What is an earth fault?
A leak from the series loop to earth — often silent on its own, because the loop stays closed and the lights stay on.
An earth fault is an unintended path between the series loop and earth, caused by failed insulation on a cable, connector or transformer primary. Because a series lighting 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.
A second earth fault elsewhere can give current a path through the ground, bypassing the section between the two faults in the same way as a short circuit. Faulted insulation can also put voltage on metalwork or the ground nearby. A single fault also ties the loop to earth at that point, so the rest of the loop then sits at a voltage to earth. A suspected earth fault is treated as a safety issue and the circuit is isolated and proved dead before it is touched.
- Insulation fails. Damage, moisture or ageing lets the conductor leak to earth.
- Loop stays closed. A single fault does not break or bypass the series circuit.
- Lights stay on. The fault is silent unless it is tested for or monitored.
- Detect. Insulation resistance falls, or earth-fault monitoring raises an alarm.
- Isolate. CCR off, locked out and tagged; prove the circuit dead and earth it at the work point (see Circuit Isolation).
- Locate & repair. Split the loop, insulation-test each section, repair, then re-test.
Frequently Asked Questions
Q1What is an earth fault in airfield lighting?
An unintended electrical path between the series loop and earth, caused by failed insulation on a cable, connector or transformer primary. Current can leak from the conductor to the ground at the fault point.
Q2Why do the lights usually stay on with one earth fault?
A series lighting circuit is not referenced to earth, so a single leak to ground does not break the loop or divert the circuit current. The fixtures keep working, which is why a single earth fault can go unnoticed unless it is monitored or tested for.
Q3What happens if a second earth fault appears?
Two faults at different points can give current a path through the ground between them, bypassing the section of loop between the faults. That behaves like a short circuit, with the bypassed fixtures going dark.
Q4How is an earth fault detected?
By insulation resistance testing of the de-energised circuit, and by earth-fault monitoring where the CCR or the control system provides it. A falling insulation resistance trend often warns of a developing fault before lights go out.
Q5Is an earth fault dangerous?
It can be. Faulted insulation can put voltage on metalwork or the ground near the fault. A single fault also ties the loop to earth at that point, so the rest of the loop then sits at a voltage to earth. It is also a sign the cable or equipment is deteriorating. The circuit is isolated, locked out and proved dead before anyone works on it, and a suspected earth fault is treated as a safety issue.
Q6How is an earth fault located?
With the loop isolated, the circuit is split at transformer pits and connectors and each section is insulation-tested to earth. The section that fails holds the fault, and splitting it again narrows the location. See Continuity Testing for the companion check.
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