BG SPD after RCD acceptable?

ValgarGrimal · Aug 4, 2023

Hello,

I was looking at installing a SPD into an existing split load RCD BG consumer unit. In the instructions for the SPD it states that it may be positioned before or after a RCD but I have always been of the idea that the SPD needs to go before the RCD due to the RCD potentially tripping from a surge leaving the SPD unable to operate correctly.

Can someone please help clear up my confusion on this?

[ElectriciansForums.net] BG SPD after RCD acceptable?

littlespark · Aug 4, 2023

Not sure why they have number 7, when number 1 states to connect to a 32A MCB.

I wouldn't connect downstream of an rcd, for the exact reason you mentioned.
Although, i guess there are some situations, where there is a 100mA or time delayed rcd some distance before any consumer unit where the SPD might be fitted....

pc1966 · Aug 4, 2023

Regulation 534.4.7 covers this, basically the RCD has to have an immunity to surges of at least 3kA and usually that is a selective type (traditionally used as incomers for TT systems, followed by 30mA per circuit (group) for additional protection).

It also advises against fitting a Type 1 SPD after any RCD, and that it might still result in the RCD tripping on a big >3kA surge (with the related issue of loss of healthy circuits).

TL;DR don't do that for RCBO or dual RCD boards.