AGL-CS-034 — CCR Undercurrent Alarm Investigation
The AGL Knowledge Hub | Engineering Case Study Series. A low-current investigation where a persistent CCR undercurrent alarm was traced to a partial open circuit from a deteriorated transformer secondary connection and a high-resistance cable joint — from impedance measurement through sectional testing and thermal inspection to field-circuit correction.
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 routine runway lighting operation, the Constant Current Regulator (CCR) generated a persistent undercurrent alarm while the regulator remained in service.
The CCR was set to 6.6 A, but the output current remained at 5.1 A, and several runway edge lights were noticeably dim.
The regulator continuously increased output voltage in an attempt to restore the selected current, but the alarm remained active.
A systematic investigation identified a partially open circuit caused by a failed isolating transformer secondary connection and a deteriorated cable joint, which significantly increased circuit impedance and reduced the achievable current.
This case demonstrates that a CCR undercurrent alarm is often an indication of partial circuit degradation rather than CCR failure.
2Learning Objectives
After completing this case study, the learner should be able to:
- Understand CCR undercurrent alarm logic.
- Diagnose low-current alarm conditions.
- Differentiate undercurrent alarms from overcurrent trips.
- Analyze partial open-circuit conditions.
- Investigate high-impedance faults.
- Correlate voltage, current, and circuit impedance.
- Apply systematic undercurrent troubleshooting procedures.
3System Background
A CCR monitors:
- Selected current.
- Actual output current.
- Output voltage.
- Regulation error.
- Alarm thresholds.
Normal operation:
Undercurrent condition:
Unlike an overcurrent trip, an undercurrent alarm often allows continued operation, making early diagnosis extremely important.
4Problem Statement
The reported condition was:
- Selected current: 6.6 A.
- Actual current: 5.1 A.
- Output voltage elevated.
- Undercurrent alarm active.
- Several lights dim.
- CCR remained energized.
Is the CCR unable to regulate current, or is the field circuit preventing normal current flow?
5Initial Observations
| CCR status | Undercurrent alarm |
| Selected current | 6.6 A |
| Actual current | 5.1 A |
| Output voltage | Elevated |
| Light intensity | Reduced |
| Circuit availability | Operational |
The combination of low current and elevated voltage indicated excessive circuit impedance.
6Initial Engineering Hypotheses
The maintenance team considered:
CCR current sensor error.
CCR power module weakness.
Partial open circuit.
Transformer secondary failure.
High-resistance cable joint.
7Undercurrent Principle
The CCR attempts to maintain:
If circuit impedance increases beyond the CCR regulation capability:
- Output voltage rises.
- Current remains low.
- Undercurrent alarm occurs.
The alarm is a symptom of insufficient current flow.
8Test Equipment
The investigation used:
- True-RMS multimeter.
- Clamp meter.
- CCR monitoring system.
- 5 kV insulation resistance tester.
- Circuit impedance measurement equipment.
- Thermal imaging camera.
- Approved AGL circuit test equipment.
- Circuit drawings.
- Airport-approved PPE.
9Investigation
Step 1: Verify CCR Performance
The CCR was tested with a calibrated test load.
Results:
| Current regulation | Normal |
| Output voltage | Normal |
| Alarm operation | Normal |
The CCR was healthy.
Step 2: Measure Circuit Impedance
Measured impedance:
34 Ω
Historical value:
23 Ω
A substantial increase was identified.
Step 3: Sectional Circuit Testing
The circuit was divided into sections.
Results:
| CCR–S1 | Normal |
| S1–S2 | Elevated impedance |
| S2–S3 | Very high impedance |
| S3–S4 | Normal |
The S2–S3 section contained the dominant fault.
Step 4: Transformer Investigation
The associated isolating transformer showed:
| Primary continuity | Normal |
| Secondary continuity | Intermittent |
| Secondary resistance | Abnormally high |
The transformer secondary connection was deteriorated.
Step 5: Cable Joint Inspection
The nearby cable joint showed:
- Corrosion.
- Moisture ingress.
- Oxidized conductor.
- Increased joint resistance.
The joint contributed additional circuit impedance.
Step 6: Thermal Inspection
Thermal imaging identified:
- Transformer secondary terminal heating.
- Cable joint heating.
- Connector temperature significantly above adjacent components.
The heating confirmed high-resistance current flow.
10Root Cause
The CCR undercurrent alarm was caused by a partial open circuit created by a deteriorated transformer secondary connection combined with a high-resistance cable joint.
