Loud pop, bright flash, tripped circuit, dead ovens - help!

[NathanM1]

Hello,
The blinding flash under my ceramic top happened with 2 halogen hobs lit just as I turned on the top oven. It destroyed the MCB in consumer unit. It turns out that the MCB being only 16A wasn't big enough for a 10KW Electrolux ECK6046W cooker anyway, and I've replaced it with a 20A breaker having done the sparky's diversity calc. But the problem with the cooker remains this: Neither oven works, but the 4 hobs and grill do work. The bottom oven is completely dead, no fan/light/heat, and the top oven causes the flash when the heat is turned on, yet the light and fan work.

I took the back panel off the cooker and there is a big burn mark on the inside of the rear panel and black charring on lots of cables, though none of the cables are actually damaged.
I've inspeced the element in the bottom oven and it appears to be fine and with a multimeter across it's terminals it has a resistance of about 250 Ohms.

I slid out the element for the top oven (not fan assisted) and it looks fine too and also has a resistance, slightly lower than the large bottom oven element.

The elements for the grill are fine too, as expected because it works still.

So can anyone take a pot-shot at what might be going on with my cooker? I'm guessing the bright electrical flash is the result of a short somewhere, backed-up by the burn marks round the back on the inside, but I can't see anything that's gone wrong so don't know now what to do troubleshoot. The cooker is only 4 years old. Thanks. Nathan.

 

[pjcomp]

The 16A breaker should have tripped out on overload, without lots of flash bang wallop. What size cable is between the consumer unit and the cooker?

You say cables are charred but not damaged - I'd count charring as damage, and could also mean that the damage is hidden from you by the charred insulation. I doubt that an underrated MCB would lead to the cooker malfunctioning - where does the big burn mark originate? That would most likely be the source of the problem ....

This one might be better directed to the appliances forum.

PJ

 

[flameport]

Your description indicates a serious fault with the cooker.
250 ohms for an element is way off, so either both elements are ruined, or your multimeter is giving totally wrong readings.

16A and 20A for a cooker circuit is also wrong, and suggests the cable to the oven is undersized, or the circuit wasn't originally designed for a cooker.

Realistically, you need to get someone to repair the cooker, test it to ensure it is safe to use, and also check/replace the circuit & MCB supplying the cooker.

 

[NathanM1]

Cable to the cooker is 6mm2 and is about 15m from the consumer unit.
I re-checked the resistance of the elements on an easier to read scale:
Grill 1: 30ohm
Grill 2: 120ohm
Top oven: 50ohm
Bottom oven: 25ohm

I've since removed all the connectors for the elements and taped them up with insulating tape to see where a fault might be by isolating one element at a time.
The bottom oven, whether it's on its own or not, will not work: no light, no fan, no heat.
The top oven (not fan assissted) works fine on it's own.
The grill elements both work fine.
The hobs all work fine.

When connecting the grill and top oven together now (they share a blue wire) I get no problem where as before there was the bright flash/pop and the circuit tripped. But I still can't use the top oven because in my ferreting around at the back in order to check the top oven element I had to remove part of the insulation material which just disintegrated between my fingers. Can you suggest where I can buy this stuff afresh as my internet searches have turned up nothing.

Looking for a local repair shop now to see if they have ideas and possibly a call-out service to try and get this sorted if I can't do it myself. I'll be properly annoyed if I can't get to the bottom of it though. But there is a lot of wiring going on round the back and under the cook top. I wish electrolux did a wiring diagram. If I can fix motorbike electrics I don't see why I shouldn't be able to fathom this and get the appropriate parts ordered. Chalk and cheese?

 

[ban-all-sheds]

Cable to the cooker is 6mm2 and is about 15m from the consumer unit.

Why was a 6mm² cable on a 16A breaker?

How did you arrive at the determination that it's 6mm²?

 

[NathanM1]

6mm2 cable on the 16A breaker since I moved in 4 years ago. The house was apparently certified as electrically safe or whatever it is when the new consumer unit was installed as a condition of sale. In doing some reading online when I was about to buy a new breaker it became apparent that 16A wasn't correct. Then I landed on the following sum from a couple of sources (I think DIYNOT was one) to arrive at 20A for the 10.6KW cooker:
10600/240 = 44.1A
44.1 - 10 = 34.1
34.1/3.3 = 10.35
10.35+10 = 20.35A breaker rating

There is no regular socket for the cooker, the 6mm cable is for the cooker alone.

 

[NathanM1]

oh, sorry, arrived at the cable being 6mm2 by refering to a chart here
//www.diynot.com/forums/viewtopic.php?p=108451#108451

image of the damage here:

 

[flameport]

That burn mark is not caused by the electrical fault - it's where cooking fumes have exited around the element terminals, probably due to a seal missing. That is also what has blackened the insulation on the wires.

As the elements appear to be intact from your earlier testing, it is more likely to be the wiring between the elements and the controls, or one of the controls itself.

 

[NathanM1]

Thanks flameport, that settles my mind a bit w.r.t. the burn mark and why I figured that the wires weren't damaged though they were appearing charred. I've put it all back together for the night now so we can get dinner sorted, but I'll see if I can spot evidence of a missing/defective seal tomorrow. The inulation material is already knackered at the back of the top oven now anyway so I won't be fiddling around trying not to disturb anything. Do you know where I can turn for new insulation material? It's like loft lagging though doubtless made for the high temperature environment. So I guess I need to try and spot the problem around the switches on top next then.

 

[bernardgreen]

Is that connector strip at the bottom left of the photo just hanging loose on the wires without being fixed the the frame work ?

Are the blue and brown wires coming up to that connector strip the "6mm" incomiing power cable. ?? Because to me they do not look like 6mm cable

Some ovens are wire internally for 3 phase supply but can be run on a single phase by a bridging link that connects the 3 "internal" phases to the one incoming phase. If that bridging link is loose or missing one or two thirds of the items ( elements, fan etc ) will not work.

 

[NathanM1]

Evening Bernard, the connector block is only loose like that because it has a tendency to come away from it's home when the back is removed, it clips neatly back into place though. The cable I think is 6mm from the CU terminates behind the cooker on one of those round, brown junction boxes. The cooker then has about 1.5m of its own cable that connects up.
I've now re-insulated the top oven at the rear after checking it's element was in tact, and it works (no idea why) along with the 4 hob and 2 grill elements, so, while I figure out how to test switches/thermostats under the hotplate I'm going to live without the bottom, fan assisted oven. I can't afford a new cooker and I can't afford day rates/time off work for someone to come looking at it, so I'm going to have to put up with it. Bit of a bummer really.