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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 22 of 30

Difficult / Intermittent Faults Troubleshooting

Diagnostic methodology for the AGL faults that will not sit still — conditions correlated with night, rain, dry weather, warm-up, heat, cold, CCR step, and switching, faults that clear on restart or cannot be reproduced at all, intermittent faults isolated to a connector, cable, transformer, fixture, CCR, or communication link, and the compound cases of multiple simultaneous intermittent faults, hidden underground faults, and chronic recurring faults that have outlasted several prior repairs. 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 · Difficult / Intermittent Fault Diagnosis (20)

01 · Difficult / Intermittent Fault Diagnosis

20 of 20 topics published
DIF-526 Fault Appears Only at Night

Symptom — A reported fault — a dark fixture, flicker, wrong intensity, or an earth-fault trip — is present only during night-time energised operation and cannot be reproduced during daytime testing or with the circuit run at reduced load in daylight.

Equipment / System

Any AGL circuit or component under its actual night-time energised operating condition, as distinct from a daytime de-energised or low-load bench check.

Possible Causes
HighThermal cycling from a full day of cooling followed by night-time full-current operation opens up a marginal connector or cable joint that reads fine cold and unloaded
HighThe fault is genuinely load- or voltage-dependent and only manifests at the circuit's actual night-time series current, which a daytime low-load or open-circuit check does not reproduce
MedA photocell- or ALCMS-scheduled step or circuit only energises at night, so a fault on that specific step or circuit is invisible to any daytime test that never exercises it
MedOvernight condensation forms inside a base can or connector as ambient temperature drops after sunset, creating a leakage path that dries out and disappears by the following daytime inspection
LowThe defect is cosmetic to a daytime observer — a dim fixture, subtle colour shift, or slight flicker — that is only perceptible against a dark background at night
LowWildlife or vehicle activity that only occurs after dark is mechanically disturbing a connector or cable at a specific location
Required Tools
CCR output/temperature datalogger for an overnight spanNight inspection vehicle and portable floodlightThermal imaging cameraTrue-RMS multimeter with min/max holdWeather/temperature log for the reporting period
Safety Precautions
  • Coordinate all night airside access through ATC/apron control before approaching any fixture on an active movement area.
  • Treat the circuit as energised at full night-time current during any inspection intended to catch the fault live; LOTO before touching any suspect connection once located.
Diagnostic Procedure
  1. ObservationCross-check every reported occurrence against the night flight-check log and any available CCR trend history to confirm the fault is genuinely absent during daytime energised checks.
  2. MeasurementFit a temporary datalogger on the affected circuit's CCR output and, if available, ambient temperature, spanning at least one full day-to-night cycle.
  3. MeasurementSchedule a night inspection timed to the specific reported condition (time of night, intensity step, weather) rather than a generic walk-through.
  4. DecisionFault correlates with the overnight temperature drop → treat as thermal/condensation-related and proceed under DIF-531 or DIF-538. Fault correlates only with actual night-time load current → treat as load-dependent and proceed under the relevant component division.
  5. Fault LocationIsolate to the specific fixture, connector, or circuit segment active at the time the fault was captured.
  6. Root CauseConfirm by reproducing the fault under controlled night conditions matching the original report before closing out the ticket.
Test Points
CCR output current/voltage trend across a full day-night cycle
Ambient temperature and humidity log for the reporting period
Connector contact resistance measured both cold (day) and after night-time thermal cycling
Night visual inspection log at the exact reported time and intensity step
Root Cause Verification

Confirm the fault does not recur over at least two further night-time operating cycles under conditions similar to the original report, and log the correlating condition (temperature swing, specific step, or load current) in the maintenance record.

Corrective Action
  • Re-terminate or re-torque a connector shown to open up under night-time thermal cycling.
  • Reseal a base can or connector found to admit condensation overnight.
  • Adjust or correct the ALCMS/photocell schedule if a specific night-only step was never being exercised or tested.
Preventive Action
  • Include a night-time energised check alongside the standard daytime PM cycle rather than relying on daytime testing alone.
  • Log ambient temperature alongside CCR trend data as standard practice to make day/night correlation possible.
  • Brief flight-check and night-inspection staff to record time, intensity step, and weather whenever an intermittent fault is observed.
Related References

SOP: SOP-DIF-01 (Night-Correlated Fault Capture Procedure)

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

Diagram: Overnight CCR Trend Log Template

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue
DIF-527 Fault Appears Only During Rain

Symptom — A fault — a dark or flickering fixture, an earth-fault trip, or reduced insulation resistance — is reported only during or shortly after rainfall and cannot be reproduced during dry-weather testing.

Equipment / System

Any below-grade or in-pavement AGL component and its cable/joint network exposed to surface or groundwater ingress — base cans, L-823 connectors, cable joints, and isolation transformers.

Possible Causes
HighMoisture ingress through a degraded gasket, cracked lens, or improperly seated L-823 connector creating a leakage path that only closes when water is present
HighA cracked or porous cable jacket admitting water at a joint or splice, dropping insulation resistance below the CCR's earth-fault threshold only while wet
MedRising groundwater or surface water pooling in an under-rated or poorly drained base can during heavy rain
MedA marginal insulation fault that is present but above the trip threshold when dry, and drops below it once moisture reduces the resistance further
LowSurface water bridging a damaged duct or handhole allowing water to track along the cable route to a distant weak point
LowA grounding/earthing conductor connection that only completes a stray path when the surrounding soil is saturated
Required Tools
1000V/2500V insulation resistance tester (megger)CCR earth-fault trend logMoisture detection meterCable route and joint location drawingWeatherproof torch and mirror for base-can inspection
Safety Precautions
  • LOTO the circuit before opening any base can, particularly during or after rain when water may be present around energised connections.
  • Treat a rain-correlated earth-fault trip as a genuine hazard indicator, not a nuisance trip, until insulation resistance is confirmed acceptable.
Diagnostic Procedure
  1. ObservationConfirm from the CCR trip log and maintenance history that the fault correlates specifically with rainfall events rather than temperature or load.
  2. MeasurementMegger the affected circuit or segment as soon as practical after a rain event, before the base cans have had time to dry out, and compare against the dry-weather baseline.
  3. MeasurementPhysically inspect base cans, connectors, and accessible joints along the affected run for standing water, a failed gasket, or a cracked lens.
  4. DecisionInsulation resistance recovers once dry and drops again after rain → confirmed moisture-related fault; isolate to the wettest base can or joint. Resistance stays low regardless of weather → treat as a standing insulation fault, not rain-dependent, and route to Division 25.
  5. Fault LocationIsolate to the specific base can, connector, or cable joint showing the greatest insulation resistance drop when wet.
  6. Root CauseConfirm the water entry point by physical inspection rather than assuming the location of the lowest reading is necessarily the entry point, since water can track along ducting.
Test Points
Insulation resistance wet vs. dry, segment by segment
Base can and connector condition (gasket, lens, seating)
Duct/handhole condition along the suspected route
CCR earth-fault trip log correlated against local rainfall records
Root Cause Verification

Re-megger the repaired segment during or immediately after the next rain event and confirm the reading holds above the acceptable threshold; do not close out on a dry-weather reading alone.

Corrective Action
  • Replace a failed gasket, cracked lens, or degraded L-823 connector seal.
  • Repair or replace the cable segment or joint found to be admitting water.
  • Improve drainage at a base can shown to be collecting standing water.
Preventive Action
  • Schedule insulation resistance testing during or immediately after the wet season rather than relying solely on dry-weather PM readings.
  • Inspect and replace ageing connector gaskets and lens seals proactively ahead of the monsoon/wet season.
  • Verify base-can drainage as a standard PM item, particularly on low-lying sections of the airfield.
Related References

SOP: SOP-DIF-02 (Weather-Correlated Insulation Fault Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: IEC 60364 (insulation testing), ICAO Annex 14 Vol I, IS 12290

Diagram: Cable Route and Joint Location Drawing

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue
DIF-528 Fault Appears Only During Dry Weather

Symptom — A fault is reported only during dry periods and clears or does not recur once rain returns — the reverse pattern of a typical moisture fault — most often surfacing as an earthing/grounding-related trip, a dust-related dimming, or a heat-related step fault.

Equipment / System

Airfield earthing/grounding system, exposed connectors and lenses subject to dust and contamination, and any component whose behaviour is temperature-linked to dry-season heat.

