AGL-CS-031 — CCR Cannot Maintain Selected Current
The AGL Knowledge Hub | Engineering Case Study Series. A current-regulation investigation where the CCR could not achieve 6.6 A due to excessive circuit impedance — from output-voltage analysis 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 evening runway lighting operation, the maintenance team observed that the Constant Current Regulator (CCR) was unable to maintain the selected 6.6 A output current.
The regulator repeatedly attempted to increase output voltage, but the circuit current remained below the selected value.
Several runway edge lights appeared dimmer than normal, and the CCR generated a current regulation alarm.
A systematic investigation identified abnormally high circuit impedance caused by deteriorated cable insulation and multiple high-resistance field connections, which prevented the CCR from achieving the required current.
This case demonstrates that a CCR current regulation problem is often a circuit problem rather than a CCR problem.
2Learning Objectives
After completing this case study, the learner should be able to:
- Understand CCR current regulation principles.
- Diagnose inability to maintain selected current.
- Differentiate CCR faults from circuit faults.
- Analyze CCR output voltage behavior.
- Investigate circuit impedance.
- Correlate insulation, connection resistance, and current regulation.
- Apply systematic CCR troubleshooting procedures.
3System Background
A Constant Current Regulator automatically adjusts its output voltage to maintain the selected current.
For example:
- Step 1: 2.8 A
- Step 2: 3.4 A
- Step 3: 4.1 A
- Step 4: 5.2 A
- Step 5: 6.6 A
The CCR increases or decreases output voltage as circuit impedance changes.
Normal regulation:
Abnormal regulation:
4Problem Statement
The reported condition was:
- CCR selected current: 6.6 A.
- Actual current: 5.8–6.0 A.
- Output voltage increasing.
- Current regulation alarm active.
- Several lights dim.
- No complete circuit outage.
Is the CCR defective, or is the field circuit preventing current regulation?
5Initial Observations
| CCR status | Alarm |
| Selected current | 6.6 A |
| Actual current | 5.9 A |
| Output voltage | Near maximum |
| Light intensity | Reduced |
| Weather | Dry |
The combination of low current and high output voltage suggested excessive circuit impedance.
6Initial Engineering Hypotheses
The maintenance team considered:
CCR power module failure.
Current feedback error.
Excessive circuit impedance.
Cable insulation deterioration.
Multiple high-resistance field connections.
7CCR Regulation Principle
The CCR attempts to satisfy:
As circuit impedance increases, the CCR must increase output voltage.
If the required voltage exceeds the CCR capability:
- Current decreases.
- Lights dim.
- Regulation alarms occur.
8Test Equipment
The investigation used:
- True-RMS multimeter.
- CCR output monitoring system.
- Clamp meter.
- 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 Input Supply
Measurements:
| Input voltage | Normal |
| Input frequency | Normal |
| Phase balance | Normal |
The CCR supply was healthy.
Step 2: Verify CCR Output Capability
With the circuit disconnected, the CCR successfully produced the required output voltage.
The regulator itself was operational.
Step 3: Measure Circuit Impedance
Measured impedance was significantly higher than historical values.
| Historical | 22 Ω |
| Current | 31 Ω |
The increase was abnormal.
Step 4: Sectional Circuit Testing
The circuit was divided into sections.
Results:
| CCR–S1 | Normal |
| S1–S2 | Elevated impedance |
| S2–S3 | Elevated impedance |
| S3–S4 | Normal |
Multiple localized resistance increases were identified.
Step 5: Insulation Resistance Testing
Results:
| Circuit to Earth | 28 MΩ |
| Historical value | Greater than 1000 MΩ |
Insulation deterioration was present.
Step 6: Thermal Inspection
Thermal imaging identified:
- Two overheated connectors.
- One cable joint with elevated temperature.
- One transformer secondary connection with abnormal heating.
These locations corresponded to the high-impedance sections.
10Root Cause
The CCR could not maintain the selected current because multiple high-resistance field connections and deteriorated cable insulation increased the total circuit impedance beyond the CCR regulation range.
The CCR attempted to compensate by increasing output voltage, but eventually reached its operational voltage limit.
The regulator was healthy; the field circuit was limiting current regulation.
11Corrective Action
The maintenance team:
- Isolated the circuit.
- Replaced deteriorated connectors.
