AGL-CS-002 — Multiple Consecutive Lights Out in a Series Circuit
The AGL Knowledge Hub | Engineering Case Study Series. A pattern-based fault-finding walkthrough for six consecutive runway edge lights extinguished between two operational sections — from CCR check through to root cause and post-repair verification.
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 routine night inspection, six consecutive runway edge lights were found extinguished between two operational sections of the runway lighting circuit.
The Constant Current Regulator (CCR) remained in normal operation, with no trip indication and normal current regulation.
The maintenance team initially suspected that several light fittings had failed simultaneously. However, the consecutive pattern of failure suggested the possibility of a common electrical problem affecting one section of the series circuit.
The investigation ultimately identified a defective connector and associated transformer connection that interrupted the affected section of the circuit.
2Learning Objectives
After completing this case study, the learner should be able to:
- Recognize the significance of consecutive light failures.
- Distinguish between isolated fixture failures and common circuit faults.
- Use failure-pattern analysis during AGL troubleshooting.
- Apply a systematic investigation sequence.
- Interpret CCR operating conditions during field faults.
- Identify connector and transformer-related failures.
- Perform post-repair verification of an AGL circuit.
3System Background
The runway edge lighting circuit operates as a constant-current series circuit.
A simplified arrangement is:
Each light is supplied through an isolating transformer connected to the primary series cable.
A defect affecting one circuit section can result in multiple consecutive lights becoming extinguished.
4Problem Statement
The reported condition was:
- Six consecutive runway edge lights OFF.
- Lights before the affected section ON.
- Lights after the affected section ON.
- CCR energized.
- No CCR trip.
- No visible cable excavation damage.
Are six individual lights defective, or is a common section fault responsible?
5Initial Observations
| CCR status | Healthy |
| Selected current | 6.6 A |
| Output current | Normal |
| Affected lights | L18–L23 |
| Adjacent lights | Operating normally |
| Weather | Heavy rainfall previous day |
| Visible damage | Not observed |
The consecutive nature of the failure immediately became the primary diagnostic clue.
6Initial Engineering Hypotheses
The maintenance team considered:
Multiple lamp or LED failures.
Multiple isolating transformer failures.
A defective connector common to the affected section.
A cable joint failure.
A circuit topology issue or maintenance error.
7Failure Pattern Analysis
The observed pattern was:
This pattern suggested a localized section fault.
The important engineering principle is that consecutive failures usually indicate a shared electrical path rather than multiple independent component failures.
8Test Equipment
The investigation used:
- True-RMS multimeter
- Insulation resistance tester
- Approved AGL circuit test equipment
- Cable continuity tester
- Circuit drawings
- Transformer identification records
- Airport-approved PPE
All testing followed approved isolation and electrical safety procedures.
9Investigation
Step 1: Verify CCR Operation
Measured values:
| Selected current | 6.6 A |
| Measured current | 6.59 A |
| CCR voltage | Within operating range |
| Alarm status | No active alarm |
The CCR was functioning correctly. The fault was therefore investigated in the field circuit.
Step 2: Inspect Affected Fixtures
Visual inspection of L18–L23 found:
- No broken lenses
- No impact damage
- No flooded fittings
- No obvious lamp failure
The simultaneous failure of six fixtures appeared unlikely.
Step 3: Check Transformer Connections
Transformer secondary continuity was verified. Most transformers tested normally. Attention then focused on the primary connector network.
Step 4: Connector Inspection
One L-823 connector located between L17 and L18 showed:
- Moisture contamination
- Partial corrosion
- Improper sealing
- Evidence of heating
Electrical testing showed a high-resistance intermittent connection.
10Root Cause
A defective moisture-affected connector created a high-resistance electrical interruption in the section feeding L18–L23.
The consecutive light failures were caused by a common connector failure, not six defective light fittings.
11Corrective Action
The maintenance team:
- Isolated the circuit.
- Removed the defective connector.
- Inspected adjacent connectors.
- Verified transformer primary connections.
- Installed an approved replacement connector.
- Properly sealed the connector assembly.
- Performed continuity verification.
- Conducted insulation resistance testing.
- Restored the circuit.
- Verified operation of all affected lights.
