High earth fault loop impedance values associated with TT earthing systems mean the earth fault current level is unlikely to be sufficient to operate the distributor’s protective device within the permitted time of 1 second, as required in Regulation 411.2.4, or at all in some cases, depending on Earth resistance values.

There is nothing to prevent a metal consumer unit being installed with a TT earthing system, but it’s important to ensure that an earth fault cannot develop between the meter tails and the metallic enclosure before the RCD.

Methods of reducing an earth faults include keeping the meter tails as short as possible, the installation of proprietary clamps and glands to secure the cables and prevent strain on the terminations, minimising the risk of damage to the meter tails and a subsequent earth fault.