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Safety-critical technical reference — read before use. Independent educational guidance for qualified AGL personnel. It is not an approved procedure or regulatory instruction and does not replace your aerodrome’s permit-to-work, isolation/LOTO and maintenance procedures or the manufacturer’s instructions. Series-circuit AGL equipment can carry lethal voltages. Read the full disclaimer ↓
Division 15 of 30

Series Circuit Electrical Faults Troubleshooting

Series circuit electrical fault diagnosis across the AGL constant-current loop — resistance and voltage-drop anomalies, current leakage and earth faults localised through light, transformer, cable, and connector, single- and multiple-point open circuits, high-resistance connections, and faults that only manifest under specific temperature, rainfall, or switching conditions. Each topic follows the full engineering diagnostic record: symptom → observation → measurement → decision → fault location → root cause → verification → corrective action → preventive action.

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01 · Series Circuit Electrical Faults (20)

01 · Series Circuit Electrical Faults

20 of 20 topics published
SCF-391 Unexpected Circuit Resistance

Symptom — A routine or pre-energisation loop resistance measurement on a series circuit reads noticeably different from the recorded as-built or last-known-good baseline, with no fixture reported dark and no CCR alarm active.

Equipment / System

Series circuit primary loop (CCR secondary output through isolation transformers and cable back to CCR), as measured loop-to-loop or loop-to-earth with the circuit isolated.

Possible Causes
MedCircuit records/baseline itself outdated or incorrectly logged after a prior modification, addition, or repair of the run
MedA recent repair, splice, or component replacement altered the loop's total resistance without being reflected in the maintenance record
MedGradual, distributed insulation or connection ageing across the whole loop rather than a single discrete fault
LowTest equipment or test lead condition, calibration drift, or a different meter/technique used than for the baseline reading
LowAmbient temperature at time of test differing significantly from the baseline test temperature, affecting conductor resistance
LowAn early-stage, not-yet-symptomatic fault developing at one point in the loop
Required Tools
Digital loop/insulation resistance tester (megger)Calibrated true-RMS multimeterAs-built circuit drawing with recorded baseline resistanceMaintenance/test history logTemperature reference for the test session
Safety Precautions
  • LOTO the CCR before taking any resistance measurement.
  • Confirm the circuit is fully isolated and discharged before connecting test leads.
  • Do not rely on a single anomalous reading to declare a fault — repeat the measurement before acting.
Diagnostic Procedure
  1. ObservationRe-verify the baseline value in the circuit record and confirm it corresponds to the correct circuit and the correct test method (loop, or loop-to-earth).
  2. MeasurementRepeat the resistance measurement at least twice, with the circuit fully isolated, to rule out a test-technique or connection error.
  3. ObservationCheck the maintenance log for any recent work on this circuit that could explain a genuine change in resistance.
  4. DecisionReading confirmed and no recorded explanation → treat as a genuine deviation and proceed to segment-by-segment testing under SCF-392/393. Reading explained by recorded work or test-method difference → update the baseline record and close out.
  5. Root CauseEstablish whether the deviation reflects an actual physical change in the circuit or a record-keeping/test-condition discrepancy.
Test Points
Current total loop resistance vs. baseline
Individual segment resistance where segmented testing is available
Ambient temperature at time of test
Maintenance log entries for the circuit
Root Cause Verification

Confirm the reading is stable and repeatable across at least two independent measurements, and that the circuit record has been reconciled with either an explanation or an updated baseline.

Corrective Action
  • Update the circuit's baseline resistance record if the change is confirmed genuine and within acceptable limits.
  • Correct the maintenance record if a prior repair was not logged.
  • Escalate to segment-by-segment testing under SCF-392 (excessive) or SCF-393 (low) if the deviation is outside acceptable tolerance.
Preventive Action
  • Log loop resistance at every scheduled test using a consistent method, meter, and reference temperature.
  • Require all splices, repairs, and component changes on series circuits to be logged with an updated resistance reading.
  • Trend resistance readings year over year rather than treating each test in isolation.
Related References

SOP: SOP-SCF-01 (Series Circuit Resistance Test Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: IEC 60364 (insulation/loop testing), ICAO Annex 14 Vol I, FAA AC 150/5340-30

Diagram: Series Circuit As-Built Drawing with Resistance Log

Case Study: Division 01 — CCR Troubleshooting (source-side verification)

Rev 0 — Initial issue
SCF-392 Excessive Circuit Resistance

Symptom — Total series circuit loop resistance measures significantly above baseline, and the CCR is having to drive a higher output voltage than normal to maintain the commanded constant current, potentially approaching the regulator's voltage limit or triggering a high-voltage alarm.

Equipment / System

Series circuit primary loop — cable, splices, connectors, and isolation transformer primary windings in series around the CCR output.

Possible Causes
HighA developing high-resistance connection at a splice, terminal, or connector somewhere in the loop
HighCorrosion at a cable joint or pull-box connection increasing contact resistance
MedOne or more isolation transformer primary windings degraded, increasing their series impedance
MedA recent repair using an incorrect cable gauge or an unnecessarily long splice run adding resistance
LowCumulative resistance from a large number of ageing connections distributed across a long circuit
LowCCR output voltage sensing or calibration issue reporting a resistance rise that is not physically present
Required Tools
Loop resistance tester (megger)Clamp-on ammeter and true-RMS multimeterCCR output voltage trend logThermal imaging cameraSeries circuit fault locator / TDRAs-built circuit drawing with segment breakpoints
Safety Precautions
  • LOTO the CCR before any loop testing or connection work.
  • Treat any connection identified as high-resistance as a potential heat/fire risk — inspect for scorching or insulation damage before re-energising.
  • Allow components to cool before handling if the circuit was recently energised at a high step.
Diagnostic Procedure
  1. ObservationReview the CCR output voltage trend to confirm the regulator is compensating with higher voltage for a normal commanded current, consistent with a resistance rise rather than a current-sensing fault.
  2. MeasurementWith the circuit isolated, megger the loop in accessible segments and compare each against baseline to localise the segment contributing the greatest resistance increase.
  3. ObservationUse thermal imaging during a brief live period, where safe to do so, to identify a hot connection consistent with high resistance under load.
  4. DecisionResistance rise isolated to one segment or connection → localised repair. Resistance rise distributed across many segments → treat as cumulative loop ageing and plan proactive maintenance.
  5. Fault LocationIsolate to the specific splice, connector, or transformer winding contributing most to the resistance rise.
  6. Root CauseConfirm with a full-loop re-test that resistance and CCR output voltage are restored to expected values.
Test Points
Segment-by-segment loop resistance vs. baseline
CCR output voltage trend for the commanded current
Thermal signature of suspect connections under load
Isolation transformer primary winding resistance
Root Cause Verification

After repair, re-test the full loop resistance and confirm the CCR output voltage for the commanded current has returned to its expected baseline range across all intensity steps.

Corrective Action
  • Repair or replace the identified high-resistance splice or connector.
  • Replace an isolation transformer with a degraded primary winding.
  • Correct an incorrectly specified splice or cable run from a prior repair.
Preventive Action
  • Include loop resistance and CCR output voltage trending in the periodic maintenance cycle.
  • Use thermal imaging on accessible connections during scheduled inspections.
  • Prioritise ageing connections for proactive replacement before they progress to a hard fault.
Related References

SOP: SOP-SCF-02 (Loop Resistance Fault Location Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: IEC 60364, ICAO Annex 14 Vol I, FAA AC 150/5345-10 (constant current regulators)

Diagram: Series Circuit As-Built Drawing with Segment Breakpoints

Case Study: Division 01 — CCR Troubleshooting (output voltage limit alarms)

Rev 0 — Initial issue
SCF-393 Low Circuit Resistance

Symptom — Total series circuit loop resistance measures significantly below baseline, without a corresponding earth-fault alarm, suggesting a partial short or reduced-impedance path has developed between conductors or across a transformer winding.

Equipment / System

Series circuit primary loop and isolation transformer primary windings, tested loop-to-loop with the circuit isolated.

