AGL-CS-015 — Fault Location From CCR to Field Fixture
The AGL Knowledge Hub | Engineering Case Study Series. An expert-level end-to-end fault-localization walkthrough demonstrating the importance of following the complete electrical path from the CCR to the field fixture — from systematic verification through repair.
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 night operational inspection, one section of the runway edge lighting system was reported inoperative. The Constant Current Regulator (CCR) indicated that the circuit was energized, but several field lights remained extinguished.
Previous maintenance teams had replaced a light fitting, an isolating transformer, and a connector without restoring the circuit.
The investigation was restarted using a complete systematic fault-location procedure beginning at the CCR and ending at the field fixture.
The investigation ultimately identified a high-resistance cable joint and a damaged transformer primary connection in the affected section.
This case demonstrates the importance of following the complete electrical path rather than replacing components based on symptoms.
2Learning Objectives
After completing this case study, the learner should be able to:
- Apply a complete CCR-to-fixture troubleshooting methodology.
- Understand systematic fault localization.
- Differentiate between component failure and circuit failure.
- Use sectional electrical testing effectively.
- Interpret CCR voltage and current behavior.
- Isolate cable, joint, connector, transformer, and fixture faults.
- Develop a professional AGL troubleshooting workflow.
3System Background
A typical AGL series circuit consists of:
A fault may occur at any point in this path.
The objective of fault location is to determine the exact location of the electrical interruption or degradation.
4Problem Statement
The reported condition was:
- Runway edge lighting section unavailable.
- CCR energized.
- Circuit current apparently normal.
- Several consecutive lights OFF.
- Previous component replacement unsuccessful.
- No obvious surface damage.
Where is the fault located between the CCR and the field fixture?
5Initial Observations
| CCR status | Healthy |
| Selected current | 6.6 A |
| Output voltage | Slightly elevated |
| Affected lights | L63–L68 |
| Previous maintenance | Fixture, transformer, connector replaced |
| Weather | Dry |
The previous unsuccessful replacements suggested that the actual fault had not been correctly localized.
6The Systematic Fault-Location Sequence
The investigation followed the following order:
- CCR
- CCR output terminals
- Primary cable
- Cable joints
- Connectors
- Transformer primary
- Transformer secondary
- Secondary leads
- Field fixture
No downstream component was tested before upstream verification.
7Test Equipment
The investigation used:
- True-RMS multimeter.
- 5 kV insulation resistance tester.
- Continuity tester.
- Approved AGL circuit test equipment.
- Cable fault locator.
- Voltage-drop measurement equipment.
- Circuit drawings.
- Airport-approved PPE.
8Investigation
Step 1: CCR Verification
Measured values:
| Selected current | 6.6 A |
| Measured current | 6.59 A |
| Output voltage | Higher than normal |
| CCR diagnostics | Healthy |
The CCR was functioning correctly. The investigation therefore moved into the field circuit.
Step 2: Primary Cable Verification
Continuity testing confirmed:
- CCR output cable healthy.
- Main primary circuit continuous.
- No complete open circuit detected.
Step 3: Sectional Voltage Analysis
Voltage measurements were taken across successive circuit sections. One section showed an abnormally high voltage drop.
This localized the investigation between Joint J4 and Transformer T17.
Step 4: Cable Joint Inspection
Joint J4 showed:
- Moisture contamination.
- Oxidized conductor surfaces.
- Incomplete compression connection.
- Elevated contact resistance.
However, continuity was still present. The joint was deteriorated but not completely open.
Step 5: Connector Testing
The associated connector tested within acceptable resistance limits. The connector was therefore not considered the primary fault source.
Step 6: Transformer Primary Investigation
Transformer T17 showed:
- Normal secondary resistance.
- Normal insulation resistance.
- Loose primary termination.
- Evidence of localized heating.
The primary connection exhibited high contact resistance.
Step 7: Secondary Circuit Verification
The secondary lead and field fixture were tested. Results:
- Secondary lead healthy.
- Fixture electrically normal.
The fixture was not the cause of the outage.
9Root Cause
The circuit failure was caused by a combination of:
- A high-resistance cable joint.
- A deteriorated transformer primary connection.
The excessive voltage loss across these two locations prevented normal operation of the affected field section.
The previously replaced fixture, transformer, and connector were not the root cause.