12Post-Repair Verification
Where required, additional photometric verification was performed according to the airport maintenance program.
13Root-Cause Analysis
Six consecutive lights OFF.
Multiple fixture failures.
Normal.
Localized section identified.
Moisture and corrosion detected.
High-resistance interruption confirmed.
Defective connector in common circuit section.
Replace connector and verify complete circuit.
14Why the Failure Pattern Was Important
If the team had replaced:
- Six light fittings
- Six lamps
- Six LED modules
- Six transformers
the actual fault would still have remained in the connector.
Pattern recognition significantly reduced troubleshooting time and unnecessary component replacement.
15Engineering Lessons
Consecutive failures indicate a common cause.
Consecutive failures usually indicate a shared electrical path rather than multiple independent component failures.
Always verify CCR operation first.
Always verify CCR operation before field component replacement.
Connectors are critical reliability components.
Connectors are critical reliability components in AGL systems.
Moisture creates high-resistance faults.
Moisture contamination can create high-resistance faults before a complete open circuit develops.
Consult the circuit drawing.
The circuit drawing should always be consulted before interpreting field observations.
16Engineer's Decision Point
You observe: Eight consecutive taxiway edge lights OFF. The CCR is operating normally. Lights before and after the section are ON. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about consecutive light failures on a series circuit, based on this case study.
Q1What is AGL-CS-002?
AGL-CS-002 is an engineering training case study on six consecutive runway edge lights (L18–L23) that were extinguished between two operational sections of a series circuit while the CCR remained healthy. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2What does a run of consecutive failed lights indicate in a series circuit?
Consecutive failures usually indicate a shared electrical path rather than several independent component failures. The failure pattern points to a localized section fault, so the common circuit path should be investigated before replacing individual fittings.
Q3Why was the CCR checked first in this case?
The CCR had to be ruled out before field troubleshooting. Selected current was 6.6 A, measured current was 6.59 A, voltage was within range and there was no active alarm, so the investigation moved to the field circuit.
Q4What was found on the connector between L17 and L18?
One L-823 connector between L17 and L18 showed moisture contamination, partial corrosion, improper sealing and evidence of heating. Electrical testing showed a high-resistance intermittent connection.
Q5What was the root cause of the six-light outage?
A defective, moisture-affected connector created a high-resistance electrical interruption in the section feeding L18–L23. The consecutive failures came from one common connector failure, not six defective light fittings.
Q6How was the circuit repaired?
The team isolated the circuit, removed the defective connector, inspected adjacent connectors, verified transformer primary connections, installed an approved replacement connector and sealed it properly. They then performed continuity and insulation resistance testing, restored the circuit and verified the affected lights.
Q7How was the repair verified?
Post-repair checks confirmed normal CCR operation, stable circuit current, L18–L23 and adjacent lights ON, normal connector temperature and completed documentation. Where the airport maintenance programme requires it, additional photometric verification was performed.
Q8What should you investigate first if eight consecutive taxiway edge lights are OFF and the CCR is normal?
Inspect the common connector and cable section first. A consecutive pattern points to a common circuit fault, so replacing all the lights or transformers, or increasing CCR current, is not the right first step.
Engineering & Expert Notes
Check the Boundary of the Dark Section First
Fault LocationThe connector feeding the first dead light and the one after the last are the best starting points. A fault usually sits at the edge of the affected group.
A High-Resistance Fault Can Mimic an Open Circuit
DiagnosisBefore declaring a total break, measure what current and voltage actually exist. Partial conduction tells you the joint is degrading, not absent.
Look at the Pit Environment Around the Connector
EnvironmentStanding water, silt and failed drainage around a connector explain why it failed and why a plain re-make may fail again.
Log the Connector Reference in the Record
RecordsRecording which connector failed lets the next team see repeat locations and decide whether to improve the pit rather than keep re-terminating.
20Case Study Conclusion
Multiple consecutive AGL lights should be treated as a potential common circuit failure until proven otherwise.
Systematic investigation of the shared electrical path can identify connector, cable, or transformer-section faults more efficiently than replacing multiple field components.
In AGL maintenance, the pattern of failure is often the fastest path to the root cause.
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