are two rcd's twice as good

[josey]

if there is a 30ma rcd fitted to the cu, covering the whole board, will it do any harm to fit an additional rcd to cover a circuit, i am wiring a power shower and am intending to fit a fcu in the airing cupboard to supply it ( only requires a 3A supply) and thought of fittind a H29 unit (with built in rcd protection), this will ensure the shower is protected bt an rcd, but which unit would you expect to trip first.

 

[andy]

30mA for a whole board? do you have a TT supply? ne ways, if the board is already RCD protected, then there is no need to add another RCD. it still has the protection

 

[Damocles]

The problem with having two is either one or both might trip. If you are creating a system which needs two for some reason, then you use one with a time delay, so that the other should trip first.

There is a slight benefit from having two, just in case one might be faulty. But this is pretty unlikely to happen, especially if you use the test button to check it from time to time.

 

[josey]

not sure what a TT supply is, my thinking was if i was fitting a fcu anyway, would it harm to fit an rcd one, it would give me more peace of mind, i know your suppost to test your rcd trips on the board, but who does, at least this way if the main cu rcd did fail at least the shower would still be protected.

 

[andy]

a TT supply has an earth rod. usually ur power will be from a overhead line. if there is an earth wire connected to the main incoming cable or the sealed fuse box then you do not have a TT system

as Damocles sid, if using 2 RCD's the one feeding he other one should be a time delay, to if there is a fault the 2nd one will trip, isolating anything past that. otherwise the first RCD could trip isolating everything

 

[josey]

ok, so all i ned then is a normal fcu in the airing cupboard. am i right in thinking i don't need a pull cord switch in the bathroom for a power shower, the fcu is all i need ?

supply from 6A mcb in cu, whole board protected by 80A 30ma rccb, (realise this is now not the most convenient way if lights go out when unit trips, but not caused any problems upto now, so don't seen the need to change it all for a split load board), i will fit a switched spur unit in airing cupboard fused at 3A and run from here to the shower.