AGL-CS-007 — Intermittent Series Circuit Fault
The AGL Knowledge Hub | Engineering Case Study Series. A difficult intermittent fault where runway edge lights extinguished and restored automatically — from fault logging and monitoring through connector movement testing to final resolution.
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
Airport maintenance personnel received repeated reports that a runway edge lighting circuit was switching between normal operation and partial failure.
During some inspections, all lights operated normally. During other inspections, a group of lights became extinguished for several minutes before returning to normal without any maintenance intervention.
The Constant Current Regulator (CCR) remained operational, and the fault could not be consistently reproduced during routine testing.
The investigation ultimately identified an intermittent electrical connection inside a moisture-damaged underground connector, which opened and closed under thermal expansion and mechanical movement.
This case demonstrates one of the most difficult fault conditions in AGL maintenance: a fault that is not continuously present.
2Learning Objectives
After completing this case study, the learner should be able to:
- Understand the characteristics of intermittent AGL faults.
- Differentiate intermittent faults from permanent open-circuit faults.
- Apply fault-reproduction techniques.
- Interpret unstable electrical measurements.
- Investigate connectors and cable joints under operating conditions.
- Use systematic isolation during intermittent fault diagnosis.
- Verify long-term reliability after repair.
3System Background
An AGL series circuit consists of:
An intermittent fault may be caused by:
- Loose electrical connections.
- Corroded connectors.
- Cracked conductors.
- Moisture contamination.
- Thermal expansion.
- Mechanical vibration.
- Cable movement.
- Partial insulation failure.
Unlike permanent faults, intermittent faults may appear and disappear unpredictably.
4Problem Statement
The reported condition was:
- Runway edge lights occasionally extinguished.
- Affected lights restored automatically.
- CCR remained energized.
- No permanent circuit failure.
- Fault more common during evening operation.
- Recent rainfall reported.
How can a fault be diagnosed when the circuit is normal during inspection?
5Initial Observations
| CCR status | Healthy |
| Selected current | 6.6 A |
| Output current | Stable during inspection |
| Fault duration | 1–10 minutes |
| Affected lights | L22–L29 |
| Automatic restoration | Yes |
| Weather | Humid evenings |
The automatic restoration of the lights indicated that the circuit interruption was not permanent.
6Initial Engineering Hypotheses
The maintenance team considered:
CCR output instability.
Intermittent cable-joint failure.
Connector movement causing circuit interruption.
Transformer primary connection intermittently opening.
Moisture-related insulation tracking.
7Diagnostic Challenge
The primary difficulty was that:
- Continuity testing was normal.
- Insulation resistance was acceptable.
- Visual inspection showed no major damage.
- The fault disappeared before measurements could be completed.
The investigation therefore focused on capturing the fault while it was occurring.
8Test Equipment
The investigation used:
- True-RMS multimeter.
- Data-logging current recorder.
- Voltage recorder.
- Insulation resistance tester.
- Approved AGL circuit test equipment.
- Connector inspection tools.
- Thermal imaging camera.
- Circuit drawings.
- Airport-approved PPE.
9Investigation
Step 1: CCR Monitoring
Continuous monitoring showed:
| Output current | Stable |
| Output voltage | Occasional brief fluctuations |
| CCR alarms | None |
The CCR did not exhibit internal instability.
Step 2: Fault Logging
A temporary voltage logger was installed. During one fault event:
- Circuit voltage increased briefly.
- Current regulation remained stable.
- The affected lights extinguished.
- Voltage returned to normal after restoration.
This suggested a temporary increase in circuit resistance or intermittent circuit opening.
Step 3: Physical Inspection During Operation
The maintenance team inspected the affected section while energized under approved procedures. One connector showed:
- Moisture ingress.
- Corrosion.
- Slight mechanical looseness.
Step 4: Movement Test
After isolation, the connector was mechanically manipulated. Results:
- Continuity repeatedly opened and closed.
- Resistance changed significantly with movement.
- The fault could now be reproduced.
Step 5: Thermal Cycling
The connector was subjected to moderate heating and cooling. Resistance varied significantly with temperature. This explained why the fault occurred mainly during evening temperature changes.
10Root Cause
A moisture-damaged connector developed a partially fractured electrical contact.
Thermal expansion and mechanical movement intermittently opened the connection, causing temporary interruption of the affected circuit section. The circuit automatically restored when the contact re-established electrical continuity.
11Corrective Action
The maintenance team:
- Isolated the circuit.
- Removed the defective connector.
- Inspected adjacent connectors.
- Verified conductor condition.
- Installed an approved replacement connector.
- Improved connector sealing.
- Checked pit drainage.
