AGL-CS-003 — Random Lights Out in an AGL Series Circuit
The AGL Knowledge Hub | Engineering Case Study Series. A pattern-based investigation into randomly distributed runway edge light failures — from CCR verification through connector-level diagnosis and environmental assessment.
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, several runway edge lights were found extinguished. Unlike a typical section failure, the failed lights were randomly distributed throughout the circuit.
The Constant Current Regulator (CCR) was operating normally, and the affected lights did not form a continuous group. Some lights were OFF, while adjacent lights remained ON.
The maintenance team initially suspected individual fixture failures. However, the random pattern suggested that multiple independent faults or intermittent electrical problems might be present within the series circuit.
The investigation ultimately identified multiple moisture-affected connectors and one deteriorated transformer secondary connection, causing intermittent loss of operation at several locations.
2Learning Objectives
After completing this case study, the learner should be able to:
- Recognize the difference between random and consecutive light failures.
- Develop a systematic troubleshooting strategy for intermittent faults.
- Differentiate between section faults and multiple localized component failures.
- Understand connector and transformer reliability issues.
- Use electrical testing to identify intermittent circuit problems.
- Appreciate the importance of environmental conditions during diagnosis.
- Verify complete circuit integrity after repair.
3System Background
An AGL series circuit operates with constant current flowing through all connected primary circuit components.
A simplified arrangement is:
Random light failures usually indicate that multiple localized defects may exist within the circuit rather than one common interruption.
4Problem Statement
The reported condition was:
- Seven runway edge lights OFF.
- Failed lights distributed randomly.
- Adjacent lights generally operating.
- CCR energized.
- No CCR trip.
- Circuit remained operational.
Does the random pattern indicate independent fixture failures, intermittent connector faults, transformer problems, or a developing circuit reliability issue?
5Initial Observations
| CCR status | Healthy |
| Selected current | 6.6 A |
| Output current | Stable |
| Affected lights | L4, L11, L19, L28, L35, L42, L50 |
| Failure pattern | Random |
| Weather | High humidity after rainfall |
| Visible damage | Minor corrosion at some pits |
Unlike a consecutive section failure, the affected lights were scattered throughout the circuit.
6Initial Engineering Hypotheses
The maintenance team considered:
Multiple independent fixture failures.
Multiple connector failures.
Transformer secondary connection deterioration.
Intermittent moisture-related electrical faults.
Primary cable insulation deterioration causing unstable operation.
7Failure Pattern Analysis
The observed pattern was:
This pattern suggested that the failures were not associated with a single common circuit section.
The investigation therefore focused on identifying multiple localized defects.
8Test Equipment
The investigation used:
- True-RMS multimeter
- Insulation resistance tester
- Approved AGL circuit test equipment
- Connector inspection tools
- Transformer test equipment
- Circuit drawings
- Maintenance 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 | 6.60 A |
| CCR voltage | Normal |
| Alarm status | No active alarm |
The CCR was operating correctly.
Step 2: Compare Failed Locations
The maintenance team compared the failed locations with:
- Previous maintenance records
- Connector replacement history
- Transformer installation dates
- Cable joint locations
- Drainage conditions
Several failed lights were located in areas with poor drainage.
Step 3: Connector Inspection
Inspection of affected locations found:
- Moisture ingress
- Corrosion
- Improper sealing
- Aged connector insulation
Some connectors operated intermittently when disturbed during inspection.
Step 4: Transformer Testing
One isolating transformer showed:
- Normal primary continuity
- Intermittent secondary connection
- Elevated contact resistance
The transformer itself was not failed, but the secondary termination was deteriorated.
Step 5: Environmental Assessment
Pit inspection found:
- Standing water
- Condensation
- Inadequate drainage
- Dirt accumulation around connector assemblies
The failures were strongly correlated with moisture exposure.
10Root Cause
The random light failures were caused by multiple localized electrical defects, including:
- Moisture-affected connectors.
