What does a cable failure look like?
Every light out and a tripped CCR, or lights that stay on while insulation quietly fails — the buried cable rarely shows its damage, so the circuit does.
A cable failure is damage to the cable itself: its conductor, its insulation or its outer jacket. The primary series cable carries the loop current from the CCR through every isolation transformer, so a fault on it shows up across the circuit. Most of it is buried, so the circuit usually reveals the damage before anyone can see it.
A conductor break opens the loop: every light goes out and the CCR trips on open circuit. Insulation damage or a wet cable leaks current to earth without breaking the loop, so the lights usually stay on and the fault is silent unless it is tested for. Excavation by others is the commonest cause of sudden damage. A suspected failure is isolated, locked out and proved dead before the cable is touched.
- Read the symptom. All lights out or an open-circuit alarm, a CCR trip or earth-fault indication, a slow fall in insulation resistance, or an intermittent fault.
- Check records and recent work. Use the cable records and ask what has been dug, drilled or crossed; damage after excavation or at a duct crossing is checked first.
- Isolate. CCR off, locked out and tagged; prove the circuit dead.
- Look first. Walk the route for settlement, digging or new works, open the pits for water and silt, and check joints, terminations and any exposed duct or jacket.
- Test by section. Continuity finds an open cable and insulation resistance finds an earth fault; split the loop at pits and connectors, or use a fault locator (see cable fault location) to narrow the length.
- Repair, re-test and record. Repair the section to the maker’s instructions, retest it, tell ATC and update the cable records.
Conductor cut or failed: all lights out, CCR trips. Found with continuity tests by section. See open circuit.
Insulation damaged or wet: lights stay on, the leak is silent. Found with insulation resistance tests. See earth fault.
Frequently Asked Questions
Q1What is a cable failure in airfield lighting?
Damage to the cable itself: its conductor, its insulation or its outer jacket. Most AGL cable is buried, so a failure usually shows first as a circuit symptom, such as lights out, a CCR trip, an earth-fault indication or falling insulation resistance, before anyone sees the damage.
Q2Why can one cable fault affect the whole circuit?
The primary series cable carries the loop current from the CCR through every isolation transformer, so one weak length is part of every light's supply path. A break opens the loop for the whole circuit, and insulation damage anywhere on the run is an earth fault on the whole circuit.
Q3How does a conductor break differ from an insulation fault?
A conductor break opens the loop: every light goes out and the CCR's open-circuit protection trips. An insulation fault leaks current to earth without breaking the loop, so the lights usually stay on and the fault is silent unless it is tested for or monitored. Continuity tests find the first; insulation resistance tests find the second.
Q4Why can a cable earth fault go unnoticed?
Because a single leak does not break the series loop. The circuit keeps running while insulation resistance quietly falls. A second earth fault elsewhere can give current a path through the ground and bypass the section between the two faults, which is why a falling reading is acted on early.
Q5What causes cable failures?
Excavation and other work by third parties is the commonest cause of sudden damage. Crushing or ground movement, moisture, ageing of the insulation, surge or electrical overstress, damage during installation and heat also contribute. Joints and terminations are made on site and fail more often than the factory cable, so they are checked early.
Q6Is it safe to work on a cable fault?
Only after the circuit is isolated, locked out and proved dead. Cable ends at a break can still be hazardous if the CCR is energised, and faulted insulation can put voltage on nearby metalwork or the ground. Follow the site's permit-to-work and the cable manufacturer's instructions.
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