Possible Causes
HighPartial insulation breakdown between the two primary conductors at a chafed or damaged cable point
HighA shorted or partially shorted isolation transformer primary winding
MedMoisture bridging exposed conductor at a poorly sealed splice or connector, without yet producing a full earth-fault path
MedAn incorrect splice or repair inadvertently reducing loop impedance (e.g. conductors bridged)
LowTest equipment fault or lead condition producing a falsely low reading
LowA parallel unintended path introduced by a temporary bonding or test connection left in place
Required Tools
Loop resistance tester (megger)True-RMS multimeterSeries circuit fault locator / TDRAs-built circuit drawingInsulation resistance tester (1000V/2500V)
Safety Precautions
  • LOTO the CCR before any testing.
  • Treat a low-resistance reading as a potential short-circuit hazard — do not re-energise the circuit until the cause is confirmed and cleared.
  • Be alert for signs of prior arcing or heat damage at the suspect location.
Diagnostic Procedure
  1. ObservationConfirm the low reading is repeatable and not an artefact of test lead placement or a temporary connection left on the circuit.
  2. MeasurementWith the circuit isolated, megger the loop in accessible segments and identify the segment showing reduced resistance relative to baseline.
  3. MeasurementTest each isolation transformer primary winding individually within the suspect segment to rule in or out a shorted transformer.
  4. DecisionFault isolated to a cable segment → proceed to cable fault location. Fault isolated to a single transformer → replace that transformer.
  5. Fault LocationNarrow to the specific cable length, splice, or transformer responsible for the reduced resistance.
  6. Root CauseConfirm with a full-loop re-test and an insulation resistance test that the reading is restored above the acceptable threshold.
Test Points
Segment-by-segment loop resistance vs. baseline
Individual isolation transformer primary winding resistance
Suspect splice/connector condition
Full-loop insulation resistance
Root Cause Verification

After repair, re-test the full loop resistance and confirm it has returned to within the expected baseline range, with no residual reduced-impedance path detected.

Corrective Action
  • Repair or replace the damaged cable section causing the partial short.
  • Replace the shorted isolation transformer.
  • Correct the faulty splice or remove any inadvertent bonding/test connection.
Preventive Action
  • Include loop resistance testing in the periodic maintenance cycle to catch a developing partial short before it becomes a hard fault.
  • Ensure temporary test connections are logged and removed before returning a circuit to service.
  • Inspect splices for correct sealing and separation during routine maintenance.
Related References

SOP: SOP-SCF-03 (Reduced-Resistance Fault Location Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: IEC 60364, ICAO Annex 14 Vol I, IS 12290 (isolating transformers)

Diagram: Series Circuit As-Built Drawing

Case Study: Division 01 — CCR Troubleshooting (current regulation behaviour under reduced impedance)

Rev 0 — Initial issue
SCF-394 Circuit Voltage Drop Excessive

Symptom — The voltage drop measured across a section of the series circuit — or the CCR's total output voltage for the commanded current — is higher than expected, without a hard open or earth fault present.

Equipment / System

Series circuit primary loop, CCR output stage, and isolation transformer primary windings across the affected section.

Possible Causes
HighA developing high-resistance connection within the measured section (see SCF-392) causing localised voltage drop
MedUndersized conductor used in a prior repair for the length of run installed
MedA long circuit run approaching its design voltage-drop limit even without a discrete fault, particularly at high intensity steps
MedCCR nearing its maximum output voltage capability for the total connected load
LowVoltage measurement taken at an incorrect point or with an incorrect meter category, giving a misleading reading
Required Tools
True-RMS multimeter (CAT III/IV as appropriate)CCR output voltage/current trend logAs-built circuit drawing with conductor gauge and length dataClamp-on ammeter
Safety Precautions
  • LOTO before disconnecting any point to insert a meter in series, or use appropriate live-measurement rated equipment and PPE if a live voltage-drop test is required.
  • Confirm meter category rating is appropriate for the circuit voltage before connecting.
Diagnostic Procedure
  1. ObservationReview the CCR output voltage and current trend to determine whether the total loop is approaching the regulator's rated output voltage for the commanded current.
  2. MeasurementMeasure voltage drop across defined sections of the loop, with the circuit energised at a safe, controlled step if live testing is required and permitted, or isolate and measure resistance per SCF-392 if not.
  3. ObservationCompare the as-built conductor gauge and run length for the section against the expected voltage-drop calculation for that design.
  4. DecisionVoltage drop consistent with a single high-resistance point → treat as SCF-392. Voltage drop consistent with conductor gauge/length across the whole run → treat as a design/loading issue rather than a discrete fault.
  5. Root CauseConfirm whether the excessive drop is due to a discrete fault, an undersized conductor, or the circuit approaching its designed capacity.
Test Points
Section-by-section voltage drop
CCR total output voltage vs. rated maximum for the commanded current
Conductor gauge and run length vs. design calculation
Root Cause Verification

After corrective action, re-measure section voltage drop and confirm CCR output voltage for the commanded current is within its normal operating range across all intensity steps.

Corrective Action
  • Repair the high-resistance point identified within the affected section.
  • Replace an undersized conductor section with correctly rated cable.
  • If the circuit is at or near its design voltage-drop limit without a fault, flag for capacity/load review rather than repeated fault-finding.
Preventive Action
  • Verify conductor sizing against calculated voltage drop for any circuit extension or repair before installation.
  • Trend CCR output voltage for the commanded current over time to catch a gradual rise early.
  • Include voltage-drop verification in commissioning and periodic testing for long circuit runs.
Related References

SOP: SOP-SCF-04 (Voltage Drop Test Procedure), SOP-CCR-02 (CCR Output Verification)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5345-10, IS 12290

Diagram: Series Circuit As-Built Drawing with Conductor Schedule

Case Study: Division 01 — CCR Troubleshooting (regulator output voltage limits)

Rev 0 — Initial issue
SCF-395 Circuit Current Imbalance

Symptom — The measured series circuit current does not consistently match the value commanded for the selected intensity step, or differs when measured at different points around the loop, when in a true series circuit the current should be identical at every point.

Equipment / System

CCR output, series circuit primary loop, and current measurement instrumentation/CTs used for monitoring.

Possible Causes
HighCCR output current regulation fault or calibration drift — verify against a calibrated reference ammeter
HighAn unintended parallel or leakage current path allowing part of the current to bypass the intended series loop
MedA clamp-on ammeter or monitoring CT reading in error due to incorrect placement, saturation, or calibration
MedA developing earth fault providing a partial alternate current path (see SCF-396 onward)
LowHarmonic or waveform distortion on the circuit affecting a true-RMS reading if a non-true-RMS meter is used
Required Tools
Calibrated true-RMS clamp-on ammeterCCR reference/calibration ammeterEarth-fault/ground-fault monitor readingOscilloscope or power quality logger (for waveform verification)
Safety Precautions
  • Treat the circuit as energised and hazardous during live current measurement — use correctly rated CT/clamp equipment.
  • Do not open a live series circuit to insert a meter in series — always use a clamp-on current measurement.
Diagnostic Procedure
  1. ObservationConfirm the discrepancy using a calibrated true-RMS clamp-on ammeter at more than one point on the loop, ruling out a single faulty monitoring instrument.
  2. MeasurementCheck the CCR's earth-fault/ground-fault monitor for any indicated leakage current that would account for a current difference around the loop.
  3. MeasurementIf instrumentation is confirmed accurate and a genuine difference exists, this indicates a parallel or leakage path is present rather than a true series condition.
  4. DecisionDiscrepancy resolves to an instrumentation error → correct or recalibrate the meter. Genuine current difference confirmed → escalate to earth-fault/leakage investigation under SCF-396.
  5. Root CauseConfirm whether the imbalance is instrumentation-related or reflects an actual leakage/parallel path in the circuit.
Test Points
Current at CCR output
Current at one or more points around the loop
Earth-fault/ground-fault monitor reading
Reference/calibration ammeter comparison
Root Cause Verification

After corrective action, confirm current readings match at multiple points around the loop and the earth-fault monitor shows no unexplained leakage, across all commanded intensity steps.

