Cooker switch shorted against metal back box

[OldKettle]

Hi,
I am in the process of wiring up a cooker switch and outlet.

After I wired the supply side to the socket (including earthing the back box) I activated the circuit at the CU, but there was a bang and flash and the fuse tripped.

Upon inspection there was blackening on the area around the screw on the live terminal and on the back box underneath the screw.

I think what I had stupidly done was to turn the power on, without either fixing the switch to the back box, or pulling it well away from the box. Instead it was loose in the back box, and I believe it was probably just in contact with the back box (or very close to it) and it simply arced from the screw to the box.

Does that sound right?

I have screwed the face plate in properly now, and all seems fine.

 

[AndyPRK]

Humm. Are you sure you are capable of doing this job

 

[donrkebab]

Cooker switches need a 47mm deep back box, how deep is yours?

 

[OldKettle]

Cooker switches need a 47mm deep back box, how deep is yours?

35

Does the cooker outlet also need a 47mm box?

I can switch out the current back box tomorrow, and I haven't done the outlet yet.

 

[dannyboi2003]

Cooker switches need a 47mm deep back box, how deep is yours?

Say's who exactly? A 35mm back box should be ample for a cooker circuit wired in 6mm T&E, I'd agree with 47mm if it was 10mm T&E.

 

[AndyPRK]

35 maybe ok. For both.

Ensure it’s ok before getting carried away

 

[JohnD]

it would be interesting to see what sort of cooker switch he's got. Some are very big.

A photo showing the wiring into the terminals would tell us if there is exposed metal.

 

[John D v2.0]

yes photo needed, but this does illustrate exactly why testing is prescribed by the regs, and not just to keep electricians in work.
Even if you're not going to do any compliant dead testing, at least use a multimeter on resistance setting to check for shorts before energising.

 

[donrkebab]

Say's who exactly?

The person you quoted

A 35mm back box should be ample for a cooker circuit wired in 6mm T&E,

"should be ample" and what do you do if it's not ample? Remove it and fit a 47mm box maybe? be a bit of a bugger if the wall has been tiled.

I'd agree with 47mm if it was 10mm T&E.

Neither you or I know the cable size.

I always find its easier to do what works, and ignore what "should "work, how many "should works" make a cock up?

 

[OldKettle]

Hi. The cable is 6mm.

I don't believe that there was any problem with the wiring itself, just that I stupidly energised it with the terminal screws very near the back box, rather than properly fixed. I'll post a picture when I redo. The only think I couldn't manage was to bend the earth wire so that I could connect it as a continuous wire to the back box earth and then the supply side earth terminal. I instead ran a separate earth wire from the back box to the supply side earth terminal (checked continuity with the earth on another circuit).

This is the switch. The Q&A section does say 47mm so I able going to redo it tomorrow.

https://www.screwfix.com/p/lap-1-gang-45a-dp-cooker-switch-white/49779

I was originally going to use a 47mm box, just to give me more room, but decided that 35mm was enough.

I'll put a 47mm on the back of the outlet too.

 

[rsgaz]

it simply arced from the screw to the box. Does that sound right?

stupidly energised it with the terminal screws very near the back box

That doesn't sound right at all. It would have to be so incredibly close. To the eye, effectively touching anyway.

Mains voltage is a 230v sine wave, so it peaks at 230*√2=325v. Dielectric breakdown of air is about 3 Megavolts per metre, so the gap works out at one tenth of a millimetre to initially create an arc from only 325 volts.

You must have a problem with the wiring. A loose strand maybe? Copper on show outside the terminals? (which gets exaggerated when you cram the wires into the box as the wire bends and the insulation moves)

 

[OldKettle]

That doesn't sound right at all. It would have to be so incredibly close. To the eye, effectively touching anyway.

Mains voltage is a 230v sine wave, so it peaks at 230*√2=325v. Dielectric breakdown of air is about 3 Megavolts per metre, so the gap works out at one tenth of a millimetre to initially create an arc from only 325 volts.

You must have a problem with the wiring. A loose strand maybe? Copper on show outside the terminals? (which gets exaggerated when you cram the wires into the box as the wire bends and the insulation moves)

Thank you.

It is entirely possible that the terminal screw was in contact with the case, as the face plate was pulled forward slightly and tilted. Certainly the blackening is around the terminal screw and the blackening on the case is directly below where the screw would have been (please see pic below).

I don't believe that there was any wire showing, but even if there was, it would have been much further away from the back box than the screws were. In any case, I will post a pic when I have rewired it with the new back box.

Thanks for the extra info.

 

[JohnD]

It seems that the copper earthwires have not been sleeved. They should have green-and-yellow plastic sleeving pushed over them.

There also seem to be knockout holes with no grommets

 

[OldKettle]

It seems that the copper earthwires have not been sleeved. They should have green-and-yellow plastic sleeving pushed over them.

There also seem to be knockout holes with no grommets

Thanks. Yes I removed the sleeving as I am dismantling the box. I knocked out the wrong hole the first time, so only put a grommet on the ones I was going to use.

 

[bernardgreen]

Is the back box deep enough for the switch ?

If a terminal of the switch can touch any part of the back box then the back box is too small.

EDIT just noticed the question has already been asked