Category 02 · Isolating Transformer & Field Equipment Faults

AGL-CS-016 — AGL Isolating Transformer Secondary Open Circuit

The AGL Knowledge Hub | Engineering Case Study Series. A single-light failure investigation where an internal transformer secondary winding open circuit mimicked a failed fixture — from primary verification through secondary continuity testing to transformer replacement.

Case ID

AGL-CS-016

1Case Study Overview

During a routine night inspection, one runway edge light was found extinguished 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.

Initial inspection suggested a failed LED fitting. However, replacement of the fixture did not restore operation.

The investigation ultimately identified an open circuit in the secondary winding circuit of the isolating transformer, preventing power from reaching the field fixture.

This case demonstrates the importance of separating primary-circuit diagnosis from secondary-circuit diagnosis in AGL systems.

2Learning Objectives

After completing this case study, the learner should be able to:

Understand the function of the isolating transformer secondary circuit.
Recognize symptoms of a secondary open-circuit fault.
Differentiate transformer secondary faults from fixture failures.
Perform systematic transformer secondary testing.
Interpret continuity and resistance measurements.
Investigate secondary cable and connector integrity.
Verify transformer performance after replacement.

3System Background

An AGL isolating transformer electrically isolates the primary series circuit from the secondary fixture circuit.

A simplified arrangement is:

Primary Series Cable → Transformer Primary → Magnetic Coupling → Transformer Secondary → Secondary Leads → Light Fixture

A secondary open circuit may occur due to:

Secondary winding failure.
Broken secondary lead.
Loose secondary termination.
Corroded secondary connector.
Internal transformer connection failure.

The primary circuit may remain fully operational while the individual light remains OFF.

4Problem Statement

The reported condition was:

One runway edge light OFF.
Adjacent lights ON.
CCR operating normally.
Primary circuit voltage normal.
Fixture replacement unsuccessful.
No visible cable damage.

Engineering Question

Is the fault located in the light fixture, the transformer secondary circuit, or the transformer itself?

5Initial Observations

CCR status	Healthy
Selected current	6.6 A
Primary circuit	Normal
Affected light	L71 OFF
Adjacent lights	ON
Fixture replacement	No improvement

The failure of only one light strongly suggested a localized secondary-circuit problem.

6Initial Engineering Hypotheses

The maintenance team considered:

Hypothesis A

LED fixture failure.

Hypothesis B

Secondary cable open circuit.

Hypothesis C

Transformer secondary winding open circuit.

Hypothesis D

Secondary connector failure.

Hypothesis E

Primary transformer failure.

7Primary vs Secondary Faults

A useful diagnostic distinction is:

Primary Fault	May affect multiple lights
Secondary Fault	Usually affects one light
Primary Fault	CCR voltage often changes
Secondary Fault	CCR often remains normal
Primary Fault	Circuit current may change
Secondary Fault	Circuit current usually remains normal
Primary Fault	Section failure common
Secondary Fault	Individual fixture failure common

This observation directed the investigation toward the secondary side of the transformer.

8Test Equipment

The investigation used:

True-RMS multimeter.
Low-resistance continuity tester.
Insulation resistance tester.
Approved AGL transformer test equipment.
Secondary cable test leads.
Circuit drawings.
Airport-approved PPE.

All testing was performed after approved circuit isolation.

9Investigation

Step 1: Verify Primary Circuit

Measurements confirmed:

CCR current	Normal
Primary continuity	Normal
Adjacent transformers	Operational

The primary series circuit was healthy.

Step 2: Verify the Fixture

The LED fixture was tested separately. Results:

Fixture continuity normal.
Insulation normal.
Bench operation satisfactory.

The fixture was not defective.

Step 3: Secondary Circuit Continuity

Continuity testing between the transformer secondary terminals and the fixture showed:

Open circuit.
Infinite resistance.
No continuity.

This confirmed a secondary-circuit interruption.

Step 4: Secondary Lead Inspection

The secondary cable showed:

No mechanical damage.
Secure terminations.
Acceptable insulation condition.

The secondary leads were healthy.

Step 5: Transformer Secondary Testing

The transformer was disconnected. Measurements:

Primary winding	Normal
Secondary winding	Open circuit
Insulation resistance	Acceptable
Core condition	Normal

The secondary winding showed infinite resistance, confirming an internal open circuit.

10Root Cause

Training-Case Finding

The isolating transformer developed an internal secondary winding open circuit caused by failure of the winding connection inside the transformer.

The primary winding remained functional, but the secondary circuit could not deliver power to the field fixture.

