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."

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."