Earth fault loop impedance, Zs, is the sum of the source transformer winding, the line conductor out to the fault and the protective return path back to the source. Nominal voltage divided by Zs gives the prospective earth fault current, which must be high enough to operate the protective device within the required disconnection time. Electricians measure it with a loop tester at the far end of each circuit and compare it with the maximum tabulated for that breaker or fuse.
Zs at the far socket is 1.9 ohms and the max for a B32 is 1.37, so the earth fault loop impedance fails, it needs bigger cable or an RCD.
Ze is the external part measured at the origin with the main earth disconnected; Zs is Ze plus the circuit's own conductors.
Also heard as
- Zs
- loop impedance
Related terms
TN system
Earthing arrangement where the source is earthed and installation metalwork connects back to it by a protective conductor.
Earth fault
Fault in which a live conductor makes unintended contact with earth or with earthed metalwork.
RCD
Residual current device: trips when the current going out on the live conductors does not all come back.
TT system
Earthing arrangement where the supply and the installation each have their own separate earth electrode.