AGL-CS-040 — CCR Fault Indication vs Actual Field Fault
The AGL Knowledge Hub | Engineering Case Study Series. A diagnostic-interpretation investigation where a persistent CCR undercurrent alarm was traced to a deteriorated secondary connector and moisture-damaged cable in the field — from symptom analysis through impedance measurement, sectional testing, and field-circuit restoration.
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 runway lighting operation, the Constant Current Regulator (CCR) repeatedly displayed an undercurrent fault indication, and the runway edge lighting circuit operated at reduced intensity.
The maintenance team initially concluded that the CCR was defective because the alarm appeared on the CCR display.
A replacement CCR was installed, but the fault indication remained unchanged.
A systematic investigation revealed that the actual defect was a deteriorated secondary connector and moisture-damaged transformer secondary cable in the field, which had significantly increased circuit impedance.
This case demonstrates that a CCR fault indication identifies an electrical symptom, not necessarily the physical location of the fault.
2Learning Objectives
After completing this case study, the learner should be able to:
- Interpret CCR fault indications correctly.
- Distinguish symptoms from root causes.
- Differentiate CCR faults from field-circuit faults.
- Analyze electrical evidence systematically.
- Correlate current, voltage, and impedance behavior.
- Perform structured fault localization.
- Avoid unnecessary CCR replacement.
3System Background
A CCR monitors:
- Output current.
- Output voltage.
- Earth leakage.
- Regulation status.
- Internal protection.
- Control conditions.
The CCR can indicate:
- Undercurrent.
- Overcurrent.
- Earth fault.
- Voltage abnormal.
- Regulation failure.
- Communication failure.
These are electrical conditions, not physical fault locations.
4Problem Statement
The reported condition was:
- CCR undercurrent alarm.
- Reduced runway light intensity.
- Stable CCR operation.
- No internal CCR diagnostic fault.
- Fault persisted after CCR replacement.
Why did the CCR fault indication remain after the regulator was replaced?
5Initial Observations
| CCR indication | Undercurrent |
| Selected current | 6.6 A |
| Actual current | 5.2 A |
| Output voltage | Elevated |
| Light intensity | Reduced |
| CCR replacement | No improvement |
The persistence of the fault after CCR replacement suggested that the regulator was not the root cause.
6Initial Engineering Hypotheses
The maintenance team considered:
Defective replacement CCR.
Current sensing error.
Field-circuit impedance increase.
Transformer secondary failure.
Connector and cable deterioration.
7Diagnostic Interpretation Principle
For example:
The CCR reports what the circuit is doing, not which component is defective.
8Test Equipment
The investigation used:
- CCR monitoring system.
- True-RMS multimeter.
- Clamp meter.
- Circuit impedance measurement equipment.
- 5 kV insulation resistance tester.
- Thermal imaging camera.
- Secondary voltage test equipment.
- Approved AGL circuit test equipment.
- Airport-approved PPE.
9Investigation
Step 1: Verify Replacement CCR
The replacement CCR showed:
| Current regulation | Normal |
| Voltage regulation | Normal |
| Internal diagnostics | Normal |
| Protection functions | Normal |
The replacement CCR was healthy.
Step 2: Analyze Electrical Symptoms
Measurements showed:
- Current below selected value.
- Elevated output voltage.
- Stable regulation attempt.
- No internal CCR instability.
These symptoms indicated high circuit impedance.
Step 3: Circuit Impedance Measurement
Measured impedance:
35 Ω
Historical value:
24 Ω
The circuit impedance had increased substantially.
Step 4: Sectional Field Investigation
The affected circuit section showed:
- Intermittent secondary continuity.
- Elevated connector resistance.
- Reduced secondary voltage.
- Localized thermal heating.
The fault was localized to one field pit.
Step 5: Connector Examination
Inspection revealed:
- Corrosion.
- Moisture ingress.
- Carbonized contact surfaces.
- Reduced contact pressure.
- High contact resistance.
The connector was severely deteriorated.
Step 6: Secondary Cable Inspection
The associated secondary cable showed:
- Insulation cracking.
- Moisture contamination.
- Conductor corrosion.
- Elevated resistance.
The cable and connector together produced a significant impedance increase.
10Root Cause
The CCR undercurrent indication was electrically correct, but the actual fault was located in the field secondary circuit.
The deteriorated connector and moisture-damaged secondary cable increased circuit impedance, causing:
- Reduced current.
