RCD trips out when oven gets hot

[Sion Whellens]

Domestic electrics installed with RCD in 2000, hasn’t been tested or recalibrated since then. Intermittently (but now every time) it trips out when I use the electric oven/grill on my cooker. Curiously though it doesn’t trip out until the oven has heated up. Is this an issue with the cooker or with the RCD?

 

[Colin Brenton]

It's the grill (or oven) at fault, not the RCD. Probably the element shorting to ground when it gets hot would be my guess. Most likely an element replacement will fix it.

 

[bernardgreen]

Another possible fault is the thermostat creating a short circuit Live to Earth when it switches OFF the supply to the element.

If the un-used OFF contact in the thermostat has developed a conductive path to Earth then when the thermostat switches this path will create a Live to Earth fault.

 

[EFLImpudence]

Is there is not an OFF contact? Surely just an ON contact no longer connected.

 

[jj4091]

As has been said the most common cause of that problem is a faulty element but can you post a pic of the rear of the machine that shows the cable entry & suppressor. I had an intermittent fault of the RCD being tripped caused by an oven that only showed when hot & it was the leg of a component on the suppressor board poking through the back of the board & shorting to the frame of the machine which distorted when oven heated up.

 

[bernardgreen]

Is there is not an OFF contact? Surely just an ON contact no longer connected.

The thermostat removes supply to the element by forcing a moving contact to move away from the fixed contact that connects to the element.

The moving contact when not touching the fixed contact rests against a back stop. The back stop maybe a second fixed contact ( as in a change-over switch ) or an insulated surface. When the back stop has a path to Earth the RCD will trip when the moving contact touches it.

 

[AdrianUK]

As others have said.... almost certainly a bad element.

I do find it rather alarming that, when an RCD trips, a large proportion of OPs seem to feel that this protective device is 'faulty' when in fact its doing the very job it was installed to do!

I hear what Bernard is saying.... obviously a thermostat can be constructed to be 'change-over' but I've never yet seen one on an oven... most manufacturers would want to save those extra few pennies (per device) instead of paying for a contact which will never be used.

 

[EFLImpudence]

I do find it rather alarming that, when an RCD trips, a large proportion of OPs seem to feel that this protective device is 'faulty' when in fact its doing the very job it was installed to do!

A very good point. It always amazes me when people say that.

Oil light comes on in car - must be faulty sensor.

 

[bernardgreen]

to be 'change-over' but I've never yet seen one on an oven

I don't think I have either in domestic appliances, The moving contact has to have a place to go when not touching the contact connected to the element. This back stop may get dirty with cooking fumes, debris from arcing and/or other contaminates enough to produce a conductive path to something in the thermostat that is Earthed

 

[JohnW2]

A very good point. It always amazes me when people say that.

Indeed - and it sometimes also makes me wonder about all the reports of 'nuisance trips' we hear about.

I don't doubt that RCDs and RCBOs do sometimes just trip spontaneously, for no legitimate reason - but, as I always say, and for what it's worth, my experience during the 30+ years I have been living with many of them is that I can't recall any occasion on which one has tripped here without there being 'a good reason', even though the reason has sometimes been difficult to identify.

Kind Regards, John

 

[JohnW2]

... obviously a thermostat can be constructed to be 'change-over' but I've never yet seen one on an oven... most manufacturers would want to save those extra few pennies (per device) instead of paying for a contact which will never be used.

That's probably true. However, some applications require NC contacts and others require NO ones (even in an oven, the element and fan ones could have 'opposite' requirements) so it is conceivable that there could sometimes be a cost benefit for a manufacturer in buying just c/o ones ("in larger bulk quantities") and using them for both applications.

Kind Regards, John

 

[roystaan67]

Another possible fault is the thermostat creating a short circuit Live to Earth when it switches OFF the supply to the element.

If the un-used OFF contact in the thermostat has developed a conductive path to Earth then when the thermostat switches this path will create a Live to Earth fault.

I had this same issue.
My Britannia range main oven was tripping the rcd when it was getting near 200c.
Having replaced a faulty heating element just a few years prior I didn't think that this would be the cause but as a process of elimination I replaced it but the problem persisted.
I then bought a replacement stat but wasn't hopeful that this was the issue as moving up through temperature settings when the oven reached it's mark and the stat cut the heating to power to the element it functioned normally, as I said the rcd would only trip when it was approaching something like 200c.
So I fitted the new stat and "hallelujah" issue resolved. I have no idea why but 3 days before the big Xmas roast up I'm just glad all is sorted

 

[ericmark]

You should start your own thread, as answer to you @roystaan67 may not be the same as answer to @Sion Whellens, although it seems he is no longer on the forum.

But we are allowed between 15 mA and 30 mA to trip the RCD within 40 ms, in the main around 27 mA is common, the background leakage can be up to 30% so 9 mA, so an oven may trip with just 6 mA if already at the limit, so we need to measure. Clearly here one meter is wrong, likely the yellow one as getting rather old, but showing 15 mA so if I was using a single RCD I would be well over the background leakage allowed, I actually have 14 RCBO and two RCD sockets so for me will within what is allowed.

The next is to see if the RCD is OK, with another tester, and if OK can test the appliance using DC but since the appliance uses AC, this is a bit hit-and-miss, and yet another tester so looking at around £150 to get the test gear, so in the main, people guess.