AGL-CS-018 — Low Insulation Resistance of Isolating Transformer
The AGL Knowledge Hub | Engineering Case Study Series. A preventive-maintenance investigation where low IR readings revealed progressive insulation deterioration in an operational transformer — from testing through trend analysis to condition-based 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 program, insulation resistance testing was performed on several runway edge light isolating transformers.
One transformer showed significantly lower insulation resistance than the remaining units, although the associated runway edge light was still operating normally.
The Constant Current Regulator (CCR) remained healthy, and no operational fault had yet been reported.
The investigation ultimately identified moisture-assisted deterioration of the transformer insulation system, which had reduced the dielectric strength of the transformer and increased leakage current.
This case demonstrates that low insulation resistance is often an early indicator of transformer deterioration before complete electrical failure occurs.
2Learning Objectives
After completing this case study, the learner should be able to:
- Understand insulation resistance testing of AGL isolating transformers.
- Recognize early signs of transformer insulation deterioration.
- Differentiate winding continuity from insulation health.
- Interpret insulation resistance measurements correctly.
- Investigate moisture-related transformer degradation.
- Evaluate transformer serviceability.
- Implement transformer condition-monitoring practices.
3System Background
An AGL isolating transformer provides:
- Electrical isolation.
- Voltage transformation.
- Current transfer to the field fixture.
The insulation system protects:
- Primary winding.
- Secondary winding.
- Winding-to-core insulation.
- Winding-to-enclosure insulation.
Low insulation resistance may indicate:
- Moisture contamination.
- Insulation aging.
- Thermal degradation.
- Dielectric deterioration.
- Internal contamination.
- Seal failure.
A transformer may continue operating despite significant insulation deterioration.
4Problem Statement
The reported condition was:
- Transformer operating normally.
- Field light operational.
- CCR healthy.
- Low insulation resistance measured during preventive maintenance.
- No visible external damage.
- Transformer installed nine years earlier.
Can a transformer remain in service if insulation resistance is significantly below historical maintenance values?
5Initial Observations
| CCR status | Healthy |
| Selected current | 6.6 A |
| Light operation | Normal |
| Transformer temperature | Normal |
| Insulation resistance | 8 MΩ |
| Historical IR | Greater than 1000 MΩ |
The important observation was that the transformer was operational but electrically deteriorated.
6Initial Engineering Hypotheses
The maintenance team considered:
Moisture contamination.
Aging insulation materials.
Seal deterioration.
Internal contamination.
Testing error.
7Transformer Insulation Resistance Principle
Insulation resistance testing evaluates the dielectric condition of the transformer insulation system.
Low insulation resistance may indicate:
- Increased leakage current.
- Reduced dielectric strength.
- Moisture absorption.
- Insulation contamination.
- Progressive aging.
A transformer with low IR is often approaching the later stages of insulation life.
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.
Testing was performed after complete transformer isolation.
9Investigation
Step 1: Verify Transformer Operation
Measured values:
| Primary continuity | Normal |
| Secondary continuity | Normal |
| Output operation | Normal |
The transformer remained electrically functional.
Step 2: Insulation Resistance Testing
Results:
| Primary to Earth | 9 MΩ |
| Secondary to Earth | 8 MΩ |
| Primary to Secondary | 12 MΩ |
All insulation values were substantially lower than expected.
Step 3: Historical Trend Review
Maintenance records showed:
| 5 years earlier | Greater than 1000 MΩ |
| 3 years earlier | 600 MΩ |
| 1 year earlier | 120 MΩ |
| Current | 8 MΩ |
The steady decline indicated progressive insulation deterioration.
Step 4: Moisture Assessment
Inspection found:
- Seal aging.
- Moisture staining.
- Condensation evidence.
- Minor resin cracking.
- Surface contamination.
These findings suggested long-term moisture exposure.
Step 5: Transformer Examination
After replacement, internal examination revealed:
- Insulation discoloration.
- Moisture absorption.
- Reduced dielectric integrity.
- Minor thermal aging.
- Contaminated insulation surfaces.
The deterioration was consistent with aging combined with moisture contamination.
10Root Cause
The transformer developed progressively low insulation resistance due to moisture-assisted aging of the insulation system.
The winding conductors remained electrically intact, but the insulation had lost a significant portion of its dielectric strength.
The transformer remained operational but had become electrically unreliable.
11Corrective Action
The maintenance team:
- Isolated the transformer.
- Removed the deteriorated unit.
- Installed a new approved isolating transformer.
- Verified winding continuity.
- Performed insulation resistance testing.
- Checked secondary operation.
- Restored the circuit.
- Confirmed normal insulation values.
