Help Please ?! - Oven and Hob electrics

[DeeBee]

Hi

After posting some questions about cable sizes on this forum a while ago, I have finally got around to wiring in my new cooker and hob.

To summarise:

Old set up was a slot in cooker / ceramic hob (one unit) with a rating of

10.2 / 11.1kw at 230/240 Volts. This has run for the past six years with 6mm cable and a 32 MCB.

New hob and oven rated at 5.4kw and 4.9kw respectively, so 10.3kw total load.. So get the new stuff up and running at the command of SWMBO, I lashed the wiring the two units with 6mm cable (could not get 4mm as suggested here) into one cooker outlet plate, all the cables fitted relatively easily, and then to the old slot in cooker isolator, then obviously back to the CU. The old isolator was hanging in the next room behind the kitchen wall. OK so this was a lash up, but no kids in the house and SWMBO was told not to touch !!!. This worked OK no problems, no tripped MCB etc.

Then yesterday, I had finished the tiled slash back etc and fitted the new

45A isolator, one with a single 13A socket in. So wired the oven and hob via the single cooker outlet plate as mentioned above to the new isolator, then put in a new run of 6mm back to the CU. The old cable was too short. When I turned the CU back on again there was a load deep electrical "pop" from the CU and the cooker 32A MCB and the split load RCD had tripped.

So the wiring now looks like this. 32A MCB - 6mm - 45A DP Isolator (new) -

6mm - single cooker outlet plate - then two 6mm cables to oven and hob.

I have not dared try again and have a lecky my builder knows coming tomorrow, but any other ideas folks. Is the above arrangement OK? Are the cable sizes really OK?

Any advice gratefully received.

BTW I have been doing domestic electric as DIY for 20 years now and I have never had this happen to me before, so I consider myself to be relatively competent and work very methodically

[Owain]

"DeeBee" wrote | Then yesterday, I had finished the tiled slash back etc and fitted | the new 45A isolator, one with a single 13A socket in. So wired | the oven and hob via the single cooker outlet plate as mentioned | above to the new isolator, then put in a new run of 6mm back to | the CU. The old cable was too short. When I turned the CU back on | again there was a load deep electrical "pop" from the | CU and the cooker 32A MCB and the split load RCD had tripped.

Have you connected the neutral to the wrong neutral bar i.e. if the cooker is on the RCD side you must connect its neutral to the RCD'd neutral bar.

Owain

[Mike Humphrey]

If that was the case, why would the MCB trip? The only way to trip both at once should be for much more than 32A to flow in the live, and *not* all return through the neutral. I can't see how that can happen other than a live-earth short - damaged insulation, loose wires or a wrong connection are all possible causes.

Mike

[Owain]

"Mike Humphrey" wrote | "Owain" wrote | > | the cooker 32A MCB and the split load RCD had tripped. | > Have you connected the neutral to the wrong neutral bar i.e. | > if the cooker is on the RCD side you must connect its neutral | > to the RCD'd neutral bar. | If that was the case, why would the MCB trip? The only way to trip | both at once should be for much more than 32A to flow in the live,

Oh yes, I didn't notice the bit about the MCB tripping too :-)

| and *not* all return through the neutral.

Yes.

| I can't see how that can happen other than a live-earth short - | damaged insulation, loose wires or a wrong connection are all | possible causes.

Either way, it reinforces the need for proper inspection and testing before energising circuits.

Owain

[Tim Mitchell]

In article , DeeBee writes

Check you haven't trapped the live conductor against the backbox in the isolator (or anywhere else it runs). I recently had a ring main socket where one of the knockouts in the rear of the backbox was slightly bent, the sharp edge cut through the insulation of the live cable when I fitted the socket plate and made a big bang when the power was turned back on.

[DeeBee]

Tim

You were right, the live had shorted onto the back box, but burnt through the live insulation and the cutout grommet. I have never had this happen before. Either I must have just knicked the insulation, or there was a shaft edge on the cutout. Still at least that has solved the mystery. The evidence was only visible when I removed the metal box.

Cheers

Duncan