11Corrective Action

The maintenance team:

Isolated the circuit.
Removed the defective transformer.
Inspected secondary connectors.
Verified secondary cable integrity.
Installed a new approved isolating transformer.
Checked secondary continuity.
Performed insulation resistance testing.
Restored the circuit.
Verified field-light operation.
Recorded transformer replacement details.

12Post-Repair Verification

CCR operationNormal

Primary circuitNormal

Secondary continuityNormal

Light outputRestored

Transformer temperatureNormal

Maintenance documentationCompleted

The affected runway edge light returned to normal operation.

13Root-Cause Analysis

Problem

Individual light OFF.

CCR checked

Normal.

Fixture tested

Healthy.

Secondary continuity tested

Open circuit.

Secondary cable tested

Healthy.

Transformer tested

Secondary winding open.

Root Cause

Internal transformer secondary open circuit.

Corrective Action

Replace isolating transformer.

14Why This Case Was Important

A common maintenance error is replacing:

LED fixtures.
Connectors.
Secondary cables.

before verifying transformer secondary continuity.

A secondary open circuit can perfectly mimic a failed light fitting.

15Engineering Lessons

Separate primary and secondary diagnosis.

Always separate primary and secondary diagnosis.

Single light failure rarely indicates a primary fault.

A single light failure rarely indicates a primary circuit fault.

Test secondary continuity before fixture replacement.

Fixture replacement should not precede secondary continuity testing.

Winding resistance measurement is valuable.

Transformer winding resistance measurement is a valuable diagnostic tool.

Systematic testing prevents unnecessary replacement.

Systematic testing prevents unnecessary component replacement.

16Engineer's Decision Point

You observe: One runway edge light OFF. Adjacent lights ON. CCR operating normally. Fixture replacement unsuccessful. What should you investigate first?

A. Replace the CCR.

B. Replace the primary cable.

C. Test transformer secondary continuity.

D. Increase CCR current.

Correct answer: C. The failure pattern indicates a localized secondary-circuit problem. Secondary continuity testing is the appropriate next step.

17Knowledge Check

Q1. What was the primary fault?

Answer: Transformer secondary winding open circuit.

Q2. Did the primary circuit fail?

Answer: No.

Q3. Why did only one light remain OFF?

Answer: The fault was confined to the transformer secondary circuit.

Q4. What test confirmed the fault?

Answer: Secondary winding continuity and resistance testing.

Q5. What was the corrective action?

Answer: Replacement of the isolating transformer.

18Interview Questions

What is the function of an AGL isolating transformer?

How does a secondary open circuit affect the AGL system?

Why can the CCR remain normal during a secondary fault?

How would you test a transformer secondary winding?

What is the difference between a fixture failure and a transformer secondary failure?

Why is winding resistance measurement important?

How would you isolate the transformer from the field fixture?

What post-replacement tests should be performed?

What documentation should be recorded after transformer replacement?

How can transformer failures be reduced through preventive maintenance?

Quick Reference: Isolating Transformer Secondary Voltage

The isolating transformer secondary voltage is the voltage delivered to the lamp or LED module. It depends on the lamp load at the rated current. With the circuit energised and the right safety procedure followed, a normal reading means the transformer works and the fault is in the lamp or its wiring. No reading with a healthy primary points to an open secondary circuit, as in this case.

19Frequently Asked Questions

Quick answers about isolating transformer secondary open circuit, based on this case study.

Q1What is AGL-CS-016?

AGL-CS-016 is an engineering training case study on a single runway edge light failure caused by an open circuit in an isolating transformer secondary winding, which mimicked a failed fixture. It is an engineering simulation created for training, not a report of an actual airport incident.

Q2Why can a transformer fault look like a fixture failure?

One light was OFF while adjacent lights were ON and the CCR and primary circuit were normal, which is the usual pattern of a fixture failure. Replacing the fixture did not restore operation, revealing the fault was elsewhere.

Q3Why separate primary and secondary diagnosis?

The primary series circuit and the secondary circuit fail in different ways. Here the primary winding was functional, but the secondary could not deliver power to the fixture, so the two sides had to be tested separately.

Q4What was the root cause?

An internal secondary winding open circuit, caused by failure of the winding connection inside the isolating transformer. The primary winding remained functional.

Q5How is a transformer secondary open circuit confirmed?

Test transformer secondary continuity after approved circuit isolation. The team used a true-RMS multimeter, low-resistance continuity tester, insulation resistance tester, approved AGL transformer test equipment and secondary cable test leads.

Q6How was the circuit repaired?

