Electric oven problem

[BruceB]

I have an AEG Competence B4140-1 built-in fan oven which I installed about 7 years ago without problems. Twice in the last few days when switching it off having been using it in fan oven mode, the rear fan element does not switch off. So I have an oven, switched off, with a cooling fan gently running and a bright red element inside the fan casing at the back. I had to isolate the oven at the wall to kill it. Restore the power again, but with the oven switched off, the element shines bright red again. Once the oven is cool and I restore the power all is OK. After the first incident I did a pyrolytic clean to get rid of gunge inside, but the latest incident is since then.

It is on the same isolator as the electric hob which is a pain - lesson learned.

Looks to me like a sticking relay somewhere. I cannot find a wiring diagram online though. Anyone had a similar problem and fixed it. I am about to delve inside the oven!

Regards Bruce

[Ronald Raygun]

Seems a silly idea to have a cooling fan (as opposed to the normal circulating fan). The usual way of letting the oven cool down after use is just to leave the door open!

Well done, and here's hoping it didn't just fall off when you opened it up, thus making you *think* you've found the problem. :-)

Potentially dangerous also from the point of view that it could have switched the element on spontaneously in your absence, causing a fire.

[BruceB]

To answer my own post I think I have found the problem. Opened up the casing and found the control pcb mounted on the underside of the roof had fallen out of its mounting and was lying on a metal/foil backed insulation area. So I suspect a connection was being made that should not which meant the element came on when the cooling fan was on. I cannot see how it would fall out of its mounting in normal use (x2 plastic clips in good condition) so I suspect it has been like that since manufacture/transit years ago. Anyway, seems to be working now. Potentially dangerous failure mode because it could conceivably have made the case live which as it was on a non-rcd protected 16th edition cooker circuit might not have tripped. Regards Bruce

[BruceB]

I think it is quite common in modern built in ovens. It means the surrounding units do not get hot, the oven electronics stay cool and you do not have to have a massive volume of insulation around the oven when cleaning itself at c. 500 degrees C.

Regards Bruce

[dennis]

I would hope the earth would take out the fuse. It is earthed?

[BruceB]

Yes it is. What I meant was that a high impedance 'short', because it was lying loosely on metal/foil, might be enough to make a case give a tingle or jolt, but not enough to trip a mcb. Regards Bruce

[ARWadsworth]

I doubt it will make the case live that you will be able to feel.

If the earth resistance back to the CU was in the region of 0.75 ohms and the metal case of the cooker rose to 50V then you would have over 66amps flowing through the cable and the PCB. Something would blow pretty quickly.

Cheers

[BruceB]

Yes, I think you are right - I thought about it a bit more after I posted. I suspect the relevant bit of the pcb would be 'self fusing'.