AGL-CS-023 — Transformer Secondary Cable Fault
The AGL Knowledge Hub | Engineering Case Study Series. A secondary cable fault investigation where a corroded conductor caused an intermittent open circuit — from transformer replacement through continuity testing to cable replacement.
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 a routine night inspection, one runway edge light was found inoperative while all adjacent lights remained fully operational.
The Constant Current Regulator (CCR) was maintaining the selected current, and the primary series circuit showed normal electrical behavior.
The isolating transformer was initially suspected and replaced. However, the light remained OFF after transformer replacement.
A systematic investigation identified a fault in the secondary cable between the isolating transformer and the field fixture, where moisture ingress and mechanical damage had created a high-resistance and intermittent connection.
This case demonstrates the importance of isolating the secondary cable before replacing the transformer or fixture.
2Learning Objectives
After completing this case study, the learner should be able to:
- Understand the function of the transformer secondary cable.
- Recognize symptoms of secondary cable faults.
- Differentiate secondary cable faults from transformer faults.
- Perform continuity and insulation testing of secondary circuits.
- Investigate moisture-related secondary cable failures.
- Locate intermittent secondary cable defects.
- Verify cable integrity after repair.
3System Background
The secondary circuit consists of:
The secondary cable is exposed to:
- Moisture.
- Mechanical movement.
- Connector stress.
- Thermal cycling.
- Installation damage.
- Pit contamination.
Common secondary cable faults include:
- Open circuit.
- High resistance.
- Insulation damage.
- Moisture ingress.
- Conductor breakage.
- Connector deterioration.
4Problem Statement
The reported condition was:
- One runway edge light OFF.
- Adjacent lights ON.
- CCR healthy.
- Transformer replaced.
- Fixture tested healthy.
- No visible cable damage.
Is the fault located in the transformer secondary cable or elsewhere in the secondary circuit?
5Initial Observations
| CCR status | Healthy |
| Selected current | 6.6 A |
| Primary circuit | Normal |
| Affected light | L84 OFF |
| Adjacent lights | ON |
| Transformer replacement | No improvement |
The unsuccessful transformer replacement suggested that the fault was external to the transformer.
6Initial Engineering Hypotheses
The maintenance team considered:
Secondary cable open circuit.
Secondary cable high resistance.
Secondary connector failure.
Fixture connector damage.
Intermittent cable conductor break.
7Secondary Cable Failure Principle
A typical failure sequence is:
The primary series circuit usually remains unaffected.
8Test Equipment
The investigation used:
- True-RMS multimeter.
- Continuity tester.
- Low-resistance ohmmeter.
- Insulation resistance tester.
- Approved AGL circuit test equipment.
- Secondary cable test leads.
- Circuit drawings.
- Airport-approved PPE.
9Investigation
Step 1: Verify Primary Circuit
Measurements confirmed:
| CCR current | Normal |
| Primary continuity | Normal |
| Transformer primary | Normal |
The primary circuit was healthy.
Step 2: Verify Transformer Secondary
The replacement transformer was tested.
Results:
| Secondary continuity | Normal |
| Secondary voltage | Normal |
| Insulation resistance | Normal |
The transformer was healthy.
Step 3: Secondary Cable Continuity
Continuity testing showed:
- Unstable resistance.
- Intermittent continuity.
- Resistance increased when the cable was flexed.
This strongly indicated cable conductor damage.
Step 4: Connector Inspection
The secondary connector showed:
- Moisture contamination.
- Corroded contacts.
- Green copper oxidation.
- Seal deterioration.
However, connector cleaning did not restore stable continuity.
Step 5: Cable Examination
The secondary cable was removed and examined.
Findings included:
- Conductor strand breakage.
- Water penetration.
- Insulation cracking.
- Corrosion.
- Localized mechanical damage near the transformer pit.
The conductor had partially fractured inside the insulation.
10Root Cause
The secondary cable developed progressive conductor corrosion and strand breakage caused by moisture ingress through a damaged cable section.
The cable initially produced a high-resistance fault and eventually became intermittently open.
The transformer and fixture were both healthy.
11Corrective Action
The maintenance team:
- Isolated the circuit.