Possible Causes
HighRising earth/ground resistance in dry, low-moisture soil pushing the airfield earthing system above its acceptable threshold, only recovering once the soil is wet again
HighDust and airborne contamination bridging a marginal connector or terminal that a damp environment would otherwise keep from arcing
MedDry-season ambient temperatures pushing equipment into a heat-related fault mode covered separately under DIF-530
MedStatic discharge from very low humidity affecting sensitive CCR control cards or an ALCMS interface
LowDust accumulation on lenses reducing apparent luminance to the point of a reported dim/failure complaint that clears once rain washes the lenses
LowSeasonal ground movement/shrinkage in dry, expansive soils placing mechanical stress on a duct or cable route
Required Tools
Earth resistance tester (fall-of-potential or clamp-on)Insulation resistance tester (megger)Luminance/photometric meterThermal imaging cameraSoil moisture reference log
Safety Precautions
  • LOTO before accessing any earthing conductor or connection for testing.
  • Treat an elevated earth resistance reading as a genuine safety and lightning-protection concern, not merely a seasonal nuisance, until corrected.
Diagnostic Procedure
  1. ObservationConfirm the fault correlates specifically with dry periods and not with a coincidental rise in ambient temperature or CCR loading.
  2. MeasurementTest earth/ground resistance at the affected location and compare against both the design threshold and the same point's wet-season reading.
  3. MeasurementInspect the affected fixture, connector, or terminal for dust, contamination, or a visibly degraded lens.
  4. DecisionEarth resistance rises well above threshold in dry soil → confirmed seasonal earthing issue; consider supplementary grounding. Reading stays within tolerance → investigate contamination or heat-related causes instead.
  5. Fault LocationIsolate to the specific earth electrode, terminal, or lens/connector responsible for the seasonal variation.
  6. Root CauseConfirm the corrected reading holds through at least one further dry spell before closing out.
Test Points
Earth/ground resistance, dry-season vs. wet-season baseline
Lens/connector contamination level
CCR control card condition where static discharge is suspected
Soil moisture and ambient temperature log for the reporting period
Root Cause Verification

Re-test earth resistance and/or luminance at the same location during the next dry spell and confirm the corrected value holds within the acceptable range.

Corrective Action
  • Install supplementary earth electrodes or a chemical earthing treatment where dry-season resistance consistently exceeds the design threshold.
  • Clean lenses and connectors as part of a dry-season-specific PM pass.
  • Add anti-static/surge protection at a control card shown to be affected by low-humidity static discharge.
Preventive Action
  • Test earth resistance at both the height of the wet season and the height of the dry season to capture the full seasonal range, not a single annual reading.
  • Increase lens-cleaning frequency during known dusty/dry periods.
  • Track dry-season earth resistance trends year over year to catch a worsening pattern before it becomes a lightning-protection risk.
Related References

SOP: SOP-DIF-03 (Seasonal Earthing Verification Procedure)

Standards: IS 3043 (earthing), ICAO Annex 14 Vol I, IEEE 142

Diagram: Airfield Earthing/Grounding Layout Drawing

Case Study: Division 27 — Lightning Protection & Earthing

Rev 0 — Initial issue
DIF-529 Fault Appears After Warm-Up

Symptom — Equipment operates correctly for a period after being switched on cold, then develops a fault — drift in output, a dropped fixture, a nuisance trip — once it has reached normal operating temperature.

Equipment / System

CCR control electronics, thyristor/SCR banks, and any AGL component whose parameters are known to drift with self-heating during sustained operation.

Possible Causes
HighThermal drift of a CCR control card, feedback sensor, or thyristor firing circuit once it reaches its normal operating temperature
HighA marginal connection whose contact resistance rises as the surrounding enclosure heats up, eventually opening under thermal expansion
MedA cooling fan or ventilation path in the CCR cabinet that is inadequate under sustained full-current operation, allowing internal temperature to climb past a component's rated limit
MedAn electronic component (capacitor, semiconductor) nearing end of life that only fails once self-heating pushes it past its degraded tolerance
LowA calibration or feedback loop that has a temperature-dependent offset not accounted for in the control algorithm
LowA cable or transformer insulation weak point that only breaks down once conductor temperature rises under load
Required Tools
Thermal imaging cameraTrue-RMS multimeter with min/max holdCCR cabinet temperature dataloggerOscilloscope for control-signal drift analysisManufacturer's thermal derating specification
Safety Precautions
  • LOTO before any internal cabinet inspection; allow the cabinet to cool if opening it live is not required for the specific test.
  • Be alert to genuinely hot internal components during a live thermal inspection and avoid contact.
Diagnostic Procedure
  1. ObservationEstablish the typical elapsed time or duration of operation before the fault appears, and whether it is consistent across occurrences.
  2. MeasurementLog CCR cabinet and key component temperatures alongside output parameters from cold start through to the point the fault appears.
  3. MeasurementUse thermal imaging at the point the fault manifests to identify any component running hotter than its neighbours or above its rated temperature.
  4. DecisionA specific component is identified as running abnormally hot → isolate to that component for replacement. Temperatures are within normal range but drift still occurs → treat as a calibration/feedback issue rather than a thermal failure.
  5. Fault LocationIsolate to the specific control card, connection, or component whose behaviour changes with temperature.
  6. Root CauseConfirm by artificially warming the isolated component (within safe limits) on the bench and reproducing the drift or fault.
Test Points
CCR cabinet internal temperature from cold start to fault onset
Thermal image of control cards and power components at fault onset
Cooling fan/ventilation operation under sustained load
Control signal drift vs. temperature on the oscilloscope
Root Cause Verification

Run the corrected equipment through a full sustained-operation cycle matching or exceeding the original time-to-fault and confirm no drift or fault recurs.

Corrective Action
  • Replace the identified drifting or degraded component (control card, capacitor, connector).
  • Improve CCR cabinet ventilation or cooling if internal temperatures are found to exceed manufacturer limits.
  • Recalibrate a feedback loop found to have a temperature-dependent offset.
Preventive Action
  • Include a sustained-operation thermal check in the periodic PM cycle rather than testing only at cold start.
  • Trend CCR cabinet temperatures over time to catch a developing cooling deficiency early.
  • Replace ageing electronic components proactively once they approach their rated service life, rather than waiting for a thermal-onset failure.
Related References

SOP: SOP-DIF-04 (Thermal Drift Diagnostic Procedure)

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

Diagram: CCR Cabinet Thermal Layout Drawing

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue
DIF-530 Fault Appears When Equipment Is Hot

Symptom — A fault appears specifically once equipment reaches an elevated operating or ambient temperature — during peak daytime heat, after extended full-load running, or in a poorly ventilated enclosure — and clears once the equipment cools.

Equipment / System

CCR cabinets, isolation transformers, and any AGL component subject to a temperature-dependent thermal cutout, derating, or component failure mode.

Possible Causes
HighA thermal protection cutout (CCR or transformer) tripping once internal temperature exceeds its set point under sustained high load or high ambient conditions
HighContact resistance at a connector or terminal increasing with heat until it crosses into an open-circuit or high-resistance fault state
MedA transformer or cable operating above its thermal rating for the ambient conditions, with insulation resistance dropping as temperature rises
MedInadequate cabinet ventilation, a blocked filter, or a failed cooling fan allowing internal temperature to climb beyond design limits
LowA semiconductor device (thyristor, diode) exhibiting a known temperature-dependent failure mode as it approaches its rated junction temperature
LowExternal heat sources — direct sun on a cabinet, proximity to other hot equipment — pushing ambient temperature at the enclosure beyond the design envelope
Required Tools
Thermal imaging cameraCabinet/component temperature dataloggerTrue-RMS multimeterManufacturer's thermal cutout and derating specificationInsulation resistance tester (megger)
Safety Precautions
  • LOTO before touching any component suspected of being at an elevated temperature; allow time to cool if practical.
  • Do not defeat or bypass a thermal protection device to keep equipment running — investigate and correct the underlying cause instead.
Diagnostic Procedure
  1. ObservationConfirm the fault correlates with elevated ambient temperature, extended run time under full load, or both, using available trend data.
  2. MeasurementLog cabinet/component temperature against time and load, capturing the point at which the fault occurs.
  3. MeasurementCheck ventilation, filters, and cooling fan operation for the affected enclosure.
  4. DecisionA thermal cutout is confirmed tripping at its set point → investigate the cooling deficiency or load condition causing the over-temperature. No cutout involved, but a connection or component still fails when hot → isolate to that specific component.
  5. Fault LocationIsolate to the specific cutout, connection, or component whose behaviour changes at elevated temperature.
  6. Root CauseConfirm by reproducing the fault under a controlled heat-soak test matching the original conditions.
Test Points
Cabinet/component temperature vs. time under sustained full load
Cooling fan and filter condition
Thermal cutout set point vs. actual trip temperature
Insulation resistance at elevated operating temperature vs. cold baseline
Root Cause Verification

Run the corrected equipment through a full heat-soak cycle under conditions equal to or more severe than the original fault conditions and confirm no recurrence.

Corrective Action
  • Clear a blocked filter or replace a failed cooling fan.
  • Replace a connector or component confirmed to fail specifically at elevated temperature.
  • Relocate or shield equipment exposed to an avoidable external heat source.
Preventive Action
  • Include a summer/peak-heat thermal check in the seasonal PM cycle.
  • Trend cabinet temperatures against ambient conditions to catch a developing cooling deficiency before it causes a trip.
  • Verify thermal cutout set points against manufacturer specification during periodic calibration checks.
Related References

SOP: SOP-DIF-05 (Elevated-Temperature Fault Diagnostic Procedure)

Standards: FAA AC 150/5345-10, IS 12290

Diagram: CCR/Transformer Thermal Rating Chart

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue
DIF-531 Fault Appears When Equipment Is Cold

Symptom — A fault — a dark fixture, a nuisance trip, or a flicker — appears specifically during cold starts, early-morning low-temperature operation, or in winter conditions, and clears once the equipment warms up or ambient temperature rises.