- Updated transformer condition records.
- Scheduled testing of similar-age transformers.
12Post-Repair Verification
The transformer insulation integrity was fully restored.
13Root-Cause Analysis
Low transformer IR detected.
Valid measurement.
Windings healthy.
Progressive deterioration.
Moisture and aging identified.
Insulation deterioration.
Replace transformer.
14Why This Case Was Important
A transformer may appear satisfactory because:
- The light operates.
- The CCR operates normally.
- The windings remain continuous.
However, low insulation resistance indicates reduced dielectric strength and increased failure risk.
Failure to replace the transformer may eventually result in:
- Earth leakage.
- Insulation breakdown.
- Primary-to-secondary failure.
- Transformer overheating.
- Complete transformer failure.
15Engineering Lessons
Low IR is an early warning indicator.
Low insulation resistance is an early warning indicator.
Operational transformers may be insulation-defective.
Operational transformers may still be insulation-defective.
Historical IR trends are more valuable than a single measurement.
Historical IR trends are more valuable than a single measurement.
Moisture and aging commonly occur together.
Moisture and aging commonly occur together.
Condition-based replacement is preferable to failure-based replacement.
Condition-based replacement is preferable to failure-based replacement.
16Engineer's Decision Point
You observe: Transformer operating normally. Light functioning correctly. Primary and secondary continuity normal. Insulation resistance reduced from 800 MΩ to 8 MΩ over several years. What should you do?
17Knowledge Check
18Interview Questions
Quick Reference: Isolating Transformer Resistance & Insulation Tests
The isolating transformer insulation test is made with an insulation tester between the windings and between each winding and earth, with the transformer disconnected from the circuit. The isolating transformer resistance test measures the DC resistance of the primary and secondary windings. A low insulation reading, as in this case, points to moisture or damage. Compare both results with the manufacturer data or the previous record.
19Frequently Asked Questions
Quick answers about low insulation resistance in isolating transformers, based on this case study.
Q1What is AGL-CS-018?
AGL-CS-018 is an engineering training case study on low insulation resistance in an operational isolating transformer, found during preventive maintenance and resolved by condition-based replacement. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Why replace a transformer that is still working?
Its insulation had lost a significant portion of its dielectric strength, so it had become electrically unreliable even though it was operating. The case shows replacement is necessary once insulation deterioration is progressive and has reached an unreliable condition.
Q3How was the problem found?
During scheduled insulation resistance testing on several runway edge light transformers, one unit read significantly lower than the others. The light was still operating, the CCR was healthy, no fault had been reported and the transformer had been installed nine years earlier.
Q4What was the root cause?
Moisture-assisted ageing of the transformer insulation system. The winding conductors were intact, but reduced dielectric strength increased leakage current.
Q5Why is low insulation resistance an early warning indicator?
Low IR is an early warning indicator because it appears before an operational failure. In the decision-point scenario IR fell from 800 MΩ to 8 MΩ over several years while the light still worked.
Q6How was the circuit repaired?
The team isolated the transformer, removed the deteriorated unit and installed a new approved isolating transformer. They verified winding continuity, tested insulation resistance, checked secondary operation, restored the circuit, updated condition records and scheduled testing of similar-age transformers.
Q7What did post-repair verification show?
Insulation resistance was greater than 1000 MΩ, primary and secondary continuity were normal, the light operated normally and documentation was completed.
Q8What should you do if a transformer works normally but insulation resistance has fallen from 800 MΩ to 8 MΩ over several years?
Replace the transformer. Leaving it in service, increasing CCR current or replacing the light fixture does not address the deteriorated insulation.
Engineering & Expert Notes
Build a Baseline for Each Transformer Type
BaselineComparing a unit against its own history or similar units shows deterioration early.
Record Conditions Alongside Readings
ConditionsTemperature and humidity affect results. Without them comparisons can mislead.
Check the Location for Moisture Sources
EnvironmentIf low insulation is found, inspect the pit and seals. Replacing the unit without fixing the environment invites a repeat.
Plan Replacement Before Failure
PlanningMarginal units can be replaced during planned work rather than waiting for a night-time outage.
20Case Study Conclusion
Low insulation resistance in an AGL isolating transformer is a progressive condition that should be treated as a developing reliability and insulation-integrity failure even when the transformer remains operational.
Systematic insulation resistance testing, historical trend analysis, moisture assessment, and condition-based replacement can prevent unexpected transformer failures and improve the long-term reliability of the airfield lighting system.
In AGL engineering, a transformer that still transfers power is not necessarily a transformer that still provides adequate insulation protection.
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Please read before relying on any content on this page or on any record downloaded from it.
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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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