AGL-CS-017 — Primary-to-Secondary Insulation Failure
The AGL Knowledge Hub | Engineering Case Study Series. A transformer insulation failure investigation where a functioning transformer was found to have lost its isolation integrity — from routine IR testing through transformer examination to 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 scheduled preventive maintenance inspection, a runway edge lighting circuit remained operational, but insulation resistance measurements produced abnormal results.
The Constant Current Regulator (CCR) was maintaining the selected current, and all associated field lights were operational. However, insulation testing between the primary and secondary circuits of one isolating transformer showed an unexpectedly low resistance value.
Further investigation identified breakdown of the insulation barrier between the transformer primary and secondary windings, compromising the transformer's electrical isolation.
This case demonstrates that an isolating transformer can continue operating while its insulation system has already become unsafe and unreliable.
2Learning Objectives
After completing this case study, the learner should be able to:
- Understand the insulation barrier function in an AGL isolating transformer.
- Recognize symptoms of primary-to-secondary insulation failure.
- Perform insulation resistance testing correctly.
- Differentiate winding continuity from insulation integrity.
- Interpret transformer insulation test results.
- Evaluate transformer serviceability.
- Apply safe transformer replacement procedures.
3System Background
An AGL isolating transformer provides electrical isolation between the primary series circuit and the secondary fixture circuit.
A simplified arrangement is:
The insulation system separates:
- Primary winding.
- Secondary winding.
- Transformer core.
- Transformer enclosure.
Failure of the insulation barrier may create:
- Leakage current.
- Reduced insulation resistance.
- Loss of electrical isolation.
- Equipment reliability risk.
- Potential safety hazards.
4Problem Statement
The reported condition was:
- Circuit operational.
- CCR healthy.
- Light functioning normally.
- Low primary-to-secondary insulation resistance.
- No visible transformer damage.
- Transformer installed eight years earlier.
Can the transformer remain in service if it is still operating but has lost primary-to-secondary insulation integrity?
5Initial Observations
| CCR status | Healthy |
| Selected current | 6.6 A |
| Light operation | Normal |
| Primary continuity | Normal |
| Secondary continuity | Normal |
| Primary-to-secondary IR | Abnormally low |
The unusual condition was that the transformer was electrically functional but insulation-defective.
6Initial Engineering Hypotheses
The maintenance team considered:
Moisture contamination.
Transformer insulation deterioration.
Internal winding insulation breakdown.
External connector contamination.
Testing error.
7Transformer Insulation Principle
A healthy isolating transformer should maintain high insulation resistance between the primary and secondary windings.
The insulation barrier prevents direct electrical conduction between:
- Primary circuit.
- Secondary circuit.
- Transformer core.
Insulation failure may occur due to:
- Thermal aging.
- Moisture ingress.
- Dielectric deterioration.
- Manufacturing defects.
- Repeated surge stress.
- Long-term electrical loading.
8Test Equipment
The investigation used:
- 5 kV insulation resistance tester.
- Winding resistance meter.
- True-RMS multimeter.
- Approved transformer test equipment.
- Circuit drawings.
- Historical maintenance records.
- Airport-approved PPE.
All testing was performed after approved circuit isolation.
9Investigation
Step 1: Verify Circuit Operation
Measured values:
| CCR current | Normal |
| Primary circuit | Normal |
| Secondary output | Normal |
The transformer continued to power the fixture.
Step 2: Winding Continuity Testing
Measurements:
| Primary | Normal |
| Secondary | Normal |
Neither winding was open or shorted.
Step 3: Primary-to-Secondary Insulation Test
Results:
| Primary to Secondary | 4 MΩ |
| Historical value | Greater than 1000 MΩ |
The insulation resistance had deteriorated severely.
Step 4: Primary-to-Core Test
Primary-to-core insulation was also reduced. This suggested deterioration of the transformer insulation system rather than an external connection problem.
Step 5: Transformer Examination
After removal, inspection revealed:
- Moisture staining.
- Insulation discoloration.
- Resin cracking.
- Thermal aging.
- Minor carbon tracking near winding insulation.
The damage was consistent with long-term insulation deterioration.
10Root Cause
The transformer developed primary-to-secondary insulation breakdown due to aging and moisture-assisted dielectric deterioration.
The winding conductors remained electrically continuous, but the insulation barrier had lost its dielectric strength.
The transformer could still operate temporarily, but its isolation function had been compromised.
