Do I need two SPD's?

Roberts7286 · May 26, 2022

Hi

I've got a summerhouse/garden bar job coming up and was wondering where I need SPD's. I think I may need to split the main supply via henley blocks and go to a switch fuse to supply the bar. My question is do I need an SPD at the switch fuse and also at the CU in the bar. Would an SPD at the supply be enough or just one at the board in the bar.

Thanks

Tom

pc1966 · May 26, 2022

It kind of depends on the risk. Typically if you have a 2nd DB that is more than 10m away then you would consider an additional SPD for that on the grounds that (literally) there could be a major current disturbance via the earth in the event of a nearby strike.

Having said that, in most domestic situations with very little risk of either building getting a direct hit, and if the summerhouse has little in the way of valuable electronics in it, I probably would not bother.

Probably the best option would be to speak to the client and ask about the use-case. If it is a few lights and maybe a cheap ghetoblaster for occasional music, no need. If they are planning a £2k TV, etc, in it then probably worth doing.

Most 'garage' style CU don't come with SPD options, usually just a 4 unit width for a mains RCD/switch and two MCBs, but a small house CU might be better suited if they have plans for anything fancy (e.g. separate heaters, sockets, lights, spare slot, etc) and you can get the smaller SPD for a tens of quid, in some case part of the CU deal.

pc1966 · May 26, 2022

Roberts7286 said:
My question is do I need an SPD at the switch fuse and also at the CU in the bar. Would an SPD at the supply be enough or just one at the board in the bar.

I kind of mis-read your question, so:

If you are not considering the house CU at all, then just put an SPD in at the new property.
However, if the house has an SPD already, then you might look at ignoring it for the out building.

If you have a significant distance between any house SPD and your power take-off point then it won't protect the outbuilding to a great degree. The UK regs say max loop 1m but ideally no more than 0.5m, that means a distance of 0.5m or 0.25m so your cable out/back come to less. Remember it is the inductance that bothers SPD performance, not so much resistance, as your typical lightning waveform has an 8us rise-time and 20us decay time, compared to 10,000us for a 50Hz supply half-cycle.