AGL-CS-022 — Incorrect Transformer VA Selection
The AGL Knowledge Hub | Engineering Case Study Series. A design and maintenance case where an incorrectly rated isolating transformer caused overheating and light instability — from nameplate verification through load calculation to proper selection.
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
Following a runway edge light maintenance activity, a newly installed isolating transformer began operating at an unusually high temperature, and the associated LED light exhibited unstable brightness.
The Constant Current Regulator (CCR) was operating normally, and the primary series circuit showed no electrical abnormalities.
Initial inspection suggested a transformer quality issue. However, detailed investigation revealed that the replacement transformer had the wrong VA (Volt-Ampere) rating for the connected AGL fixture.
This case demonstrates how incorrect transformer VA selection can create thermal stress, voltage instability, reduced lighting performance, and premature transformer failure.
2Learning Objectives
After completing this case study, the learner should be able to:
- Understand the purpose of transformer VA ratings.
- Select the correct transformer VA for AGL fixtures.
- Recognize symptoms of incorrect VA selection.
- Calculate transformer loading.
- Evaluate thermal and electrical effects of undersized transformers.
- Understand the risks of oversized and undersized transformers.
- Apply proper transformer replacement procedures.
3System Background
An AGL isolating transformer transfers power from the 6.6 A primary series circuit to the secondary lighting fixture.
The transformer VA rating must be compatible with the fixture load.
Typical transformer ratings may include:
- 30 VA
- 45 VA
- 65 VA
- 100 VA
- Other approved ratings
Incorrect VA selection can affect:
- Secondary voltage.
- Temperature rise.
- Regulation.
- Efficiency.
- Insulation life.
- Fixture performance.
4Problem Statement
The reported condition was:
- Transformer recently replaced.
- Associated LED light unstable.
- Transformer temperature elevated.
- CCR operating normally.
- No cable or connector fault detected.
- Replacement transformer installed from local stock.
Has the transformer been incorrectly selected for the connected AGL fixture load?
5Initial Observations
| CCR status | Healthy |
| Selected current | 6.6 A |
| Light operation | Unstable brightness |
| Transformer temperature | 78°C |
| Adjacent transformers | 50–55°C |
| Transformer age | Newly installed |
The problem began immediately after transformer replacement.
6Initial Engineering Hypotheses
The maintenance team considered:
Defective new transformer.
Incorrect transformer VA rating.
Connector resistance.
Fixture malfunction.
Secondary cable fault.
7Transformer VA Principle
Transformer loading should satisfy:
Transformer VA rating ≥ Fixture VA requirement
If the transformer is undersized:
- Higher temperature rise.
- Reduced voltage regulation.
- Increased losses.
- Accelerated insulation aging.
If significantly oversized:
- Regulation characteristics may differ.
- Inventory standardization issues arise.
- Replacement errors become more likely.
Correct VA selection is therefore a design and maintenance requirement.
8Test Equipment
The investigation used:
- True-RMS multimeter.
- Transformer nameplate verification.
- Thermal imaging camera.
- Load measurement equipment.
- Approved AGL circuit test equipment.
- Manufacturer data sheets.
- Circuit drawings.
- Airport-approved PPE.
9Investigation
Step 1: Verify Circuit Operation
Measured values:
| CCR current | Normal |
| Primary circuit | Healthy |
| Adjacent lights | Stable |
The fault was localized to one transformer-fixture combination.
Step 2: Nameplate Verification
Installed transformer:
- 30 VA
Design drawing requirement:
- 65 VA
A significant rating mismatch existed.
Step 3: Fixture Load Assessment
The connected LED fixture required approximately:
- 52 VA under operating conditions
The installed transformer was therefore operating well above its intended loading range.
Step 4: Temperature Analysis
Thermal measurements showed:
| Correct 65 VA transformer | 52°C |
| Installed 30 VA transformer | 78°C |
The undersized transformer exhibited excessive heating.
Step 5: Secondary Voltage Measurement
The undersized transformer showed:
- Voltage instability.
- Increased regulation error.
- Reduced output stability under load.
These measurements correlated with the observed light flicker.
10Root Cause
A 30 VA transformer had been installed in place of the required 65 VA transformer.
The transformer was significantly undersized for the connected LED fixture.
This caused:
- Excessive temperature rise.
- Poor voltage regulation.