Equipment / System

Any AGL component subject to cold-related contraction, condensation, or low-temperature material and electronic behaviour — connectors, cable jackets, isolation transformers, and CCR electronics.

Possible Causes
HighThermal contraction of a connector or terminal at low temperature loosening a contact that was adequately seated when warm
HighCondensation forming inside a base can or cabinet as temperature drops overnight, then freezing or creating a leakage path at cold start
MedA cable jacket or connector seal becoming brittle and cracking at low temperature, admitting moisture that would not otherwise enter
MedElectronic components (capacitors, control cards) exhibiting known low-temperature parameter drift or start-up instability
LowIce or frost accumulation on a lens or photocell affecting apparent output or triggering activation logic
LowA lubricant or grease in a mechanical connector stiffening at low temperature and preventing full seating
Required Tools
True-RMS multimeter with min/max holdThermal imaging cameraInsulation resistance tester (megger)Ambient temperature log for the reporting periodManufacturer's low-temperature operating specification
Safety Precautions
  • LOTO before opening any base can or cabinet, particularly where ice or frost is present, since moisture and cold both increase shock risk.
  • Handle cold, brittle cable jackets and seals carefully during inspection to avoid causing additional damage.
Diagnostic Procedure
  1. ObservationConfirm the fault correlates specifically with low ambient temperature or cold-start conditions rather than a coincidental factor such as time of day.
  2. MeasurementInspect suspect connectors and terminals for looseness at low temperature, comparing torque/seating against the same point once warmed.
  3. MeasurementMegger the affected circuit at cold-start conditions and again once the circuit and ambient have warmed, to isolate a condensation- or contraction-related insulation drop.
  4. DecisionFault clears once equipment warms and connections re-seat → confirmed cold-contraction issue; re-terminate affected connectors. Fault persists regardless of temperature once triggered → treat as a separate standing fault rather than cold-related.
  5. Fault LocationIsolate to the specific connector, seal, or electronic component showing cold-dependent behaviour.
  6. Root CauseConfirm by reproducing the fault under controlled cold-start conditions matching the original report.
Test Points
Connector torque/seating, cold vs. warmed
Insulation resistance, cold-start vs. warmed
Condensation/frost presence inside base cans and cabinets
CCR control card behaviour through a cold-start cycle
Root Cause Verification

Confirm the fault does not recur through at least two further cold-start cycles under conditions similar to or colder than the original report.

Corrective Action
  • Re-terminate and properly torque connectors shown to loosen under cold contraction.
  • Replace a cracked or brittle cable jacket or connector seal.
  • Add or improve base-can/cabinet insulation or heating where condensation and freezing are a recurring issue.
Preventive Action
  • Include a cold-start energised check in the winter-season PM cycle.
  • Use low-temperature-rated seals and connectors in locations with a known cold-season history.
  • Trend cold-start faults by location year over year to identify sites needing a permanent fix rather than repeated seasonal repair.
Related References

SOP: SOP-DIF-06 (Cold-Start Fault Diagnostic Procedure)

Standards: FAA AC 150/5345-10, IS 12290

Diagram: Base Can Winterisation Detail Drawing

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue
DIF-532 Fault Appears at Higher CCR Step

Symptom — A fault — a fixture dropping out, an earth-fault trip, or abnormal harmonics — appears only when the CCR is commanded to a higher intensity step and is absent at lower steps.

Equipment / System

CCR output stage and the full series circuit under it, evaluated across its full range of commanded intensity steps.

Possible Causes
HighA marginal insulation weak point on the cable or a transformer winding that only breaks down once voltage/current rises to the level associated with a higher step
HighA connector or joint with elevated contact resistance overheating and failing only once current reaches the higher-step level
MedA CCR output stage (thyristor bank, tap) with a fault specific to its higher-output configuration
MedHarmonic or transient stress at higher steps stressing a marginal component that tolerates lower-step operation without issue
LowA series circuit loop with a marginal joint that only draws enough current at a higher step to generate a detectable heating or arcing signature
LowAn earth-fault protection threshold calibrated close to the normal leakage current at higher steps, tripping on a marginal but not yet failed insulation reading
Required Tools
Clamp-on series circuit ammeterCCR output trend log across all intensity steps1000V/2500V insulation resistance tester (megger)Thermal imaging cameraPower quality/harmonics analyser
Safety Precautions
  • LOTO before any hands-on component testing; do not attempt to trace a live higher-step fault by touch.
  • Coordinate step-by-step live testing with ATC/apron control given the circuit will be run at full commanded output during diagnosis.
Diagnostic Procedure
  1. ObservationConfirm from the CCR trend log or trip history the specific step(s) at which the fault reliably appears and the steps at which it does not.
  2. MeasurementWith the circuit isolated, megger the full loop and individual accessible segments, noting any reading that is marginal rather than a hard failure.
  3. MeasurementRe-energise under controlled conditions and step the CCR incrementally, using thermal imaging to spot a connector or joint that heats disproportionately as output rises.
  4. DecisionA specific segment or connector is confirmed heating/failing only at the higher step → isolate and repair that point. Fault is CCR-internal rather than circuit-side → route to Division 01 for output-stage investigation.
  5. Fault LocationIsolate to the specific cable segment, joint, transformer, or CCR output stage responsible for the step-dependent behaviour.
  6. Root CauseConfirm by re-testing across the full step range after repair, holding at the previously faulting step for an extended period.
Test Points
Insulation resistance across accessible segments, marginal vs. hard-failed
Thermal image of connectors/joints stepped from low to high output
CCR output current and harmonics at each intensity step
Earth-fault trip threshold vs. actual leakage current at the higher step
Root Cause Verification

Hold the circuit at the previously faulting step for an extended period after repair and confirm no recurrence, then re-verify across the full step range.

Corrective Action
  • Repair or replace the cable segment, joint, or transformer confirmed to fail at higher current/voltage.
  • Re-terminate a connector confirmed to overheat only at higher-step current.
  • Adjust CCR earth-fault threshold only if confirmed to be miscalibrated relative to actual leakage current, not as a substitute for fixing the insulation fault.
Preventive Action
  • Include a full step-range test, not just a single intensity check, in the periodic PM cycle.
  • Trend insulation resistance readings to catch a segment trending toward a step-dependent failure before it becomes a live fault.
  • Verify CCR earth-fault thresholds are correctly calibrated to expected leakage current at full output.
Related References

SOP: SOP-DIF-07 (Step-Dependent Fault Diagnostic Procedure), SOP-CCR-04 (CCR LOTO Procedure)

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

Diagram: CCR Step-Output Trend Template

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue
DIF-533 Fault Appears at Lower CCR Step

Symptom — A fault — flicker, dimming below spec, or an unstable output — appears only when the CCR is commanded to a lower intensity step and is absent at higher steps.

Equipment / System

CCR output/regulation stage and the series circuit under it, particularly its behaviour at minimum or reduced-output configurations.

Possible Causes
HighThyristor/SCR firing-angle instability at the shallow conduction angles required for low-output steps, common as a control card ages
HighA minimum-load condition on the series circuit close to the CCR's rated minimum, causing regulation instability at the lowest steps
MedA dimming/regulation feedback loop with reduced accuracy or hunting behaviour at the low end of its control range
MedAn LED driver or lamp/LED module with poor low-current performance, flickering or dropping out at reduced drive current
LowElectrical noise or interference proportionally more significant relative to a low control signal than to a high one
LowA standby/emergency step configuration that is rarely exercised and has developed an undetected fault
Required Tools
Oscilloscope for firing-angle/waveform analysisCCR output trend log across all intensity stepsClamp-on series circuit ammeterTrue-RMS multimeter with min/max holdManufacturer's CCR regulation specification
Safety Precautions
  • LOTO before any internal CCR inspection.
  • Coordinate live step-by-step testing with ATC/apron control since low-step operation is often used during specific traffic conditions.
Diagnostic Procedure
  1. ObservationConfirm from the CCR trend log the specific low step(s) at which the fault appears and its absence at higher steps.
  2. MeasurementCapture the CCR output waveform on an oscilloscope at the faulting low step to check for firing-angle instability or hunting.
  3. MeasurementConfirm the series circuit's total connected load is within the CCR's rated minimum for the affected step.
  4. DecisionWaveform shows firing-angle instability or hunting → CCR control card/regulation issue, route to Division 01. Waveform is stable but individual fixtures flicker → investigate lamp/LED module low-current performance instead.
  5. Fault LocationIsolate to the CCR control card, the specific fixture/module with poor low-current behaviour, or a marginal circuit load condition.
  6. Root CauseConfirm by holding the circuit at the faulting low step for an extended period after correction and verifying stable output.
Test Points
CCR output waveform at each low intensity step
Total connected circuit load vs. CCR rated minimum
Individual fixture behaviour at the lowest commanded step
Control card firing-angle stability under sustained low-step operation
Root Cause Verification

Hold the circuit at the previously faulting low step for an extended period and confirm stable, flicker-free output before closing out.