Corrective Action
  • Recalibrate or replace faulty monitoring instrumentation.
  • Investigate and repair a confirmed leakage or parallel path per SCF-396 through SCF-400.
  • Correct CCR current regulation calibration if drift is confirmed at the regulator itself.
Preventive Action
  • Periodically cross-check CCR current readings against an independent calibrated reference ammeter.
  • Include earth-fault/ground-fault monitor verification in the periodic maintenance cycle.
  • Maintain calibration records for all series-circuit current measurement instruments.
Related References

SOP: SOP-SCF-05 (Circuit Current Verification Procedure), SOP-CCR-02 (CCR Output Verification)

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

Diagram: CCR Panel Wiring and Instrumentation Diagram

Case Study: Division 01 — CCR Troubleshooting (current regulation faults)

Rev 0 — Initial issue
SCF-396 Current Leakage to Earth

Symptom — The CCR's earth-fault (ground-fault) monitor indicates a measurable leakage current from the series circuit to earth, below the level that trips protection, without any fixture reported dark.

Equipment / System

Series circuit primary loop, isolation transformer insulation systems, and CCR earth-fault/ground-fault monitoring circuit.

Possible Causes
HighGradual moisture ingress at one or more cable joints, pull boxes, or base cans reducing insulation resistance
HighA single degrading isolation transformer developing a partial insulation breakdown to its core/earth
MedAged cable jacket allowing minor moisture or conductive contamination to bridge to earth at one point
MedA connector with a corroded or partially seated pin providing a high-resistance leakage path to earth
LowCCR earth-fault monitor sensitivity or calibration issue reporting leakage current that is not physically significant
LowCumulative distributed insulation ageing across an older circuit rather than one discrete point
Required Tools
CCR earth-fault/ground-fault trend log1000V/2500V insulation resistance tester (megger)Segment-by-segment test plan and as-built drawingMoisture detection meter
Safety Precautions
  • LOTO the circuit before insulation testing.
  • Treat any confirmed leakage current as a developing hazard rather than a nuisance reading — do not disable or desensitise earth-fault protection to suppress the reading.
Diagnostic Procedure
  1. ObservationReview the CCR earth-fault/ground-fault trend to determine whether leakage is stable, increasing, or correlates with weather (see SCF-409).
  2. MeasurementWith the circuit isolated, megger the loop in accessible segments from pull boxes or base cans, comparing each against baseline.
  3. MeasurementIdentify the segment or transformer contributing the greatest reduction in insulation resistance.
  4. DecisionLeakage isolated to one segment/transformer → localised repair per SCF-397 through SCF-400. Leakage distributed across many segments → treat as cumulative circuit ageing.
  5. Fault LocationNarrow to the specific light, transformer, cable section, or connector contributing most to the leakage.
  6. Root CauseConfirm with a full-loop re-test that insulation resistance is restored to, or above, the acceptable threshold.
Test Points
Segment-by-segment insulation resistance vs. baseline
Individual isolation transformer insulation resistance
CCR earth-fault trend log
Joint/connector/base-can condition in the lowest-reading segment
Root Cause Verification

After repair, re-megger the full loop and confirm the reading is restored above the acceptable threshold, and monitor the CCR earth-fault trend over subsequent operating cycles.

Corrective Action
  • Repair or replace the identified degraded cable segment, joint, or connector.
  • Replace the transformer with the leaking insulation.
  • Reseal an identified moisture entry point at a pull box, base can, or connector.
Preventive Action
  • Schedule periodic full-loop insulation resistance testing and trend results rather than waiting for a nuisance trip.
  • Prioritise ageing circuit sections for proactive cable or connector replacement.
  • Seal known moisture entry points identified during preventive maintenance.
Related References

SOP: SOP-SCF-06 (Earth Leakage Fault Location Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: IEC 60364, ICAO Annex 14 Vol I, FAA AC 150/5340-30

Diagram: Series Circuit Route Drawing with Insulation Test Log

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue
SCF-397 Earth Fault at Light

Symptom — Segment-by-segment insulation testing has localised a confirmed earth fault to an individual light fixture, distinguishing it from a fault in the cable, transformer, or connector serving that fixture.

Equipment / System

Light fixture body, base can, and the internal wiring between the fixture's secondary connection and its lamp/LED module.

Possible Causes
HighWater ingress into the fixture body through a failed or degraded gasket/seal
HighInternal wiring insulation damage inside the fixture from heat, vibration, or age
MedA conductive foreign body or debris bridging live wiring to the fixture's earthed body inside the base can
MedPhysical damage to the fixture from vehicle/aircraft wheel loading or a ground strike compromising internal insulation
LowManufacturing defect or latent fault in the fixture's internal insulation
Required Tools
1000V/2500V insulation resistance tester (megger)Fixture-specific service/overhaul documentationReplacement gaskets/sealsBase-can inspection kit
Safety Precautions
  • LOTO the circuit before opening the fixture.
  • Treat the fixture as a potential shock hazard until confirmed isolated and the fault verified de-energised.
  • Allow the fixture to cool before handling if recently energised.
Diagnostic Procedure
  1. MeasurementWith the circuit isolated and the fixture disconnected at its secondary connector, megger the fixture body and internal wiring independently of the base can and cable.
  2. ObservationOpen the fixture and inspect for water ingress, gasket/seal condition, internal wiring damage, or foreign debris.
  3. DecisionInsulation fault confirmed within the fixture itself, isolated from cable/connector/transformer → proceed to repair or replace the fixture. Fault clears when fixture is disconnected but base can/connector remains suspect → re-route to SCF-400.
  4. Root CauseIdentify the specific ingress point, damaged wiring run, or debris path causing the earth fault.
Test Points
Fixture internal insulation resistance (isolated from cable/base can)
Gasket/seal condition
Internal wiring condition
Base can water/debris presence
Root Cause Verification

After repair or replacement, re-megger the fixture in isolation and confirm the reading is restored above the acceptable threshold before reconnecting to the circuit.

Corrective Action
  • Replace failed gaskets/seals and confirm fixture is watertight before reinstallation.
  • Repair or replace damaged internal wiring.
  • Replace the fixture if internal damage is extensive or the fixture is beyond economical repair.
  • Clear debris and confirm the base can is properly drained.
Preventive Action
  • Include fixture seal/gasket condition in the periodic maintenance and relamping cycle.
  • Confirm base can drainage is functioning during routine inspection.
  • Track fixtures with recurring earth-fault history for proactive replacement.
Related References

SOP: SOP-SCF-07 (Fixture-Level Earth Fault Isolation Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5345-46, IEC 60364

Diagram: Fixture Assembly and Seal Detail Drawing

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue
SCF-398 Earth Fault at Transformer

Symptom — Segment-by-segment insulation testing has localised a confirmed earth fault to an isolation transformer, with the associated fixture, cable, and connector tested clear.

Equipment / System

Individual isolation transformer serving one light fixture on the series circuit, including its primary and secondary windings and core.

Possible Causes
HighMoisture ingress into the transformer housing degrading winding-to-core insulation
HighInternal winding insulation breakdown from thermal ageing or an internal short
MedTransformer physically damaged (impact, cracked housing) allowing moisture or contamination entry
MedIncorrect transformer type/rating installed at a prior replacement, operating outside its designed thermal/insulation margin
LowManufacturing defect or latent insulation fault in the transformer
Required Tools
1000V/2500V insulation resistance tester (megger)Isolation transformer test setReplacement transformer of correct ratingBase-can inspection kit
Safety Precautions
  • LOTO the circuit before disconnecting or testing the transformer.
  • Treat a confirmed transformer earth fault as a shock/fire hazard until the unit is removed from service.
Diagnostic Procedure
  1. MeasurementWith the circuit isolated, disconnect the transformer from both the primary loop and the fixture, and megger the primary winding, secondary winding, and core independently.
  2. ObservationInspect the transformer housing for cracking, corrosion, or evidence of moisture ingress.
  3. DecisionInsulation fault confirmed within the transformer, isolated from fixture and cable → replace the transformer with a correctly rated unit. Reading remains marginal after removal → re-check the base can and connector per SCF-400.
  4. Root CauseConfirm whether the fault is due to moisture ingress, thermal ageing, physical damage, or an incorrect unit.
Test Points
Transformer primary winding insulation resistance
Transformer secondary winding insulation resistance
Transformer core condition
Base can/housing seal condition
Root Cause Verification

After replacement, re-megger the new transformer in isolation and confirm the reading meets specification before reconnecting to the circuit, then re-test the full loop.