- Corroded electrical contacts.
- One deteriorated transformer secondary connection.
This was not a single circuit interruption, but a developing reliability problem affecting several independent locations.
11Corrective Action
The maintenance team:
- Isolated the circuit.
- Replaced defective connectors.
- Re-terminated damaged transformer connections.
- Cleaned affected connector assemblies.
- Improved connector sealing.
- Removed moisture contamination.
- Verified pit drainage.
- Performed continuity testing.
- Conducted insulation resistance testing.
- Restored and monitored the circuit.
12Post-Repair Verification
The circuit was monitored during subsequent night inspections to confirm stable operation.
13Root-Cause Analysis
Random lights OFF.
Multiple fixture failures.
Normal.
Non-consecutive distribution.
Moisture and corrosion identified.
Intermittent secondary connection found.
Multiple localized electrical defects.
Replace connectors, repair connections, improve sealing.
14Why This Case Was Different
Unlike a consecutive section failure, this case required the engineer to recognize that:
- One fault could not explain all failures.
- Multiple independent defects can exist simultaneously.
- Environmental degradation may affect several locations at once.
Random failures often require location-by-location investigation.
15Engineering Lessons
Random failures indicate multiple problems.
Random failures usually indicate multiple localized problems rather than one common interruption.
Connector reliability is critical.
Connector reliability is critical in underground AGL installations.
Moisture creates intermittent faults.
Moisture can produce intermittent faults before complete component failure occurs.
Historical data improves efficiency.
Historical maintenance data can significantly improve troubleshooting efficiency.
Environmental inspection is essential.
Environmental inspection should be part of every AGL fault investigation.
16Engineer's Decision Point
You observe: Nine runway edge lights OFF. The failed lights are randomly distributed. The CCR is operating normally. No continuous affected section is evident. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about random light failures on a series circuit, based on this case study.
Q1What is AGL-CS-003?
AGL-CS-003 is an engineering training case study on seven randomly distributed runway edge light failures (L4, L11, L19, L28, L35, L42 and L50) in a series circuit with a healthy CCR. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2How does a random failure pattern differ from a consecutive one?
Consecutive failures usually point to one shared circuit section. Random failures usually indicate multiple localized defects, or intermittent electrical problems, at separate locations rather than one common interruption.
Q3Why not look for a single fault in this case?
The failed lights did not form a continuous group and adjacent lights were operating, so no single circuit section explained the pattern. The investigation therefore focused on finding several localized defects.
Q4What did the connector and transformer inspections find?
Inspection found moisture ingress, corrosion, improper sealing and aged connector insulation, and some connectors operated intermittently when disturbed. One isolating transformer secondary connection was also found deteriorated.
Q5Why were maintenance records and drainage conditions reviewed?
The team compared failed locations with maintenance records, connector replacement history, transformer installation dates, cable joint locations and drainage conditions. Several failed lights were in areas with poor drainage, which supported a moisture-related cause.
Q6What was the root cause?
Multiple localized electrical defects: moisture-affected connectors, corroded contacts and one deteriorated transformer secondary connection. It was a developing reliability problem across several locations, not a single circuit interruption.
Q7How was the circuit restored and monitored?
The team replaced defective connectors, re-terminated damaged transformer connections, cleaned and better sealed connector assemblies, removed moisture and verified pit drainage. After continuity and insulation resistance testing, the circuit was monitored during subsequent night inspections to confirm stable operation.
Q8What should you investigate first if nine randomly distributed runway edge lights are OFF and the CCR is normal?
Investigate each failed location independently for localized connector, transformer or environmental faults. Replacing all nine lights, replacing the CCR or increasing circuit current is not the right first step.
Engineering & Expert Notes
Plot the Failures on the Layout Drawing
PatternRandom-looking failures often cluster by pit, cable run or age once plotted. The map decides where to inspect first.
