Interesting stuff, thank you all. So it would seem likely it was a 'low' over voltage. I did also wonder if the SPD being in the consumer unit in the house and not at the source of the installation had anything to do with it.

How long before the regs start asking us to fit voltage monitoring relays (like https://uk.rs-online.com/web/p/monitoring-relays/1026131) (and a thumping big contactor) to avoid this problem?

In a sensible world it could probably just be done with a software update on smart meters (as they already have voltage measurement and a contactor built-in) and arguably would be appropriate for the supply industry to bear the responsibility of ensuring that what they deliver is within spec. however...

   - Andy.

That's an interesting device. 

Northern Spark said:

I did also wonder if the SPD being in the consumer unit in the house and not at the source of the installation had anything to do with it.

I am not sure that that question has been answered. If your supply comes in at the garage and then goes > 20 m to the house (surely not unique) do you need 2 SPDs, or will one at the house suffice?

Northern Spark said:

I did also wonder if the SPD being in the consumer unit in the house and not at the source of the installation had anything to do with it.

I am not sure that that question has been answered. If your supply comes in at the garage and then goes > 20 m to the house (surely not unique) do you need 2 SPDs, or will one at the house suffice?

If your supply comes in at the garage and then goes > 20 m to the house (surely not unique) do you need 2 SPDs, or will one at the house suffice?

I'd say (unhelpfully) it depends. There's nothing magical about who owns a cable (customer or DNO), so in principle a SPD in the CU should protect that part of the installation just the same as if it it were fed direct from the cut-out. The garage end of the installation won't benefit to the same extent though. Also as when a SPD activates it'll "pull up" the voltage on protective conductors just as much (sometimes a lot more) than it "pulls down" the voltage on the live conductors, the relative positions of main bonding and SPDs can have an effect - if main bonding/MET was connected at the intake position, but the SPD was on the end of a submain, downstream equipment that was in contact with extraneous-conductive-parts (e.g. boiler PCBs, pumps, showers) could be subject to a much larger voltage difference than the SPD initially limited it to.

   - Andy.