Earth-fault loop impedance
The path a fault current takes back to the source, and why its impedance decides whether protection trips in time.
The loop
When a live conductor touches earth (or an exposed metal part), the fault current flows out along the live conductor, through the fault, and back to the supply transformer via the earthing arrangement. The whole round trip is the earth-fault loop, and its total opposition is the loop impedance.
The lower that impedance, the bigger the fault current — and a big fault current is what makes a breaker or fuse trip quickly. So the loop impedance directly sets how fast protection disconnects a fault.
Why disconnection time matters
A fault to an exposed metal part puts that part at a dangerous voltage until the protection clears it. The protection has to clear it fast enough that nobody touching it gets a harmful shock. A loop impedance that's too high means the fault current is too low to operate the device in the required time — the fault, and the touch voltage, persist.
That's why loop impedance is measured and verified on testing: a high reading points to a poor earth path, a long or undersized conductor, or a loose connection somewhere in the loop. RCDs provide a backstop where the loop impedance alone can't guarantee fast disconnection.
Safety first
A high loop impedance can mean protection won't clear an earth fault in time — exposed metalwork can stay live. Loop impedance is verified by testing; a poor result is investigated, not ignored.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
High earth-fault loop impedance / earthing concern
Testing shows a high earth-fault loop impedance (or an earthing concern) on a circuit — meaning protection might not operate fast enough on a fault, which is a safety issue to resolve.
RCD passes test but doesn't trip on a real leakage test
The RCD's own button trips it, but a proper instrument test shows it doesn't trip within the required time/current — so it may not protect adequately in a real fault.
Loose connection overheating (discolouration / smell)
A terminal or connection is overheating — discoloured insulation, a burning smell, or heat you can feel — a common cause of nuisance faults and a real fire risk.
Related definitions
Earthing & bonding
Connecting exposed metal to earth so a fault blows protection fast and metalwork can't become live.
How an RCD protects you
An RCD compares current out and back; any imbalance means leakage to earth, so it disconnects fast.
Circuit breaker (MCB)
Automatically disconnects a circuit on overload or short circuit, and can be reset rather than replaced.