AGL-CS-041 — Runway Edge Light Complete Circuit Failure
The AGL Knowledge Hub | Engineering Case Study Series. A complete circuit outage investigation where all runway edge lights extinguished simultaneously due to a major primary cable joint failure — from CCR verification through continuity testing, TDR localization, excavation, and joint 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 evening runway operations, the airport control tower reported that the entire runway edge lighting circuit became unavailable.
All runway edge lights extinguished simultaneously, making the runway unavailable for night operations.
The Constant Current Regulator (CCR) was operational, but the runway edge lighting circuit could not be energized successfully.
A systematic investigation identified a complete primary circuit interruption caused by failure of a major underground cable joint, which disconnected the runway edge lighting series circuit.
This case demonstrates that complete runway edge light circuit failure requires structured system-level diagnosis, rapid fault localization, and safe operational restoration.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose complete runway edge light circuit failures.
- Distinguish CCR faults from primary circuit failures.
- Perform systematic circuit isolation.
- Locate complete series-circuit interruptions.
- Investigate cable joints and underground primary cables.
- Restore runway edge lighting safely.
- Document complete circuit failure investigations.
3System Background
A typical runway edge lighting circuit consists of:
Complete circuit failure may result from:
- Primary cable open circuit.
- Major cable joint failure.
- Excavation damage.
- Earth fault.
- Lightning damage.
- Multiple simultaneous failures.
- Incorrect circuit modification.
Because runway edge lights are connected in a series circuit, one major interruption can disable the entire circuit.
4Problem Statement
The reported condition was:
- All runway edge lights OFF.
- CCR operational.
- Circuit could not be energized.
- No scheduled maintenance in progress.
- Runway unavailable for night operation.
Has the CCR failed, or has the primary runway edge lighting circuit become completely interrupted?
5Initial Observations
| CCR status | Healthy |
| Selected current | 6.6 A |
| Actual current | 0 A |
| Output voltage | Rising rapidly |
| Runway edge lights | Entire circuit OFF |
| Circuit energization | Unsuccessful |
The absence of output current suggested a complete circuit interruption.
6Initial Engineering Hypotheses
The maintenance team considered:
CCR output failure.
Primary cable open circuit.
Major cable joint failure.
Complete transformer circuit interruption.
Switching or isolation error.
7Complete Circuit Failure Principle
A series AGL circuit requires:
If the primary loop is interrupted:
The CCR may remain electrically healthy.
8Test Equipment
The investigation used:
- 5 kV insulation resistance tester.
- Continuity tester.
- Cable fault location equipment.
- Time-domain reflectometer (TDR).
- Low-resistance ohmmeter.
- Thermal imaging camera.
- Approved AGL circuit test equipment.
- Circuit drawings.
- Airport-approved PPE.
9Investigation
Step 1: Verify CCR Operation
Testing confirmed:
| CCR startup | Normal |
| Internal diagnostics | Normal |
| Output capability | Normal |
| Protection system | Normal |
The CCR was healthy.
Step 2: Measure Circuit Continuity
The primary circuit showed:
- No continuity.
- Infinite resistance.
- No measurable loop current.
The circuit was open.
Step 3: Sectional Isolation
The circuit was divided into major sections.
Results:
| CCR–J1 | Continuous |
| J1–J2 | Continuous |
| J2–J3 | Open circuit |
| J3–End | Not tested |
The interruption was localized to the J2–J3 section.
Step 4: Cable Fault Location
TDR measurements indicated:
- Reflection characteristic of an open circuit.
- Estimated distance matching cable joint J3.
- No evidence of a short circuit.
The fault was localized accurately.
Step 5: Cable Joint Excavation
Excavation revealed:
- Water ingress.
- Joint insulation failure.
- Corroded conductor connection.
- Mechanical separation of the primary conductor.
- Carbonized joint insulation.
The joint had completely opened the series circuit.
Step 6: Transformer Verification
Transformers downstream of the joint showed:
- No primary current.
- No secondary voltage.
- No transformer defects.
The transformers were healthy but de-energized.
10Root Cause
The complete runway edge light circuit failure was caused by catastrophic failure of a major underground primary cable joint, which completely interrupted the series circuit.
The joint failed because of:
- Long-term moisture ingress.
- Corrosion.
- Insulation deterioration.
- Mechanical conductor separation.