- Repaired the cable joint.
- Replaced one damaged secondary cable.
- Restored connector sealing.
- Performed insulation resistance testing.
- Measured circuit impedance.
- Verified CCR current regulation.
- Restored full operation.
- Updated maintenance records.
12Post-Repair Verification
The CCR maintained the selected current without approaching the voltage limit.
13Root-Cause Analysis
CCR cannot maintain current
Normal
Healthy
High
Localized resistance found
Connector and joint heating identified
Excessive circuit impedance
Repair field connections and cable deterioration
14Why This Case Was Important
A common maintenance error is replacing the CCR when:
- The regulator is healthy.
- The circuit impedance is excessive.
- The field equipment is deteriorated.
The CCR is often reporting the circuit problem, not causing it.
15Engineering Lessons
High output voltage with low current indicates excessive circuit impedance.
High output voltage with low current usually indicates excessive circuit impedance.
Always verify CCR capability before replacing the regulator.
Always verify CCR capability before replacing the regulator.
Circuit impedance measurement is a critical CCR diagnostic tool.
Circuit impedance measurement is a critical CCR diagnostic tool.
Multiple small resistance increases can create a major regulation problem.
Multiple small resistance increases can create a major regulation problem.
Thermal inspection is highly effective for locating high-resistance connections.
Thermal inspection is highly effective for locating high-resistance connections.
16Engineer's Decision Point
You observe: Selected current: 6.6 A. Actual current: 5.9 A. CCR output voltage near maximum. Several lights dim. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about a CCR that cannot maintain the selected current, based on this case study.
Q1What is AGL-CS-031?
AGL-CS-031 is an engineering training case study on a CCR that could not maintain its selected 6.6 A output current. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2What were the symptoms?
Actual current was 5.8–6.0 A (5.9 A observed) against the selected 6.6 A, output voltage was near maximum, several lights were dim and a current regulation alarm was active. There was no complete circuit outage.
Q3Does low current with high output voltage mean the CCR is faulty?
Not necessarily. Low current combined with near-maximum voltage suggests excessive circuit impedance. The regulator was healthy and the field circuit was limiting current regulation.
Q4What was the root cause?
Multiple high-resistance field connections and deteriorated cable insulation raised total circuit impedance beyond the CCR's regulation range. The CCR raised its output voltage to compensate and eventually reached its operational voltage limit.
Q5What should you investigate first?
Measure circuit impedance and inspect field connections. Replacing the CCR, increasing the current setting or replacing all transformers is not the right first step.
Q6How was the circuit repaired?
The team isolated the circuit, replaced deteriorated connectors, repaired the cable joint and replaced one damaged secondary cable. They restored connector sealing, tested insulation resistance, measured circuit impedance, verified CCR regulation and updated maintenance records.
Q7How was the repair verified?
Actual current returned to 6.6 A with normal output voltage and circuit impedance of 22 Ω. Light intensity was normal, the CCR alarm cleared and the CCR maintained current without approaching the voltage limit.
Q8Which test equipment was used?
The investigation used a true-RMS multimeter, CCR output monitoring system, clamp meter, 5 kV insulation resistance tester, circuit impedance measurement equipment, thermal imaging camera and circuit drawings.
Engineering & Expert Notes
Compare Output Voltage With the Rated Range
DiagnosisIf the regulator is working at the top of its voltage range yet cannot reach current, the load is the problem rather than the regulator.
Estimate Expected Circuit Resistance From the Design
BaselineComparing measured impedance to the design figure shows how much has been added by degradation.
Test the Regulator Separately Under Approved Procedure
MethodDisconnecting the field circuit lets you prove the CCR independently, provided the procedure and permits are followed.
Keep Voltage-at-Current Records
TrendingRecording output voltage at each step gives an early warning of rising resistance.
20Case Study Conclusion
A CCR that cannot maintain the selected current is often responding correctly to excessive field-circuit impedance caused by deteriorated insulation, high-resistance connections, cable joints, or secondary circuit defects.
Systematic CCR verification, circuit impedance measurement, sectional testing, insulation assessment, and thermal inspection can accurately distinguish regulator faults from field-circuit problems and restore reliable current regulation.
In AGL engineering, when the CCR raises voltage but cannot achieve current, the regulator is often telling the engineer to investigate the circuit, not the CCR itself.
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