Surge protection devices for PME supplies?

[SJD]

I've been installing Type 2 SPDs in consumer units for several years now, and they always come as a pair, connected across L & E, and across N & E. Typically they plug into a holder, sometimes as two separate SPDs (e.g. Hager, with separate E connections), more often now as one combined SPD (e.g. Fusebox) with one each of L, N & E connections.

For a TN-C-S supply where N & E are typically connected a very short distance way (e.g. well under 3m), can someone tell me why the SPD across N & E is actually needed?

I can see the merit for TT and maybe TN-S supplies, and wonder is it just to simplify making only one devices that will serve all applications?

Do single module Type 2 SPDs for use across just L & E actually exist?

 

[Pretty Mouth]

Good question. At a guess, I'd say the voltage drop between the SPD and the N-E connection at the cutout could be significant. So there could be a potential difference between N and E within the installation.

 

[pc1966]

Do single module Type 2 SPDs for use across just L & E actually exist?

Yes, for example:

Type 2 Surge Arrester (KM2-20-1+0 SC) - Kingsmill Industries

System: Single Phase TN-C/1 Pole Nominal Voltage: 230V AC

kingsmillindustries.com

However, the reality is the L-N-E style are far more useful in general as they can be used for all style of earthing (as the risk of L shorting via N to E is negligible for TT use) and it reduces the need to worry about the inductance of the cabling from, for example, the N-E joint in a TN-C-S system to the point where the SPD is fitted.

As you need to keep the inductance down for the very high dI/dt of a lightning surge, and so total loop distance from L via SPD to N/E to no more than 1m and ideally less than 0.5m, you can only usefully deploy a single module SPD if the cutout is very close to the CU or similar where the SPD is fitted.