding.
MeasurementWhere the damage is significant, bench-test winding resistance and no-load current to check for any resulting internal damage.
DecisionCase damage confirmed to penetrate to the potting compound, or insulation/bench tests show degradation → reject and replace the transformer. Damage confirmed superficial only, with normal insulation and bench test results → note for monitoring.
Fault LocationIsolate the damage to case-only versus damage that has progressed to affect internal insulation or the winding/core.
Root CauseConfirm whether the damage originated from impact, material fatigue, or improper seating in the light base/pit.

Test Points

Visual extent and depth of case damage

Insulation resistance (bench)

Winding resistance and no-load current (bench, if damage is significant)

Root Cause Verification

After replacement, confirm the new unit's case is intact and that it passes insulation resistance and bench acceptance criteria before returning to service.

Corrective Action

Replace a transformer with case damage that penetrates to the potting compound or that has already led to insulation degradation.
Correct an undersized or poorly maintained light base/pit if it is found to be causing case compression damage.

Preventive Action

Handle transformers carefully during pit works to avoid impact damage from tools or dropped objects.
Inspect case condition as a standard item whenever a unit is removed for any reason.
Verify light base/pit dimensions and condition are adequate for the installed transformer type.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Isolating Transformer Wiring Diagram

Case Study: —

Rev 0 — Initial issue

IT-154 Seal Failure

Symptom — A dedicated case seal, gasket, or sealed joint on the transformer (distinct from connector moulding) is found to have failed or degraded, identified either through a visible gap/gasket defect on inspection or through a moisture-related electrical symptom traced back to this specific seal.

Equipment / System

Transformer case gasket, sealed joint, or dedicated seal component, as distinct from connector boot moulding (see IT-152).

Possible Causes

HighGasket material has hardened, shrunk, or perished with age, no longer maintaining full compression at the joint

MediumThe joint was not correctly reassembled or torqued after a previous field repair or inspection, leaving the seal only partially effective

MediumContamination (grit, debris) trapped in the seal interface during a previous assembly, preventing full closure

LowerSeal is intact and effective, and the moisture symptom that prompted investigation originates elsewhere (case crack, connector moulding, or lead damage)

Required Tools

Visual/borescope inspection kitInsulation resistance tester (5kV)Manufacturer torque specification for sealed jointsReplacement gasket/seal kit (if field-serviceable)

Safety Precautions

Apply LOTO on the CCR before opening or inspecting a sealed joint.
Do not reassemble a sealed joint without the correct gasket and torque specification — an improperly resealed unit can appear closed while providing little actual moisture protection.

Diagnostic Procedure

ObservationVisually inspect the seal/gasket for visible gaps, hardening, shrinkage, or trapped debris preventing full closure.
MeasurementPerform an insulation resistance test to determine whether moisture has already entered via this seal.
ObservationCheck whether the joint shows signs of having been previously opened and reassembled without correct torque or a fresh gasket.
DecisionSeal confirmed degraded or improperly reassembled, with normal insulation resistance → reseal using manufacturer-specified gasket and torque, monitor afterward. Seal degraded and insulation resistance already low → treat as a combined moisture ingress fault, cross-reference IT-134/IT-135/IT-130. Seal found intact and effective → investigate case, connector moulding, or lead as the actual moisture path.
Fault LocationIsolate the failure to the gasket/seal material itself, or to an improperly reassembled joint.
Root CauseConfirm whether the cause is age-related gasket degradation or a workmanship issue from a previous reassembly.

Test Points

Visual seal/gasket condition and joint closure

Insulation resistance (bench)

Joint torque/reassembly history where known

Root Cause Verification

After reseal, confirm insulation resistance is stable on a follow-up test and that the joint shows correct torque and full closure.

Corrective Action

Reseal the joint using the manufacturer-specified gasket and torque value.
Replace the transformer if seal failure has already led to confirmed moisture-related insulation degradation.
Retrain or brief personnel on correct sealed-joint reassembly procedure if a workmanship cause is identified.

