AGL-CS-030 — Failure Analysis of a Damaged AGL Transformer
The AGL Knowledge Hub | Engineering Case Study Series. A forensic investigation of a failed isolating transformer — from external examination through electrical testing, thermal assessment, internal disassembly, and failure-sequence reconstruction.
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
An AGL isolating transformer removed from a runway edge lighting circuit after repeated operational failures was submitted for detailed engineering examination.
The transformer had experienced:
- Intermittent light operation.
- Progressive overheating.
- Low insulation resistance.
- Eventual complete failure.
Previous maintenance records suggested possible moisture ingress, but no formal failure analysis had been performed.
A comprehensive forensic investigation was conducted to determine:
- The failure mechanism.
- The sequence of deterioration.
- The initiating cause.
- The final failure mode.
The investigation revealed moisture-assisted insulation deterioration accelerated by high-resistance secondary connections, leading to thermal degradation, insulation breakdown, and internal winding failure.
This case demonstrates that successful transformer failure analysis requires evidence-based engineering rather than assumption-based replacement.
2Learning Objectives
After completing this case study, the learner should be able to:
- Conduct systematic transformer failure analysis.
- Preserve and interpret physical evidence.
- Differentiate primary and secondary failure mechanisms.
- Correlate thermal, insulation, and electrical damage.
- Identify moisture-related deterioration.
- Analyze winding damage patterns.
- Develop evidence-based root-cause conclusions.
3System Background
An AGL isolating transformer may fail due to:
- Insulation deterioration.
- Moisture ingress.
- Thermal aging.
- Overloading.
- High-resistance connections.
- Connector failure.
- Surge damage.
- Manufacturing defects.
- Environmental contamination.
Failure analysis seeks to determine which mechanism occurred first.
4Problem Statement
The reported condition was:
- Transformer failed after repeated intermittent operation.
- Low insulation resistance measured before failure.
- Elevated operating temperature.
- No CCR fault.
- Transformer removed from a water-prone pit.
- Previous connector repairs documented.
What was the actual root cause of the transformer failure, and what evidence supports that conclusion?
5Initial Observations
| CCR status | Healthy |
| Transformer operation | Failed |
| Insulation resistance | Very low |
| External damage | Moderate |
| Pit environment | Moisture-prone |
| Maintenance history | Repeated connector repairs |
The maintenance history suggested that connection problems may have preceded the transformer failure.
6Failure Analysis Methodology
The investigation followed a forensic sequence:
- Visual examination.
- Documentation review.
- External inspection.
- Electrical testing.
- Thermal evidence assessment.
- Internal disassembly.
- Winding examination.
- Insulation analysis.
- Corrosion assessment.
- Root-cause reconstruction.
7Test Equipment
The investigation used:
- High-resolution photography.
- Thermal imaging records.
- Winding resistance meter.
- 5 kV insulation resistance tester.
- Low-resistance ohmmeter.
- Microscopic inspection equipment.
- Approved transformer test equipment.
- Maintenance records.
- Airport-approved PPE.
8Investigation
Step 1: External Examination
Inspection revealed:
- Resin discoloration.
- Localized burn marks.
- Connector corrosion.
- Moisture staining.
- Surface contamination.
The damage was concentrated near the secondary termination area.
Step 2: Electrical Testing
Results:
| Primary continuity | Abnormal |
| Secondary continuity | Open circuit |
| Primary-to-earth IR | 1.8 MΩ |
| Secondary-to-earth IR | 1.2 MΩ |
| Primary-to-secondary IR | 2.5 MΩ |
Both insulation and winding integrity had deteriorated.
Step 3: Thermal Evidence Review
Historical thermal records showed:
| 6 months earlier | 52°C |
| 3 months earlier | 61°C |
| 1 month earlier | 74°C |
| Before failure | 86°C |
The temperature rise was progressive.
Step 4: Connector Analysis
The secondary connector showed:
- Severe oxidation.
- Reduced contact pressure.
- Carbon tracking.
- Moisture contamination.
- High contact resistance.
The connector had clearly experienced prolonged overheating.
Step 5: Internal Disassembly
The transformer was sectioned.
Findings included:
- Carbonized secondary insulation.
- Melted winding insulation.
- Corroded secondary terminations.
- Moisture contamination.
- Resin cracking.
- Thermal decomposition of insulation materials.
The damage was concentrated around the secondary winding region.
Step 6: Winding Analysis
The secondary winding exhibited:
- Conductor discoloration.
- Localized hot spots.
- Insulation embrittlement.
- Inter-turn damage.
- Final winding separation.
The primary winding was comparatively less damaged.
9Failure Sequence Reconstruction
The evidence supported the following sequence:
This was a progressive thermally accelerated failure, not a sudden catastrophic event.
10Root Cause
The transformer failed because a deteriorated secondary connector created persistent high contact resistance, generating excessive localized heating.
The elevated temperature accelerated insulation aging, while moisture ingress further reduced dielectric strength.
The combined thermal and moisture stress caused:
- Insulation deterioration.
- Leakage current.
- Secondary winding damage.
