Single pole rcbo on TT

[stilllearning]

Hi, could anyone explain why in a TT system, if the consumer unit has Single Pole RCBO's that it's also necessary to fit a 100ma TD RCD as a main incomer. Sorry but I can't get my head around this one!

Thanks

 

[TicklyT]

In the absence of any replies from anyone who knows what they are talking about, my take on it is there's little, if anything, holding neutral and earth at at the same potential on a TT system. As an RCBO is single pole, there is no means of disconnecting the neutral, which may be at a significantly different voltage to earth.

 

[flameport]

... there's little, if anything, holding neutral and earth at at the same potential on a TT system.

That is correct.

You don't have to use a 100mA RCD however, other options can be:
Double pole RCBOs for all circuits
Split board with an RCD for some circuits and separate double pole RCBOs for others
Boards with 2 or more RCDs supplying several MCBs (such as most of the ones sold as '17th edition' types)