Preventive Action

Use only manufacturer-specified gasket material and torque values when reassembling any sealed joint.
Inspect seal condition whenever a unit is opened for any other maintenance reason.
Keep a stock of correct replacement gaskets on hand rather than reusing a degraded one at reassembly.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Isolating Transformer Wiring Diagram

Case Study: —

Rev 0 — Initial issue

IT-155 Mechanical Damage

Symptom — The transformer shows general physical/mechanical damage not specifically confined to the case, seal, or lead — for example, a broken mounting lug, a deformed body, or impact damage affecting its overall structural integrity — found during inspection or while investigating another symptom.

Equipment / System

Isolating transformer body, mounting features, and overall structural integrity.

Possible Causes

HighImpact or crushing damage from pit works, vehicle loading over an inadequately protected light base, or dropped equipment during handling

MediumDamage sustained during transport or storage prior to installation, not identified at incoming inspection

MediumStress from an incorrect or undersized light base/pit forcing the unit into a position it was not designed to occupy

LowerDamage is cosmetic only and does not affect the unit's electrical or structural function

Required Tools

Visual/borescope inspection kitInsulation resistance tester (5kV)Isolating transformer bench test set

Safety Precautions

Apply LOTO on the CCR before removing a mechanically damaged transformer for inspection or testing.
Handle a structurally compromised unit carefully during removal, since a broken mounting or deformed body may fail further under normal handling loads.

Diagnostic Procedure

ObservationVisually document the type, location, and extent of mechanical damage, distinguishing structural damage from purely cosmetic marks.
MeasurementBench-test insulation resistance, winding resistance, and no-load current to check whether the mechanical damage has affected internal electrical integrity.
ObservationAssess whether the light base/pit itself is correctly sized and in good condition, in case it is contributing to ongoing mechanical stress on the unit.
DecisionStructural damage confirmed with any bench test outside acceptance criteria → reject and replace the transformer. Structural damage confirmed but bench tests remain within acceptance criteria → replace as a precaution given compromised mechanical integrity, or monitor per site policy for cosmetic-only cases. Light base/pit found to be contributing stress → correct the base/pit condition in addition to any transformer action.
Fault LocationIsolate the damage to the transformer body/mounting itself, or to a contributing light base/pit condition.
Root CauseConfirm whether the damage originated from impact, transport/storage, or ongoing installation stress.

Test Points

Visual extent of mechanical/structural damage

Insulation resistance, winding resistance, and no-load current (bench)

Light base/pit size and condition

Root Cause Verification

After replacement, confirm the new unit's bench test results are within acceptance criteria and that it seats correctly in a sound light base/pit before returning to service.

Corrective Action

Replace a transformer with confirmed structural damage or any bench result outside acceptance criteria.
Correct an undersized or damaged light base/pit if found to be contributing to mechanical stress.
Review incoming inspection procedure if damage is traced to transport or storage prior to installation.

Preventive Action

Protect light bases and pits from vehicle loading or heavy equipment where not designed for it.
Include a mechanical condition check as part of incoming inspection for new and spare transformers.
Handle transformers with appropriate care during removal and installation to avoid impact or crushing damage.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Isolating Transformer Wiring Diagram

Case Study: —

Rev 0 — Initial issue

05 · Recurring & Environmental Patterns

5 of 5 topics published

IT-156 Transformer Repeated Failure

Symptom — A transformer at the same fixture location, or transformers of the same type/batch across a circuit, fail more than once within a short service interval — a pattern that points toward a recurring systemic cause rather than an isolated, unrelated failure each time.

Equipment / System

Transformer replacement and failure history at a specific fixture location, or across a circuit/batch.

Possible Causes

HighAn underlying non-transformer condition at this location (chronic moisture, a marginal connection, a loop overcurrent condition) is damaging each replacement unit in turn

HighA batch or manufacturing defect is causing multiple units of the same production run to fail independently of site conditions

MediumIncorrect transformer rating repeatedly being installed at this location without the design schedule being corrected (see IT-144)

MediumA recurring installation/workmanship error (connection, torque, sealing) being repeated at each replacement rather than being corrected

LowerCoincidental unrelated failures being misread as a pattern — verify the failure mode is actually consistent across the repeat events before concluding a systemic cause

Required Tools

Transformer replacement/failure history log or CMMS recordsIsolating transformer bench test setInsulation resistance tester (5kV)Transformer schedule (VA rating by fixture type)

Safety Precautions

Apply LOTO on the CCR before removing any transformer for inspection during a repeated-failure investigation.
Treat a location with repeated failures as an elevated-risk point — prioritise root-cause investigation over simply replacing the unit again.

