How do you troubleshoot an earth fault?
An earth fault is silent until it is detected, so the work runs in order: spot the alarm or the falling reading, make the circuit safe, then split the loop and test each section until one is left.
An earth fault is an unintended path between the series loop and earth, caused by failed insulation on a cable, connector or transformer primary. A series circuit is not referenced to earth, so a single fault does not break the loop: the lights usually stay on and the fault can go unnoticed without testing or monitoring. It shows as a CCR earth-fault alarm or trip, or as insulation resistance that falls from test to test, often well before any alarm. A second fault elsewhere can give current a path through the ground and bypass the section between the two faults.
Faulted insulation can put voltage on metalwork or the ground nearby, so a suspected earth fault is a safety issue. ATC and airport operations are told so the outage can be agreed, the CCR code and readings are noted before any reset, and the CCR is switched off, isolated, locked out and tagged, with the circuit proved dead. The loop is then divided at accessible points such as pits and tested by insulation resistance using the half-split: test about half, set the good half aside, split the bad half again until one section or device remains. The circuit is discharged after each insulation test, since cable holds charge. Wet connectors, joints and terminations, damaged cable and degraded transformer primaries are the usual sites, and areas disturbed by works are checked first. After repair the section and the whole circuit are retested and the readings recorded.
Frequently Asked Questions
Q1What is an earth fault in a series lighting circuit?
An unintended path between the series loop and earth, caused by failed insulation on a cable, connector or transformer primary. Because the circuit is not referenced to earth, a single leak does not break the loop, so the lights usually stay on and the fault can go unnoticed without testing or monitoring.
Q2Why is a single earth fault still taken seriously?
A second earth fault elsewhere can give current a path through the ground, bypassing the section between the two faults, much like a short circuit. Faulted insulation can also put voltage on metalwork or the ground nearby, so a suspected earth fault is a safety issue.
Q3What is done first when an earth-fault alarm or CCR trip appears?
ATC and airport operations are told so the outage can be agreed, the code and readings are noted before any reset, and the CCR is switched off, isolated, locked out and tagged. The circuit is proved dead before anyone touches it.
Q4How is the fault located?
By insulation resistance and the half-split method. The loop is divided at accessible points such as pits, about half is tested, the good half is set aside, and the bad half is split again until one section or device remains. The circuit is discharged after each insulation test, since cable holds charge.
Q5Where do earth faults most often occur?
At made-up seals and places that suffer water or damage: wet connectors, cable joints and terminations, damaged cable, and transformer primaries with degraded insulation. Areas disturbed by excavation or work at a crossing or duct are checked first.
Q6What happens after the repair?
The repaired section and then the whole circuit are retested for insulation resistance, the CCR is restored and status confirmed, and the readings are recorded as a new baseline. A low reading, or one that keeps falling, is trended because it often appears well before a fault.
See It. Understand It. Then Learn How to Engineer It.
Recognising an AGL system visually is the first step. The next step is knowing how to design, install, test, commission and maintain them correctly.
Build Practical AGL Engineering Skills
Go deeper with structured training from Precision AGL Academy, covering:
Learn how each light, sign and circuit works as part of a complete airport lighting system, not just as an isolated diagram.