These defects increased total circuit impedance, preventing the CCR from achieving the selected 6.6 A current despite increased output voltage.
The CCR was functioning correctly.
11Corrective Action
The maintenance team:
- Isolated the circuit.
- Replaced the defective transformer secondary connection.
- Replaced the damaged cable joint.
- Installed new approved connectors.
- Restored environmental sealing.
- Performed insulation resistance testing.
- Measured circuit impedance.
- Verified CCR current regulation.
- Cleared the undercurrent alarm.
- Updated maintenance records.
12Post-Repair Verification
The CCR returned to stable current regulation.
13Root-Cause Analysis
Undercurrent alarm detected
Healthy
High
Fault localized
Secondary connection defective
High resistance identified
Partial open circuit and high-resistance joint
Repair secondary connection and cable joint
14Why This Case Was Important
A CCR undercurrent alarm should not automatically be treated as a CCR fault.
It often indicates:
- Partial open circuits.
- High-resistance connections.
- Transformer secondary defects.
- Cable joint deterioration.
- Progressive circuit degradation.
Early investigation can prevent complete circuit failure.
15Engineering Lessons
Undercurrent alarms usually indicate insufficient circuit current.
Undercurrent alarms usually indicate insufficient circuit current.
Elevated CCR output voltage is an important diagnostic clue.
Elevated CCR output voltage is an important diagnostic clue.
Partial open circuits are often difficult to detect visually.
Partial open circuits are often difficult to detect visually.
Transformer secondary connections can create major impedance increases.
Transformer secondary connections can create major impedance increases.
Thermal inspection is highly effective for locating high-resistance components.
Thermal inspection is highly effective for locating high-resistance components.
16Engineer's Decision Point
You observe: Selected current: 6.6 A. Actual current: 5.1 A. Elevated output voltage. Undercurrent alarm active. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about CCR undercurrent alarms, based on this case study.
Q1What is AGL-CS-034?
AGL-CS-034 is an engineering training case study on a persistent CCR undercurrent alarm with reduced runway light brightness. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2What were the symptoms?
The CCR was set to 6.6 A but output current was only 5.1 A, output voltage was elevated, several lights were dim and the undercurrent alarm stayed active. The CCR kept increasing voltage but could not restore the current.
Q3What does low current with elevated voltage indicate?
Excessive circuit impedance in the field circuit. The CCR was functioning correctly and could not reach the selected current because the circuit was preventing normal flow.
Q4What was the root cause?
A partial open circuit created by a deteriorated transformer secondary connection combined with a high-resistance cable joint. Together they increased total impedance so the CCR could not achieve 6.6 A despite higher voltage.
Q5What should you investigate first?
Measure circuit impedance and inspect transformer secondary connections and cable joints. Replacing the CCR, increasing the current setting or replacing all field transformers is not the right first step.
Q6How was the circuit repaired?
The team isolated the circuit, replaced the defective transformer secondary connection and the damaged cable joint, and installed new approved connectors. They restored sealing, tested insulation resistance, measured impedance, verified CCR regulation and cleared the alarm.
Q7How was the repair verified?
Actual current returned to 6.6 A with normal output voltage and circuit impedance of 23 Ω. Light intensity was normal and the undercurrent alarm cleared.
Q8Which test equipment was used?
The team used a true-RMS multimeter, clamp meter, CCR monitoring system, 5 kV insulation resistance tester, circuit impedance measurement equipment and thermal imaging camera, with circuit drawings.
Engineering & Expert Notes
Read Current and Voltage Together
InterpretationHigh output voltage with low current points to excess resistance. One number alone can mislead.
Check Secondary Terminations Early
TerminationsPartial opens at transformer secondaries are easy to miss visually. Test them rather than inspect only.
Treat the Alarm as a Starting Point
AlarmsAn undercurrent alarm tells you the circuit cannot reach set current. It does not say where.
Verify at All Brightness Steps After Repair
VerificationConfirm the circuit reaches each set current before closing the job.
20Case Study Conclusion
A CCR undercurrent alarm is usually a protective indication of reduced circuit current caused by excessive impedance from partial open circuits, high-resistance connections, transformer secondary defects, or cable joint deterioration rather than CCR malfunction.
Systematic CCR verification, circuit impedance measurement, sectional fault localization, transformer secondary testing, cable joint inspection, and thermal analysis can accurately identify the source of the undercurrent condition and restore reliable current regulation.
In AGL engineering, an undercurrent alarm is often the first warning that a circuit is gradually losing its ability to carry the required constant current.
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