AGL-CS-005 — High Circuit Resistance Investigation
The AGL Knowledge Hub | Engineering Case Study Series. A progressive high-resistance fault causing reduced light intensity and elevated CCR voltage — from voltage-trend analysis through cable-joint inspection and corrective action.
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 night operation, the runway edge lighting circuit appeared operational, but several lights were noticeably dimmer than expected. The Constant Current Regulator (CCR) maintained the selected current, yet the output voltage was significantly higher than historical operating values.
No circuit trip occurred, and all lights remained illuminated. However, maintenance personnel reported inconsistent brightness and reduced visual performance in one runway section.
The investigation ultimately identified high-resistance electrical connections in multiple underground cable joints, causing excessive voltage losses within the field circuit.
This case demonstrates how high circuit resistance can degrade AGL performance even when the circuit remains energized.
2Learning Objectives
After completing this case study, the learner should be able to:
- Understand the effect of high circuit resistance in an AGL series circuit.
- Interpret abnormal CCR voltage behavior.
- Differentiate high resistance from open-circuit and short-circuit faults.
- Apply voltage-drop and resistance-based troubleshooting techniques.
- Identify connector and cable-joint deterioration.
- Perform systematic resistance localization.
- Verify circuit performance after corrective action.
3System Background
An AGL series circuit operates with constant current supplied by the CCR. The CCR automatically adjusts its output voltage to maintain the selected current. When circuit resistance increases, the CCR must increase its output voltage to maintain the same current.
A simplified arrangement is:
Excessive resistance may occur at:
- Cable joints.
- Connectors.
- Transformer primary connections.
- Cable terminations.
- Corroded electrical contacts.
4Problem Statement
The reported condition was:
- All runway edge lights ON.
- Several lights dimmer than normal.
- CCR maintaining 6.6 A.
- CCR output voltage unusually high.
- No active alarm.
- Increased energy consumption observed.
Why is the CCR output voltage significantly higher than normal while the circuit remains operational?
5Initial Observations
| CCR status | Healthy |
| Selected current | 6.6 A |
| Output current | Stable |
| Output voltage | Higher than historical value |
| Light operation | All lights ON |
| Light intensity | Uneven brightness |
| Weather | Humid conditions |
The most important observation was normal current with abnormally high voltage.
6Initial Engineering Hypotheses
The maintenance team considered:
Normal seasonal voltage variation.
Additional circuit loading.
High-resistance cable joints.
Corroded connector assemblies.
Transformer primary connection deterioration.
7Electrical Principle
Using Ohm's law: V = I × R
For a constant-current circuit:
- Current remains essentially constant.
- Increased resistance causes increased voltage.
- The CCR compensates by raising output voltage.
This makes CCR voltage trend analysis a valuable diagnostic tool.
8Test Equipment
The investigation used:
- True-RMS multimeter.
- Micro-ohmmeter where applicable.
- Insulation resistance tester.
- Approved AGL circuit test equipment.
- Voltage-drop measurement equipment.
- Circuit drawings.
- Historical CCR voltage records.
- Airport-approved PPE.
9Investigation
Step 1: Review Historical CCR Data
Historical operating records showed:
| Six months earlier | 1,850 V |
| Three months earlier | 1,920 V |
| Current condition | 2,260 V |
The gradual voltage increase suggested progressive resistance growth rather than a sudden fault.
Step 2: Verify Circuit Current
| Selected current | 6.6 A |
| Measured current | 6.59 A |
| CCR regulation | Normal |
Current regulation was satisfactory. The investigation therefore focused on voltage losses in the field circuit.
Step 3: Sectional Voltage Analysis
Voltage measurements were taken at approved field locations. One circuit section showed a significantly higher voltage drop than adjacent sections. This localized the investigation to a defined cable-joint area.
Step 4: Cable Joint Inspection
Inspection revealed:
- Corrosion.
- Oxidized conductor surfaces.
- Incomplete crimping.
- Moisture contamination.
- Thermal discoloration.
Micro-resistance measurements showed elevated contact resistance.
Step 5: Thermal Assessment
Infrared inspection identified elevated temperatures at two cable joints. The heating confirmed that power loss was occurring at the high-resistance connections.
10Root Cause
Multiple deteriorated cable joints developed high-resistance electrical connections.
The increased resistance caused:
- Excessive voltage drop.
- Higher CCR operating voltage.
- Uneven light intensity.
- Localized connector heating.
The circuit remained operational because the resistance had not yet progressed to a complete open circuit.
