What is the correct course of action if a high earth fault loop impedance (Ze) value is measured at the origin of an existing supply?

Answer:

There are normally two situations where this can arise:

1) Where a high value is obtained from an existing supply earth terminal
2) Where there is no existing supply earth terminal and the only means of earthing is through extraneous conductive parts such as water pipes etc.

For 1) above, the correct course of action is to contact the supply distributor and arrange for them to rectify the fault. The distributor has a duty to maintain their system in a safe condition, normally with a maximum value of the earth fault loop impedance outside the consumer’s installation of 0.35 ohms for a TN-C-S system and 0.8 ohms for a TN-S system.

For 2) above, the recommended course of action is first to contact the distributor and arrange for an earth terminal to be provided. There is normally a charge for this service. If the distributor is unable to provide an earth terminal the installation should be provided with an earth electrode and RCD protection at the mains position, i.e. a TT system.