10Corrective Action
The maintenance team:
- Isolated the circuit.
- Rebuilt cable joint J4.
- Re-terminated the transformer primary connection.
- Verified connector integrity.
- Performed continuity testing.
- Conducted insulation resistance testing.
- Measured section voltage drops.
- Restored the circuit.
- Verified field operation.
- Updated maintenance documentation.
11Post-Repair Verification
The affected runway section returned to normal operation.
12Root-Cause Analysis
Field lights OFF.
Healthy.
Healthy.
Abnormal section identified.
High resistance found.
Loose termination identified.
Healthy.
High-resistance joint and transformer primary connection.
Repair both connection points.
13The Complete Fault-Location Workflow
This workflow minimizes unnecessary component replacement.
14Why This Case Was Important
This investigation demonstrates that:
- Symptoms are not fault locations.
- Component replacement is not fault diagnosis.
- Electrical testing should follow circuit topology.
- Upstream verification prevents downstream misdiagnosis.
- Systematic fault location reduces maintenance time.
15Engineering Lessons
Always begin at the CCR.
Always begin at the CCR.
Verify upstream before downstream.
Verify upstream components before downstream components.
Voltage-drop analysis is powerful.
Voltage-drop analysis is a powerful localization tool.
High-resistance faults may involve multiple components.
High-resistance faults may involve multiple components simultaneously.
Define an investigation sequence.
Successful troubleshooting requires a defined investigation sequence.
16Engineer's Decision Point
You arrive at an airport where: Several runway lights are OFF. CCR operating normally. A fixture has already been replaced. The fault remains. What should you do first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about fault location from the CCR to the field fixture, based on this case study.
Q1What is AGL-CS-015?
AGL-CS-015 is an expert-level engineering training case study on fault location from the CCR to the field fixture, following the complete electrical path after earlier repairs had failed. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Why had the earlier repairs not worked?
Previous teams had replaced a light fitting, an isolating transformer and a connector without restoring the circuit. None of those components was the root cause, so the replacements did not address the fault.
Q3Why should fault finding always begin at the CCR?
Starting at the CCR and following the circuit path systematically avoids assumptions and repeated component swapping. When a fixture replacement fails to fix the problem, the correct approach is to restart systematic fault localization from the CCR.
Q4What was the root cause?
A high-resistance cable joint combined with a deteriorated transformer primary connection. The excessive voltage loss across these two locations prevented normal operation of the affected field section.
Q5How was the circuit repaired?
The team isolated the circuit, rebuilt cable joint J4 and re-terminated the transformer primary connection. They verified connector integrity, tested continuity and insulation resistance, measured section voltage drops, restored the circuit and updated documentation.
Q6How was the repair verified?
CCR operation and output voltage were in the normal range, continuity and voltage distribution were normal, the field lights were operational and documentation was completed.
Q7What does this case teach about replacing components without diagnosis?
Replacing parts without locating the fault wastes time and can leave the real defect in place. Systematic fault localization finds the actual cause before any component is replaced.
Q8What should you do first if lights are OFF, the CCR is normal and a replaced fixture did not fix the fault?
Begin systematic fault localization from the CCR. Replacing all transformers or the cable, or increasing CCR current, is not the right first step.
Engineering & Expert Notes
Work in a Fixed Sequence From the Source
MethodStarting at the CCR and moving outward prevents skipped steps and makes the record reproducible.
Past Replacements Are Evidence Too
HistoryIf parts were replaced without effect, the fault lies elsewhere. Record what was changed and why it did not help.
Two Small Resistances Can Add Up
Combined FaultsSeveral marginal connections can together exceed what the circuit tolerates. Fixing only one may improve but not cure.
Verify at the Fixture After Fixing the Source
VerificationConfirm operation at the field end, across brightness steps, to be sure the whole path is sound.
20Case Study Conclusion
Every AGL fault should be investigated through a complete CCR-to-field-fixture diagnostic sequence, verifying each electrical section before replacing components.
Systematic CCR verification, sectional testing, voltage-drop analysis, connector inspection, transformer testing, and fixture verification can accurately locate the true fault while avoiding unnecessary replacement of healthy components.
In AGL engineering, the fastest repair is usually the one that begins with the correct fault-location methodology rather than the first suspected component.
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