RCD tripping when earth core touched

[Badged01]

Maybe i've misread the OP but i took it that it was 2 seperate unrelated questions...

Maybe Ive assume too much into the question then ....... just take my reply as answering some random question that may have been asked!

 

[BillyTheSparks]

An RCD works on the balance of current flowing through it (only through the line and neutral). If you isolate the circuit via the MCB and then cut the cable, you will join the earth conductor and the neutral conductor, thus allowing some of the neutral current (from the neutral bar in the CU) to travel via the earth conductor in your isolated circuit.
This then causes an imbalance in the RCD and trips it off.

The reason why an RCD won't trip when you touch the casing of a metallic appliance, is due to not causing any imbalance of current through the RCD.

 

[Lucien Nunes]

Classic case of overthinking the question.

@OP: Your original assumption (and hence the title) is wrong and misleading. Touching or cutting the CPC ('earth') of a circuit, regardless of whether the circuit is energised or not, does not trip an RCD. Therefore, there is no question to answer about why touching the case of an appliance does not cause this. As others have explained, it is making contact with the neutral that is the issue.

Re post 15, the assumption here is not correct either. If there is a potential between neutral and earth at the origin, e.g. with a TN-S system, even without load on the system the RCD might still trip when N & CPC are shorted by cutting the cable. If the installation has load on other circuits (which need not be on the same RCD or CU) it is more likely that there will be enough p.d. between N & E to pass a current above the trip threshold through the N-CPC short.