11Corrective Action
The maintenance team:
- Isolated the circuit.
- Removed defective cable joints.
- Re-terminated conductor connections.
- Installed approved replacement joints.
- Verified conductor integrity.
- Performed continuity testing.
- Conducted insulation resistance testing.
- Measured joint resistance.
- Restored the circuit.
- Verified CCR voltage and field performance.
12Post-Repair Verification
The CCR operating voltage returned close to historical values.
13Root-Cause Analysis
Lights dimmer than normal.
Normal.
Abnormally high.
Progressive increase.
Fault section localized.
High-resistance connections identified.
Deteriorated cable joints.
Replace joints and verify circuit resistance.
14Why This Case Was Important
High-resistance faults are often overlooked because:
- Lights may still operate.
- The circuit may not trip.
- The CCR may appear healthy.
- Failures develop gradually.
Without investigation, high resistance can eventually lead to:
- Connector overheating.
- Transformer stress.
- Reduced photometric performance.
- Increased energy consumption.
- Complete circuit failure.
15Engineering Lessons
Rising CCR voltage is a warning.
A rising CCR operating voltage is an important maintenance indicator.
High resistance can exist without failure.
High resistance can exist without a complete circuit failure.
Cable joints are long-term reliability points.
Cable joints are major long-term reliability points.
Thermal inspection identifies resistive heating.
Thermal inspection is valuable for identifying resistive heating.
Historical trend analysis reveals developing problems.
Historical trend analysis can reveal developing electrical problems before operational failure occurs.
16Engineer's Decision Point
You observe: CCR maintaining 6.6 A. Output voltage increasing over several months. Lights operating but some appear dim. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about high circuit resistance and rising CCR voltage, based on this case study.
Q1What is AGL-CS-005?
AGL-CS-005 is an engineering training case study on a progressive high-resistance fault that left runway edge lights dim and the CCR output voltage well above historical values. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Why does a rising CCR voltage matter when the current is stable?
The CCR was still maintaining 6.6 A, but its output voltage was significantly higher than historical values. Higher voltage at stable current indicates rising circuit resistance, so a rising CCR operating voltage is an important maintenance warning.
Q3Why did the circuit keep working despite the fault?
The resistance had not yet progressed to a complete open circuit. All lights stayed on, but the excess resistance caused voltage drop, uneven brightness and localized connector heating.
Q4What was the root cause?
Multiple deteriorated underground cable joints developed high-resistance connections. This caused excessive voltage drop, higher CCR operating voltage, uneven light intensity and localized connector heating.
Q5Which measurements help find a high-resistance fault?
The investigation used a true-RMS multimeter, micro-ohmmeter where applicable, insulation resistance tester, voltage-drop measurement equipment, circuit drawings and historical CCR voltage records. Voltage-drop measurements and joint inspection were the appropriate first steps.
Q6How was the circuit repaired?
The team isolated the circuit, removed the defective cable joints, re-terminated conductor connections and installed approved replacement joints. They verified conductor integrity, tested continuity and insulation resistance, measured joint resistance, restored the circuit and verified CCR voltage and field performance.
Q7What did post-repair verification show?
CCR output voltage returned to the expected range, close to historical values, and light intensity became uniform. Circuit current was stable, joint temperature normal and documentation completed.
Q8What should you do if CCR voltage keeps rising over several months while some lights look dim?
Investigate circuit resistance and voltage drops. Increasing CCR current, replacing all light fittings or replacing the CCR is not the right response to steadily rising voltage at stable current.
Engineering & Expert Notes
Watch the CCR Output Voltage as a Health Indicator
TrendingA circuit working at current but needing more voltage than before is telling you resistance has grown. Log the figure at each step.
Use Thermal Inspection Where Safe and Approved
ThermographyResistive joints run warmer than their neighbours. Compare like with like under similar load and record the conditions.
Rank the Joints by Age and Exposure
PrioritisingNot every joint can be reopened at once. Target the oldest, wettest and most previously repaired joints first.
Re-Measure After Repair to Confirm the Improvement
VerificationCompare post-repair voltage and current with the earlier readings. The change proves the fix worked rather than assuming it did.
20Case Study Conclusion
High circuit resistance is often a progressive reliability fault that can significantly affect AGL performance before complete circuit failure occurs.
Systematic analysis of CCR voltage trends, voltage drops, connector resistance, and cable-joint condition can identify developing problems early, reducing maintenance costs and preventing future operational failures.
In AGL engineering, an abnormal voltage trend is often the first warning that circuit resistance is increasing.
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