Bad heating element, or something else ?

[guix]

Hello!

I have a problem with a multi function electric oven Whirlpool MT66 which is at least 10 years old, it can do microwave, grill, and "pulsed heat".

When I use this "pulsed heat" mode (a fan with a circular heater at the back of the oven), it will trip the RCD, but not really randomly, so I doubt that it's the heating element!


Apparently, I can repeat this scenario indefinitely: I preheat at 200°C, or 250, for 10 minutes... When countdown reach 0:00, RCD trips. I reset the RCD, insert food in the oven and turn it on again at 200 or 250°C, now it will trip the RCD every ~2-3 minutes (the hotter, the quicker), and the interval between each trip is so constant that I could predict, within 10 seconds, when it was going to trip.

Then finally, when countdown reach 0:00 and my meal is ready, this time it will NOT trip the RCD but ends normally..! But I did not test this last part carefully, because generally I don't bother turning the oven again for the last few minutes, in case it could trip again at 0:00, I prefer to not attempt...


So, I'm not sure but I think it's more a problem with an electronic component like a relay or transistor, or maybe a thermostat..? I'm not too bad at electronics so I'm not affraid of opening the case, I know I must not touch the big capacitor, etc, in fact I opened the back and checked the heating element with my multimeter and it looks fine... I think! But I will wait to read your answers before opening the entire thing.

Other than that, every other function of the oven, grill, microwave, plate rotation motor, fan on the back, buttons, display.. everything else than "pulsed heat", works great since so many year... I like it so much, so I would like to keep it and repair it if possible.

The RCD (Merlin Gerin Multi 9 ID, 40A, 30mA) begins to show mechanical issues, when it trips, sometimes I have to try 3-5 times to get the lever to lock in the ON position.

Also, I know I shouldn't have done that because from what I read, it's EXTREMELY DANGEROUS, DO NOT ATTEMPT THIS AT HOME, IT CAN KILL YOU, but I tried to remove the Ground wire from the plug of the oven, and yes the issue does not appear when I do this, the oven can run at 250°C for hours.


What do you think it could be? If you are convinced that it's most likely the heating element that is failing, how do you explain the non randomness of the scenario above?

Thanks in advance to help me save this oven!

[R_J]

It could be that the heating element is shorting inside as the outside of the element is at ground. It can happen as the element inside heats up, expands, and shorts to the elements tube. I don't know how you can be 100% sure, If there is a way to remove the element mounting screws to isolate the element from ground that would tell you if it shorting internally. also inspect the element sometimes the outer tube of the element will show where it was shorting,
By temporary removing the ground that would suggest that is the problem.
You could also disconnect the element, that way it won't heat up and short confirming its not something else in the oven

[guix]

Hello,

Problem solved... Long story short, I had to invert L and N in the plug of the oven, in fact reverting a previous attempt by myself to fix the same issue a few months ago... Don't ask, too long to explain

[stj]

it has a plug?
photo?
normally we hardwire stuff that big.

[guix]

It's not much bigger than a microwave.

Not my picture but same oven with original plug.

[stj]

ok, i thought it was a big 12KW floor standing oven!

[R_J]

Just for the hell of it, check what A/C voltage you get between the mwo cabinet and any other appliance or sink with in arms reach.

[stj]

france is single-phase, the only thing you could find on a sink is static from capacitive coupling if his earth failed.

[guix]

The case of the oven is properly grounded. Thanks, I didn't think about verifying this

[R_J]

Originally Posted by guix

The case of the oven is properly grounded. Thanks, I didn't think about verifying this

But by looking at the plug you have removed the ground If the heat element is shorting to ground, it will apply full ac to the cabinet, without the ground it will NOT blow any breakers/fuses etc.

[stj]

ground on a schuko is by side-clips or an inverted pin(pin on socket side) on older french stuff.
he has ground there.

[kaboom]

Originally Posted by R_J

But by looking at the plug you have removed the ground If the heat element is shorting to ground, it will apply full ac to the cabinet, without the ground it will NOT blow any breakers/fuses etc.

If the chassis is grounded, it'll trip the GFCI/RCD if present (as in the OP's case), OR energize and heat up if on conventional ckt bkr AND switch contacts for element are in the neutral- what fuckery reversible polarity is in combination with single-switch-contact (or relay contact) controls.

If cabinet is floating and GFCI present, it'll float up till someone touches it; the leakage should(?) trip the ground-fault protective device, with a "bite" during the time it takes to open.


That element is leaking to its sheath.