Achieving separation of circuits in small CU

Smugley · Jul 9, 2009

Hi, I'm specifying an upgrade from an old rewirable fusebox to CU. To satisfy 17th ed requirement for separation of circuits in a house I'd have two RCDs and the usual upstairs lights together with downstairs ring and vice versa on each RCD. However this is a small flat with very basic electrics (currently only three circuits - lighting, ring main and oven - gas hob/boiler and mixer shower).

My plan was just to replace it with a small single RCD CU and put the lighting through a separate RCBO. This would achieve separation of lighting/power circuits. Is this the way you'd do it or is there another way to interpret the requirement?

Thanks all.

Lou7 · Jul 9, 2009

Use a board with 3 rcbo's. The way you are thinking of ,you will have 2 rcd's on the lighting which is not advisable.

Guest123 · Jul 9, 2009

RCBO's, deffo the way to go on this one.

Smugley · Jul 9, 2009

Lou, I meant that the RCBO for lighting would be in a separate (non-RCD) way, and the other two MCBs would come off the RCD protected ways. The reason for this setup is cheaper expansion at a later date if she wants to add any additional circuits (could do with outside lights for example).

However, as you and Lenny pointed out, individual RCBOs for each circuit in place of any RCD would be nice and simple and not that much different in price.

Thanks guys.

By the way, one question I had about this approach.. The RCD will normally be a DP device. In the regs I haven't found any clear indication that an RCD needs to break both live conductors. Most RCBOs only break the line conductor. Is there any requirement for DP isolation in the case of an earth fault or can the whole installation be SP protected?