- Performed continuity testing.
- Conducted insulation resistance testing.
- Monitored the circuit for several operational cycles.
12Post-Repair Verification
The circuit was observed for multiple night operations with no recurrence.
13Root-Cause Analysis
Lights intermittently OFF.
Normal.
Temporary voltage fluctuations.
Affected section identified.
Movement-sensitive resistance.
Fault reproduced.
Intermittent connector connection.
Replace connector and verify long-term stability.
14Why This Case Was Difficult
Intermittent faults often:
- Pass routine testing.
- Disappear before measurement.
- Mimic CCR problems.
- Recur unpredictably.
- Consume significant maintenance time.
The key engineering skill was fault reproduction. Without reproducing the fault, component replacement becomes guesswork.
15Engineering Lessons
A fault that disappears is not necessarily repaired.
A fault that disappears is not necessarily repaired.
Intermittent faults require monitoring.
Intermittent faults require monitoring, not just inspection.
Connector movement testing can reveal hidden defects.
Connector movement testing can reveal hidden electrical defects.
Thermal expansion changes contact resistance.
Thermal expansion can change contact resistance significantly.
Long-term post-repair monitoring is essential.
Long-term post-repair monitoring is essential for intermittent faults.
16Engineer's Decision Point
You observe: Lights occasionally extinguish. They restore automatically. CCR appears normal. Routine continuity tests are satisfactory. What should you investigate first?
17Knowledge Check
18Interview Questions
Quick Reference: Airport Series Circuit Fault
An airport series circuit fault can be an open circuit, an earth fault or a high-resistance connection. Intermittent faults, as in this case, often come from a loose or damp connection that changes with temperature, vibration or moisture. Logging the CCR current and alarms against time helps to show the pattern.
19Frequently Asked Questions
Quick answers about intermittent series circuit faults, based on this case study.
Q1What is AGL-CS-007?
AGL-CS-007 is an engineering training case study on an intermittent series circuit fault in which runway edge lights extinguished and then restored themselves. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Why are intermittent faults hard to diagnose?
The circuit was normal during many inspections, and the fault could not be consistently reproduced during routine testing. Lights went out for several minutes and came back without any maintenance, and the fault was more common during evening operation.
Q3How did the team catch a fault that kept disappearing?
The team used data-logging current and voltage recorders, a thermal imaging camera, connector inspection tools and insulation resistance testing. Monitoring the circuit and reproducing the fault were more effective than guessing at component replacement.
Q4What was the root cause?
A moisture-damaged underground connector developed a partially fractured electrical contact. Thermal expansion and mechanical movement intermittently opened the connection, and the circuit restored itself when the contact re-established continuity.
Q5Does a fault that disappears mean it has been repaired?
No. A fault that disappears is not necessarily repaired. The underlying defect remains until it is found and corrected, which is why this case stresses monitoring and fault reproduction.
Q6How was the circuit repaired?
The team isolated the circuit, removed the defective connector, inspected adjacent connectors and verified conductor condition. They installed an approved replacement connector, improved sealing, checked pit drainage, tested continuity and insulation resistance, and monitored the circuit for several operational cycles.
Q7How long was the circuit monitored after repair?
The circuit was observed over multiple night operations with no recurrence. CCR operation, circuit current and voltage stability were normal and the intermittent fault was not observed.
Q8What should you do if lights occasionally extinguish, restore automatically and routine continuity tests pass?
Monitor the circuit and reproduce the intermittent fault. Replacing the CCR or all affected fittings, or increasing CCR current, is guesswork and not the recommended approach.
Engineering & Expert Notes
Intermittent Faults Cluster Around Conditions
PatternNote time of day, temperature, rain and brightness step for each occurrence. The pattern points to expansion, moisture or vibration.
Gentle Movement Testing Can Expose a Weak Contact
TechniqueOnly under an approved, isolated procedure, controlled flexing of a suspect connection may reveal instability. Never do so on energised equipment.
Do Not Close the Job on a Self-Clearing Fault
ClosureA fault that went away has not been fixed. Keep it open for monitoring until the cause is found and corrected.
Replace Suspect Contacts Rather Than Cleaning Them
RepairA partly fractured or corroded contact rarely recovers. Re-make the termination with new parts and seals.
20Case Study Conclusion
Intermittent series-circuit faults must be diagnosed through monitoring, fault reproduction, and dynamic testing rather than static electrical measurements alone.
Systematic analysis of connector movement, thermal behavior, voltage trends, and operational conditions can identify hidden intermittent defects that are invisible during routine inspections.
In AGL engineering, the ability to reproduce an intermittent fault is often the turning point between repeated temporary repairs and permanent fault elimination.
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