Corrective Action
  • Replace the faulty isolation transformer with a unit of the correct rating.
  • Confirm the replacement transformer is correctly sealed and seated in the base can.
  • Verify the transformer tap/rating matches the fixture's design requirement.
Preventive Action
  • Include isolation transformer insulation testing in the periodic maintenance cycle.
  • Confirm transformer ratings match design specification at every replacement.
  • Track transformers with recurring earth-fault history by manufacturer/batch for possible systemic issues.
Related References

SOP: SOP-SCF-08 (Transformer Earth Fault Isolation Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: IS 12290, IEC 60364, FAA AC 150/5345-47 (isolation transformers)

Diagram: Isolation Transformer Test Data Sheet

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue
SCF-399 Earth Fault at Cable

Symptom — Segment-by-segment insulation testing has localised a confirmed earth fault to a section of the series circuit primary cable itself, with the fixtures, transformers, and connectors serving that section tested clear.

Equipment / System

Series circuit primary cable and its insulation system between two accessible test points (pull boxes, base cans, or transformer connections).

Possible Causes
HighCable jacket damage from a prior excavation, trenching, or ground-vehicle activity allowing moisture ingress
HighAged cable jacket cracking from UV/thermal cycling, particularly at pavement expansion joints or exposed sections
MedRodent or wildlife damage to the cable jacket
MedA poorly made or unsealed splice within the section acting as an insulation weak point
LowManufacturing defect in the original cable
Required Tools
1000V/2500V insulation resistance tester (megger)Time-domain reflectometer (TDR) or cable fault locatorCable route drawing with pull box/joint locationsExcavation and cable repair kit
Safety Precautions
  • LOTO the circuit before insulation testing or excavation.
  • Confirm underground service locates are obtained before any excavation near the cable route.
  • Treat the exposed cable end as energisable until confirmed isolated.
Diagnostic Procedure
  1. MeasurementWith the circuit isolated, megger the loop in segments between accessible pull boxes/base cans to bracket the faulty section between two known-good test points.
  2. MeasurementUse a TDR or cable fault locator referenced to the cable route drawing to estimate the fault's physical distance within the bracketed section.
  3. ObservationExcavate at the estimated location, following correct utility-locate procedure, and visually inspect the cable jacket.
  4. DecisionFault located and visually confirmed → repair or replace the damaged section. Fault distance estimate uncertain → re-test from the opposite end to cross-reference the location before excavating.
  5. Root CauseConfirm the specific mechanism of jacket damage (impact, ageing, wildlife, splice failure).
Test Points
Segment-by-segment insulation resistance bracketing the fault
TDR fault distance estimate
Cable jacket condition at the excavated point
Splice condition within the section
Root Cause Verification

After repair, re-megger the full section and the full loop, and confirm the reading is restored above the acceptable threshold before backfilling and reinstating pavement.

Corrective Action
  • Repair the damaged cable section using a correctly rated, sealed splice kit, or replace the full section if damage is extensive.
  • Reinstate pavement/trench per airport engineering standards.
  • Update the as-built cable route drawing to reflect the repair location.
Preventive Action
  • Include periodic segment insulation testing in the maintenance cycle to catch developing jacket damage before a hard fault.
  • Maintain accurate cable route records to speed future fault location.
  • Coordinate with airport works/excavation teams to avoid future accidental cable damage near known routes.
Related References

SOP: SOP-SCF-09 (Cable Fault Location and Repair Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: IEC 60364, ICAO Annex 14 Vol I, FAA AC 150/5340-30

Diagram: Series Circuit Cable Route Drawing

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue
SCF-400 Earth Fault at Connector

Symptom — Segment-by-segment insulation testing has localised a confirmed earth fault to an L-823 (or equivalent) connector, with the fixture, transformer, and cable on either side of it tested clear.

Equipment / System

L-823 primary or secondary connector and its immediate base can environment.

Possible Causes
HighMoisture ingress into a connector that is not fully seated, has a degraded O-ring/seal, or was not properly greased at installation
HighCorroded connector pins providing a leakage path to earth via base can moisture
MedIncorrect connector type or a damaged connector body used at a prior repair
MedBase can flooding due to blocked or failed drainage, submerging the connector
LowMechanical damage to the connector from base can lid handling or prior maintenance access
Required Tools
1000V/2500V insulation resistance tester (megger)L-823 connector test adaptorConnector grease/sealant rated for the applicationReplacement connector kitBase-can drainage inspection tool
Safety Precautions
  • LOTO the circuit before disconnecting or testing the connector.
  • Treat standing water in a base can as potentially energised until the circuit is confirmed isolated.
Diagnostic Procedure
  1. MeasurementWith the circuit isolated, disconnect the connector and megger each side (fixture/transformer side and cable side) independently to confirm the fault is within the connector itself rather than either component.
  2. ObservationInspect the connector for a degraded or missing seal, corrosion on the pins, incomplete seating, or physical damage.
  3. ObservationCheck the base can for standing water and confirm whether drainage is functioning.
  4. DecisionConnector confirmed as the fault source, both sides testing clear → clean, reseal, or replace the connector. Fault persists after connector replacement → re-check the adjoining fixture or cable.
  5. Root CauseConfirm whether the root cause is a sealing/installation defect or a base can drainage failure allowing prolonged water exposure.
Test Points
Connector pin insulation resistance (both sides independently)
Seal/O-ring condition
Base can water level and drainage condition
Connector seating/torque
Root Cause Verification

After repair, re-megger the connector in circuit and confirm the reading is restored above the acceptable threshold, and re-check base can drainage before closing out.

Corrective Action
  • Clean, re-grease, and properly reseat the connector where the seal is intact.
  • Replace the connector where the seal, body, or pins are damaged or corroded.
  • Clear and restore base can drainage if flooding is the underlying cause.
Preventive Action
  • Include connector seating, greasing, and seal condition in the periodic maintenance cycle.
  • Verify base can drainage during routine inspection, particularly ahead of the monsoon/wet season.
  • Use only manufacturer-specified connector grease and torque procedure at every reconnection.
Related References

SOP: SOP-SCF-10 (Connector Earth Fault Isolation Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: FAA AC 150/5345-26 (L-823 connectors), ICAO Annex 14 Vol I, IEC 60364

Diagram: Base Can and Connector Detail Drawing

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue
SCF-401 Multiple Simultaneous Light Failure

Symptom — Several lights on the same series circuit go dark at the same time, reported together rather than as separate individual failures, distinguishing this from an isolated single-light fault.

Equipment / System

Series circuit primary loop feeding the affected run, and the isolation transformers/fixtures downstream of the suspected fault point.

Possible Causes
HighAn open circuit in the primary cable upstream of the affected lights, interrupting current to every fixture downstream of that point
HighA failed or tripped CCR (or one output channel of a multi-circuit CCR) removing current from the entire circuit
MedA damaged section of cable affecting a defined run (e.g. from recent excavation, pavement work, or a vehicle strike)
MedA CCR earth-fault/ground-fault trip taking the whole circuit offline in response to a developing fault
LowCoincidental independent failure of multiple individual fixtures at the same time — verify this is genuinely unlikely before assuming a shared cause
Required Tools
Clamp-on series circuit ammeterCCR panel/ALCMS status logAs-built circuit and base-can layout drawingSeries circuit fault locator / TDR
Safety Precautions
  • Coordinate airside access through ATC/apron control before approaching the affected run.
  • Treat the circuit as energised until confirmed isolated and locked out at the CCR.
  • LOTO before any cable work.
Diagnostic Procedure
  1. ObservationCheck the CCR/ALCMS panel to confirm whether the circuit is showing zero output, a partial fault, or a trip condition, rather than assuming a cable-only cause.
  2. ObservationConfirm from the field or as-built drawing whether the dark lights form a continuous downstream run (pointing to an upstream cable break) or are scattered (pointing to coincidental individual faults or a shared upstream component).
  3. MeasurementIf the CCR is confirmed energised and outputting correctly, isolate the circuit and megger/fault-locate the cable feeding the start of the dark run to bracket the open point.
  4. DecisionDark lights form a continuous run from a bracketed point onward → primary cable open circuit at that point, proceed under SCF-402/404. CCR itself at fault → route to Division 01.
  5. Fault LocationIsolate to the specific point in the primary cable, or to the CCR/output channel, responsible for the loss of the downstream section.
  6. Root CauseConfirm the physical cause of the interruption — cable break, connector failure, or CCR fault.
Test Points
CCR output current and status
Cable continuity/fault location bracketing the start of the dark run
Pattern of affected fixtures (continuous run vs. scattered)
Root Cause Verification

After repair, re-energise the circuit and confirm every previously dark fixture on the run illuminates correctly at all intensity steps, with CCR output current and status normal.

