AGL-CS-083 — Approach Lighting Circuit Fault
The AGL Knowledge Hub | Engineering Case Study Series. An approach-lighting circuit fault investigation where primary-cable insulation breakdown produced intermittent earth leakage and unstable CCR operation — from CCR analysis through insulation testing, earth-leakage detection, section isolation, TDR verification, cable replacement, and electrical restoration.
Important: This is an engineering simulation created for training. It is not presented as an actual airport incident. All readings, values, dates and outcomes are illustrative, and no airport, organisation, person or manufacturer is identified or implied.
1Case Study Overview
During routine night operations, the approach lighting system began exhibiting abnormal circuit behavior.
Several approach lights flickered intermittently, the CCR output voltage fluctuated, and maintenance personnel observed unstable circuit loading.
Unlike a normal segment failure, the affected lights were not confined to one section, and the circuit condition changed during operation.
A detailed investigation was initiated covering:
- CCR electrical parameters.
- Circuit current stability.
- Output voltage behavior.
- Insulation resistance.
- Earth leakage.
- Cable integrity.
- Isolating transformers.
- Circuit impedance.
The investigation identified a deteriorated primary cable with moisture-related insulation breakdown, producing intermittent earth leakage and unstable series-circuit operation.
This case demonstrates that approach-lighting circuit faults are electrical-system problems requiring analysis of the complete series circuit rather than individual light fittings.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose ALS circuit faults.
- Interpret CCR electrical behavior.
- Analyze voltage and current abnormalities.
- Investigate insulation deterioration.
- Locate earth leakage.
- Evaluate circuit impedance.
- Restore stable ALS electrical operation.
3System Background
An ALS circuit consists of:
- CCR.
- Series primary cable.
- Isolating transformers.
- Secondary circuits.
- Connectors.
- Cable joints.
- Light fittings.
- Grounding system.
Circuit faults may include:
- Open circuit.
- Earth fault.
- Low insulation resistance.
- Intermittent cable failure.
- Water ingress.
- Connector breakdown.
- Transformer insulation failure.
- Cable-joint deterioration.
Circuit performance depends on stable series-current operation.
4Problem Statement
The reported condition was:
- Intermittent light operation.
- CCR voltage fluctuation.
- Stable current not maintained.
- No complete segment failure.
- Random operational disturbances.
What electrical fault is causing unstable approach-lighting circuit behavior?
5Initial Observations
| CCR current | Fluctuating |
| CCR voltage | Unstable |
| Light operation | Intermittent |
| Failure pattern | Distributed |
| Circuit trips | Occasional |
The symptoms suggested an electrical circuit defect.
6Initial Engineering Hypotheses
The investigation considered:
CCR regulation failure.
Primary-cable insulation deterioration.
Earth leakage.
Transformer insulation failure.
Connector breakdown.
7Circuit Fault Principle
Circuit faults affect the electrical behavior of the entire series circuit.
8Test Equipment
The investigation used:
- Digital multimeter.
- Insulation resistance tester.
- Earth-fault locator.
- Time-domain reflectometer.
- CCR monitoring equipment.
- Power-quality analyzer.
- Clamp meter.
- Cable fault locator.
9Investigation
Step 1: CCR Performance Analysis
Measurements showed:
| Parameter | Observation |
| Output current | Oscillating |
| Output voltage | 2.1-3.4 kV |
| Regulation response | Unstable |
| Fault alarms | Intermittent |
The CCR was responding to changing circuit conditions.
Step 2: Circuit Insulation Resistance
Measured insulation resistance:
0.9 MOhm
Expected:
Greater than 50 MOhm
The circuit had severe insulation deterioration.
Step 3: Earth Leakage Test
Earth leakage was detected during:
- High humidity.
- Night operation.
- Full brightness operation.
Leakage current varied with environmental conditions.
Step 4: Circuit Section Isolation
The circuit was divided into four sections.
Results:
| Section | IR |
| S1 | > 100 MOhm |
| S2 | > 100 MOhm |
| S3 | 1.2 MOhm |
| S4 | > 100 MOhm |
The fault was localized to Section S3.
Step 5: TDR Analysis
TDR identified an abnormal impedance change at:
312 m from the reference point
The location corresponded to a buried primary-cable section.
Step 6: Excavation Inspection
Inspection revealed:
- Water ingress.
- Outer sheath damage.
- Insulation cracking.
- Partial conductor corrosion.
The cable had deteriorated significantly.
Step 7: Controlled Replacement Test
The damaged cable section was replaced.
Post-repair measurements:
| Parameter | Before | After |
| IR | 0.9 MOhm | > 200 MOhm |
| Voltage stability | Poor | Stable |
| Current stability | Poor | Stable |
| Light operation | Intermittent | Normal |
Circuit behavior returned to normal.