Corrective Action
  • Replace or recalibrate a CCR control card exhibiting firing-angle instability at low output.
  • Replace an LED module or lamp confirmed to have poor low-current performance.
  • Adjust circuit configuration if total load is found to be below the CCR's rated minimum for stable regulation.
Preventive Action
  • Include a full low-step stability check, not just a nominal-output check, in the periodic PM cycle.
  • Trend low-step waveform stability over time to catch a control card degrading before it produces a visible flicker.
  • Verify circuit loading remains within the CCR's rated operating range whenever fixtures are added or removed.
Related References

SOP: SOP-DIF-08 (Low-Step Regulation Diagnostic Procedure)

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

Diagram: CCR Regulation Range Specification Sheet

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue
DIF-534 Fault Appears During Switching

Symptom — A fault — a nuisance trip, a fixture dropout, or a visible flash/flicker — occurs specifically during a CCR intensity-step change, circuit changeover, or on/off switching event, rather than during steady-state operation at any step.

Equipment / System

CCR switching/contactor stage, transfer/changeover equipment, and any downstream component sensitive to switching transients.

Possible Causes
HighInrush or transient current during step change or energisation stressing a marginal connector, joint, or component that tolerates steady-state current without issue
HighContactor or relay contact bounce/wear generating a transient during switching that a protection device interprets as a fault
MedThyristor commutation issues during a step transition producing a brief but detectable disturbance
MedA changeover/transfer switch (e.g., standby power or circuit selector) with worn or misaligned contacts generating a transient during operation
LowElectromagnetic interference generated by the switching event itself affecting a nearby sensitive control or communication circuit
LowA protection relay/earth-fault device with a trip threshold too close to the normal transient signature of a switching event
Required Tools
Oscilloscope with transient capture capabilityCCR/relay event log with timestamped switching recordsClamp-on series circuit ammeter with inrush captureContact resistance meter for contactor/relay inspectionManufacturer's switching transient specification
Safety Precautions
  • LOTO before any hands-on inspection of contactors, relays, or transfer switches.
  • Treat repeated switching-related trips as a developing protection or contactor issue rather than a routine nuisance, and investigate before disabling any protection function.
Diagnostic Procedure
  1. ObservationConfirm from event logs that the fault timestamp consistently aligns with a step change, energisation, or changeover event rather than steady-state operation.
  2. MeasurementCapture the switching event on an oscilloscope or transient recorder to characterise the disturbance — duration, magnitude, and affected phase/conductor.
  3. MeasurementInspect the relevant contactor, relay, or transfer switch contacts for wear, pitting, or misalignment consistent with bounce or poor make/break.
  4. DecisionTransient is traced to a specific contactor/relay with worn contacts → replace or service that device. Transient is inherent to the switching event and affecting a marginal downstream component → isolate and repair that component instead.
  5. Fault LocationIsolate to the specific contactor, relay, transfer switch, or downstream component responsible for the switching-correlated fault.
  6. Root CauseConfirm by cycling the switching event repeatedly after repair and verifying no recurrence of the transient or trip.
Test Points
Transient waveform capture at the moment of switching
Contactor/relay contact condition and resistance
Protection relay/earth-fault trip threshold vs. normal switching transient magnitude
Repeated switching cycle test post-repair
Root Cause Verification

Cycle the switching event (step change, energisation, or changeover) repeatedly under representative conditions and confirm no recurrence of the fault or trip.

Corrective Action
  • Service or replace a contactor/relay exhibiting contact bounce or wear.
  • Repair or replace the downstream component confirmed to be stressed by the switching transient.
  • Adjust a protection threshold only if confirmed to be miscalibrated relative to the normal switching transient, not as a substitute for fixing a genuine fault.
Preventive Action
  • Include a contactor/relay contact inspection in the periodic PM cycle, particularly for frequently switched circuits.
  • Trend switching-event logs to catch a developing contactor issue before it produces a hard trip.
  • Verify protection thresholds are set with adequate margin above normal switching transients.
Related References

SOP: SOP-DIF-09 (Switching Transient Diagnostic Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: IEC 60364, FAA AC 150/5345-10

Diagram: CCR Switching/Contactor Schematic

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue
DIF-535 Fault Disappears After Restart

Symptom — A fault clears completely as soon as the CCR, ALCMS panel, or affected fixture is power-cycled or restarted, with no component replacement, and the equipment then runs normally — until the fault eventually recurs, often unpredictably.

Equipment / System

CCR/ALCMS control electronics and firmware, and any component whose fault signature is consistent with a transient upset rather than a hard failure.

Possible Causes
HighA firmware or software latch/flag in the CCR or ALCMS controller that becomes set by a transient condition and is only cleared by a power cycle or reset command
HighA transient environmental condition (a brief voltage sag, an EMI event, a momentary short) that has genuinely cleared on its own by the time of restart, with the restart itself not the actual fix
MedA marginal connection that happened to be jarred back into contact by the physical act of accessing the equipment to restart it
MedA control processor or memory fault requiring a reset to recover, potentially indicative of a developing hardware issue rather than a software-only one
LowA communication link (fieldbus, Modbus, fibre) losing synchronisation and re-establishing cleanly only after a restart
LowA protection device (surge, thermal, earth-fault) that legitimately tripped on a real but transient event and simply needs to be reset rather than indicating equipment damage
Required Tools
CCR/ALCMS event and fault log with timestampsPower quality logger for upstream supply eventsFirmware/software version and changelog referenceTrue-RMS multimeter with min/max holdCommunication link diagnostic tool (where applicable)
Safety Precautions
  • LOTO before any physical inspection prompted by a restart-clearable fault, even though the fault itself has cleared.
  • Do not adopt "restart and move on" as a standing practice — log every occurrence and investigate the underlying cause before it develops into a hard failure.
Diagnostic Procedure
  1. ObservationPull the full CCR/ALCMS event log for the fault occurrence, noting the exact fault code, timestamp, and any preceding events.
  2. MeasurementCheck upstream power quality logs for a voltage sag, surge, or interruption coinciding with the fault onset.
  3. MeasurementReview firmware/software version history for any known issue matching the observed fault code that is resolved by a later update.
  4. DecisionA known firmware issue with a documented fix is identified → schedule the firmware update. No known software cause, and a power quality event coincides → investigate the upstream supply. Neither applies and the pattern is recurring → treat as a developing hardware fault and monitor closely rather than continuing to simply restart.
  5. Fault LocationIsolate to the specific control processor, firmware version, communication link, or upstream supply condition responsible.
  6. Root CauseDo not close out on "cleared by restart" alone — identify and document the underlying trigger before considering the fault resolved.
Test Points
CCR/ALCMS event and fault log detail at each occurrence
Upstream power quality log at the time of fault onset
Firmware/software version vs. known issue list
Communication link status through a fault-and-recovery cycle
Root Cause Verification

Monitor the equipment closely over an extended period after the underlying cause is addressed (firmware update, supply correction, or connection repair) and confirm the fault does not recur without a restart.

Corrective Action
  • Apply a firmware/software update if a known matching issue and fix exist.
  • Investigate and correct an upstream power quality issue if power events are confirmed to coincide with fault onset.
  • Escalate to hardware replacement if a control processor fault pattern persists despite ruling out software and power causes.
Preventive Action
  • Log every restart-clearable fault occurrence, not just recurring or severe ones, to build a pattern history.
  • Keep CCR/ALCMS firmware current and review manufacturer bulletins for known intermittent-fault fixes.
  • Install or verify power quality monitoring upstream of the CCR to catch supply-side transients independently.
Related References

SOP: SOP-DIF-10 (Restart-Clearable Fault Logging Procedure)

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

Diagram: CCR/ALCMS Event Log Template

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue
DIF-536 Fault Cannot Be Reproduced

Symptom — A fault has been reliably reported — by a pilot, vehicle operator, or night inspection — but cannot be reproduced during a subsequent maintenance visit despite equipment appearing to test normally on every parameter checked.

Equipment / System

Any AGL component or circuit where a genuine reported fault does not correlate with any abnormal reading during a standard follow-up test.