Corrective Action
  • Repair the identified primary cable open circuit or damaged section.
  • Reset/repair the CCR or affected output channel if it was the root cause.
  • Address the underlying earth fault if a protective trip was the cause, rather than simply resetting the CCR.
Preventive Action
  • Maintain accurate as-built drawings to speed identification of downstream-run patterns during fault reports.
  • Coordinate with airport works teams to prevent accidental cable damage during excavation near known routes.
  • Include CCR/circuit status monitoring in the routine inspection to catch a developing trip condition early.
Related References

SOP: SOP-SCF-11 (Multiple Light Failure Triage Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5340-30, FAA AC 150/5345-10

Diagram: Series Circuit As-Built Drawing with Run Sequence

Case Study: Division 24 — Complete Runway Lighting Failure

Rev 0 — Initial issue
SCF-402 Single Light Failure

Symptom — A single light on an otherwise correctly functioning series circuit fails to illuminate, with every other fixture on the same circuit operating normally — the classic isolated single-point fault characteristic of a constant-current series circuit.

Equipment / System

The individual fixture, its isolation transformer, and its L-823 connectors, isolated from the rest of the series loop.

Possible Causes
HighIsolation transformer failure at the fixture (open primary or secondary winding)
HighLamp/LED module failure inside the fixture
MedL-823 connector not fully seated, corroded, or damaged, breaking the secondary connection
MedInternal fixture wiring open between the connector and the lamp/LED module
LowFixture physically damaged or displaced, disrupting an internal connection
Required Tools
True-RMS multimeter (CAT III)1000V insulation resistance tester (megger)L-823 connector test adaptorBase-can lifting tool and PPE for airside work
Safety Precautions
  • Coordinate airside access through ATC/apron control before approaching the fixture.
  • LOTO the circuit before disconnecting the fixture or opening the base can.
Diagnostic Procedure
  1. ObservationConfirm from the CCR/ALCMS panel and a run inspection that this is genuinely an isolated single-fixture fault, with every other light on the circuit operating normally.
  2. MeasurementWith the circuit isolated, disconnect the fixture's L-823 plug and megger the isolation transformer primary and secondary windings independently.
  3. ObservationInspect the base can for water or debris and check the connector and lamp/LED module for visible failure.
  4. DecisionTransformer or lamp confirmed faulty at this fixture only → proceed to component replacement. Transformer and lamp both test clear → re-check the connector and internal fixture wiring.
  5. Fault LocationIsolate the fault to the transformer, lamp/LED module, connector, or internal fixture wiring.
  6. Root CauseConfirm the specific failed component before replacement rather than swapping parts speculatively.
Test Points
Isolation transformer primary/secondary insulation and continuity
L-823 connector condition and seating
Lamp/LED module condition
Internal fixture wiring continuity
Root Cause Verification

After repair, re-energise the circuit and confirm the fixture lights correctly at all intensity steps, and verify no adjacent fixtures were disturbed during access.

Corrective Action
  • Replace the failed isolation transformer or lamp/LED module.
  • Reseat or replace a corroded/damaged connector.
  • Repair identified internal fixture wiring.
Preventive Action
  • Include fixture and connector inspection in the routine flight-check and periodic maintenance cycle.
  • Maintain an accurate base-can and circuit layout drawing to speed fault location.
  • Stock common transformer and lamp/LED spares to minimise time-to-repair for this most common fault type.
Related References

SOP: SOP-SCF-12 (Single Fixture Fault Isolation Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5345-46, FAA AC 150/5340-30

Diagram: Fixture and Base-Can Layout Drawing

Case Study: Division 01 — CCR Troubleshooting (verify series circuit output)

Rev 0 — Initial issue
SCF-403 Two-Point Fault

Symptom — Standard single-fault localisation techniques (bracketing, halving, single-segment isolation) fail to produce a consistent result, because two separate faults exist simultaneously on the same series circuit, each partially masking or confusing the diagnosis of the other.

Equipment / System

Series circuit primary loop, considered as a whole where two independent fault locations must be identified rather than one.

Possible Causes
MedTwo independent earth faults at different points on the loop, producing a combined leakage reading that does not localise cleanly to either
MedAn open circuit at one point combined with an earth fault at a different point, producing an inconsistent or contradictory test pattern
MedTwo independent high-resistance connections developing at different times, coincidentally both becoming significant around the same test cycle
LowA genuine single fault being misread as two due to test method limitations or an incorrect as-built drawing
Required Tools
Loop resistance and insulation resistance tester (megger)Series circuit fault locator / TDRAs-built circuit drawingCCR earth-fault trend logSegment isolation test plan
Safety Precautions
  • LOTO the circuit before any testing.
  • Treat the circuit as having an unresolved and potentially hazardous fault throughout the extended diagnostic process — do not re-energise between test stages without confirming isolation is still valid.
Diagnostic Procedure
  1. ObservationRecognise the pattern of a two-point fault: bracketing tests that give inconsistent or contradictory results, or a fault that appears to move location between test sessions.
  2. MeasurementDivide the loop into the smallest practically testable segments (using every accessible pull box, base can, and transformer connection) and test each segment individually rather than relying on whole-loop bracketing.
  3. MeasurementRecord and map every segment's individual reading against the as-built drawing to identify two distinct anomalous segments rather than assuming a single midpoint.
  4. DecisionTwo distinct segments confirmed with independent anomalous readings → treat as two separate faults and resolve each independently under the relevant fault type (SCF-393/396–400/404 etc.). Only one segment confirmed anomalous on granular testing → the original inconsistency was a single-fault test-method artefact.
  5. Fault LocationIsolate and resolve each of the two fault locations independently, re-testing the loop after each individual repair.
  6. Root CauseConfirm both faults have been independently identified, repaired, and verified before considering the loop resolved.
Test Points
Individual segment readings across the full loop, mapped against the as-built drawing
CCR earth-fault trend pattern over the diagnostic period
Result consistency between successive bracketing attempts
Root Cause Verification

After repairing the first identified fault, re-test the full loop before assuming resolution — confirm whether a second anomaly remains, and only close out once both are independently verified clear.

Corrective Action
  • Repair each of the two identified faults independently, following the relevant corrective action for each fault type.
  • Re-test the full loop after each individual repair rather than only after both are believed complete.
Preventive Action
  • When a fault does not localise cleanly on first attempt, default to full granular segment testing rather than repeated whole-loop bracketing.
  • Maintain accurate, up-to-date as-built drawings with every accessible test point marked, to make granular segment testing practical.
  • Log and review cases where an initial repair did not fully resolve a reported symptom, to catch two-point faults promptly.
Related References

SOP: SOP-SCF-13 (Multi-Fault / Two-Point Fault Diagnostic Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: IEC 60364, ICAO Annex 14 Vol I

Diagram: Series Circuit As-Built Drawing with All Test Points Marked

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue
SCF-404 Multiple Open Circuits

Symptom — More than one genuinely independent open-circuit fault exists on the same series circuit at the same time, extinguishing more than one section of the loop and requiring each open point to be located and repaired separately.

Equipment / System

Series circuit primary loop, considered across its full length where more than one break must be identified.

