Electrical mystery - any ideas?

[Penners]

I've got an electrical problem in the house, that I just can't fathom out. If any electrical experts have any ideas, I'd be extremely grateful.

First some background: Our Belling 4-ring electric cooker (bought new 4 years ago, and in good nick) is powered by its own dedicated radial circuit. There's a 45A MCB in the consumer unit, supplying a 6mm^2 TWE cable that runs exclusively to the cooker. There's a double-pole cooker switch on the kitchen wall, with an integral plug socket that's almost never used.

The cable from the cooker switch runs down inside the wall and emerges at a 30A junction box on the wall behind the cooker, and the 6mm cooker cable is joined to it within that junction box.

The whole consumer unit is protected by a master RCD. All circuits are supplied through MCBs.

Now the problem: three times in the last couple of months, there's been a bang in the kitchen (from the area of the cooker), and all the power has gone off. On checking the consumer unit, I have found that the cooker MCB has tripped off and the master RCD has done the same.

On every occasion, none of the heating elements on the cooker has been switched on. The main cooker switch on the wall has been on, hence the cooker's clock has been running, but that's all. And once the power is restored, everything on the cooker works normally.

I know that some cookers have rather dodgy heating element controls, which makes it quite difficult to tell whether they are properly switched off or not. Ours isn't like that - it has very clear off positions on all functions.

I've double-checked all the supply cable connections inside the back of the cooker and in the junction box - they're all well made.

As I've said, when the incident occurs, there is an audible bang from the area of the cooker.

Has anyone got any ideas what's going on, please? It's got me totally foxed!

 

[Flyfisher]

Is there an electrical burning smell in the vicinity of the cooker after the bang?

It sounds like you've already checked the obvious things. I would also have immediately thought of the connection into the cooker, especially the (usually) metal cover that normally covers the access point to check for possible short.

I've heard of heating elements gradually failing and causing earth leakage trips but the fact that you have a 45A breaker tripping out suggests that there is something a bit more drastic than that. I think it implies a live-neutral fault because I suspect that is the only way to draw the current required to trip the MCB. Other faults should, I think, trip the RCD well before the MCB would trip. Hopefully a qualified expert can confirm (or correct) this assumption.

Is there any possibility of cable damage along the radial circuit? We've discussed potential rodent damage before now.

 

[MdB]

Nothing else springs to my mind (sorry). You do want to get to the cause of it though.

 

[Penners]

Thanks for the response, FF.

No, there's no electrical burning smell at all. I did wonder about some kind of damage somewhere along the run of the cable, but it's the bang from the direction of the cooker that has made me sceptical about that.

I've checked to see whether there's any sign of insulation damage where the cable enters the cooker - definitely nothing. I've also checked to make sure that the connection terminals aren't close to anything that might short them out, and I can't see anything at all.

If it was one of the heating elements breaking down, any suggestions as to how I could isolate which one? I'm reluctant to try disconnecting everything except one element - that process of elimination could take weeks or months, given that the problem only occurs every few weeks. And I've got my stomach to think of in the meantime!

 

[Flyfisher]

I can't see how it could be a heating element, though, because the problem occurs when the cooker is off, unless there is some sort of switch fault.

We once had a steam iron that would cause the RCB to trip if not used regularly (a common occurence in our household), but after a few minutes it would be fine. I was told that this sort of fault arises where the heating element becomes cracked and moisture enters the clay-like insulation around the heating element. This can cause a leakage current to the earthed outer metal, causing an RCD to trip. Once the iron 'gets-going' it drys out the clay and the problem disappears.

But I can't see how this type of fault could affect something that's switched off. And, again, you're getting 45A of fault current to trip the MCB, which is certainly consistent with a 'bang'.

I can only suggest turning off the cooker at the wall between meals and see if the problem ever occurs again. If it does, at least you'll have eliminated the cooker.

 

[Penners]

That's an interesting idea, and one which makes me realise that I have no idea how a cooker ring is constructed.

Ours has the flat plate type of rings, rather than the curley-whirley jobs (tell me if I'm getting too technical for you). Do you know how they are constructed? I'm wondering whether they could be similar to an iron element.

 

[chuckey]

I once had to rewire a 24 pole connector that burnt up on our Belling cooker. I was under whelmed by the standard of construction, to say the least. I would isolate the cooker (pull the fuse) and take the back of it, and have a poke about to see if you can see any scorch marks or flappy bits of silver foil that could get into the works.
Frank

 

[Penners]

Thanks Frank - I have done that. It all looks pristine inside. No scorch-marks or anything that would indicate a problem.

Mind you, it has just occurred to me that I've been spending all my time concentrating on the hob rings. I wonder whether the oven element could be the source of the problem (and where the bl**dy thing is!)

It's a fan oven, and the fan is in the back face of the oven "box". Is it probable that the element will surround the fan?

 

[Flyfisher]

Ours has the flat plate type of rings, rather than the curley-whirley jobs (tell me if I'm getting too technical for you). Do you know how they are constructed? I'm wondering whether they could be similar to an iron element.

I don't know for sure but I'd guess that the heating element within the ring must be separated from the earthed outer metalwork by some form of insulator - perhaps it is some sort of clay/fullers earth (or whatever they use)?

But I'd still bet against it being a leakage problem because of the bang and the MCB tripping. I'd also bet against it being a heater problem because it occurs when they are switched off (unless, perhaps, the switches are only single pole?).

Regarding the oven elements, I' expect them to be in the sides of the oven - perhaps behind the side panels that typically house the grooves for the shelves? I don't think the location of the fan and/or elements is as important as the fact that all the hot air is wafted evenly around.

All guesswork though.

 

[chuckey]

The way the temperature of most of the elements is controlled is thus:- The knob is the actually a thermostat type arrangement and in its housing is a small heater that is in series with the main element. So when you turn the temperature up, it takes longer for the small heater to trip the "thermostat" and so the current flows for longer in the main element. Each controller has to have its heater matched to the main element so each is rated for its main element., i.e. 1.8kW,2.2 kW etc. All this is about is, that the BIG current flows behind those dinky little knobs on the front panel, so water getting in here could cause a big bang.
As most elements get up to red heat I would think that they would not be wrapped around a fan, unless it was made of some really exotic materials especially the motor wiring!
Frank

 

[Penners]

I can't speak for the oven or grill controls, but the controls to each of the four hob rings actually consist if a series of finger contactors - about eight of them. The rotating part of the control switch consists of a shaft with a series of cams on it. These cams activate different permutations of the contactors as you rotate the shaft,

I therefore assume that each hob ring actually consists of a series of independently controlled elements, with more and more of these cutting in as you demand a higher and higher temperature.

However I imagine the grill and oven controls are rather more as Frank described, and my suspicions are falling increasingly on one of them. I just wish they were as easy to get to as the hob units!

 

[andrew gayton]

condensation from cooking getting into the cooker socket................... this has happened to me..............

 

[Penners]

Thanks Andrew - do you mean the combined master switch and plug socket on the wall near the cooker?

 

[andrew gayton]

yes penn, the one and the same.

 

[Me!]

Sorry if I'm asking the obvious, but can you confirm that the MCBs and RCDs are absolutely nowhere near the kitchen, and that it's not the sound of them tripping that you are hearing?

 

[Nemesis]

It's a pertinent point - ours certainly makes a loud bang when it trips.

 

[Penners]

That's not at all obvious, Me! But yes, I can confirm that. The consumer unit is the other side of the house.