Possible Causes
HighThe fault is condition-dependent (temperature, load step, weather, time of day) and the follow-up test was not conducted under the same conditions as the original report
HighA genuinely intermittent connection or component that happened to be in its good state at the time of the follow-up test
MedThe original report was accurate but described a transient event (a single flicker, a momentary trip) that has since self-cleared and left no trace in available logs
MedA perception or observation issue — the reported condition was real but its cause was misattributed to the wrong fixture, circuit, or division
LowInsufficient logging/monitoring capability to have captured the state of the equipment at the actual time of the report
LowA rare coincidence of factors (a specific combination of step, weather, and load) that is statistically unlikely to recur during a scheduled test window
Required Tools
CCR/ALCMS full event and trend logStructured intermittent-fault report form (time, date, weather, step, location)Temporary datalogger for extended monitoringNight inspection vehicle for condition-matched re-testingCross-reference to related divisions' as-built drawings
Safety Precautions
  • Do not close out a reported fault as "not found" without a structured investigation — treat the original report as valid until disproven, particularly if it has safety implications.
Diagnostic Procedure
  1. ObservationInterview the original reporter (pilot, vehicle operator, inspector) if possible to capture the exact time, location, intensity step, and weather conditions of the observation.
  2. MeasurementPull the full CCR/ALCMS event and trend log covering the reported time window, even where no explicit fault code was logged, looking for any anomaly in current, voltage, or trip history.
  3. MeasurementFit a temporary datalogger and, where practical, schedule a follow-up inspection matched as closely as possible to the original reported conditions rather than a generic daytime check.
  4. DecisionA condition match reproduces the fault → route to the relevant condition-specific division (DIF-526 to DIF-534). No anomaly found even under matched conditions after repeated attempts → log as monitored/not reproduced, cross-reference with DIF-545, and keep the ticket open for pattern tracking rather than closing it outright.
  5. Fault LocationWhere reproduced, isolate as per the relevant fault-specific procedure. Where not reproduced, isolate the investigation to the specific location and condition window reported.
  6. Root CauseOnly close out with a confirmed cause; a "could not reproduce" outcome should remain flagged for trend tracking, not treated as resolved.
Test Points
CCR/ALCMS trend log for the exact reported time window
Original reporter's account of time, location, step, and weather
Condition-matched re-test results
Cross-reference against other open intermittent-fault tickets for the same location
Root Cause Verification

Where the fault is eventually reproduced, verify per the relevant condition-specific procedure. Where it remains unreproduced, verify only that the location has been placed under active monitoring rather than treating the absence of a repeat as confirmation of no fault.

Corrective Action
  • Where a cause is eventually identified, apply the corrective action specified under the matching condition-specific division.
  • Where no cause is found, install temporary or permanent monitoring at the reported location rather than taking no action.
Preventive Action
  • Standardise a fault report form capturing time, location, weather, and intensity step for every pilot/vehicle/inspector report, to make future correlation possible.
  • Maintain rolling trend logging on CCRs and ALCMS panels rather than only event-triggered logging, to increase the chance of capturing a transient after the fact.
  • Track "not reproduced" tickets by location over time to catch a recurring pattern that individual visits miss.
Related References

SOP: SOP-DIF-11 (Non-Reproducible Fault Investigation Procedure)

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

Diagram: Structured Intermittent Fault Report Form

Case Study: Division 22 — DIF-545 Recurring Unidentified Fault

Rev 0 — Initial issue
DIF-537 Intermittent Connector Fault

Symptom — A fixture or circuit segment exhibits an intermittent open, high-resistance, or short condition traced specifically to an L-823 or similar field connector, rather than the cable, transformer, or fixture itself.

Equipment / System

L-823 series/secondary connectors and equivalent field-mate connectors throughout the AGL network, including their base-can mounting and strain relief.

Possible Causes
HighThe connector is not fully seated or latched, allowing vibration from ground traffic to intermittently break contact
HighCorrosion or oxidation on the connector pins from prior moisture ingress creating a high-resistance contact that varies with vibration or thermal expansion
MedA damaged or degraded gasket allowing intermittent moisture ingress that creates a leakage path only under certain conditions
MedMechanical damage to the connector shell or pins from a previous rough handling or installation, creating a marginal contact
LowIncorrect or mismatched connector type/rating for the application, resulting in a poor mechanical fit
LowGrease or contamination inside the connector interfering with full pin engagement
Required Tools
L-823 connector test adaptorContact resistance meterThermal imaging camera for live hot-spot detectionTorque/seating verification tool per manufacturer specReplacement connector and gasket stock
Safety Precautions
  • LOTO before disconnecting or inspecting any connector; treat every connector as energised until confirmed isolated.
  • Wear appropriate PPE when handling a connector suspected of internal arcing or heat damage.
Diagnostic Procedure
  1. ObservationConfirm the fault is isolated to a specific connector location rather than the cable or transformer on either side of it, using segment-by-segment testing.
  2. MeasurementWith the circuit isolated, disconnect and inspect the connector for corrosion, damage, contamination, or an incompletely seated interface.
  3. MeasurementReconnect and measure contact resistance under mechanical flex to attempt to reproduce the intermittent condition; use thermal imaging live if the connector must be tested under load.
  4. DecisionContact resistance varies noticeably under flex → confirmed connector-level intermittent fault; clean, reseat, or replace. Resistance remains stable under flex → the fault is more likely upstream or downstream; re-scope to the adjoining cable or transformer.
  5. Fault LocationIsolate to the specific connector pair (plug and receptacle) responsible.
  6. Root CauseConfirm by replacing the connector and re-testing under the same flex/vibration conditions that originally reproduced the fault.
Test Points
Contact resistance under mechanical flex
Pin/socket condition and corrosion level
Gasket condition and seating
Thermal signature under live load, where safe to test
Root Cause Verification

Re-test the replaced or reseated connector under mechanical flex and, where possible, monitor over several operating cycles including varying weather conditions before closing out.

Corrective Action
  • Clean, re-grease, and properly reseat the connector per manufacturer torque/seating specification.
  • Replace a connector with corroded, damaged, or contaminated pins.
  • Replace a degraded gasket to restore the moisture seal.
Preventive Action
  • Include connector seating and torque verification as a standard PM step, particularly on runs with continuous ground-traffic vibration.
  • Use dielectric grease appropriate to the connector type at every reconnection to reduce future corrosion.
  • Track repeat connector failures by location to identify a site needing a more robust connector or installation method.
Related References

SOP: SOP-DIF-12 (Intermittent Connector Diagnostic Procedure)

Standards: FAA AC 150/5345-46 (L-823/L-824), ICAO Annex 14 Vol I

Diagram: L-823 Connector Assembly Detail

Case Study: Division 07 — Taxiway Lighting Troubleshooting

Rev 0 — Initial issue
DIF-538 Intermittent Cable Fault

Symptom — A circuit exhibits an intermittent open, short, or insulation-resistance drop traced to the primary series cable itself — not a connector or joint — typically appearing under specific mechanical, thermal, or moisture conditions and clearing otherwise.

Equipment / System

Primary series circuit cable between base cans, including any buried, ducted, or exposed sections along the affected run.

Possible Causes
HighA chafed or nicked cable jacket from a previous excavation, rodent damage, or installation defect making and breaking contact with a moisture path or adjacent conductor
HighA cable route flex point (e.g., near a pavement expansion joint or where the cable crosses a settled section) with repeated mechanical stress opening a marginal fault under movement
MedInsulation degradation from age or UV exposure at an exposed section, breaking down intermittently under voltage or thermal stress
MedA splice or repair joint with a marginal connection that behaves like a cable fault rather than a discrete joint failure
LowRodent or wildlife damage creating an intermittent short that varies with the exact position of the damaged insulation
LowManufacturing or installation defect in the cable itself, such as a void or inclusion in the insulation, degrading intermittently under load
Required Tools
TDR (time-domain reflectometer) or equivalent cable fault locator1000V/2500V insulation resistance tester (megger)Cable route and as-built drawingLoad-box or flex-test rig to mechanically stress a suspect sectionGround-penetrating locator for buried sections
Safety Precautions
  • LOTO before any cable testing or excavation.
  • Call for utility/services location clearance before any excavation near the suspected fault, in line with standard airside works procedures.
Diagnostic Procedure
  1. ObservationEstablish which segment of the loop the fault is confined to using segment-by-segment isolation from accessible base cans or pull boxes.
  2. MeasurementRun a TDR trace on the isolated segment to identify the approximate distance to any discontinuity or impedance change.
  3. MeasurementWhere the TDR result is inconclusive, apply controlled mechanical flex or thermal cycling to the suspect segment while monitoring insulation resistance or continuity to reproduce the intermittent condition.
  4. DecisionTDR and flex/thermal testing consistently point to a specific location → excavate and repair at that point. Fault will not reproduce under any controlled condition → log per DIF-536 and continue monitoring rather than excavating speculatively.
  5. Fault LocationIsolate to the specific cable segment and, where possible, the approximate distance/location within it.
  6. Root CauseConfirm by physical inspection of the excavated or exposed section once located, identifying the specific mechanism (chafe, splice, degradation).
Test Points
TDR trace distance-to-fault reading
Insulation resistance under mechanical flex and thermal cycling
Segment-by-segment isolation results
Physical condition of the cable at the identified location once exposed
Root Cause Verification

Re-test the repaired segment by megger and, where the original fault was condition-dependent, under the same flex or thermal conditions that originally reproduced it, before backfilling or closing out.

Corrective Action
  • Repair or replace the identified cable segment and reinstate with a jointing method rated for the application.
  • Correct the root mechanical cause (e.g., inadequate cable slack at a flex point) as part of the repair, not just the immediate fault.
Preventive Action
  • Maintain accurate cable route and joint records to speed future fault location.
  • Provide adequate slack and protection at known flex points (expansion joints, settlement-prone sections) during any future cable installation or repair.
  • Schedule periodic TDR baseline traces on ageing circuit sections to catch a developing fault before it becomes a live intermittent problem.
Related References

SOP: SOP-DIF-13 (Intermittent Cable Fault Location Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: IEC 60364, ICAO Annex 14 Vol I, IS 12290

Diagram: Cable Route and Joint Location Drawing

Case Study: Division 08 — Taxiway Centreline / Guidance Lighting Troubleshooting

Rev 0 — Initial issue
DIF-539 Intermittent Transformer Fault

Symptom — A fixture or group of fixtures exhibits an intermittent dropout, flicker, or earth-fault contribution traced specifically to an isolation transformer, which tests within tolerance on a static bench check but fails under live or mechanically disturbed conditions.

