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Did the SPD work?

Northern Spark

Domestic installation. PME supply. 50A type b mcb in meter cupboard, 16mm armoured to consumer unit in house. BG consumer unit with type 2 SPD.

There was a power cut, and seemingly a network born over voltage(a neighbour reported that their type 3 spd incorporated into a plug board operated). Unclear wether the over voltage was when the power failed or when it was restored. The 50A mcb in the meter cupboard and a 20A type b rcbo in the consumer unit protecting the kitchen socket circuit both tripped. The 32A type b mcb protecting the spd did not trip. A day or two later the customer noticed that their google nest mini smart speaker(other smart speakers are available) was not working and discovered its plug and the socket it was plugged in to were charred. It was plugged into a socket that is probably the closest one to the consumer unit. The spd indicator remained green. No other signs of damage to anything.

Did the spd work?

Why did the 50A and 20A breakers operate, but not the 32A breaker protecting the spd? If the spd works, shouldn't it shunt the fault current and therefore trip its breaker?

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  • 0

    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.

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  • 0

    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.

Children
  • 0 in reply to Northern Spark

    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.

  • 0 in reply to AJJewsbury

    That's an interesting device. 

  • 0 in reply to Northern Spark
    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?

  • 0 in reply to Chris Pearson

    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.

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