- Final open-circuit failure.
The connector problem was the initiating cause, and the winding failure was the final failure mode.
11Corrective Action
The maintenance team:
- Replaced the transformer.
- Replaced the secondary connector.
- Replaced the secondary cable.
- Improved pit drainage.
- Restored connector sealing.
- Verified contact resistance.
- Performed insulation resistance testing.
- Conducted thermal verification.
- Updated inspection procedures.
- Implemented connector condition monitoring.
12Post-Repair Verification
The failure did not recur after correction of the connector and environmental conditions.
13Root-Cause Analysis
Transformer failed
Insulation and winding abnormal
Progressive overheating
High resistance identified
Secondary insulation damage
Connector → heating → insulation → winding
High-resistance secondary connector with moisture-assisted thermal deterioration
Replace transformer, connector, cable, and improve environmental protection
14Why This Case Was Important
Failure analysis differs from troubleshooting.
Troubleshooting asks:
What is faulty?
Failure analysis asks:
- Why did it fail?
- What failed first?
- What evidence proves it?
- How can recurrence be prevented?
This distinction is essential for long-term reliability engineering.
15Engineering Lessons
The first failed component is not always the root cause.
The first failed component is not always the root cause.
Thermal evidence can reveal the failure sequence.
Thermal evidence can reveal the failure sequence.
Connector resistance can destroy a healthy transformer.
Connector resistance can destroy a healthy transformer.
Moisture and heat often act together.
Moisture and heat often act together.
Failure analysis should always reconstruct the sequence of deterioration.
Failure analysis should always reconstruct the sequence of deterioration.
16Engineer's Decision Point
A transformer has failed with: Burned secondary winding. Low insulation resistance. Connector corrosion. Thermal discoloration. Moisture contamination. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about failure analysis of a damaged AGL transformer, based on this case study.
Q1What is AGL-CS-030?
AGL-CS-030 is an engineering training case study on the forensic failure analysis of a damaged AGL isolating transformer removed after repeated failures. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2What symptoms did the transformer show before failing?
It showed intermittent light operation, progressive overheating, low insulation resistance and eventual complete failure. It came from a moisture-prone pit, and previous connector repairs were documented.
Q3What questions did the failure analysis answer?
It examined the failure mechanism, the sequence of deterioration, the initiating cause and the final failure mode. Previous records suggested moisture ingress, but no formal analysis had been done.
Q4What was the root cause?
A deteriorated secondary connector created persistent high contact resistance and excessive localized heating. The heat accelerated insulation ageing while moisture ingress reduced dielectric strength, leading to leakage current, secondary winding damage and a final open-circuit failure.
Q5What is the difference between the initiating cause and the final failure mode here?
The connector problem was the initiating cause, and the winding failure was the final failure mode. Replacing only the transformer would not address the initiating cause.
Q6Which techniques were used in the examination?
The team used high-resolution photography, thermal imaging records, a winding resistance meter, 5 kV insulation resistance tester, low-resistance ohmmeter, microscopic inspection equipment, approved transformer test equipment and maintenance records.
Q7How was the circuit corrected?
The team replaced the transformer, secondary connector and secondary cable, improved pit drainage and restored connector sealing. They verified contact resistance, tested insulation resistance, conducted thermal verification, updated inspection procedures and implemented connector condition monitoring.
Q8What should you investigate first when a transformer fails with a burned winding, corrosion and moisture?
Reconstruct the failure sequence and identify the initiating cause. Replacing the transformer only, replacing the CCR or increasing CCR current is not the right first step.
Engineering & Expert Notes
Do Not Stop at the First Failed Part
Root CauseThe transformer was the casualty, not the cause. Trace the sequence backward to the connector.
Preserve the Failed Unit and Photograph It
EvidenceHeat marks and tracking show the failure sequence. Keep the unit until analysis is complete.
Heat Evidence Points to Resistance
ThermalLocalised discolouration near a connector suggests a resistive joint. Look at where heat started.
Repair the Cause Before Fitting the Replacement
RepairFitting a new transformer onto a bad connector repeats the failure.
20Case Study Conclusion
Transformer failure analysis should identify the initiating cause, the sequence of deterioration, and the final failure mode through systematic examination of electrical, thermal, mechanical, and environmental evidence.
Systematic connector analysis, thermal assessment, insulation testing, winding examination, moisture investigation, and failure-sequence reconstruction can accurately identify the true root cause and prevent repeated transformer failures.
In AGL engineering, the failed winding is often the final chapter of the story; the connector, moisture, temperature, and insulation history usually write the earlier chapters.
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- Fictional training simulation. This case study is an engineering simulation created for training. It is not a report of an actual airport incident. Scenarios, sequences, dates, readings, measurements, outcomes and numerical values are illustrative and invented for teaching. No fault, failure, cause or liability is attributed to any person, company, airport, regulator, manufacturer or product, and any resemblance to a real event is coincidental. The content is not a forensic, legal, insurance, regulatory or accident-investigation finding and must not be cited as evidence, as a record of an event, or as a basis for disciplinary, contractual, insurance or legal action.
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