Installation of SPDs in metal CUs with TT earths: what

Jan 19, 2024

Question:

When installing a metal domestic consumer unit with a TT earth and all RCBO configuration we are allowed to install without an incoming RCD because the tails are clamped and there is limited risk of a line conductor coming loose.

But if we now look at putting an SPD in the same unit, it's typically protected by a MCB and not RCBO. Although there are a few that just have links from the incoming switch. Then the wire from the mcb to the SPD is only protected by a MCB and may presumably come loose. Typically a type 2 SPD is protected by a 32A device and type 1 63A.

I generally work with fuse box CU's because I like their SP+N RCBO's and their type 1/2/3 SPD's are much lower cost than others.

This is for a EV install and installing a second CU for the EV, hoping to do it in a way that circuits from the main CU are also protected by the SPD

So my initial thought was to use a time delayed RCD as the incoming device with the type 1 SPD downstream of the RCD. Fusebox support have stated this is ok. But I wonder what impact the coils etc in the RCD will have on the SPD's ability to protect circuits from the other consumer unit.

My alternative is to continue with the normal incoming switch so that the SPD's protection is as effective as possible. But then I have the small risk of it's line connection coming loose and touching the case, making it and everything connected to earth live.

I know the best solution is probably just to use a supply isolation switch with built in SPD like the Proteus EMIST12, I think Wylex have a similar product. My issue with this is mainly commercial as it costs an extra £100. As it is when I include a type 1 SPD my win rate on projects goes from about 40% down to about 10%, presumably most companies are ignoring the fact that they should be including type 1 SPD's when there are over head supply lines.

As a side note it's interesting that the people writing the regs don't seem to be worried about the RCBO neutral fly lead coming loose.