The CCR was healthy and correctly indicated loss of circuit current.
11Corrective Action
The maintenance team:
- Isolated the circuit.
- Excavated the failed joint.
- Removed the damaged cable section.
- Installed a new approved cable joint.
- Restored conductor continuity.
- Performed insulation resistance testing.
- Verified complete circuit continuity.
- Energized the CCR.
- Confirmed runway edge light operation.
- Returned the runway to operational service.
12Post-Repair Verification
The complete runway edge lighting circuit was restored successfully.
13Root-Cause Analysis
Complete circuit outage
Healthy
Open circuit
Fault localized
Joint location identified
Cable joint failure confirmed
Primary cable joint open circuit
Replace joint and restore continuity
14Why This Case Was Important
Complete runway edge circuit failure has major operational consequences:
- Runway closure.
- Flight delays.
- Reduced operational availability.
- Increased maintenance urgency.
- Potential safety implications.
Rapid fault localization is essential.
15Engineering Lessons
A healthy CCR can exist with a completely failed runway lighting circuit.
A healthy CCR can exist with a completely failed runway lighting circuit.
Zero output current usually indicates a complete circuit interruption.
Zero output current usually indicates a complete circuit interruption.
Sectional isolation dramatically reduces fault-location time.
Sectional isolation dramatically reduces fault-location time.
TDR is highly effective for open-circuit localization.
TDR is highly effective for open-circuit localization.
Cable joints are critical reliability components in AGL systems.
Cable joints are critical reliability components in AGL systems.
16Engineer's Decision Point
You observe: Entire runway edge lighting circuit OFF. CCR healthy. Output current 0 A. Output voltage rising rapidly. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about complete runway edge light circuit failure, based on this case study.
Q1What is AGL-CS-041?
AGL-CS-041 is an engineering training case study on a complete runway edge light circuit failure in which every runway edge light went out at once. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2What did the CCR readings show?
The CCR was healthy and operational, but actual current was 0 A against the selected 6.6 A and output voltage was rising rapidly. The absence of output current suggested a complete circuit interruption.
Q3Does zero current mean the CCR has failed?
Not necessarily. Zero current with a healthy CCR points to a complete series-circuit interruption, and the CCR correctly indicated the loss of circuit current.
Q4What was the root cause?
Catastrophic failure of a major underground primary cable joint completely interrupted the series circuit. The joint failed because of long-term moisture ingress, corrosion, insulation deterioration and mechanical conductor separation.
Q5Which equipment helped locate the open circuit?
The team used a 5 kV insulation resistance tester, continuity tester, cable fault location equipment, a time-domain reflectometer (TDR), a low-resistance ohmmeter and a thermal imaging camera, with circuit drawings.
Q6How was the circuit restored?
The team isolated the circuit, excavated the failed joint and removed the damaged cable section. They installed a new approved joint, restored conductor continuity, tested insulation resistance, verified complete circuit continuity, energized the CCR and returned the runway to operational service.
Q7How was the repair verified?
Output current returned to 6.6 A with normal output voltage, circuit continuity was restored and the runway edge lights were operational.
Q8What should you investigate first if the whole circuit is OFF, the CCR is healthy, current is 0 A and voltage rises rapidly?
Verify primary circuit continuity and localize the open circuit. Replacing the CCR, increasing the current setting or replacing all transformers is not the right first step.
Engineering & Expert Notes
Use Fault-Location Tools Where Available
TechniqueA time-domain reflectometer can narrow the search to a few metres. Combine it with route records.
Split the Circuit Rapidly
MethodSectional isolation reduces the search quickly even without special instruments.
Plan Safe Excavation Before Digging
SafetyConfirm the cable position and isolation, and treat the exposed cable with caution.
Update the Joint Record After Repair
RecordsAdd the new joint position and method so future searches are faster.
20Case Study Conclusion
Complete runway edge light circuit failure is usually a primary series-circuit problem that requires systematic continuity verification, sectional isolation, cable fault localization, and field-joint investigation rather than immediate CCR replacement.
Systematic CCR verification, continuity testing, TDR analysis, cable-joint excavation, insulation testing, and post-repair circuit verification can rapidly restore runway edge lighting operation and minimize operational disruption.
In AGL engineering, when an entire runway edge circuit goes dark, the fastest restoration begins with proving where the primary loop has been broken.
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