Diagnostic Procedure

ObservationReview the replacement/failure history for this specific location, and separately for this transformer batch/type across other circuits, to establish the actual pattern.
ObservationCompare the failure mode across each repeat event (e.g., always insulation-related, always winding-related, always moisture-related) to determine whether a single systemic cause is consistent.
MeasurementInspect the site condition at this location — pit drainage, connection quality, sealing — for a chronic condition that could be damaging each replacement in turn.
ObservationVerify the correct transformer rating and installation procedure have been used at each replacement, per the design schedule.
DecisionConsistent failure mode traced to a chronic site condition → correct the underlying condition (moisture, connection, loading) rather than continuing to simply replace units. Consistent failure mode traced to a batch defect → escalate to the supplier and consider proactive replacement of the remaining batch. Wrong rating or workmanship error repeated at each install → correct the schedule/procedure and retrain as needed. Failures found not actually consistent in mode → treat as coincidental, no systemic cause confirmed.
Fault LocationIsolate the recurring cause to the site/location condition, a batch defect, a rating/schedule error, or a repeated workmanship issue.
Root CauseConfirm the specific systemic cause through history review and comparison of failure mode across repeat events.

Test Points

Replacement/failure history by location and by batch

Failure mode consistency across repeat events

Site condition at the recurring location (drainage, connections, sealing)

Installed rating vs. design schedule at each replacement

Root Cause Verification

After the underlying cause is corrected, monitor this location (or affected batch) over subsequent maintenance cycles to confirm the repeated-failure pattern does not recur.

Corrective Action

Correct the underlying chronic site condition (drainage, connection quality, sealing) rather than only replacing the transformer again.
Escalate a confirmed batch/manufacturing defect to the supplier and proactively assess or replace remaining units from that batch.
Correct the design schedule or installation procedure if a repeated rating or workmanship error is identified.

Preventive Action

Maintain a transformer replacement/failure history log by location and by batch to make recurring patterns visible.
Review failure history for a location or batch whenever a second failure occurs within a short interval, rather than waiting for a third.
Feed confirmed batch defects back into procurement/acceptance criteria.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Bench Test Procedure Reference Sheet

Case Study: —

Rev 0 — Initial issue

IT-157 Transformer Failure After Rain

Symptom — One or more transformer-related faults (low insulation resistance, earth leakage, open/short winding) appear shortly after a rain event, with a time correlation to the weather that points toward water-related ingress rather than an unrelated coincidental failure.

Equipment / System

Isolating transformer case, seals, and connectors at locations affected by recent rainfall.

Possible Causes

HighWater ingress through a compromised case, seal, or connector at a pit that was not adequately protected during the rain event (see IT-134)

HighA previously marginal seal or connector (see IT-152/IT-154) was pushed past its limit by standing water accumulating during heavy rain

MediumPoor pit drainage allowing water to pool and remain in contact with the transformer for longer than a well-drained pit would

LowerThe failure timing is coincidental and the transformer would have failed regardless of the rain — verify a genuine water-related fault signature (e.g., insulation resistance recovering somewhat on drying) before concluding a rain-related cause

Required Tools

Insulation resistance tester (5kV)Isolating transformer bench test setVisual/borescope inspection kitManufacturer field-drying guidance (where applicable)

Safety Precautions

Apply LOTO on the CCR before opening a flooded or wet pit for inspection.
Assume standing water in a pit may be energised until the circuit is confirmed isolated — do not reach into a wet pit before LOTO is applied.
Follow airside work permit and NOTAM procedures before accessing an affected light base after weather-related access restrictions, if applicable.