Equipment / System

Series-circuit isolation transformers at individual fixtures, including their primary/secondary windings, potting/encapsulation, and base-can mounting.

Possible Causes
HighA developing internal winding short or partial breakdown that only manifests under load current or mechanical vibration, not on a static unloaded megger test
HighMoisture ingress into the transformer housing or potting compound creating an intermittent leakage path that varies with humidity
MedA degraded or cracked potting compound allowing partial internal arcing under specific thermal or load conditions
MedA loose internal connection between the winding and the external terminal, intermittent under vibration
LowEnd-of-life insulation degradation across the winding that has not yet progressed to a permanent fault
LowAn incorrectly rated transformer for the fixture's actual load, causing intermittent overheating under specific conditions
Required Tools
1000V/2500V insulation resistance tester (megger)Transformer turns-ratio/winding testerThermal imaging cameraLoad-box for under-load testingVibration/flex test rig
Safety Precautions
  • LOTO before disconnecting or testing any transformer.
  • Handle a transformer suspected of internal breakdown carefully, as internal arcing can generate heat and gas pressure even in a sealed unit.
Diagnostic Procedure
  1. ObservationConfirm the fault is isolated to a specific fixture's transformer through segment isolation, ruling out the connector and cable feeding it.
  2. MeasurementMegger the transformer statically, unloaded, as a baseline, then repeat under mechanical flex and, where safe, under actual load current.
  3. MeasurementUse thermal imaging under load to identify abnormal heating in the transformer housing consistent with internal partial breakdown.
  4. DecisionInsulation resistance drops or the unit heats abnormally only under load/flex → confirmed intermittent transformer fault; replace. Static and dynamic tests both pass → re-scope to the connector or cable feeding the fixture.
  5. Fault LocationIsolate to the specific transformer unit.
  6. Root CauseConfirm the replaced unit resolves the fault under the same load and flex conditions that reproduced the original fault.
Test Points
Insulation resistance, static unloaded vs. under load/flex
Turns ratio and winding continuity
Thermal signature under load
Base-can moisture condition around the transformer
Root Cause Verification

Re-test the replacement transformer under load and mechanical flex, and monitor the fixture over several operating cycles before closing out.

Corrective Action
  • Replace the transformer confirmed to have an intermittent internal fault.
  • Correct any moisture ingress path into the base can housing the transformer.
  • Verify and correct the transformer rating if found to be mismatched to the fixture's actual load.
Preventive Action
  • Include an under-load or flex-test component in periodic transformer testing, not a static megger reading alone.
  • Track transformer age and replace proactively as units approach end of rated service life on high-traffic circuits.
  • Ensure base cans are properly sealed and drained to limit moisture exposure to the transformer.
Related References

SOP: SOP-DIF-14 (Intermittent Transformer Diagnostic Procedure)

Standards: FAA AC 150/5345-46, IS 12290

Diagram: Isolation Transformer Wiring Detail

Case Study: Division 05 — Runway Centreline Lighting Troubleshooting

Rev 0 — Initial issue
DIF-540 Intermittent Fixture Fault

Symptom — A fixture flickers, drops out, or varies in output intermittently, with the transformer, connector, and cable feeding it all confirmed sound, pointing to the fixture body, lamp/LED module, or internal wiring itself.

Equipment / System

AGL fixture body — lamp/LED module, internal wiring, lens/optic assembly, and driver electronics where applicable — with the upstream circuit confirmed fault-free.

Possible Causes
HighAn LED driver nearing end of life exhibiting intermittent instability under load or thermal cycling
HighA loose internal wiring connection within the fixture body, intermittent under vibration from ground traffic or aircraft operations
MedA failing lamp filament or LED die exhibiting an intermittent open under thermal expansion
MedMoisture ingress into the fixture body through a degraded lens seal, gasket, or housing crack
LowA driver or lamp incompatible with the actual supply characteristics of the circuit, causing marginal intermittent operation
LowPhysical damage to the fixture body from a previous impact, creating an intermittent internal short or open
Required Tools
Fixture bench-test rig matching the series circuit characteristicsTrue-RMS multimeter with min/max holdThermal imaging cameraVibration/flex test capability for the fixture bodyManufacturer's fixture wiring diagram
Safety Precautions
  • LOTO before removing or opening any fixture.
  • Confirm the fixture is fully de-energised and, where removed for bench testing, transported and handled per FOD-safe procedures.
Diagnostic Procedure
  1. ObservationConfirm the upstream transformer, connector, and cable have already tested sound, isolating the investigation to the fixture itself.
  2. MeasurementBench-test the removed fixture on a rig replicating the series circuit's actual supply characteristics, applying mechanical flex/vibration to attempt to reproduce the intermittent fault.
  3. MeasurementInspect the fixture body internally for a loose connection, moisture ingress, or physical damage once opened.
  4. DecisionFault reproduces under bench flex/vibration → confirmed fixture-internal fault; repair or replace as appropriate. Fault does not reproduce on the bench → reconsider whether the upstream isolation was complete, or log per DIF-536.
  5. Fault LocationIsolate to the specific internal component — driver, lamp/LED module, internal wiring, or lens seal.
  6. Root CauseConfirm the repaired or replacement fixture holds stable under the same bench flex/vibration conditions that originally reproduced the fault.
Test Points
Fixture output under bench flex/vibration testing
Internal wiring connection integrity
Lens seal and housing condition
Driver/lamp behaviour under simulated thermal cycling
Root Cause Verification

Reinstall the repaired or replacement fixture and monitor over several operating cycles, including varying intensity steps and weather, before closing out.

Corrective Action
  • Replace a failing LED driver, lamp, or LED module.
  • Re-terminate a loose internal wiring connection.
  • Replace a degraded lens seal or gasket, or the fixture body if physically damaged.
Preventive Action
  • Include a bench flex/vibration test as part of fixture overhaul or repair verification, not just a static bench check.
  • Track fixture failure patterns by type and location to identify a batch or design issue.
  • Replace lamps/LED modules and drivers proactively as they approach end of rated service life.
Related References

SOP: SOP-DIF-15 (Intermittent Fixture Diagnostic Procedure)

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

Diagram: Fixture Internal Wiring Diagram

Case Study: Division 06 — Touchdown Zone Lighting Troubleshooting

Rev 0 — Initial issue
DIF-541 Intermittent CCR Fault

Symptom — A CCR exhibits an intermittent fault — an output drop, an unexpected trip, a step-command error, or an internal alarm — that does not correlate with a specific external circuit condition and appears to originate within the CCR itself.

Equipment / System

CCR power stage (thyristor/SCR bank, transformer), control card, and internal protection/monitoring circuitry.

Possible Causes
HighA thyristor/SCR nearing end of life exhibiting intermittent misfiring or partial conduction failure
HighA control card component (capacitor, connector, solder joint) degrading and producing intermittent control signal errors
MedAn internal protection relay or sensor (current transformer, voltage sensor) with a marginal or intermittent fault producing false trips
MedA cooling or ventilation deficiency causing the CCR's own internal components to fail intermittently under thermal stress, overlapping with DIF-529/DIF-530
LowA firmware/software issue producing an intermittent control fault, overlapping with DIF-535
LowLoose internal wiring or a connector within the CCR cabinet itself, intermittent under vibration or thermal cycling
Required Tools
Oscilloscope for thyristor firing/waveform analysisCCR internal event/alarm logThermal imaging cameraContact resistance meter for internal terminals and connectorsManufacturer's CCR internal wiring and control schematic
Safety Precautions
  • LOTO and follow manufacturer arc-flash precautions before any internal CCR inspection given the presence of high-current power components.
  • Allow capacitors and power components to discharge per manufacturer procedure before hands-on internal work.
Diagnostic Procedure
  1. ObservationPull the CCR's internal event/alarm log to characterise the fault — frequency, associated alarm codes, and any correlating external condition already ruled out.
  2. MeasurementCapture thyristor firing waveforms and control card signals on an oscilloscope during normal and, where possible, faulting operation.
  3. MeasurementInspect and test internal terminals, connectors, and protection sensors for looseness, corrosion, or drift from specification.
  4. DecisionA specific thyristor, control card component, or sensor is confirmed faulty → replace. No internal component fault found despite thorough testing → reconsider external correlating conditions (thermal, switching, firmware) before concluding the CCR itself is at fault.
  5. Fault LocationIsolate to the specific power stage component, control card element, or internal connection responsible.
  6. Root CauseConfirm by running the repaired CCR through an extended operating cycle covering its full step range and typical duty conditions.
Test Points
Thyristor firing waveform under normal and stressed conditions
Control card signal integrity
Internal terminal/connector contact resistance
Protection sensor (CT, voltage sensor) calibration and stability
Root Cause Verification

Run the repaired CCR through an extended operating cycle across its full step range, including switching events and, where practical, extended full-load duration, confirming no recurrence.

