In a TT system what are the requirements of BS 7671 in relation to the resistance of the earth electrode and the rated residual operating current of the RCD at the mains position.

Answer:

For a TT system Regulation 413-02-20 requires that the resistance of the earth electrode and associated protective conductors (Ra) multiplied by the current causing automatic operation of the protective device, which in this instance is the rated residual operating current of the RCD (I?n), should not exceed 50Volts or 25Volts for certain special locations.

i.e. Ra x I?n = 50V or 25V.

Note Guidance on this subject states that the resistance of an earth electrode in this situation should not exceed 200 ohms, as the electrode may be unreliable at values above this level.

The actual recommended operating current of an RCD in a TT system is not given, but it is normal to use 300mA or 100mA, both with time delay to provide discrimination with other RCDs and prevent nuisance tripping. It should be noted that in addition Regulation 471-08-06 requires every socket-outlet circuit in a TT system to be protected by an RCD satisfying the above formula.