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Excuse any incorrect terminology in this question (I will correct if advised).

I'm in the UK. I have a house with relatively new wiring (past 5 years) which has circuit breakers on the power and lighting circuits and two RCD devices (I think these are GFCI in the US) for the downstairs and upstairs circuits.

I have a very basic built-in electric oven with fan that runs off a standard 13A plug.

I unplugged the oven from the socket, moved the shelf in the housing that the oven sits on to make room for something else. I then plugged the oven back into its original socket (two outlet, dual pole, switched shared with microwave). When I switched on the thermostat on the oven, it tripped the "downstairs" RCD. Just switching on the lamp or the fan did not trip the RCD. I looked at the wiring in the plug and the earth wire looked to be just a few strands of copper. I stripped insulation and trimmed and refitted all three wires (E,L,N) to the plug, plugged the device in, switched on the element and thermostat and no RCD tripping. Hooray! The device has been ok for about two weeks then yesterday it tripped the RCD after running at 180C for 15 minutes.

Now, the device is probably faulty in some way but I am concerned that this is some "cumulative" effect and that there is a "baseline" current drain happening from other devices and the oven is just adding to that and causing the RCD to trip.

Is there some "plug in" tester that provides a standard UK plug and socket that can be inserted between the device and the power outlet where I can check any current drain for every device in my house? I'm trying to isolate the problem to the oven - if indeed that is the only source of the problem. I'm willing to pay £30 for such a device.

Is there any link to a systematic check I can perform on the oven if the current drain problem is isolated to that? I'm trying to decide whether there is an easy diagnosis / fix I can achieve for under £50 plus some time investment and decide whether to get the oven serviced or to junk it and buy a replacement. I chose £50 as the target expenditure as that is the charge for a call out plus half hour work around here. Also if it is possible to "residual current" test all devices that will unearth other problems and I can replace faulty devices or make me sleep better.

I don't want to replace the oven if the problem is (partly) somewhere else and replacing the oven doesn't actually fix the RCD trip.

  • Heating elements do fail and may be leaking current to ground causing your tripping. If it is an element they usually get worse prior to total failure. several devices could have leakage that adds up to enough to trip your RCD. Although I am in the U.S. we sometimes have similar problems on branch circuits, I normally use a megohm meter or megger. I place 1 lead on the hot wire and one on the frame and run the voltage up to 1000v when I get a low reading that’s where the problem is. This can check the insulation, and plug or just the elements. This is one way to find the leakage point. – Ed Beal Aug 14 at 16:25
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Yes. Get an extension cord (I assume they are all 3-prong in the UK). Slice down the edge of the cable and remove the outer sheath for about a foot. Don't cut into any of the insulation on the individual wires (though if you nick ground's insulation, don't worry about it).

Bind the hot and neutral (brown and blue) wires tightly together with electrical tape.

Now get a clamp ammeter and clamp the hot/neutral bundle. The amp reading should be zero. If that is not zero, that is a ground fault, which explains the RCD trip. Now clamp the ground wire. The reading should be zero. If it is not zero and the same number, you have an internal ground fault that has been caught by the ground wire.

If the non-zero figure on the conductors isn't accounted for by the figure on the ground, you have current leaking in a third direction.

Either leakage implies a problem with the oven. Try giving it a good clean, especially in the wiring areas.

  • I have seen a couple of YouTube videos on using a clamp meter - I guess they work by detecting the magnetic field around each separate conductor in the lead when current is flowing. I can get a fairly cheap meter on Amazon. Am I right in thinking that the device will only read a value when the oven is switched on or is it if there is a current shown in the love and neutral pair when the oven isn't "officially" switched on then there is a fault? Thanks for your help. – Clive Long Aug 16 at 12:01
  • @CliveLong a clamp meter detects EMFs from current moving through a cable. (Yes, EMFs are real). For the two tightly bound wires, their EMFs will be considerable since they are powering an oven, but they will be equal and opposite and cancel each other out. If the oven is working. This is the same way RCDs detect. – Harper - Reinstate Monica Aug 16 at 15:51
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There are a variety of test.

The simplest and cheapest is to test for continuity between the earth/chassis of the oven and the live/neutral. This you can do with any cheap multimeter. However it won't reveal groundfaults that only reveal themselves under high voltage which breaks down the insulation.

Or you can DIY a short extension cord which runs the earth wire separately and then use a current clamp meter around both the neutral and live (but not earth) to measure how much leakage current there is under normal operation.

The next step up is using a megaOhm meter that is designed to test for these faults in appliances. This is essentially the same test as the continuity test above but the mega-ohm meter will use high voltage to ensure there is no dielectric breakdown under high voltage conditions.

Any electrician will be able to do the first two tests though the second requires a bit of prep. But the mega-ohm meter is rather specialized.

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I would re-examine the plug. It gave you problems before and maybe the insulation and conductors are in worse shape than you thought. Next, try plugging the oven into a different outlet and see if the problem travels to the new outlet. You stated that it runs off a standard 13Amp outlet, not actually sure what that is as I'm in the U.S.A.

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    In the UK the receptacles are supplied with a high limit breaker (30 A or so) often in a ring topology. To protect an appliance and the wiring cord to the appliance there is a fuse or a breaker in the plug for the appliance. One standard size is 13 A for heavy draw appliances. en.wikipedia.org/wiki/Ring_circuit – Jim Stewart Aug 14 at 13:48
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    Since voltage is 240V, 13A is nearly 3000 watts. It's a stupid large amount of power, fit for proper ranges/ovens. Their plug-in kettles and grills are also much stronger than US versions. This gives people fits when they import a "much better" euro expresso machine to the US. – Harper - Reinstate Monica Aug 14 at 21:43

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