Corrective Action
  • Replace a failing thyristor/SCR or power stage component.
  • Replace or repair a degraded control card.
  • Re-terminate a loose internal connection or replace a drifting protection sensor.
Preventive Action
  • Include internal terminal and connector inspection in the periodic CCR PM cycle.
  • Track thyristor and control card age against manufacturer-recommended service life.
  • Trend CCR internal alarm logs over time to catch a developing intermittent fault before it produces a hard failure.
Related References

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

Standards: FAA AC 150/5345-10, IEEE C57 (transformer/power electronics)

Diagram: CCR Internal Power and Control Schematic

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue
DIF-542 Intermittent Communication Fault

Symptom — A CCR, ALCMS panel, or field device intermittently loses or corrupts its communication link — fieldbus, Modbus/SCADA, or fibre — producing sporadic status errors, lost commands, or false alarms without a corresponding physical output fault.

Equipment / System

ALCMS/SCADA communication network, including fieldbus wiring or fibre runs, media converters, network switches, and communication cards at the CCR or remote I/O end.

Possible Causes
HighA degraded or intermittent physical connection at a communication terminal, patch panel, or fibre splice
HighElectromagnetic interference from nearby high-current AGL circuits or switching equipment corrupting the communication signal intermittently
MedA failing communication card, media converter, or network switch port exhibiting intermittent errors
MedA grounding or earthing issue on the communication network creating a ground loop that intermittently disrupts signal integrity
LowFirmware or protocol timing issues at the CCR or ALCMS end producing intermittent handshake failures, overlapping with DIF-535
LowA fibre run with a marginal bend radius violation or connector contamination causing intermittent signal attenuation
Required Tools
Network/protocol analyser appropriate to the communication medium (Modbus, fieldbus, fibre)Optical power meter and fibre inspection scope, where fibre is usedCable/connector continuity and contact resistance testerEMI survey meterALCMS/SCADA communication error log
Safety Precautions
  • LOTO before any physical inspection of communication terminals located within or near energised CCR cabinets.
  • Follow laser safety precautions when inspecting or testing live fibre optic connections.
Diagnostic Procedure
  1. ObservationPull the communication error log from the ALCMS/SCADA system to characterise the fault — frequency, affected device(s), and error type.
  2. MeasurementTest physical layer integrity at each accessible termination point — continuity, contact resistance, or optical power — comparing against specification.
  3. MeasurementWhere EMI is suspected, correlate communication error timestamps against nearby CCR switching events or high-current circuit operation.
  4. DecisionA specific termination, card, or fibre segment tests outside specification → isolate and repair/replace at that point. Physical layer tests pass but errors correlate with nearby switching → investigate EMI shielding/routing or grounding.
  5. Fault LocationIsolate to the specific termination point, card, or cable/fibre segment responsible.
  6. Root CauseConfirm by monitoring the communication error log over an extended period after repair, including through conditions that previously correlated with the fault.
Test Points
Physical layer continuity/contact resistance or optical power at each termination
Communication error log frequency and correlation with other equipment operation
EMI levels near suspect cable/fibre routing
Grounding/earthing continuity of the communication network
Root Cause Verification

Monitor the communication error log over an extended period after repair, including through the specific conditions (switching events, weather) that previously correlated with the fault, and confirm no recurrence.

Corrective Action
  • Repair or replace a degraded termination, connector, or fibre splice.
  • Replace a failing communication card, media converter, or switch port.
  • Reroute or add shielding to a communication cable run confirmed to be affected by EMI, or correct a grounding/ground-loop issue.
Preventive Action
  • Route communication cabling with adequate separation from high-current AGL circuits per manufacturer and IEC guidance.
  • Include physical layer testing (continuity, optical power) in the periodic PM cycle for the communication network.
  • Keep ALCMS/SCADA firmware current and review manufacturer bulletins for known intermittent communication issues.
Related References

SOP: SOP-DIF-17 (Intermittent Communication Fault Diagnostic Procedure)

Standards: IEC 61850 (where applicable), FAA AC 150/5345-10

Diagram: ALCMS/SCADA Communication Network Schematic

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue
DIF-543 Multiple Simultaneous Intermittent Faults

Symptom — Two or more separate circuits, fixtures, or systems develop intermittent faults at overlapping times, raising the question of whether they share a common root cause rather than being coincidental independent failures.

Equipment / System

Multiple AGL circuits, systems, or components potentially sharing a common upstream supply, earthing system, environmental exposure, or maintenance history.

Possible Causes
HighA shared upstream power supply issue (voltage sag, harmonics, an unstable transformer or substation feed) affecting every downstream circuit fed from it
HighA common environmental trigger (a weather event, a temperature swing, heavy rainfall) affecting multiple circuits with similar installation vulnerabilities at the same time
MedA shared earthing/grounding system fault raising the reference potential across multiple connected circuits simultaneously
MedA common installation batch (cable, connector, or transformer from the same lot) exhibiting a shared manufacturing or installation defect surfacing under similar conditions
LowEMI from a single source affecting multiple nearby circuits or communication links at once
LowCoincidental, genuinely independent faults with no common cause, which must still be ruled out rather than assumed
Required Tools
Single-line diagram showing shared supply, earthing, and routing relationships between affected circuitsPower quality logger at the shared upstream supply pointEarth resistance tester for the shared earthing systemWeather/environmental log for the reporting periodCable/component batch and installation records
Safety Precautions
  • LOTO each affected circuit individually before investigation, even where a common cause is suspected — do not assume isolating one circuit protects work on another.
Diagnostic Procedure
  1. ObservationMap every affected circuit/system against the single-line diagram to identify any shared upstream supply, earthing point, cable route, or installation batch.
  2. MeasurementLog power quality at the shared upstream supply point and earth resistance at the shared earthing system, correlating timestamps against each individual fault occurrence.
  3. MeasurementCross-reference installation and maintenance records for the affected circuits to check for a shared cable, connector, or transformer batch.
  4. DecisionA shared supply, earthing, or batch link is confirmed → treat as a single common-cause investigation and address the shared root cause. No shared link found → treat each fault as independent and route each to its own relevant division/procedure.
  5. Fault LocationIsolate to the shared upstream point, earthing system, or batch if a common cause is confirmed; otherwise isolate each fault individually.
  6. Root CauseConfirm by observing whether correcting the shared root cause resolves all affected circuits simultaneously, which is strong evidence the link was real.
Test Points
Power quality at the shared upstream supply point
Earth resistance across the shared earthing system
Timestamp correlation across all affected circuits' fault logs
Installation batch records for cable, connectors, and transformers across affected circuits
Root Cause Verification

After addressing the shared root cause, monitor all previously affected circuits simultaneously over an extended period and confirm all resolve together rather than one recurring independently.

Corrective Action
  • Correct the shared upstream supply issue (voltage regulation, harmonics filtering) if confirmed as the common cause.
  • Repair the shared earthing system if a common ground-reference fault is confirmed.
  • Proactively inspect or replace remaining components from a confirmed defective installation batch, even where they have not yet faulted.
Preventive Action
  • Maintain an up-to-date single-line diagram showing shared supply, earthing, and routing relationships to speed future common-cause investigation.
  • Log installation batch information (cable reel, connector lot, transformer serial) at the time of installation to enable future batch tracing.
  • Monitor upstream power quality and shared earthing resistance continuously rather than only when multiple faults prompt an investigation.
Related References

SOP: SOP-DIF-18 (Common-Cause Multi-Fault Investigation Procedure)

Standards: IEC 60364, IS 3043 (earthing), ICAO Annex 14 Vol I

Diagram: Airfield Single-Line Diagram (Supply, Earthing, and Routing)

Case Study: Division 24 — Complete Runway Lighting Failure

Rev 0 — Initial issue
DIF-544 Hidden Underground Fault

Symptom — A confirmed insulation, continuity, or earth-fault condition exists somewhere within a buried or ducted cable run, but the fault location cannot be determined from accessible base cans, pull boxes, or surface inspection alone.

Equipment / System

Buried or ducted primary series cable, joints, and duct/handhole infrastructure between accessible above-grade access points.

