question here is whether lowering the rating of the MCB might make the SPD pointless please?

Not at all - the SPD carries no current at all most of the time, until it carries potentially huge currents (kA )for times that are  very very short compared with the time it takes a breaker to yawn, stretch and think about tripping - the effect of the actual rating of that  MCB on the waveforms that fast is pretty much negligible.
I suppose if the over-voltage is due to say an HV transformer failure rather than a short transient, so the SPD conducts for more than a fraction of a msec, the breaker may operate, but then we would probably like it to do  so. Until it is reset the SPD is then out of the loop of course. But for all the kind of events an SPD is supposed to trap, no normal MCB will get anywhere near de-latching before it is all over, so the exact MCB ratng is not so important. The main use of a fuse or breaker with an SPD is to disconnect safely it once it fails and goes short circuit - which they do tend to do if overheated by an extra long over-voltage, or if 'lifed' by too many transients at or near the upper rating.

Mike.

The part of the installation to be protected is served by an SP DB served by a 30 m long distribution circuit through a B63 MCB.

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My second question concerned the MCB for the SPD. The kit includes a C63 MCB. Clearly, this will provide no discrimination. The BEAMA Guide says that a 20 A MCB should be adequate.

MCBs are pretty rubbish at discrimination anyway - if the fault current is reasonably high then both MCBs will likely de-latch before either breaks the current - so typically both end up tripping.  Better solution (SPDs or not) might be a HBC fuse for the submain rather than a B63.

   - Andy.

Mike, many thanks. Yes hopefully, the SPD lives to fight another day, but I was aware that they can fail short circuit. I had wondered whether energy let through was an important consideration on the basis that you want as much as possible to be diverted through the SPD.

Any thoughts on the enclosure?

Andy, thank you. It would be nice if one could get fuse holders to go in standard DBs, but at, least with the ones that I have, they are not.

I think that discrimination in the domestic context is over-played and I am I am not too worried about it. I don't really see why a fault should develop in the fixed wiring - we don't go around banging nails into walls. The PFC at sockets will be around 250 A so a B32 may win the race. More likely is a fault in a flex or appliance in which case we can rely upon the BS 1362 fuse.

It would be nice if one could get fuse holders to go in standard DBs

You're not the first to think that - it was one of the features of the fictitious "Concept" range...

PDF

I think that discrimination in the domestic context is over-played and I am I am not too worried about it. I don't really see why a fault should develop in the fixed wiring - we don't go around banging nails into walls. The PFC at sockets will be around 250 A so a B32 may win the race. More likely is a fault in a flex or appliance in which case we can rely upon the BS 1362 fuse.

As a B-type can trip at anything over 3x it's rating, so 250A can easily de-latch a B63 (as well as the B32). Indeed common experience is that a B32 trips and the 13A fuse remains intact on appliance faults. It was all some much nicer before MCBs - 13A plug fused discriminated very nicely with 30A fusewire, which in turn discriminated with a 60A DNO fuse. simples.

   - Andy.