The team isolated the circuit, removed the defective transformer, inspected secondary connectors and verified secondary cable integrity. They installed a new approved isolating transformer, checked secondary continuity, tested insulation resistance, restored the circuit and recorded the replacement details.

Q7How was the repair verified?

CCR operation, the primary circuit and secondary continuity were normal, light output was restored and transformer temperature was normal. Documentation was completed.

Q8What should you investigate first if one light is OFF, adjacent lights are ON and a fixture replacement failed?

Test transformer secondary continuity. Replacing the CCR or primary cable, or increasing CCR current, is not the right next step.

Engineering & Expert Notes

By Rahul Sharma

Airfield Ground Lighting Specialist

Separate Primary and Secondary Tests

Method

Test each side independently so the result points to the winding rather than the cable or fixture.

Check Secondary Connections Before Opening the Case

Terminations

Most secondary faults are at the lead or connector. Confirm those before condemning the transformer.

Treat the Primary as Live Until Proved Isolated

Safety

The primary circuit carries series current and voltage. Follow isolation and permit procedures before any disconnection.

Record Transformer Type and Rating at Replacement

Records

The replacement must match the original rating. Note it on the register to avoid mismatches later.

20Case Study Conclusion

Key Engineering Principle

A single AGL light failure should be investigated through systematic secondary-circuit testing before replacing field fixtures or primary-circuit components.

Verification of secondary continuity, winding resistance, connector integrity, and transformer isolation can accurately identify secondary open-circuit faults and prevent unnecessary maintenance actions.

In AGL engineering, a healthy primary circuit does not guarantee a healthy transformer secondary circuit, and the correct diagnosis often begins at the transformer terminals.

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Important Notice, Disclaimer & Limitation of Liability

Please read before relying on any content on this page or on any record downloaded from it.

Specific to this case study — Electrical fault-finding, testing, transformer and CCR content. Insulation-resistance, continuity, earth-fault, current, voltage and VA figures, and any test-voltage values, are illustrative for the scenario and depend on the circuit, equipment, temperature and test method. They are not acceptance limits, pass/fail criteria or test instructions. Insulation testing at high voltage, work on CCR outputs and series circuits, and work on isolating transformers can be fatal. Where a case narrates opening, testing, measuring or inspecting a circuit, it is narrative only and is not an endorsement of working on energised equipment. Do not open, short, bridge or test any series circuit or CCR unless it is isolated, locked out, proved dead and covered by your own approved procedures.

Purpose and intended users. This page is an independent educational and informational reference for competent, suitably trained and authorised airfield ground lighting (AGL) and aerodrome electrical personnel. It is not professional engineering, legal or regulatory advice, and using it does not create any professional or client relationship.
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.
Safety. Work on series circuits, constant current regulators (CCRs), isolating transformers, cables, connectors and control equipment can be fatal. Always apply the aerodrome’s permit-to-work and isolation/LOTO procedures, prove dead, use correctly rated instruments and PPE, and follow the manufacturer’s instructions. Nothing here authorises work on live equipment or the bypassing of any safety device or interlock. Where a case narrates an inspection, measurement or test, that narrative is not an instruction or an endorsement of the method.
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Notice v1.0 · 20 October 2026 · Full terms: Legal & Disclaimer

AGL-CS-016 — AGL Isolating Transformer Secondary Open Circuit

1Case Study Overview

2Learning Objectives

3System Background

4Problem Statement

5Initial Observations

6Initial Engineering Hypotheses

7Primary vs Secondary Faults

8Test Equipment

9Investigation

Step 1: Verify Primary Circuit

Step 2: Verify the Fixture

Step 3: Secondary Circuit Continuity

Step 4: Secondary Lead Inspection

Step 5: Transformer Secondary Testing

10Root Cause

11Corrective Action

12Post-Repair Verification

13Root-Cause Analysis

14Why This Case Was Important

15Engineering Lessons

Separate primary and secondary diagnosis.

Single light failure rarely indicates a primary fault.

Test secondary continuity before fixture replacement.

Winding resistance measurement is valuable.

Systematic testing prevents unnecessary replacement.

16Engineer's Decision Point

17Knowledge Check

18Interview Questions

Quick Reference: Isolating Transformer Secondary Voltage

19Frequently Asked Questions

Engineering & Expert Notes

Separate Primary and Secondary Tests

Check Secondary Connections Before Opening the Case

Treat the Primary as Live Until Proved Isolated

Record Transformer Type and Rating at Replacement

20Case Study Conclusion

Turn Case Studies Into Engineering Capability

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Important Notice, Disclaimer & Limitation of Liability