- Elevated CCR output voltage.
- Undercurrent alarm.
- Reduced light intensity.
The CCR was accurately reporting the circuit condition.
11Corrective Action
The maintenance team:
- Replaced the damaged connector.
- Replaced the affected secondary cable.
- Restored connector sealing.
- Verified secondary continuity.
- Performed insulation resistance testing.
- Measured circuit impedance.
- Verified CCR current regulation.
- Cleared the fault indication.
- Restored normal light intensity.
- Updated maintenance records.
12Post-Repair Verification
The CCR fault indication disappeared immediately after field repairs.
13Root-Cause Analysis
CCR undercurrent indication
Healthy
No improvement
High
Secondary fault localized
Corrosion found
Moisture damage found
Field secondary connector and cable deterioration
Replace connector and cable
14Why This Case Was Important
A common maintenance mistake is assuming:
CCR alarm = CCR failure
In reality:
- CCR alarms often originate in the field.
- Electrical symptoms must be interpreted.
- Fault localization must be systematic.
Replacing healthy CCRs increases cost without solving the problem.
15Engineering Lessons
CCR indications describe electrical conditions, not fault locations.
CCR indications describe electrical conditions, not fault locations.
Always distinguish symptoms from root causes.
Always distinguish symptoms from root causes.
Current and voltage should be interpreted together.
Current and voltage should be interpreted together.
Field-circuit investigation is essential after CCR verification.
Field-circuit investigation is essential after CCR verification.
Systematic diagnostics prevent unnecessary equipment replacement.
Systematic diagnostics prevent unnecessary equipment replacement.
16Engineer's Decision Point
You observe: CCR undercurrent alarm. Elevated output voltage. Reduced light intensity. No improvement after CCR replacement. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about CCR fault indications versus actual field faults, based on this case study.
Q1What is AGL-CS-040?
AGL-CS-040 is an engineering training case study on a CCR undercurrent fault indication that stayed after the CCR was replaced, because the real fault was in the field. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Why did the alarm remain after the CCR was replaced?
The persistence of the fault after replacement showed the regulator was not the root cause. The CCR undercurrent indication was electrically correct and it was accurately reporting the circuit condition.
Q3What were the symptoms?
Selected current was 6.6 A but actual current was 5.2 A, output voltage was elevated and light intensity was reduced. There was no internal CCR diagnostic fault.
Q4What was the root cause?
A deteriorated secondary connector and a moisture-damaged transformer secondary cable increased circuit impedance in the field. This caused reduced current, elevated CCR output voltage, the undercurrent alarm and reduced light intensity.
Q5What should you investigate first?
Measure circuit impedance and investigate field connectors, transformers and secondary cables. Replacing the CCR again, increasing the current setting or replacing all field fixtures is not the right first step.
Q6How was the circuit repaired?
The team replaced the damaged connector and the affected secondary cable and restored connector sealing. They verified secondary continuity, tested insulation resistance, measured circuit impedance, verified CCR regulation, cleared the fault indication and restored normal light intensity.
Q7How was the repair verified?
Actual current returned to 6.6 A with normal output voltage and circuit impedance of 24 Ω. Light intensity was normal and the CCR fault indication disappeared immediately after field repairs.
Q8What is the key lesson about CCR fault indications?
A fault shown on the CCR display is not necessarily a CCR fault. The indication may correctly report a field problem, so diagnose the field circuit before replacing the regulator.
Engineering & Expert Notes
Treat the CCR Display as a Symptom Report
InterpretationThe CCR tells you the electrical condition it sees, not where the problem is.
Compare Current and Voltage Against Normal
MethodLook at both readings together to estimate whether resistance or leakage is at fault.
Move the Search to the Field Secondary
Fault LocationAfter proving the CCR, concentrate on connectors and secondary cables.
Record the CCR Indication With the Field Findings
RecordsLinking the alarm to the actual cause helps future diagnosis.
20Case Study Conclusion
A CCR fault indication is an electrical symptom that must be interpreted through systematic circuit analysis, because the actual physical fault may be located anywhere in the primary circuit, transformer, secondary circuit, connector, cable, or field equipment.
Systematic CCR verification, voltage and current interpretation, circuit impedance measurement, sectional field testing, connector inspection, and secondary-circuit investigation can accurately identify the true root cause and prevent unnecessary CCR replacement.
In AGL engineering, the CCR tells you what the circuit is experiencing; the field investigation tells you why it is experiencing it.
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