Diagnostic Procedure

ObservationConfirm the timing correlation between the rain event and the onset of the fault, using CCR alarm logs or fault-report timestamps where available.
ObservationInspect the pit for standing water, drainage condition, and any visible case/seal/connector defect that could explain water entry.
MeasurementPerform an insulation resistance test; where practical and permitted, dry the unit per manufacturer guidance and re-test to check for recovery consistent with a water-related cause.
DecisionInsulation resistance recovers after drying, with a confirmed seal/case/connector defect or pit drainage issue → water ingress confirmed, address per IT-134/IT-152/IT-154 as applicable and correct drainage. Insulation resistance does not recover after drying → treat as genuine internal degradation, reject and replace regardless of the rain timing. No water entry point or drainage issue found → treat the rain timing as coincidental, investigate other causes.
Fault LocationIsolate to a specific water entry point (case, seal, connector) or to poor pit drainage as a contributing factor.
Root CauseConfirm the water-related cause through drying-recovery testing and physical inspection, rather than assuming causation from timing alone.

Test Points

Insulation resistance before and after drying

Pit standing water level and drainage condition

Visible case/seal/connector entry point

Fault timing versus rain event timing

Root Cause Verification

After correction and drying (or replacement), confirm insulation resistance is stable on a follow-up test, ideally after a subsequent rain event, to verify the entry point has been fully addressed.

Corrective Action

Dry the unit per manufacturer guidance and return to service if insulation resistance recovers and no entry point defect is found.
Repair or replace the case, seal, or connector identified as the water entry point.
Correct pit drainage where poor drainage is confirmed as a contributing factor.
Replace the transformer if insulation resistance does not recover after drying.

Preventive Action

Improve pit drainage at locations with a history of standing water after rain.
Inspect and proactively reseal or replace ageing seals/connectors before the rainy season where a seasonal pattern is known.
Prioritise post-rain-event inspection of pits with a prior history of water-related faults.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Bench Test Procedure Reference Sheet

Case Study: —

Rev 0 — Initial issue

IT-158 Transformer Failure After Flooding

Symptom — One or more transformers fail, or show significantly degraded insulation resistance, following confirmed flooding of a pit, duct bank, or light base area — distinguished from IT-157 by full or near-full submersion of the unit rather than incidental rain-driven moisture ingress.

Equipment / System

Isolating transformer(s) and associated wiring within a confirmed flooded pit, duct, or light base area.

Possible Causes

HighFull or partial submersion allowed bulk water to enter the transformer case directly, well beyond what any seal or moulding is designed to resist

HighProlonged submersion time allowed water to fully penetrate the winding insulation system, not just the case, making the internal insulation damage severe

MediumFlood water carried contaminants (silt, fuel, de-icing fluid) that compound the insulation damage beyond clean water alone

MediumMultiple transformers on the same low-lying section of circuit are affected simultaneously, indicating a drainage design or storm-event severity issue rather than a per-unit defect

Required Tools

Insulation resistance tester (5kV)Isolating transformer bench test setVisual/borescope inspection kitSite flood extent records/photos where available

Safety Precautions

Apply LOTO on the CCR before any work in or around a flooded or recently flooded pit or duct area.
Treat all flood-affected transformers and wiring as unsafe to energise until fully inspected and tested — do not attempt to restore power to a flooded section on a time-pressure basis alone.
Follow airside work permit, NOTAM, and any site flood-response procedures before accessing affected areas.

Diagnostic Procedure

ObservationConfirm the extent and duration of flooding at this location, including whether the transformer was fully or partially submerged.
MeasurementPerform an insulation resistance test on every transformer within the confirmed flood-affected area, not only the one that triggered the original alarm.
ObservationInspect for contaminant residue (silt, fuel sheen, de-icing fluid) in addition to plain water, which may indicate additional insulation risk.
DecisionInsulation resistance confirmed severely degraded and non-recoverable on drying, for a fully or near-fully submerged unit → reject and replace; do not attempt to dry and return a submerged unit to service. Insulation resistance only mildly affected on a partially exposed unit → attempt manufacturer-permitted drying and re-test, treating per IT-157 if recovery is achieved. Multiple units affected on the same section → treat as a drainage design issue requiring correction beyond individual transformer replacement.
Fault LocationIsolate to individually affected transformers within the confirmed flood extent, and to the drainage/design condition that allowed flooding to occur.
Root CauseConfirm the flood extent, duration, and any contaminant involvement, and assess whether the site's drainage design is adequate for the storm event experienced.