- Removed the damaged secondary cable.
- Replaced the cable with an approved secondary lead.
- Installed new secondary connectors.
- Verified connector sealing.
- Performed continuity testing.
- Conducted insulation resistance testing.
- Restored the circuit.
- Verified stable light operation.
- Updated maintenance records.
12Post-Repair Verification
The affected runway edge light returned to normal operation.
13Root-Cause Analysis
Individual light OFF.
Normal.
Healthy.
Unstable resistance.
Contamination found.
Conductor damage identified.
Secondary cable deterioration.
Replace secondary cable and connectors.
14Why This Case Was Important
A common maintenance error is replacing:
- Transformers.
- LED fixtures.
- Primary components.
before testing the secondary cable.
Secondary cable faults can perfectly imitate:
- Transformer failure.
- Secondary winding failure.
- Fixture failure.
- Connector failure.
15Engineering Lessons
Test secondary continuity before replacing transformers.
Always test secondary continuity before replacing transformers.
Cable flex testing reveals hidden conductor damage.
Cable flex testing can reveal hidden conductor damage.
Moisture ingress commonly causes secondary cable corrosion.
Moisture ingress commonly causes secondary cable corrosion.
Intermittent faults often indicate conductor strand failure.
Intermittent faults often indicate conductor strand failure.
Secondary connector condition should always be verified.
Secondary connector condition should always be verified.
16Engineer's Decision Point
You observe: One runway edge light OFF. Transformer replaced. Fixture healthy. CCR operating normally. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about transformer secondary cable faults, based on this case study.
Q1What is AGL-CS-023?
AGL-CS-023 is an engineering training case study on a transformer secondary cable fault that left one runway edge light OFF even after the transformer had been replaced. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Why did replacing the transformer not fix the light?
The unsuccessful replacement showed the fault was external to the transformer. The fixture also tested healthy, which left the secondary cable between the transformer and the fixture as the remaining suspect.
Q3What was the root cause?
The secondary cable developed progressive conductor corrosion and strand breakage from moisture ingress through a damaged cable section. It first produced a high-resistance fault and eventually became intermittently open. The transformer and fixture were both healthy.
Q4What should you check when a single light is OFF, the transformer is replaced and the fixture is healthy?
Test secondary cable continuity and connector integrity. Replacing the CCR or primary cable, or increasing CCR current, is not the right next step.
Q5Which test equipment was used?
The investigation used a true-RMS multimeter, continuity tester, low-resistance ohmmeter, insulation resistance tester, approved AGL circuit test equipment and secondary cable test leads, with circuit drawings.
Q6How was the circuit repaired?
The team isolated the circuit, removed the damaged secondary cable and replaced it with an approved secondary lead. They installed new secondary connectors, verified sealing, tested continuity and insulation resistance, restored the circuit and verified stable operation.
Q7How was the repair verified?
CCR and transformer operation were normal, secondary continuity was stable, insulation resistance was normal and the light for L84 returned to normal operation.
Q8Why does this case emphasize a systematic approach?
Swapping the transformer without diagnosis did not solve the problem. Working through the primary circuit, transformer, secondary cable and connectors in turn located the actual fault.
Engineering & Expert Notes
Flex the Cable Only Under Approved Isolation
TechniqueMovement testing can reveal strand breakage but must be done under an isolated, approved procedure.
Strip Back Damaged Sections to Inspect Conductors
InspectionCorrosion may extend beyond the visible damage. Check along the cable until clean copper is found.
Find How the Water Got In
CauseRepair the sheath or entry point so the replacement cable does not suffer the same fate.
Test Continuity and Insulation After Replacement
VerificationConfirm the repaired secondary circuit under the operating conditions, not just at rest.
20Case Study Conclusion
Secondary cable faults should be systematically isolated and tested before replacing transformers or field fixtures, because localized cable deterioration can produce identical operational symptoms.
Systematic secondary continuity testing, cable movement testing, connector inspection, and insulation assessment can accurately identify transformer secondary cable faults and prevent unnecessary replacement of healthy equipment.
In AGL engineering, the most reliable transformer diagnosis is often achieved only after the secondary cable has been electrically verified.
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