But surprised to see an SPD supplied via a 30mA RCBO.

   - Andy.

Is this the way then Andy?

Z.

Certainly a MCB rather than an RCBO seems better.

Not sure about putting the Earth conductor through a different hole in the steel box to to the meter tails (obviously the thermal effects would be minimal, but I can't help wondering about the extra impedance for the spikes we're trying to divert), nor about the loss of a single main switch for the installation.

   - Andy.

AJJewsbury: 
 

Certainly a MCB rather than an RCBO seems better.

   - Andy.

i recently fitted a pre-populated board with all RCBOs and an SPD. The SPD was fed from the outgoing terminals of the first RCBO, 32A used also for sockets iirc. The main switch in between. I queried the arrangement of using an RCBO, asking whether a dedicated MCB would also be needed. Here is their reply;

"The type 2 SPD is independently connected to both line and neutral internally and is looking for transient voltages, or voltage spikes. Whereas the RCBO is looking for a current in-balance greater than 20mA. The only instance that the SPD could trip the RCBO is when it does come to end of service life it may fail open or closed circuit. If it fails closed circuit the RCBO would trip, as would an MCB. 

The SPD is given to be a limited life device, we guarantee them for 7 years.  I am yet to see one fail closed circuit. 

You may remember several manufacturers fitting very similar SPD's straight off the main switch, most have now changed to provide an MCB. I believe the misunderstanding came from the issue of OCP not really being required in the running life of the SPD."

"The type 2 SPD is independently connected to both line and neutral internally and is looking for transient voltages, or voltage spikes. Whereas the RCBO is looking for a current in-balance greater than 20mA. The only instance that the SPD could trip the RCBO is when it does come to end of service life it may fail open or closed circuit. If it fails closed circuit the RCBO would trip, as would an MCB. 

Aren't their SPDs also connected to PE? with the possibility of >>20mA flowing from L/N to PE when dealing with a surge? “I'm not entirely convinced” is perhaps the polite reply!

   - Andy.

I think it has to do with the duration of the overvoltage that type 2 SPDs are designed to deal with being so short that RCDs won't trip in that time.

I posted their reply here to get others' thoughts on this.

Edit; yes, CPC of SPD connected with a very short lead to the metal casing

Still not convinced. Certainly doesn't seem to be in compliance with 534.4.7 … unless their 30mA RCBOs can claim immunity to at least 3kA 8/20.

Even using S-type RCDs comes with a warning that some surge currents can still cause a trip.

   - Andy.

Also seems less than ideal to leave the RCBO itself unprotected from the surge.

   - Andy.