How do you troubleshoot an AGL cable?
Match the symptom to a place along the cable run, then test section by section with the circuit isolated.
The primary series cable carries the loop current from the CCR through every isolation transformer, so a cable fault shows up across the circuit. The symptom points to a place along the run. All lights out means the cable is open: cut, or failed at a joint or termination, found with continuity tests by section. A CCR trip or earth-fault alarm points to insulation damage or a wet joint, found with insulation resistance tests by section. A single earth fault on a floating series loop often does not stop the lights and shows only on earth-fault monitoring or an insulation test; a second fault can short out lights or trip the CCR. A slow fall in insulation resistance shows ageing or moisture, and an intermittent fault often traces to a loose or wet joint or termination.
The cable run is divided at terminations, joints and pits, and each section is tested. Cable records show where the cable runs and where the joints are, and fault-locating equipment can help pinpoint a position. Joints and terminations are checked early because they are made-up seals and fail more often than the factory cable. Damage after excavation or work at a crossing or duct is checked first at those places. After a repair the section is retested, ATC is told and the cable records are updated. Test methods and limits come from the cable specification, the manufacturer and the airport’s safe-working rules.
Frequently Asked Questions
Q1What does a primary cable carry in an AGL circuit?
The primary series cable carries the constant current from the CCR round the whole loop, passing through each isolation transformer's primary. It is usually single-core, insulated for the circuit voltage, and laid in ducts or buried along the airfield.
Q2How is a cable fault located?
The circuit is isolated, then divided into sections at joints, pits or terminations. Continuity tests find an open section and insulation resistance tests find an earth fault. Cable route drawings and, where used, cable fault locating equipment help to pinpoint the position.
Q3What most often damages airfield cable?
Excavation and construction work, crossings and ducts where the cable is stressed or abraded, water ingress at joints, and ageing of the insulation. Rodents and ground movement can also play a part.
Q4Why can insulation resistance fall over time without a fault alarm?
Moisture, ageing or minor damage can lower insulation gradually while the CCR still runs. Trending the test readings shows the fall early, so the weak section can be found and repaired before it trips the circuit.
Q5Why are joints and terminations often the cause?
They are the points where the factory insulation is replaced by a made-up seal. Poor workmanship, water entry or movement can fail them first, so they are checked early when a cable fault is suspected.
Q6What must happen before anyone digs near a cable?
The cable route is checked against the records and located and marked on site, and the circuit is treated as live unless proven isolated. After any repair the section is retested and the cable records are updated.
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