Possible Causes
HighTwo separate cable damage events (e.g. from two unrelated excavation incidents) occurring on the same circuit before either was reported or repaired
MedA cable break at one point combined with a separately failed connector or transformer primary winding elsewhere on the loop
MedAge-related conductor fatigue producing near-simultaneous failures at two separate weak points
LowA single break being misread as two due to an incomplete or outdated as-built drawing
Required Tools
Series circuit fault locator / TDRLoop resistance/continuity testerAs-built circuit drawing with all accessible test pointsExcavation and cable repair kit
Safety Precautions
  • LOTO the circuit before any testing or cable work.
  • Confirm underground service locates before excavating at any suspected fault point.
  • Treat all fault points as potentially hazardous until each is confirmed repaired and the full loop re-verified.
Diagnostic Procedure
  1. ObservationEstablish from the field which sections of the circuit are dark and whether the pattern is consistent with a single break or suggests more than one interruption (e.g. two separate dark runs with a lit section between them).
  2. MeasurementTest continuity/resistance from each accessible test point toward both directions of the loop to bracket each suspected open independently.
  3. MeasurementUse a TDR or fault locator to estimate the physical distance to each bracketed open point.
  4. DecisionTwo independently bracketed open points confirmed → treat as two separate repairs. Only one open point found despite an apparently unusual dark pattern → re-check for an earth fault or CCR-side cause producing the pattern instead.
  5. Fault LocationLocate and repair each open circuit independently, re-testing loop continuity after each repair.
  6. Root CauseConfirm the cause of each break independently — they may have entirely unrelated root causes.
Test Points
Continuity/resistance from each accessible test point in both directions
TDR distance estimate to each bracketed open
Pattern of dark sections across the full run
Root Cause Verification

After each repair, re-test full loop continuity and confirm every previously dark section is restored before considering the circuit fully resolved.

Corrective Action
  • Repair or splice each identified open-circuit location independently.
  • Replace any failed connector or transformer winding identified as one of the open points.
  • Reinstate pavement/trench per airport engineering standards at each excavated location.
Preventive Action
  • Maintain accurate, current as-built drawings so that unusual dark-section patterns can be quickly interpreted.
  • Coordinate with airport works teams to log all excavation activity near AGL cable routes.
  • Investigate promptly when a reported dark pattern does not match a simple single-break profile.
Related References

SOP: SOP-SCF-14 (Multiple Open Circuit Diagnostic Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5340-30

Diagram: Series Circuit As-Built Drawing with Run Sequence

Case Study: Division 24 — Complete Runway Lighting Failure

Rev 0 — Initial issue
SCF-405 Hidden Open Circuit

Symptom — An open circuit exists somewhere on the series loop, but standard visual inspection at accessible points (base cans, pull boxes, connectors) shows no obvious damage — the break is internal to a component or cable that appears externally intact.

Equipment / System

Series circuit primary cable, splices, and isolation transformer windings, where the physical break is not visible without further testing or disassembly.

Possible Causes
HighAn internal conductor strand break inside an intact-looking cable jacket, from repeated flexing, a manufacturing defect, or a prior over-tension event during installation
HighInternal corrosion inside a sealed splice or connector that appears externally sound but has failed internally
MedAn internal winding break inside an isolation transformer with no external sign of damage
MedA cable crushed or damaged below grade with the jacket remaining intact at the surface while the conductor inside has separated
LowA very fine crack in a solder or crimp joint that only fully separates under specific load or temperature conditions
Required Tools
Series circuit fault locator / TDRLoop resistance/continuity tester1000V/2500V insulation resistance testerThermal imaging camera (for suspect connections under load)
Safety Precautions
  • LOTO before any testing.
  • Do not assume a visually intact cable or component is electrically sound — verify by test.
Diagnostic Procedure
  1. ObservationConfirm the open circuit exists via loop continuity testing, since visual inspection at accessible points has already shown no obvious external damage.
  2. MeasurementUse a TDR or fault locator to estimate the physical distance to the open point as precisely as possible before considering excavation or component removal.
  3. MeasurementAt the estimated location, test continuity across each individual component in turn (cable segment, each splice, each transformer winding) rather than relying on external inspection alone.
  4. DecisionOpen isolated to a specific internal component → replace or repair that component. Open still not isolated after component-level testing → extend the bracketed search area and repeat.
  5. Fault LocationNarrow the search progressively from whole-loop bracketing to segment, to individual component, until the specific break is found.
  6. Root CauseOnce found, determine the physical mechanism — strand fatigue, internal corrosion, or a latent manufacturing defect — to inform whether similar components nearby should be checked.
Test Points
TDR/fault locator distance estimate
Continuity of each individual component within the bracketed area
Insulation resistance of suspect components
Root Cause Verification

After repair, re-test full loop continuity and resistance, and confirm the reading is stable and matches expectations under normal operating conditions.

Corrective Action
  • Repair or replace the specific cable segment, splice, or transformer identified as internally open.
  • If a latent manufacturing defect is suspected, flag the batch/supplier for other installed units to be checked.
Preventive Action
  • Where a hidden open circuit is found, inspect nearby components of the same age/batch/installation for similar latent faults.
  • Include internal continuity testing (not just visual inspection) of accessible splices in the periodic maintenance cycle.
  • Maintain installation records (batch numbers, installation dates) to support tracing latent defects.
Related References

SOP: SOP-SCF-15 (Hidden Fault Location Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: IEC 60364, ICAO Annex 14 Vol I

Diagram: Series Circuit As-Built Drawing

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue
SCF-406 Intermittent Open Circuit

Symptom — One or more lights on a series circuit go dark and then recover, or the CCR shows a momentary current drop, without a sustained fault being present at the time of inspection — the open circuit is making and breaking contact rather than remaining permanently open.

Equipment / System

Series circuit primary cable, splices, connectors, and isolation transformer connections, particularly at points subject to vibration, thermal cycling, or moisture ingress.

Possible Causes
HighA loose or partially seated connector making intermittent contact, worsened by ground-vehicle and aircraft wheel-loading vibration
HighA chafed cable making and breaking contact at the point of insulation damage, particularly under thermal expansion/contraction
MedA splice with a marginal crimp or solder joint that opens under thermal or mechanical stress and recloses when conditions change
MedMoisture inside a base can causing intermittent tracking or a temperature-dependent contact issue
LowA degrading isolation transformer with an internal intermittent winding fault
Required Tools
True-RMS multimeter with min/max holdCCR output current trend log/data loggerThermal imaging cameraConnector test adaptor
Safety Precautions
  • Treat intermittent faults as live until proven otherwise — LOTO the circuit before opening any base can even if the light appears steady at the time of access.
  • Do not assume a fault has resolved itself simply because it is not present during inspection.
Diagnostic Procedure
  1. ObservationConfirm whether the intermittent behaviour is on a single fixture or several, and whether it correlates with a particular intensity step, ground-traffic movement, temperature, or weather condition.
  2. MeasurementLog CCR output current over a period spanning the reported intermittent event to characterise the pattern and rule in/out a source-side cause.
  3. MeasurementWith the circuit isolated, inspect and re-torque suspect connectors, and test cable/splices under light mechanical flexing to try to reproduce the intermittent fault.
  4. ObservationUse thermal imaging during a live period to spot a hot connection consistent with an intermittent high-resistance contact.
  5. DecisionIntermittent behaviour follows CCR current logging → source-side issue, route to Division 01. Intermittent behaviour isolated to the fixture/segment and reproducible by flexing → connector, splice, or cable fault at that point.
  6. Fault LocationIsolate to the specific connector, splice, cable point, or transformer producing the intermittent contact.
  7. Root CauseConfirm by replacing or repairing the suspect component and re-testing under the conditions that previously produced the fault.
Test Points
Connector/splice contact resistance under mechanical flex
CCR output current trend log
Suspect connection thermal signature under load
Component insulation resistance
Root Cause Verification

Monitor the repaired point over several operating cycles, including varying intensity steps and, if possible, the conditions (temperature, vibration) that previously produced the fault, before closing out.

Corrective Action
  • Clean, re-grease, and properly seat the affected connector.
  • Repair or replace the chafed cable or marginal splice.
  • Replace a transformer confirmed to have an intermittent internal fault.
Preventive Action
  • Torque-check connectors as a standard preventive maintenance step given continuous ground-traffic vibration exposure.
  • Include thermal imaging of connectors and splices in the periodic maintenance cycle.
  • Trend CCR output current logs to distinguish source-side from fixture-side intermittent issues early.
Related References

SOP: SOP-SCF-16 (Intermittent Fault Diagnostic Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5340-30

Diagram: Series Circuit As-Built Drawing

Case Study: Division 01 — CCR Troubleshooting (output current logging)

Rev 0 — Initial issue
SCF-407 High Resistance Fault

Symptom — A connection point on the series circuit has developed measurably elevated resistance without progressing to a full open circuit, producing localised heating, a voltage-drop anomaly, or gradual light dimming rather than an outright failure.

