How do you find a ground fault when the lights stay on?
A series circuit floats clear of earth, so one insulation fault changes nothing you can see. Ground-fault detection and insulation monitoring are how that silent leak gets found early.
Because an AGL series circuit is not referenced to earth, a single insulation fault does not stop the lights. Ground-fault detection and insulation monitoring exist to find that silent leak early, before a second fault turns it into an outage or a hazard.
1Why the first fault matters
One leak leaves the loop closed, so nothing looks wrong. A second earth fault elsewhere can give current a path through the ground and bypass 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, so a suspected fault is a safety matter.
2Ground-fault detection
The CCR, or a separate unit, senses leakage from the loop to earth and raises a ground-fault alarm when it passes a set level. The alarm goes out through the CCR’s fault indication to the control system. How the leakage is sensed varies between manufacturers.
3Insulation monitoring
Monitoring measures how good the insulation is and follows it over time. A slow fall points to ageing, damage or moisture ingress, and gives a warning before the alarm level. It complements, and does not replace, the periodic insulation resistance test done on an isolated circuit.
4What it cannot do, and what goes wrong
The monitor sees the circuit as a whole, so it says there is a leak, not where. Locating it means isolating the circuit, splitting the loop and testing each section. Wet weather can lower readings and then recover, so trends and sensible levels matter more than one reading. Levels set too loosely hide a real fault, and levels set too tightly cause nuisance alarms. A circuit with no monitor relies entirely on periodic testing.
| Method | Circuit state | What it tells you | Limit |
|---|---|---|---|
| Ground-fault detection | Energised | Leakage to earth has passed the set level | Does not say where |
| Insulation monitoring | Energised | Insulation level and its trend | Weather affects readings |
| Insulation resistance test | Isolated, locked out, proved dead | Insulation of the loop or a section | Periodic only |
| Section testing | Isolated, loop split | Which section is faulty | Takes time and access |
Frequently Asked Questions
Q1Why does an AGL series circuit need ground-fault detection?
The series loop is not referenced to earth, so a single insulation fault does not stop the lights and nothing looks wrong. A second fault elsewhere can give current a path through the ground and bypass the section between the two faults. Detection finds the first fault while it is still harmless to the lighting.
Q2What is the difference between ground-fault detection and insulation monitoring?
Ground-fault detection raises an alarm when leakage from the loop to earth passes a set level. Insulation monitoring measures the state of the insulation and follows it over time, so a slow decline can be seen and acted on before the alarm level is reached. Some equipment does both.
Q3Does the monitor say where the fault is?
No. It sees the circuit as a whole, so it tells you there is a leak and how serious it is, not which cable, connector or transformer is at fault. Locating it means isolating the circuit, splitting the loop and insulation-testing each section.
Q4Why can rain trigger an insulation alarm?
Moisture can lower the measured insulation for a while and then recover as things dry. That is why a trend, and sensible warning and alarm levels, are more reliable than a single reading. A fall that does not recover is treated as a real fault.
Q5Can insulation be tested with the circuit running?
Monitoring equipment measures an energised circuit, but a full insulation resistance test is done with the circuit isolated, locked out and proved dead. Never test or touch a series circuit that has not been isolated, because the loop carries a hazardous current and voltage.
Q6What should be done when a warning or alarm appears?
Record it, check the trend and the weather, and follow the airport's maintenance procedure. If the fault is real, the circuit is isolated, the section is located by testing, and the repair is re-tested before the circuit returns to service. Warning and alarm levels come from the CCR manufacturer and the project specification.
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