11Corrective Action
The maintenance team:
- Isolated the circuit.
- Removed the defective transformer.
- Tested adjacent transformers.
- Installed a new approved isolating transformer.
- Performed winding continuity testing.
- Conducted insulation resistance testing.
- Verified secondary operation.
- Restored the circuit.
- Confirmed normal insulation values.
- Updated transformer maintenance records.
12Post-Repair Verification
The transformer isolation integrity was fully restored.
13Root-Cause Analysis
Low primary-to-secondary IR detected.
Valid measurement.
Windings healthy.
Insulation deterioration confirmed.
Aging and moisture damage identified.
Primary-to-secondary insulation breakdown.
Replace isolating transformer.
14Why This Case Was Important
A transformer may appear healthy because:
- The light operates normally.
- The CCR operates normally.
- The windings remain continuous.
However, insulation failure means the transformer has lost its fundamental isolation function.
Continued operation may increase the risk of:
- Leakage current.
- Insulation collapse.
- Transformer failure.
- Surge vulnerability.
- Electrical safety concerns.
15Engineering Lessons
Operation does not guarantee insulation integrity.
Transformer operation does not guarantee insulation integrity.
IR testing is essential preventive maintenance.
Insulation resistance testing is essential preventive maintenance.
Isolation is the transformer's most important safety function.
Primary-to-secondary isolation is the transformer's most important safety function.
Moisture and aging accelerate dielectric deterioration.
Moisture and aging often combine to accelerate dielectric deterioration.
Low IR requires replacement.
Low insulation resistance requires transformer replacement, not continued operation.
16Engineer's Decision Point
You observe: Light operating normally. CCR healthy. Primary continuity normal. Secondary continuity normal. Primary-to-secondary insulation resistance only 5 MΩ. What should you do?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about primary-to-secondary transformer insulation failure, based on this case study.
Q1What is AGL-CS-017?
AGL-CS-017 is an engineering training case study on a primary-to-secondary insulation failure in an isolating transformer that was still operating normally. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Can a transformer stay in service if it works but has lost isolation integrity?
No. Transformer operation does not guarantee insulation integrity. In this case a working transformer with primary-to-secondary insulation resistance of only 5 MΩ must be replaced regardless of continued functionality.
Q3How was the problem discovered?
During scheduled preventive maintenance the circuit, CCR and light all appeared normal, but insulation testing between the primary and secondary of one isolating transformer gave an unexpectedly low resistance. No transformer damage was visible and the unit had been installed eight years earlier.
Q4What was the root cause?
Ageing and moisture-assisted dielectric deterioration broke down the insulation barrier between the primary and secondary windings. The winding conductors stayed electrically continuous, but the barrier had lost its dielectric strength.
Q5Why does primary-to-secondary insulation matter?
The barrier provides the transformer's electrical isolation. Once compromised, the transformer may keep operating temporarily, but its isolation function is no longer reliable.
Q6How was the circuit repaired?
The team isolated the circuit, removed the defective transformer and tested adjacent transformers. They installed a new approved isolating transformer, tested winding continuity and insulation resistance, verified secondary operation, restored the circuit and updated transformer records.
Q7What did post-repair verification show?
Primary and secondary continuity were normal, primary-to-secondary insulation resistance was greater than 1000 MΩ, the light operated normally and documentation was completed.
Q8Why were adjacent transformers tested?
The team tested adjacent transformers as part of the corrective action, since ageing and moisture-related deterioration may affect other units in similar condition.
Engineering & Expert Notes
Insulation Between Windings Is a Safety Barrier
FunctionIsolation protects both people and the circuit. Treat a weakened barrier as a defect even if lights still work.
Test With the Correct Method and Voltage
TestingUse the approved test voltage and procedure for transformers. Wrong methods can damage good units or miss weak ones.
Schedule Insulation Tests on Older Transformers
PlanningAgeing units benefit from periodic testing. Prioritise older and wetter locations.
Remove Suspect Units From Service
DispositionA transformer with a failed barrier should be replaced and tagged, not returned to stock.
20Case Study Conclusion
A functioning AGL isolating transformer may still be electrically unsafe if primary-to-secondary insulation integrity has deteriorated.
Systematic insulation resistance testing, winding verification, and isolation assessment are essential for evaluating transformer health and maintaining the electrical safety and reliability of the airfield lighting system.
In AGL engineering, the transformer's ability to isolate the primary and secondary circuits is as important as its ability to transfer power.
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