Oven tripping RCD when turning off

[Lee]

Please no arguments about why it's on an RCD protected circuit, I can't easily/practically change that at the moment*. It turns on (it's a fancy one with a SMPSU and LCD display) and if no heating functions are selected it can be turned off again with no issues. If a heating mode is selected it will heat up but if it's then turned OFF it will trip the RCD. It will, rarely, also trip when the control board/thermostat turns off the elements, but mostly it's when it's switched off.

It's passing 2mA to ground when it's on (guessing that's the class Y cap in the PSU) but my meter isn't quick enough to catch the fault current.

I've measured the elements with a Megger (with them disconnected) and they all check out fine cold and hot so now open to suggestions...

It has umpteen relays on the control board is it possible one of them could be arcing and upsetting the RCD?

*If it is the RCD at fault it will require changing the CU, as it's obsolete :(

[harry]

Some equipment generates "transients" (or spikes in the voltage) which upsets some RCBs. Especially older ones. When they first came out, there were lots of problems in this respect. The problem seems to be overcome these days. If after testing out there's no apparent fault, you are down to checking the RCB by substitution. ie, put a new one in and hope the problem disappears.

[harry]

Oh, another possibility with cookers. If left unused, condensation in the ends of the elements can cause a temporary fault. The insulators are slightly hygroscopic. When they are heated, the problem disappears. Can be a bit baffling.

[Lee]

It's an obsolete whole house RCD in the CU And therein lies the problem.

Also I can't claim the seal fairy has visited because there is a large "do not remove this fuse" sticker on the cutout :(

VDE gloves and PPE time maybe... wouldn't be the first time :) But older and maybe a little less eager to take risks these days...

[John Rumm]

It could be a case of a sensitised RCD - i.e. there is enough total leakage being seen by the RCD from a combination of circuits, that its close to its trip limit. At that point, switching transients can be enough to push it over the edge.

[Chris Green]

Just pull the fuse. It's odds on no one will notice and if they do they probably won't care. Keeping yourself safe is more important.

[The Natural Philosopher]

May be irrelevant, but a high impedance mains supply, will place a BIG step on any live to earth RF caps when a BIG load is switched on.

And that can alter the earth leakage dramatically for a part cycle.

Suggest 150mA main trip and RCBOs where it matters

[Roger Hayter]

The people who redid our pole transformer just cut off *all* the seals in our meter box - I have no idea why the touched anything apart from the main fuse, but they did - and made no attempt to replace them. So I reckon there is plausible deniablity there, in the absence of any other evidence of rewiring.

[tabbypurr]

If a 2 pole switch disconnects neutral before live, and the fault is at the neutral end of the element, that end goes from about 0v to 240v momentaril y, and earth leakage current spikes until the live switch pole is also open . I can't say if that's what's happening without seeing the oven.

NT

[Lee]

Well now... they are using DP relays to switch the elements (why?)* so that sounds very plausible.

*Unless I'm missing something, I can't see any advantage in doing this, but they are switching both "ends" of the elements.

What really confused me to start with is that the negative rail of the

12v SMPSU is grounded to the oven and all the control circuitry is 12v so there are voltages to ground on all the switches and sensors...

[Graham.]

[Graham.]

Especially those cheap hobs with solid hotplates. The problem is so prevalent, I wouldn't be surprised if it was the source of the "Cookers not on RCDs" advice.

[Lee]

It might be now but it never used to be...that's my excuse anyway ;)

[Graham.]

I can understand why you would think that, because many cookers have a simple electronic time-clock with a capacitive wattless dropper PSU, and the LT rail will be referenced to mains neutral, and not earth.

[ARW]

Don't you mean there WAS a "do not remove this fuse" sticker?

[ARW]

Would that not only cause a problem with the RCD when the element is calling for power.

[tabbypurr]

the neutral end of the element, that end goes from about 0v to 240v moment arily, and earth leakage current spikes until the live switch pole is also open. I can't say if that's what's happening without seeing the oven.

Sounds like an easy workaround. Running the elements on max usually fixes a ny earth leakage - if not it's new element time.

NT

[Mike Tomlinson]

En el artículo , Lee escribió:

Guessing here, but maybe to spread the load across two sets of relay contacts.

[Mike Tomlinson]

En el artículo , Lee escribió:

Fit a little sub-CU and move the oven to that?

e.g.

No one will care.

[Brian Gaff]

But he says its obsolete

If its a capacitor then try changing these or even put a resistor in series if all they are for is to suppress junk affecting the logic in the device. I would also hope any relay system would be so designed not to arc. Something an old portable radio with medium way will tell you I'd imagine Brian