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RCD trips when appliance turned off

Peterd

Very strange fault, I have a couple of catering ovens which plug into normal 13A sockets. Ovens are very basic - normal oven elements, electric fan, neon indicators, rotary switch  & electromechanical timeswitch, no electronics or surge suppressors etc. Insulation resistance is up in the many megohms, so problem is not an earth fault in the ovens themselves. All works fine, except that occasionally the rcbo serving the circuit trips when the switch on the wall outlet is turned OFF. As I say, during operation, no problems, it's only when they are turned off. 

Any ideas on this rather mystifying problem?

Parents
  • 0

    Yes, well 'maybe' anyway. IF the switch is double pole and breaks neutral first, or indeed somewhere a load is neutral-side switched, at the moment of switch off the voltage to earth on the neutrally side of the elements etc is greatly increased, and any capacitance to earth on that node needs to be charged up to live voltage. That additional twitch of L-E current "leakage" even if short lived, may well take any RCD already near trip point over the top.

    How much headroom does the RCD  have before tripping ? (a clamp meter test of L-N through a meter that can read mA AC is often good for this.)

    What else is on the same RCD ?

    The other funny is that some RCDs do not respond well to sudden load shocks, but that is equally likely switching on or off.

    Mike.

    PS for RCD read RCBO as well.

Reply
  • 0

    Yes, well 'maybe' anyway. IF the switch is double pole and breaks neutral first, or indeed somewhere a load is neutral-side switched, at the moment of switch off the voltage to earth on the neutrally side of the elements etc is greatly increased, and any capacitance to earth on that node needs to be charged up to live voltage. That additional twitch of L-E current "leakage" even if short lived, may well take any RCD already near trip point over the top.

    How much headroom does the RCD  have before tripping ? (a clamp meter test of L-N through a meter that can read mA AC is often good for this.)

    What else is on the same RCD ?

    The other funny is that some RCDs do not respond well to sudden load shocks, but that is equally likely switching on or off.

    Mike.

    PS for RCD read RCBO as well.

Children
  • 0 in reply to mapj1

    Hi Mike, thanks for very useful suggestions. I'm not on site again until later next week so will follow up on your suggestions then and give some feedback

    Peter

  • 0 in reply to Peterd

    Yes, I had a similar thing with some pub lights. When the car park floods were turned off, all the lights in the building tripped as they were all protected by a single RCD. I presume it would be the capacitor in one of the SON/SOX lights discharging slightly, along with a DP switch.

    Linking out the Neutral on the switch cured it. His poorly designed lighting board wasnt upgraded as he didnt want to spend any money.

  • 0 in reply to alanblaby

    I have come across this with some appliances where stored energy is released somehow when power is removed. Sometimes NOT switching the Neutral can cause it to happen (where current flows in the N-PE circuit through the supply neutral-earth link, so if the N is monitored through the RCD, it operates the RCD).

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