How can I avoid unwanted tripping in a recently built

Aug 06, 2023

Question:

Hi, I’ll try to ask this question the shortest way - I’m asking for a friend who has just had a house built and it is a problem he is facing. The electrian who wired the house it a struggling to rectify the fault.

TT system from suppliers fuse 100amps.

This then goes through a 80amp RCD 30ma , then goes to a TPN 24 way DB. which also has 30ma TP RCD.

There is a problem that when all the CB are switched on, the RCD is ok. There are 2 ovens, a hob, under floor heating etc.

None have a fault, it's just an accumulation of loads being switched on which makes the RCD trip as the earth leakage goes over 30ma.

Now my question as it's a TT system: can both the RCDs be replaced by a switch and then replace the CB to RCBOs to each circuit?

What other ways could this be resolved?

Can the TT feed to the DB board - as I believe this has to be protected?

Thanks for any suggestion in advance.

Answer:

To avoid unwanted tripping regulation 531.3.2 (i), requires circuits to be subdivided with individual RCDs in such a way that any earth leakage likely to occur during normal operation of the connected load will not cause unwanted tripping.

531.3.2(ii) requires the protective conductor currents and/or earth leakage shall be not more than 30% of the rated residual operating current.

One method of achieving this can be the use of RCBOs for each induvial circuit.

- NICEIC Technical Standards Engineer