Equipment / System

Any series-circuit connection point — splice, connector, terminal, or transformer connection — where contact resistance has increased above design values.

Possible Causes
HighCorrosion building up at a connection point over time, particularly in a poorly sealed splice or connector
HighA connection that was under-torqued or improperly made at installation, loosening further under thermal cycling and vibration
MedOxidation on a conductor or terminal surface not properly cleaned before a prior repair
MedThermal damage from a previous fault event leaving a connection partially degraded rather than fully failed
LowA connection type or material mismatch (e.g. dissimilar metals) producing galvanic corrosion over time
Required Tools
Thermal imaging cameraTrue-RMS multimeter for section voltage dropLoop resistance tester (megger)Torque wrench/screwdriver appropriate to the connection type
Safety Precautions
  • LOTO before any connection work.
  • Allow a suspect high-resistance connection to cool before handling — it may be significantly hotter than ambient under load.
  • Treat a confirmed high-resistance point as a fire risk until repaired.
Diagnostic Procedure
  1. ObservationUse thermal imaging during a controlled live period to identify any connection running measurably hotter than comparable connections elsewhere on the circuit.
  2. MeasurementMeasure voltage drop across the suspect connection specifically, isolating it from the surrounding cable run, to quantify the resistance contribution.
  3. MeasurementWith the circuit isolated, inspect the connection for corrosion, under-torquing, or oxidation, and megger/resistance-test the point directly.
  4. DecisionElevated resistance confirmed and localised to one connection → repair or remake that connection. Elevated resistance found to be distributed across several points → treat as broader circuit ageing per SCF-392.
  5. Fault LocationConfirm the exact connection responsible before disturbing adjacent, correctly functioning connections.
  6. Root CauseDetermine whether the cause is corrosion, installation quality, thermal history, or material mismatch, to prevent recurrence at similar points.
Test Points
Thermal signature of the suspect connection under load
Voltage drop across the specific connection
Direct resistance measurement of the connection
Visual condition of contact surfaces
Root Cause Verification

After remaking the connection, re-test its resistance and voltage drop under load, and re-check with thermal imaging on a subsequent live period to confirm normal temperature.

Corrective Action
  • Clean, remake, and correctly torque the affected connection using manufacturer-specified procedure.
  • Replace corroded terminals, lugs, or connector components rather than reusing degraded hardware.
  • Address any dissimilar-metal or material mismatch identified as a contributing cause.
Preventive Action
  • Include thermal imaging of accessible connections in the periodic maintenance cycle.
  • Verify torque specification is followed and documented at every connection made or remade.
  • Use only manufacturer-specified terminal/connector materials to avoid galvanic corrosion.
Related References

SOP: SOP-SCF-17 (High Resistance Connection Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: IEC 60364, ICAO Annex 14 Vol I

Diagram: Series Circuit As-Built Drawing

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue
SCF-408 Temperature-Dependent Fault

Symptom — A series circuit fault appears, disappears, or changes in severity in correlation with ambient or component temperature — for example only manifesting on cold mornings, only after several hours of continuous energisation, or only during peak summer heat.

Equipment / System

Any series-circuit component whose electrical or mechanical behaviour changes measurably with temperature — cable, splices, connectors, or transformer windings.

Possible Causes
HighA marginal crack or gap that closes (making contact) when a component contracts in cold, or opens (breaking contact) when it expands in heat, or vice versa
HighA connection with a temperature-dependent contact resistance that only becomes significant once the circuit has been energised long enough to reach thermal equilibrium
MedCable jacket cracking that only allows moisture ingress, and therefore only produces an earth-fault reading, when temperature/humidity conditions coincide
MedA component operating close to a design temperature threshold, with a fault only manifesting once that threshold is crossed
LowCCR or protection device with a temperature-sensitive calibration drift
Required Tools
Data logger capable of correlating fault events with temperatureThermal imaging cameraTrue-RMS multimeterWeather/temperature history for the reported fault period
Safety Precautions
  • LOTO before any testing, regardless of whether the fault is currently present.
  • Where testing must be timed to a specific temperature condition to reproduce the fault, plan the access and safety arrangements for that condition in advance.
Diagnostic Procedure
  1. ObservationCorrelate the timing of every reported occurrence of the fault against ambient temperature records, time of day, and duration of energisation, to characterise the temperature dependency.
  2. MeasurementIf practical and safe, arrange testing during conditions likely to reproduce the fault (e.g. early morning cold, or after several hours of continuous energisation) rather than only testing in comfortable conditions that may not reproduce it.
  3. MeasurementUse thermal imaging and/or a data logger during the reproducing conditions to capture the fault as it develops.
  4. DecisionFault reproduced and localised under the identified temperature condition → proceed to component-level repair per the underlying fault type (open, high-resistance, or earth fault). Fault not reproduced despite matching conditions → extend monitoring period and reconsider correlation.
  5. Root CauseConfirm the specific thermal mechanism — expansion/contraction at a crack, moisture correlation, or a threshold effect — to guide a durable repair rather than one that only holds at the test temperature.
Test Points
Fault occurrence log correlated with temperature/time
Component condition under the reproducing temperature condition
Thermal imaging captured during the reproducing condition
Root Cause Verification

Confirm the repair holds under the same temperature/duration conditions that previously produced the fault, not only under comfortable test conditions, before closing out.

Corrective Action
  • Repair the underlying open, high-resistance, or earth fault identified once reproduced.
  • Where a design/material threshold is the cause, consider a component upgrade rated for the actual temperature range experienced at the site.
Preventive Action
  • Log fault reports with time and approximate temperature to build a pattern history for the circuit.
  • Include testing under varied temperature conditions in the periodic maintenance cycle for circuits with a history of temperature-dependent faults.
  • Specify components rated for the full seasonal temperature range expected at the airport.
Related References

SOP: SOP-SCF-18 (Temperature-Correlated Fault Diagnostic Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: IEC 60364, ICAO Annex 14 Vol I

Diagram: Series Circuit As-Built Drawing with Fault Occurrence Log

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue
SCF-409 Rain-Dependent Fault

Symptom — A series circuit fault — typically an earth-fault trip, leakage reading, or intermittent light — appears specifically during or shortly after rainfall, and clears once the site dries out, without being present during dry-weather testing.

Equipment / System

Series circuit primary cable, splices, connectors, base cans, and transformer housings, at any point where water can bridge or enter an otherwise intact insulation system.

Possible Causes
HighA base can with failed or blocked drainage that floods during rain, submerging a connector or transformer
HighA cracked cable jacket or degraded splice seal that only allows moisture ingress once standing water or saturated ground is present
MedA connector that is nominally sealed but marginal, allowing capillary moisture entry only under sustained wet conditions
MedSurface water tracking into a fixture through a degraded gasket during heavy rain
LowCCR/earth-fault monitor sited in a location itself affected by moisture, giving a false reading unrelated to the field circuit
Required Tools
CCR earth-fault trend log correlated with weather/rainfall records1000V/2500V insulation resistance testerMoisture detection meterBase-can drainage inspection tool
Safety Precautions
  • LOTO before any testing.
  • Treat any base can with standing water as potentially energised until the circuit is confirmed isolated.
  • Exercise caution working in wet conditions with electrical test equipment — use appropriately rated PPE.
Diagnostic Procedure
  1. ObservationCorrelate CCR earth-fault trend/trip history against rainfall records to confirm the fault genuinely tracks with rain events rather than an unrelated coincidence.
  2. ObservationDuring or immediately after a rain event, inspect accessible base cans along the circuit for standing water and check drainage function.
  3. MeasurementWhere a specific base can, connector, or fixture is suspected, test its insulation resistance during or immediately after a wet period, since a dry-weather test alone will likely not reproduce the fault.
  4. DecisionWet-condition testing localises the leakage to a specific base can, connector, or fixture → proceed to repair per SCF-397/398/399/400 as applicable. Leakage present broadly across the loop only when wet → treat as widespread drainage/sealing issue requiring a systematic review.
  5. Fault LocationConfirm the specific water ingress point using wet-condition testing rather than relying on dry-weather inspection alone.
  6. Root CauseIdentify whether the mechanism is base can flooding, a failed seal, or surface water tracking, to guide the correct repair.
Test Points
Earth-fault/leakage reading during or immediately after rain vs. dry-weather baseline
Base can water level and drainage function
Suspect connector/gasket condition when wet
Root Cause Verification

Re-test insulation resistance during a subsequent rain event (or with a controlled water application if a genuine rain event is not practical to wait for) to confirm the repair holds under the conditions that previously produced the fault.

