RCD after MCB

Jimmyb · Sep 13, 2011

Hello, Is it ok to fit a encloser with RCD in it which is fed of a mcb.

The reason for this is that i want to protect the ring main and cant source the old rcbo's or bigger mcb as i was going to put a 45a mcb to feed encloser then a 32a mcb with the rcd but with the orginal mcb being 32a i dont want to put a 25a with rcd.

Many thanks jimmy

telectrix · Sep 13, 2011

no problem with that. just because the MCB is before the RCD, you've still got overcurrent protection for the circuit and are making it safer with adding the RCD. ( similar to fitting an RCD socket. the RCD is after the OCPD there.)

malcolmsanford · Sep 13, 2011

Remember Jimmy the 32 amp on that RCD is just it's rating for loading, if you so wished you could put a 63amp RCD with that 32amp MCB, it is the 30mA that is the critical part for additional protection.

As soon as you fit the RCD the MCB is only used for overload protection, though always nice in a TN system to make sure it can trip in the 0.4secs for fault.

Jimmyb · Sep 13, 2011

malcolmsanford said:
Remember Jimmy the 32 amp on that RCD is just it's rating for loading, if you so wished you could put a 63amp RCD with that 32amp MCB, it is the 30mA that is the critical part for additional protection.

As soon as you fit the RCD the MCB is only used for overload protection, though always nice in a TN system to make sure it can trip in the 0.4secs for fault.

Yes i know about rcd rating, i was referring to the mcb in the old board being 32 and discrimation between that and the 32 mcb i was going to fit after the rcd.

I have decided to fit rcd and use orginal mcb as the ocpd just didnt know if was ok that way around as i couldnt see a problem with it and wanted to see what other thought and if there was a reason not to.