10Root Cause
The ALS circuit fault was caused by primary-cable insulation breakdown due to long-term moisture ingress, resulting in:
- Low insulation resistance.
- Intermittent earth leakage.
- Circuit impedance variation.
- CCR voltage fluctuation.
The primary failure mechanism was deterioration of the buried series cable.
11Corrective Action
The maintenance team:
- Isolated the circuit.
- Located the defective cable section.
- Removed the damaged cable.
- Installed new primary cable.
- Replaced affected connectors.
- Performed insulation testing.
- Conducted earth-leakage testing.
- Verified CCR stability.
- Energized the circuit.
- Performed operational observation.
12Post-Repair Verification
Cable replacement restored stable circuit operation.
13Root-Cause Analysis
ALS circuit unstable
Voltage fluctuating
Low
Section S3 defective
Cable location identified
Insulation breakdown found
Circuit retested
Primary-cable insulation failure
Replace cable and verify electrical integrity
14Why This Case Was Important
Circuit faults can cause:
- Intermittent lighting.
- CCR instability.
- Voltage fluctuations.
- Reduced operational reliability.
- Difficult fault diagnosis.
- Progressive cable deterioration.
System-level electrical analysis is essential.
15Engineering Lessons
Distributed intermittent failures often indicate circuit-level problems.
Distributed intermittent failures often indicate circuit-level problems.
CCR voltage behavior provides valuable diagnostic information.
CCR voltage behavior provides valuable diagnostic information.
Insulation resistance is a primary ALS health indicator.
Insulation resistance is a primary ALS health indicator.
TDR significantly improves cable-fault localization.
TDR significantly improves cable-fault localization.
Environmental moisture can create intermittent electrical faults.
Environmental moisture can create intermittent electrical faults.
16Engineer's Decision Point
You observe: Approach lights flickering. CCR voltage fluctuating. No single failed segment. Circuit behavior changing with humidity. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about approach lighting circuit faults, insulation resistance and earth leakage.
Q1What is AGL-CS-083?
AGL-CS-083 is an engineering training case study. It covers an approach lighting circuit with flickering lights and a fluctuating CCR output voltage caused by a deteriorated buried cable. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Why did the lights flicker and the CCR voltage fluctuate?
A moisture-damaged primary cable caused intermittent earth leakage, which varied the circuit impedance. The CCR responded to the changing load, so voltage oscillated between 2.1 and 3.4 kV and regulation was unstable.
Q3How low was the insulation resistance?
It measured 0.9 MΩ against an expected value above 50 MΩ, showing severe insulation deterioration.
Q4When did earth leakage appear?
It appeared during high humidity, night operation and full-brightness operation, and varied with environmental conditions.
Q5How was the fault localized?
The circuit was divided into four sections. S1, S2 and S4 read above 100 MΩ, while S3 read 1.2 MΩ. TDR then found an abnormal impedance change 312 m from the reference point, on a buried primary-cable section.
Q6What did excavation find?
Water ingress, outer sheath damage, insulation cracking and partial conductor corrosion.
Q7What was the root cause of AGL-CS-083?
Primary-cable insulation breakdown from long-term moisture ingress, leading to low insulation resistance, intermittent earth leakage, impedance variation and CCR voltage fluctuation.
Q8How was the repair verified?
After replacing the damaged section, insulation resistance rose from 0.9 MΩ to above 200 MΩ, and voltage and current became stable. Light operation returned from intermittent to normal.
Q9What is the key lesson from this case?
Intermittent, changing symptoms call for analysis of the whole series circuit. Insulation resistance, earth-leakage tests and section isolation find faults that fitting replacement would miss.
Engineering & Expert Notes
Correlate CCR Voltage Variation With Weather
PatternCompare logs against rainfall to separate moisture from other causes.
Test Insulation Section by Section
Fault LocationNarrow the leaking length before excavating.
Combine Insulation Results With Cable Locators
TechniqueUsing more than one method gives better accuracy.
Plan the Repair Before the Fault Becomes Permanent
PlanningIntermittent leakage tends to worsen. Schedule the repair.
20Case Study Conclusion
Approach-lighting circuit faults are system-level electrical problems caused by insulation deterioration, moisture ingress, earth leakage, and circuit impedance instability, and effective diagnosis requires analysis of CCR electrical behavior, insulation resistance, circuit isolation, and cable-fault localization rather than replacement of individual field components.
Systematic CCR analysis, insulation testing, earth-leakage investigation, section isolation, TDR verification, cable replacement, and post-repair electrical validation provide a complete engineering methodology for restoring stable ALS circuit operation and ICAO-compliant approach-lighting performance.
In AGL engineering, the most important fault is often not the light that is dark, but the cable that is changing the behavior of the entire circuit.
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