Possible Causes
HighA cable fault located mid-span between accessible pull boxes or base cans, with no surface indication of its position
HighA duct or handhole collapse or blockage preventing physical access to inspect a suspected joint or splice location
MedA fault location technique giving an ambiguous or low-confidence reading due to cable characteristics (splices, varying cable types, or branch points) that complicate reflectometry
MedMultiple candidate fault locations along a long run, requiring systematic narrowing rather than a single test pinpointing the exact spot
LowA fault whose signature (very high resistance, intermittent) is inherently difficult for standard TDR or bridge methods to localise precisely
LowIncomplete or inaccurate as-built records making it unclear exactly where the cable route runs relative to surface features
Required Tools
TDR (time-domain reflectometer)Cable fault bridge / Murray loop or equivalent method for high-resistance faultsGround-penetrating locator / cable route tracerAcoustic/thumper fault pinpointing method, where appropriate and permittedAs-built drawings and any available GPR/utility survey records
Safety Precautions
  • Obtain full utility/services clearance and airside works permits before any excavation.
  • LOTO the circuit before any fault-locating method that involves applying test voltage to the cable.
  • Coordinate excavation timing and location with airfield operations given the proximity to active pavement.
Diagnostic Procedure
  1. ObservationCompile all available records — as-built drawings, previous repair history, and any prior fault-location data — for the affected run before starting a new investigation.
  2. MeasurementRun TDR and, for high-resistance faults where TDR is inconclusive, a bridge-method test from both accessible ends of the suspected segment to triangulate the fault distance.
  3. MeasurementUse a ground-penetrating locator or route tracer to confirm the actual physical cable route on the surface, since the calculated distance is only useful once matched to the real route.
  4. DecisionTDR/bridge results converge on a specific, physically accessible location → proceed to targeted excavation. Results remain ambiguous or point to an inaccessible/obstructed area → consider acoustic pinpointing or a wider excavation window, and re-verify the as-built route accuracy first.
  5. Fault LocationMark the calculated fault location on the surface using the confirmed physical route before excavating.
  6. Root CauseConfirm visually once the cable is exposed, and verify the fault type and location match the electrical test predictions before repairing.
Test Points
TDR distance-to-fault from both accessible ends
Bridge-method resistance-based distance calculation
Physical route trace vs. as-built drawing
Post-excavation visual confirmation of the fault
Root Cause Verification

Re-megger and load-test the repaired and backfilled segment, and update the as-built drawing with the confirmed route and repair location before closing out.

Corrective Action
  • Excavate, expose, and repair the identified cable fault using a jointing method rated for the buried application.
  • Repair or replace a collapsed or blocked duct section obstructing access.
Preventive Action
  • Update as-built drawings immediately after any excavation or repair, including the exact GPS/measured location of the fault and repair.
  • Consider periodic TDR baseline surveys on ageing buried circuits to have a healthy reference trace on file before a fault occurs.
  • Maintain duct and handhole infrastructure proactively to preserve future access for fault location.
Related References

SOP: SOP-DIF-19 (Buried Cable Fault Location Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: IEC 60364, ICAO Annex 14 Vol I, IS 12290

Diagram: Underground Cable Route and Duct Layout Drawing

Case Study: Division 08 — Taxiway Centreline / Guidance Lighting Troubleshooting

Rev 0 — Initial issue
DIF-545 Recurring Unidentified Fault

Symptom — The same fault symptom recurs repeatedly at the same location or circuit over an extended period despite multiple maintenance visits, component replacements, or repairs, with no single corrective action yet producing a lasting fix.

Equipment / System

Any AGL circuit or component with an open, chronic fault history spanning multiple maintenance cycles and, typically, more than one prior corrective attempt.

Possible Causes
HighPrevious repairs addressed a symptom or the most recent failed component rather than the true underlying root cause, allowing the same failure mode to recur
HighAn underlying design, installation, or routing issue (inadequate cable slack, poor drainage, a marginal earthing design) that will keep producing failures until corrected at the systemic level, not the component level
MedMultiple independent fault mechanisms are present at the same location, so fixing one leaves another to surface and appear as a "recurrence"
MedA component or material genuinely unsuited to the specific environmental exposure at that location (e.g., standard-rated equipment in an unusually wet or high-traffic-vibration area)
LowInconsistent repair quality or specification across multiple maintenance visits, with different technicians addressing the symptom differently each time
LowIncomplete diagnostic investigation at each prior visit, closing out on a plausible cause without full verification
Required Tools
Complete maintenance and repair history for the location, compiled chronologicallyAs-built and revision-history drawings for the affected circuitFull diagnostic toolkit relevant to the fault type (megger, TDR, thermal camera, oscilloscope, as applicable)Environmental and load history log for the locationRoot-cause analysis / fishbone or 5-why documentation template
Safety Precautions
  • LOTO before any investigation, as with any live AGL work.
  • Treat a chronic recurring fault as a priority investigation rather than a routine repeat repair, given the cumulative safety exposure of an unresolved recurring issue.
Diagnostic Procedure
  1. ObservationCompile the complete history of every prior occurrence, repair, and component replacement at this location, looking for a pattern in timing, conditions, or the specific component replaced each time.
  2. MeasurementConduct a full, unbiased diagnostic investigation from first principles — not assuming the previous diagnosis was correct — covering every plausible cause category (connector, cable, transformer, fixture, CCR, environmental, design).
  3. MeasurementAssess whether the location has any distinguishing environmental or design factor (drainage, traffic loading, cable slack, earthing) relative to comparable locations that have not had recurring failures.
  4. DecisionA systemic design/installation issue is identified → escalate for a permanent design-level correction, not another like-for-like component swap. Multiple independent mechanisms are found → address all identified causes together rather than one at a time across further visits.
  5. Fault LocationIsolate to every contributing factor identified, not just the most recent or most obvious one.
  6. Root CauseDo not close out until the specific factor that differentiates this location from non-recurring comparable locations has been identified and addressed.
Test Points
Full chronological repair and replacement history for the location
Comparative environmental/design factors vs. non-recurring comparable locations
Complete first-principles diagnostic sweep across all plausible cause categories
Post-repair extended monitoring log
Root Cause Verification

Monitor the location through at least one full seasonal cycle and multiple representative operating conditions after the systemic correction, and confirm no recurrence before considering the chronic ticket closed.

Corrective Action
  • Implement the systemic design or installation correction identified (improved drainage, cable slack, upgraded component rating, earthing improvement) rather than repeating a prior like-for-like repair.
  • Address every contributing cause identified during the full diagnostic sweep, not only the most recent failure mode.
Preventive Action
  • Maintain a chronic-fault register flagging any location with more than a defined number of repeat visits for a mandatory root-cause-level investigation rather than continued routine repair.
  • Review and update design/installation standards where a recurring fault is traced to a systemic rather than component-level cause.
  • Ensure diagnostic findings and the specific corrective action taken are fully documented at every visit, not just "fault cleared", to make future pattern analysis possible.
Related References

SOP: SOP-DIF-20 (Chronic Recurring Fault Root-Cause Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5340-30, IS 12290

Diagram: Chronic Fault Register Template

Case Study: Division 22 — DIF-536 Fault Cannot Be Reproduced

Rev 0 — Initial issue
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Frequently Asked Questions

Quick answers about Division 22 — Difficult / Intermittent Faults Troubleshooting in the AGL Troubleshooting Atlas.

Q1What does Division 22 of the AGL Troubleshooting Atlas cover?

Division 22 is the Difficult / Intermittent Faults Troubleshooting reference. It publishes 20 diagnostic topics, DIF-526 to DIF-545, in a single subsection.

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 are in Division 22?

The 20 topics are DIF-526 to DIF-545: Fault Appears Only at Night; Fault Appears Only During Rain; Fault Appears Only During Dry Weather; Fault Appears After Warm-Up; Fault Appears When Equipment Is Hot; Fault Appears When Equipment Is Cold; Fault Appears at Higher CCR Step; Fault Appears at Lower CCR Step; Fault Appears During Switching; Fault Disappears After Restart; Fault Cannot Be Reproduced; Intermittent Connector Fault; Intermittent Cable Fault; Intermittent Transformer Fault; Intermittent Fixture Fault; Intermittent CCR Fault; Intermittent Communication Fault; Multiple Simultaneous Intermittent Faults; Hidden Underground Fault; Recurring Unidentified Fault.

Q4How does a diagnostic procedure begin in Division 22?

Each procedure starts with observation before any intervention. For example, DIF-526 (Fault Appears Only at Night) begins: Cross-check every reported occurrence against the night flight-check log and any available CCR trend history to confirm the fault is genuinely absent during daytime energised checks.

Q5What safety precautions apply in Division 22?

Each topic lists its own precautions. For example, DIF-526 (Fault Appears Only at Night) starts with: Coordinate all night airside access through ATC/apron control before approaching any fixture on an active movement area.

Q6Which SOPs are referenced in Division 22?

Related references in the topics include SOP-DIF-01 (Night-Correlated Fault Capture Procedure), SOP-DIF-02 (Weather-Correlated Insulation Fault Procedure), SOP-CCR-04 (CCR LOTO Procedure), SOP-DIF-03 (Seasonal Earthing Verification Procedure), SOP-DIF-04 (Thermal Drift Diagnostic Procedure), SOP-DIF-05 (Elevated-Temperature Fault Diagnostic Procedure), SOP-DIF-06 (Cold-Start Fault Diagnostic Procedure) and SOP-DIF-07 (Step-Dependent Fault Diagnostic Procedure), and 13 more.

Q7Which standards are referenced in Division 22?

Where a standard applies, it appears in the topic's Related References. Those cited in this division include ICAO Annex 14 Vol I, FAA AC 150/5340-30, IEC 60364, IS 12290, IS 3043, IEEE 142, FAA AC 150/5345-10, FAA AC 150/5345-46, IEEE C57 and IEC 61850.

Q8How can I find a specific topic in Division 22 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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