I moved from Canada to the UK and for convenience I would like to use some of my Canadian aquired extension leads (for light power items such as ethernet switches).

Problem is that the surge-protected extension leads seem to always trip my circuit breaker, so this pretty much means I cannot use them.

However I am not satisfied to just accept this fact without knowing why.

So I'd appreciate if some knowledgeable electrician would be able to explain to me why a surge-protected extension lead designed for the USA/Canada (110V) trips my UK circuit breakers on 220V.

  • 1
    Don't assume that every wallwart can take 220v. Please don't ask me how I know... Commented Oct 5, 2015 at 5:02
  • Interestingly enough when I tried this it didn't trip the circuit breakers (I think, it was a while ago), but did instantly burn out the MOV.
    – Ian
    Commented Jan 11, 2018 at 0:01

1 Answer 1


The surge suppression in most consumer grade power strips includes a metal oxide varistor (MOV) placed across the line at the supply side of the power strip:

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These devices suppress voltage spikes by conducting energy across the line when the incoming voltage exceeds a preset threshold determined by the manufacturer. Wikipedia has a good description of varistors here:


MOVs in a surge suppressor designed for 120v are selected to conduct at roughly 200v. This ensures the MOVs do not interfere with normal operating conditions, which include peak voltages up to 55% greater than the nominal line voltage. If you connect one of these across 220VRMS where the nominal peak voltage is 310v, far into the conduction region of the device, then the MOV conducts during normal operating conditions shorting out your extension lead.

  • Regarding your surge suppressor voltage calculation, the 1.41 factor is there to convert sine wave RMS to peak voltage, right? And the 1.20 factor is there as an estimated manufacturer choice of voltage danger zone at 20+%?
    – kdbanman
    Commented Oct 5, 2015 at 16:12
  • 1
    Note that if they tripped your breaker the surge suppression capability is destroyed by the power that went through the MOV. Commented Oct 5, 2015 at 18:23

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