Repair Systematically, Not One Lamp at a Time
StrategyChasing individual dark lights leaves the underlying degradation untouched. Plan a sweep of connectors and secondary terminations in the affected area.
Use Fault History to Set the Inspection Scope
TrendingWhere the log shows repeated call-outs, extend inspection beyond the failed item to its neighbours of similar age and exposure.
Stock Spares for the Dominant Failure Type
SparesIf connectors and secondary terminations dominate the failures, hold the matching seals and kits so repairs are completed properly the first time.
20Case Study Conclusion
Random AGL light failures should be investigated as a potential multiple-defect reliability problem rather than a single circuit interruption.
A systematic, location-by-location investigation of connectors, transformer connections, and environmental conditions can identify intermittent faults that may otherwise lead to repeated maintenance interventions.
In AGL systems, the distribution of failed lights is one of the most valuable diagnostic tools available to the maintenance engineer.
Turn Case Studies Into Engineering Capability
A case study shows you how a problem is investigated. Precision AGL Academy helps you build the knowledge to understand why it happens, diagnose it systematically, and apply the right engineering approach.
Study real-world-style engineering scenarios, identify failure patterns, understand root causes, and develop a systematic approach to AGL troubleshooting.
Study the case. Understand the system. Build the skill.
Follow the Academy
Important Notice, Disclaimer & Limitation of Liability
Please read before relying on any content on this page or on any record downloaded from it.
- 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.
- Values, test results and acceptance. Test voltages, insulation-resistance and current values, tolerances, thresholds, photometric and angular figures and pass/fail or “acceptable” wording in a case are specific to its invented scenario. They are not acceptance criteria, limits or targets. Use the project specification, the applicable standard and the manufacturer’s data for real work. Completing or following a case produces no test certificate, calibration record, acceptance or evidence of compliance.
- Aviation operations. Nothing here authorises return to service, a change of operating category (CAT I/II/III), a NOTAM, a flight inspection outcome or any aerodrome certification decision. These rest with the aerodrome operator, the air navigation service provider and the competent regulator.
- Standards and regulations. References to ICAO, FAA, IEC, IEEE, ISO and other standards are for orientation only. They are summarised, may be incomplete or out of date, and standards are revised from time to time. Consult the current official editions. Following this content does not guarantee compliance with any law, standard or regulatory approval. Official documents, regulations and manufacturers’ instructions always prevail.
- Learning aids. Knowledge checks, decision points, interview questions, FAQs and suggested answers are teaching aids with illustrative answers. They are not an assessment, certification, accreditation or competency or authorisation to work on any aerodrome, and they do not predict or guarantee the outcome of any job interview or application.
- Generic content; no warranty. Diagnostic sequences, cause rankings, tolerances and values are generic and indicative; real installations differ in design, equipment make and model, age and condition. Content is provided “as is” and “as available”, without warranty of any kind, express or implied, including accuracy, completeness, currency or fitness for a particular purpose.
- Limitation of liability. To the fullest extent permitted by applicable law, Precision AGL Academy and its owners, authors and contributors are not liable for any injury, death, loss, damage, downtime, penalty or regulatory consequence, whether direct or indirect, arising from the use of, reliance on, or inability to use this content. Nothing in this notice excludes or limits any liability that cannot lawfully be excluded or limited.
- Independence and trademarks. This is an independent publication, not affiliated with or endorsed by ICAO, the FAA, the IEC, DGCA, any airport, regulator or equipment manufacturer. Third-party names, standard designations and trademarks belong to their respective owners and are used for identification only.
- Copyright and use. © 2026 Precision AGL Academy. All rights reserved. You may read this page and use it for personal reference and internal training, keeping this notice attached. Any other copying, republication, redistribution, scraping or commercial use requires our prior written permission.
- Corrections. Content is reviewed periodically, with no obligation to update. To report an error, safety concern or rights issue, write to precisionaglsolutions@gmail.com.