Corrective Action
  • Clear and restore base can drainage where flooding is the cause.
  • Replace or reseal the degraded connector, splice, or gasket identified as the moisture entry point.
  • Reseal or replace the cracked cable jacket section where identified.
Preventive Action
  • Verify base can drainage ahead of the wet season as a standing preventive maintenance item.
  • Include wet-condition insulation testing (or scheduled testing shortly after a rain event) for circuits with a history of rain-dependent faults.
  • Correlate the earth-fault trend log against weather data routinely rather than only after a fault is reported.
Related References

SOP: SOP-SCF-19 (Rain-Dependent Fault Diagnostic Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: IEC 60364, ICAO Annex 14 Vol I, FAA AC 150/5340-30

Diagram: Base Can Drainage Detail Drawing

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue
SCF-410 Switching-Dependent Fault

Symptom — A series circuit fault manifests specifically at the moment of energisation, de-energisation, or an intensity-step change, rather than during steady-state operation, suggesting the fault is related to switching transients rather than continuous loading.

Equipment / System

CCR output switching stage, series circuit primary loop, and isolation transformers, considered specifically around the moment of a switching event.

Possible Causes
HighA marginal insulation or connection weak point that only breaks down under the voltage/current transient of switching, not under steady-state current
HighCCR step-change or soft-start behaviour producing a transient inrush that stresses a weak point in the loop
MedA transformer with marginal insulation that is stressed specifically by the switching transient rather than steady operation
MedA protective device (earth-fault/ground-fault trip) with a sensitivity setting that responds to a normal switching transient as if it were a fault
LowContactor or CCR internal switching component wear producing an inconsistent transient at the moment of switching
Required Tools
CCR output waveform capture / power quality loggerTrue-RMS multimeter with min/max or peak holdOscilloscope (for detailed transient capture where available)CCR earth-fault trend log correlated with switching events
Safety Precautions
  • LOTO before any testing not specifically requiring the circuit to be live.
  • Where live transient capture is required, ensure equipment is correctly rated and connected by a competent person before switching.
Diagnostic Procedure
  1. ObservationConfirm the fault correlates specifically with switching events (energisation, de-energisation, or intensity-step changes) rather than occurring at random during steady-state operation.
  2. MeasurementCapture the circuit's current/voltage waveform through several switching events using a power quality logger or oscilloscope to characterise the transient.
  3. MeasurementCompare the transient profile against the CCR's rated soft-start/step-change behaviour to determine whether the transient itself is abnormal or a weak point in the loop is reacting to a normal transient.
  4. DecisionTransient found abnormal at the CCR → route to Division 01 for CCR investigation. Transient normal but a specific loop component reacts to it → isolate and test that component's tolerance to a simulated switching transient.
  5. Fault LocationIdentify the specific component (transformer, connection, or protective device setting) that reacts to the switching transient.
  6. Root CauseConfirm whether the underlying issue is a genuinely marginal component or an over-sensitive protective device setting responding to normal switching behaviour.
Test Points
Waveform capture across several switching events
CCR soft-start/step-change transient profile vs. specification
Component condition/insulation at the point identified as reacting to the transient
Root Cause Verification

Re-test across multiple subsequent switching events, including energisation, de-energisation, and step changes, to confirm the fault no longer recurs under normal switching operation.

Corrective Action
  • Replace or repair the specific component confirmed to fail under normal switching transients.
  • Adjust protective device sensitivity settings if confirmed to be responding to a normal transient rather than a genuine fault, only after engineering review.
  • Escalate to CCR-level investigation and correction if the transient itself is confirmed abnormal.
Preventive Action
  • Capture and retain a reference switching transient profile for each circuit at commissioning for future comparison.
  • Include switching-event behaviour in periodic testing for circuits with a history of switching-dependent faults.
  • Review protective device sensitivity settings against the CCR's actual normal transient profile during commissioning.
Related References

SOP: SOP-SCF-20 (Switching Transient Fault Diagnostic Procedure), SOP-CCR-02 (CCR Output Verification)

Standards: FAA AC 150/5345-10, ICAO Annex 14 Vol I, IEC 60364

Diagram: CCR Output Waveform Reference Profile

Case Study: Division 01 — CCR Troubleshooting (soft-start and step-change behaviour)

Rev 0 — Initial issue
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Constant Current Regulator Diagram

A constant current regulator diagram shows the CCR's supply input, its regulated output into the series loop, and the brightness-step control. In series circuit fault work it tells you where to measure CCR output current and loop resistance, and which alarms or protection indications to expect from the regulator. Use it with the circuit's own as-built drawings, and see the AGL diagrams and schematics page for general circuit diagrams.

Frequently Asked Questions

Quick answers about Series Circuit Electrical Faults troubleshooting in the AGL Troubleshooting Atlas.

Q1What does Division 15 of the AGL Troubleshooting Atlas cover?

Division 15 is the Series Circuit Electrical Faults Troubleshooting reference. It publishes 20 diagnostic topics, SCF-391 to SCF-410, for Series Circuit Electrical Faults on airfield ground lighting systems.

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 Series Circuit Electrical Faults are covered?

The 20 topics are SCF-391 to SCF-410: Unexpected Circuit Resistance; Excessive Circuit Resistance; Low Circuit Resistance; Circuit Voltage Drop Excessive; Circuit Current Imbalance; Current Leakage to Earth; Earth Fault at Light; Earth Fault at Transformer; Earth Fault at Cable; Earth Fault at Connector; Multiple Simultaneous Light Failure; Single Light Failure; Two-Point Fault; Multiple Open Circuits; Hidden Open Circuit; Intermittent Open Circuit; High Resistance Fault; Temperature-Dependent Fault; Rain-Dependent Fault; Switching-Dependent Fault.

Q4How does a diagnostic procedure begin in Division 15?

Each procedure starts with observation before any intervention. For example, SCF-391 (Unexpected Circuit Resistance) begins: Re-verify the baseline value in the circuit record and confirm it corresponds to the correct circuit and the correct test method (loop, or loop-to-earth).

Q5What safety precautions apply when troubleshooting Series Circuit Electrical Faults?

Each topic lists its own precautions. For example, SCF-391 (Unexpected Circuit Resistance) starts with: LOTO the CCR before taking any resistance measurement.

Q6Which SOPs are referenced in Division 15?

Related references in the topics include SOP-SCF-01 (Series Circuit Resistance Test Procedure), SOP-CCR-04 (CCR LOTO Procedure), SOP-SCF-02 (Loop Resistance Fault Location Procedure), SOP-SCF-03 (Reduced-Resistance Fault Location Procedure), SOP-SCF-04 (Voltage Drop Test Procedure), SOP-CCR-02 (CCR Output Verification), SOP-SCF-05 (Circuit Current Verification Procedure) and SOP-SCF-06 (Earth Leakage Fault Location Procedure), and 14 more.

Q7Which standards are referenced in the Series Circuit Electrical Faults topics?

Where a standard applies, it appears in the topic's Related References. Those cited in this division include IEC 60364, ICAO Annex 14 Vol I, FAA AC 150/5340-30, FAA AC 150/5345-10, IS 12290, FAA AC 150/5345-46, FAA AC 150/5345-47 and FAA AC 150/5345-26.

Q8How can I find a specific fault in Division 15 quickly?

Use the search box at the top of the page to filter the 20 published topics by code, fault name or keyword; matching topics open automatically. Each diagnostic record can also be downloaded as a Word (.doc) file.

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