Test Points

Insulation resistance on every unit within the flood extent

Flood extent, depth, and duration at this location

Contaminant presence in addition to water

Drainage design adequacy for the section affected

Root Cause Verification

After replacement of non-recoverable units and drying/re-testing of marginally affected units, confirm all units in the previously flooded section show stable, acceptable insulation resistance before re-energising the section.

Corrective Action

Replace any transformer confirmed to have been fully or near-fully submerged — do not attempt to return a submerged unit to service.
Attempt manufacturer-permitted drying only for units with mild, partial exposure and confirm recovery before return to service.
Investigate and correct drainage design for any section that experiences flooding, rather than only addressing individual transformers.

Preventive Action

Review and improve drainage design for low-lying circuit sections identified as flood-prone.
Maintain a flood-event log by circuit section to identify recurring flood-prone areas.
Establish a standard post-flood inspection and test procedure covering the full affected section, not just the initially reported fixture.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Bench Test Procedure Reference Sheet

Case Study: —

Rev 0 — Initial issue

IT-159 Transformer Failure Under High Load

Symptom — A transformer fails, or a related symptom (overheating, excessive current, tripped protection) appears specifically when the circuit is commanded to a high-intensity step, while the same unit performs acceptably at lower intensity steps — indicating the fault is load-dependent rather than a constant condition.

Equipment / System

Isolating transformer winding and secondary circuit, evaluated specifically across CCR intensity steps rather than at a single fixed load point.

Possible Causes

HighA developing winding or inter-turn fault (see IT-138/IT-139) that is only thermally or electrically significant at the higher current level of a high-intensity step

HighThe transformer or fixture is correctly rated for a lower intensity step but undersized relative to the actual current at the highest commanded step for this circuit

MediumA marginal connection (terminal, connector, or lead) that only develops enough resistance/heating to cause a symptom at higher current

LowerA CCR-side issue specific to the high-intensity step (see Division 01) rather than a transformer fault — verify against other transformers on the same circuit before concluding this unit is at fault

Required Tools

True-RMS clamp meterIsolating transformer bench test setInfrared thermal cameraTransformer schedule (VA rating by fixture type)

Safety Precautions

Apply LOTO on the CCR before removing the transformer for bench testing.
Do not repeatedly command a circuit to a high-intensity step to reproduce a suspected fault without adequate monitoring — a developing fault under test can progress to a more severe failure.

Diagnostic Procedure

ObservationConfirm the symptom's correlation with CCR intensity step, noting at which step(s) it appears and whether it is absent at lower steps.
MeasurementWith appropriate monitoring, observe secondary current and/or case temperature across the full range of intensity steps to characterise the load-dependency.
MeasurementBench-test the transformer's winding resistance, insulation resistance, and no-load current to check for a developing internal fault.
ObservationVerify the transformer's VA rating is adequate for the actual current at the highest commanded step on this circuit, per the design schedule.
DecisionBench test reveals a developing winding/insulation fault → replace the transformer. Bench test normal but the unit is confirmed undersized for the actual high-step current → correct with the design-specified rating. Other transformers on the same circuit show the same step-dependent symptom → redirect to a Division 01 CCR-side investigation.
Fault LocationIsolate to a load-dependent internal transformer fault, an undersized rating, a marginal connection, or a CCR-side cause common to the circuit.
Root CauseConfirm the specific cause through step-correlated measurement and bench testing.

Test Points

Secondary current and/or case temperature across intensity steps

Winding resistance, insulation resistance, and no-load current (bench)

Transformer VA rating vs. actual current at highest commanded step

Symptom presence on other transformers on the same circuit

Root Cause Verification

After correction, confirm the fixture operates normally with no recurrence of the symptom across the full range of intensity steps, including the highest step.

Corrective Action

Replace a transformer confirmed to have a load-dependent internal fault.
Install the design-specified VA rating if the unit is confirmed undersized for the highest commanded step.
Correct a marginal connection identified as the cause.
Redirect to Division 01 if the cause is confirmed to be CCR-side and common to the circuit.

Preventive Action

Verify transformer VA rating against the actual highest-step current for the circuit, not just the fixture's nominal rating.
Include a full-range intensity-step check as part of commissioning and periodic testing, not only a single fixed-load check.
Track units with any step-dependent symptom for closer monitoring even if the fault has not yet caused an outage.

