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[ted]

I am having a new (replacement) supply installed to an existing building. The electrician tells me that I must have individual RCD's for every cct as the cables are concealed in the walls at less than 50mm depth. Surly a single RCD supplying the distribution board will be enough with individual MCB's on the ccts?

[ericmark]

It is all to do with likely hood that it will trip. If it is very unlikely then a single RCD would do. 314 is the regulation Every installation shall be divided into circuits (iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced.
It is sometimes done by using a split board and two RCD's but an RCD able to take the 80 amp available at the board costs a lot more than a RCBO and the RCBO is far less likely to trip so with a PME (TN-C-S) supply often it is better to use RCBO on all circuits. However most RCBO's only switch the Line supply so with a TT supply not sure on which is best. Also two way wiring on stairs can cause problems.
You are also aware that using Ali-tube cable you don't need RCD protection?
Eric

[Sara H]

Ted

Whilst Ericmark is right with his statement about new installations having to incorporate RCD's or RCBO's to fall in line with the 17th Edition, you state that you are having a new supply fitted to an existing installation.

If what you are saying is correct and its a new supply (installed by an Electrical Service Provider) to the incoming main 100A or 80A fuse within your property the C/U (Consumer Unit) does not need to accomodate RCD's or RCBO's as your installation remains unchanged. Yes you may have to re-test the installation as the new cable may have a differing Ze to your previous cable but apart from that nothing else has to be improved.

If no board exists and the new supply is a total new board and installation then everything has to be in accordance with BS7671:2008 (17th Edition Wiring Regulations).

HTH

Sara