RCD fused spur unit fault

[lim88]

A customer has a RCD spur unit spurred off a ring supplying their garage. It is all wired in 2.5mm twin, and in the garage there is only two sockets with nothing plugged in.

The RCD spur unit would not reset so have replaced with an identical one. When the power was turned back on the the spur unit was already on and everything was working fine, but when i tested it via the test button there was a lovely flash and the RCD in the ccu tripped too. And that broke the spur unit.

I have dead tested all the cables and all the readings are perfect.

Is it a problem with the RCD spur units or is there anything else I can check that I have missed?

Thanks

 

[truckster]

not quite i think you'll find the 30 ma would trip before the 100ma so no need for time delayed if you have a fault drawing more than 100ma then both will trip , but think youll find that a 30 ma RCD will trip at around 24ma so way before the 100ma

Disagree, unless you have a fault of less than 50mA it is likely that both will operate, agree with Londonlec the upstream RCD would need to be time delayed.