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Bench Test Procedure Reference Sheet

Case Study: —

Rev 0 — Initial issue

IT-160 Transformer Failure During High Intensity Operation

Symptom — A transformer fails outright (open, short, or thermal failure) while the circuit is actively operating at a sustained high-intensity step — for example, during a period of low visibility requiring maximum runway/taxiway light output — distinguished from IT-159 by being a confirmed in-service failure event during high-intensity operation rather than a reproducible step-correlated symptom found during testing.

Equipment / System

Isolating transformer and its associated fixture, at the point of an in-service failure occurring during sustained high-intensity operation.

Possible Causes

HighA pre-existing developing fault (winding, insulation, or core) reached its failure point under the sustained thermal and electrical stress of high-intensity operation

HighThe transformer or fixture was undersized for sustained operation at the highest intensity step, having previously only been exposed to shorter or less frequent high-step periods

MediumSustained high-intensity operation coincided with an environmental stressor (high ambient temperature, restricted pit ventilation, or recent moisture exposure) that compounded normal thermal stress

LowerThe failure is unrelated to the high-intensity operation and coincidentally occurred during that period — verify against the unit's prior test/service history before concluding load was the precipitating factor

Required Tools

True-RMS clamp meterIsolating transformer bench test setInsulation resistance tester (5kV)Infrared thermal cameraTransformer schedule (VA rating by fixture type)

Safety Precautions

Apply LOTO on the CCR before removing the failed transformer, coordinating with ATC/airside operations given the operational significance of a failure during low-visibility high-intensity operation.
Handle a unit that failed under sustained high load as a potential thermal hazard — allow it to cool and confirm temperature before handling.

Diagnostic Procedure

ObservationRecord the operational context of the failure — intensity step, duration of sustained operation prior to failure, and any concurrent environmental conditions.
MeasurementBench-test the failed transformer for winding resistance, insulation resistance, and no-load current to characterise the failure mode.
ObservationReview the unit's prior test/service history for any pre-existing developing fault indicator (declining insulation trend, marginal no-load current, prior thermal survey flag) that preceded this failure.
ObservationVerify the transformer's VA rating and duty expectation are adequate for sustained operation at the highest intensity step used on this circuit, not just intermittent exposure.
DecisionBench test confirms a specific failure mode consistent with a pre-existing developing fault → treat as that fault progressing to failure under sustained load (cross-reference the specific IT code for the failure mode found, e.g. IT-136/IT-137/IT-138). No pre-existing indicator found and the unit is confirmed undersized for sustained high-step duty → correct with the design-specified rating for sustained operation. Environmental stressor confirmed as a compounding factor → address the environmental condition in addition to replacing the unit.
Fault LocationIsolate to the specific winding/insulation/core failure mode revealed by bench testing, in the context of sustained high-intensity operation.
Root CauseConfirm whether the precipitating factor was a pre-existing fault reaching its limit, an inadequate duty rating, or a compounding environmental stressor.

Test Points

Winding resistance, insulation resistance, and no-load current (bench, post-failure)

Prior service/test history for pre-existing fault indicators

Operational context — intensity step, duration, and environmental conditions at time of failure

Transformer VA/duty rating vs. sustained high-step requirement

Root Cause Verification

After replacement, confirm the new unit's bench test results are within acceptance criteria, and monitor performance during subsequent sustained high-intensity operations to confirm no recurrence.

Corrective Action

Replace the failed transformer, addressing the underlying failure mode identified by bench testing (see the relevant specific IT code).
Install a transformer with an adequate duty rating for sustained high-intensity operation if the failed unit is confirmed undersized for that duty.
Address any compounding environmental stressor (ventilation, moisture, ambient temperature) identified as a contributing factor.

Preventive Action

Include sustained high-intensity duty capability as an explicit acceptance criterion for transformer specification, not just peak current rating.
Trend insulation resistance, no-load current, and thermal survey results for units on circuits with frequent low-visibility (high-intensity) operation, to catch developing faults before they fail in service.
Prioritise proactive replacement of units showing any developing-fault indicator ahead of anticipated high-intensity-demand periods (e.g., seasonal low-visibility conditions).

