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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

    Some network faults (e.g. broken Ns) can results in sustained overvoltages across single phase loads - say 300V or 400V for many minutes - which aren't the kind of short duration (μs) much higher voltage (kV) surges (from lightning or switching) that SPDs are meant to deal with.

    My guess was that the installation was presented by something in the region of 400V that caused the electronics to fail, causing a short that took out the MCBs (MCBs supplying MCBs is never a good idea from a discrimination point of view) - and the SPD didn't get called into action.

       - Andy.

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  • 0

    Some network faults (e.g. broken Ns) can results in sustained overvoltages across single phase loads - say 300V or 400V for many minutes - which aren't the kind of short duration (μs) much higher voltage (kV) surges (from lightning or switching) that SPDs are meant to deal with.

    My guess was that the installation was presented by something in the region of 400V that caused the electronics to fail, causing a short that took out the MCBs (MCBs supplying MCBs is never a good idea from a discrimination point of view) - and the SPD didn't get called into action.

       - Andy.

Children
  • 0 in reply to AJJewsbury

    I would concur, given that the neighbours type 3 SPD operated while the OP's customer's device did not have one... I also wouldn't be suprised if the smart speaker's withstand rating was did not coordinate with a type 2 (which was all the way back at the CU so ringing/resonance might have occurred to lessen protection further).

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