Earth leakage woes pt 2

But doing a live test with a clamp meter on a house with a TN earthing system is inherently inaccurate, unless all other forms of earth leakage are completely disconnected. If next door, or 3 doors up, have an earth- neutral fault and a poor connection to earth, then it could affect the readings you are getting. The only way you can get a true reading is via a low ohm reading across the terminals, and domestic meters are , again, not likely to be accurate enough to give a true reading. You say you are getting 7mA with no load on the circuit. There are limited ways such an earth leakage can happen, I'd still be suspecting a poor element, though induction hobs have a transfomrer in them that could be leaking via a N-E fault too. I'd also suspect that the on/off switch only isolates the line conductor, so there woudl always be a path between N to E even when off at the internal cooker switch.

I don't think you've ruled out a neutral to earth fault, if the elements are live-side switched as is likely. ie your fault looks like:

element L ---- thermostat -----OOOOOOOOOOO---- N ______V____ | | E

You have a low(ish) resistance path to earth at point V. If the live is connected and the thermostat is on, current flows from L to E.

If the thermostat is off or the live is disconnected, the circuit looks like:

OOOOOOOOOOO---- N ______V____ | | E which is a neutral to earth fault. Current flowing in the neutral from some other device in the house is diverted to earth which trips the RCD.

Separate from any discussion about the cooker, it sounds like your CU is elderly and would be better upgraded to modern standards, which includes more discrimination about earth leakage faults (in particular, with so many SMPSUs around it's easy to exceed 30mA whole-house leakage aside from the cooker).

I imagine a separate CPC busbar could be arranged if they are too short, or they could just relocate the CU closer to the tails. They could also use a 'CU relocation box':

FWIW replacing the CU is notifiable under Part P, so if you DIY you are supposed to get Building Control to sign it off. The main things that cause trouble is incorrect screwing down onto terminals (wire or busbar underneath the clamp terminal, not inside it) and incorrectly torqued terminals causing resistance heating (torque screwdriver recommended).

Theo

My last bathroom extension did not get building control approval because the electrician failed to deliver a certificate (complicated reasons) and the plumber failed to turn up at all after the first day so I did virtually all the plumbing myself. I suppose I could have asked the BCO to look at it himself but the toilet seat bidet was not WRAS approved (though it did meet specs) and anyway that was 11 years ago ...

If there is any neutral to earth leakage it must be flowing in the whole length of the neutral wire (Kirchoff's Theorem) whether it is via the circuit earth or other parts of the planet. And there was none in the clamp test. Couldn't be an equal balancing current in the live wire, which was disconnected (Kirchoff again).

Not if there is a ground short.

I appreciate yours is a combine oven / hob but induction hobs don’t tend to have exposed metal parts - ours is just a glass sheet. I don’t think there is even exposed metal on the underside.

That would suggest you may be able to isolate the two parts ( or three, I assume there is a grill?)

I’d isolate the hob and check the leakage again.

If it is in the oven / grill area, it is probably easier to fix. I’d start with charred insulation, terminals, then check elements ( a mix of experience and wishful thinking, replacing an element is more expensive).

If is is within the hob, again charring and terminals but add failing Capacitors as easy hits.

You mentioned ‘drying out’ or something similar.

If there has been water, looking for ‘salt like’ deposits and removing them is worth it.

Ok. Just explain to me where the current in the neutral to earth fault can go to complete the circuit other than along the circuit neutral connection when live is unconnected. What other route is available to it? I again invoke Kirchhoff's Law! No other mesh is available.

In a ring there are two neutral and two earth paths paths to the CU and they are not necessarily equal resistance

Current dies not flow "the whole length of the neutral wire" Different currents flow along different parts (Kirchoff's Theorem)

No ring is involved in the cooker circuit. No other appliances are connected to the cooker circuit.

Looking at the problem from afar (I haven't got an induction cooker) if the RCD trips with no heating or induction element turned on, then one of the few things still on is the clock/timer. Is that what is causing the leakage current?

Your local mains earth is connected somewhere to the supply earth. This could be via a local earth rod, if you have one. An earth wire if you have an incoming Earth wire. Or the ‘outer’ of your incomer if you have a PME supply. In all three systems, at some point the Neutral and earth are ‘eventually’ bonded.

Imagine you are measuring the currents at your consumer point.

The currents should balance. What comes from the supply comply should return, ideally via the Live and Neutral. If you have an earth route at that point, then there is a third route.

BUT, a fault path in, say your bathroom, is a leakage.

RCDs check for LN imbalance.

Everything has touch controls nowadays, and I guess that this makes it convenient in class 1 appliances to reference all the control circuitry (power supply, logic, relays etc) to earth rather than neutral. Otherwise touch screens would be bit unreliable. So a few milliamps is unsurprising. There are also probably mains filter capacitors between earth and the mains inputs.

I've changed the heating elements in Two fan assisted ovens in recent times a a Beko for one.

It is possible that its got "some" leakage and theres other stuff around the house thats leaking a bit as well, and when it gets too much...

Course a RCBO on each circuit is a very nice idea, one ill get around to one day..