- Unstable light output.
- Accelerated insulation stress.
The transformer itself was not defective; it was incorrectly selected.
11Corrective Action
The maintenance team:
- Isolated the circuit.
- Removed the incorrect transformer.
- Installed the specified 65 VA transformer.
- Verified fixture compatibility.
- Checked secondary voltage.
- Performed thermal verification.
- Confirmed stable light output.
- Updated transformer inventory records.
- Reviewed replacement procedures.
- Conducted maintenance personnel retraining.
12Post-Repair Verification
The overheating and light instability were completely eliminated.
13Root-Cause Analysis
Transformer overheating.
Healthy.
Incorrect VA identified.
Transformer undersized.
Overloading confirmed.
Incorrect transformer VA selection.
Install correct transformer rating.
14Why This Case Was Important
Incorrect transformer selection can occur due to:
- Maintenance stock shortages.
- Nameplate misreading.
- LED fixture upgrades.
- Documentation errors.
- Field replacement without design verification.
The transformer may operate temporarily but suffer progressive thermal deterioration.
15Engineering Lessons
VA rating must match the fixture load.
Transformer VA rating must match the connected fixture load.
Incorrect selection, not defect, can cause failure.
A newly installed transformer can fail due to incorrect selection, not manufacturing defects.
Thermal comparison is highly valuable.
Thermal comparison with adjacent transformers is highly valuable.
LED upgrades may require transformer verification.
LED fixture upgrades may require transformer verification.
Nameplate verification is essential.
Nameplate verification should be part of every transformer replacement.
16Engineer's Decision Point
You observe: New transformer installed. Light flickering. Transformer overheating. CCR operating normally. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about incorrect transformer VA selection, based on this case study.
Q1What is AGL-CS-022?
AGL-CS-022 is an engineering training case study on an incorrectly selected transformer VA rating, which caused overheating and unstable LED brightness after a transformer replacement. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2What symptoms appeared after the transformer was replaced?
The new transformer ran at 78°C against 50–55°C for adjacent units, and the LED light showed unstable brightness. The CCR was healthy at 6.6 A and no cable or connector fault was detected. The problem began immediately after replacement.
Q3What was the root cause?
A 30 VA transformer had been installed in place of the required 65 VA transformer, and it was significantly undersized for the connected LED fixture. The replacement came from local stock. The transformer was not defective; it was incorrectly selected.
Q4What did the undersized transformer cause?
It caused excessive temperature rise, poor voltage regulation, unstable light output and accelerated insulation stress.
Q5Why suspect VA rating when a fault starts right after a transformer replacement?
Because the timing points to the replacement itself. Nameplate verification and load calculation against the fixture are the correct first steps, before suspecting the CCR or the fixture.
Q6How was the problem corrected?
The team isolated the circuit, removed the incorrect transformer and installed the specified 65 VA unit. They verified fixture compatibility, checked secondary voltage, confirmed stable light output, updated inventory records, reviewed replacement procedures and retrained maintenance personnel.
Q7How was the repair verified?
Transformer temperature fell to 53°C, and light stability, voltage regulation and insulation condition were normal. The overheating and light instability were completely eliminated.
Q8What should you check first if a new transformer overheats and the light flickers with a normal CCR?
Verify the transformer VA rating against the fixture load. Replacing the CCR or the fixture, or increasing CCR current, is not the right first step.
Engineering & Expert Notes
Verify VA Rating Against the Fixture Load
SelectionCheck the fixture's actual load against the transformer rating, particularly after LED upgrades.
Include Transformer Rating in the Change Record
Change ControlAny fixture change should trigger a transformer check. Record the decision.
Treat Overheating as a Selection Warning
DiagnosisA transformer running hot may be undersized rather than faulty. Check the specification before replacing like for like.
Control Spares to Prevent Mix-Ups
SparesStore and label transformers by rating so the wrong unit is not fitted in the field.
20Case Study Conclusion
Correct transformer VA selection is a critical design and maintenance requirement that directly affects temperature rise, voltage regulation, insulation life, and AGL system reliability.
Systematic nameplate verification, load calculation, thermal analysis, and manufacturer compatibility checking can prevent overheating, unstable lighting performance, and premature transformer failure.
In AGL engineering, a transformer that fits physically is not necessarily a transformer that is electrically compatible with the connected airfield lighting load.
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