Related References

SOP: SOP-AGL-03 (Fixture Isolation Transformer Testing)

Standards: ICAO Annex 14 Vol I, IS 7785

Diagram: Bench Test Procedure Reference Sheet

Case Study: —

Rev 0 — Initial issue

Frequently Asked Questions

Quick answers about Isolating Transformer troubleshooting in the AGL Troubleshooting Atlas.

Q1What does Division 03 of the AGL Troubleshooting Atlas cover?

Division 03 is the Isolating Transformer Troubleshooting reference. It publishes 40 diagnostic topics, IT-121 to IT-160, across 5 subsections: Electrical & Winding Continuity (9); Insulation & Earth Faults (6); Thermal & Mechanical (11); Connections & Physical Integrity (9); Recurring & Environmental Patterns (5).

Q2What information does each topic include?

Every topic follows the same diagnostic record: the symptom, equipment/system, possible causes ranked by likelihood (high, medium or low), required tools, safety precautions, diagnostic procedure, test points, root cause verification, corrective action, preventive action and related references (SOPs, standards, diagrams and case studies).

Q3Which topics cover Electrical & Winding Continuity?

IT-121 to IT-129 (9 topics): Transformer Open Circuit; Transformer Primary Open Circuit; Transformer Secondary Open Circuit; Low Secondary Voltage; High Secondary Voltage; Incorrect Transformation Ratio; High Primary Current; Abnormal No-Load Current; Excessive Load Current.

Q4Which topics cover Insulation & Earth Faults?

IT-130 to IT-135 (6 topics): Low Insulation Resistance; Insulation Breakdown; HV Test Failure; Earth Leakage; Water Ingress; Moisture Inside Transformer.

Q5Which topics cover Thermal & Mechanical?

IT-136 to IT-146 (11 topics): Overheating; Burnt Winding; Winding Short Circuit; Inter-Turn Fault; Core Damage; Core Heating; Abnormal Transformer Noise; Transformer Vibration; Incorrect Wattage; Transformer Overloaded; Transformer Underloaded.

Q6Which topics cover Connections & Physical Integrity?

IT-147 to IT-155 (9 topics): Wrong Polarity; Incorrect Primary Connection; Incorrect Secondary Connection; Terminal Damage; Cable Lead Failure; Rubber Moulding Damage; Case Damage; Seal Failure; Mechanical Damage.

Q7Which topics cover Recurring & Environmental Patterns?

IT-156 to IT-160 (5 topics): Transformer Repeated Failure; Transformer Failure After Rain; Transformer Failure After Flooding; Transformer Failure Under High Load; Transformer Failure During High Intensity Operation.

Q8How does a diagnostic procedure begin in Division 03?

Each procedure starts with observation before any intervention. For example, IT-121 (Transformer Open Circuit) begins: Confirm the CCR is delivering rated loop current at the commanded step (per CCR-021/CCR-031 in Division 01) before concluding the fault is local to this transformer rather than a CCR-side issue.

Q9What safety precautions apply when troubleshooting Isolating Transformer?

Each topic lists its own precautions. For example, IT-121 (Transformer Open Circuit) starts with: Never open or disconnect an isolating transformer while the CCR is running — isolate the CCR and apply LOTO first, since the series loop can present hazardous induced voltage at any break point.

Q10Which SOPs are referenced in Division 03?

Related references in the topics include SOP-AGL-03 (Fixture Isolation Transformer Testing), SOP-CCR-06 (Series Circuit Loop Testing), SOP-CCR-10 (Earth Fault Location Procedure) and SOP-AGL-07 (Light Base & Pit Maintenance).

Q11Which standards are referenced in the Isolating Transformer topics?

Where a standard applies, it appears in the topic's Related References. Those cited in this division include ICAO Annex 14 Vol I, IS 7785, FAA AC 150/5345-10, IEC 60076 and IEC 61820-3-2.

Q12How can I find a specific fault in Division 03 quickly?

Use the search box at the top of the page to filter the 40 published topics by code, fault name or keyword; matching topics open automatically. Each diagnostic record can also be downloaded as a Word (.doc) file.