cooker and hob circuit

eagleeye · Oct 1, 2011

I went to a customers (a friend) house today to inspect a faulty oven. He said that the mcb kept tripping so i disconnected the oven and turned circuit back on. It stayed on for about 30 seconds then tripped with quite a big bang. I did and insulation resistance test on the circuit and between all conductors i got the same reading of 0.01 meg ohm. A dead short somewhere ? I have had something like this before where the cable had a jb on it and was laying on a wet floor below floor boards. could this be the case of this fault ? Think so.on a little further inspection the 6mm t + e cable that is on a 45amp mcbis actually suppling a 5.5kW oven and a 5.5kW electirc hob. not good me thinks. I am now gonna renew circuit and put in 2. on for oven and 1 for hob. i can drill straight though the wall under stairs to outside and run along 8 metres and go back though the wall to double pole switches in kitchen. W Normally use 6mm t + e but was just looking on a website regarding cable capacties and its saying that 2.5 will take about 30 or so amps. this cant be right can it. I thought 2.5 was about 18 - 20 amp, 6mm 30amp and 10mm 50amp. below i have put the link to what it says on the web site. Let me know your thoughts

http://www.eland.co.uk/documents/SWA Cables - BS5467.pdf

page 4

telectrix · Oct 1, 2011

6mm clipped direct is 47A. your load 11kw is 11,000/230 = 48A. applying diversity, 10A + 13A = 23A, so 6mm is more than adequate for both ( add 5A if a socket outlet is incorporated. ) . the problem you have is a short circuit , either in the cabling or the appliance/s.

Engineer54 · Oct 1, 2011

eagleeye said:
I went to a customers (a friend) house today to inspect a faulty oven. He said that the mcb kept tripping so i disconnected the oven and turned circuit back on. It stayed on for about 30 seconds then tripped with quite a big bang. I did and insulation resistance test on the circuit and between all conductors i got the same reading of 0.01 meg ohm. A dead short somewhere ? I have had something like this before where the cable had a jb on it and was laying on a wet floor below floor boards. could this be the case of this fault ? Think so.on a little further inspection the 6mm t + e cable that is on a 45amp mcbis actually suppling a 5.5kW oven and a 5.5kW electirc hob. not good me thinks. I am now gonna renew circuit and put in 2. on for oven and 1 for hob. i can drill straight though the wall under stairs to outside and run along 8 metres and go back though the wall to double pole switches in kitchen. W Normally use 6mm t + e but was just looking on a website regarding cable capacties and its saying that 2.5 will take about 30 or so amps. this cant be right can it. I thought 2.5 was about 18 - 20 amp, 6mm 30amp and 10mm 50amp. below i have put the link to what it says on the web site. Let me know your thoughts

http://www.eland.co.uk/documents/SWA Cables - BS5467.pdf

page 4

Correct, the MCB should be rated at 32A or 40A!!

You really should look at past threads on this subject. You may find find that your over rating your cooker loads. 11KW is a tiny load when diversity is taken into account!!!

eagleeye · Oct 2, 2011

Thanks for the reply telectrix. yeah understand about appling diversity but i never really do this with oven and hob as i believe that quite often all the hob rings and both ovens could be on at the same time. I know its rare but i would rather be safe than sorry. Christmas time and parties when alot of cooking is going on ????

So really if im running armoured for 7 metres from CU to cooker isolator i could do it in 2.5mm 3 core armoured as regs say at the worst it can take 28 amp clipped direct. And with each appiance being rated at between 24 - 28 amps this should be fine ?? Dont seem right to me.

Also got to fit a new CU as nothing is protected by RCD (even sockets and lights outside). I much prefer to fit a board with seperate RCBO's for each circuit. Whats your opinion between this way and dual RCD boards. Apart from the work involved (which is not alot)

telectrix · Oct 2, 2011

you have to weigh up the cost of RCBOs against 2 RCDs and MCBs. generally a CU will come fully loaded with 2 RCDs and , say, 10 MCBs for around £60- £80. which is a lot less than buying 10 RCBOs. don't forget, with cooking appliances, even if everything is turned on that can be, all elements are thermostatically controlled so will be switching on and off automatically. the diversity formula for cookers has been in use for donkey's years and works perfectly well.

Engineer54 · Oct 2, 2011

eagleeye said:
Thanks for the reply telectrix. yeah understand about appling diversity but i never really do this with oven and hob as i believe that quite often all the hob rings and both ovens could be on at the same time. I know its rare but i would rather be safe than sorry. Christmas time and parties when alot of cooking is going on ????

So what your saying here, is that a system that has been proving itself for over 60 odd years for cooking loads up to 15KW (7/.044 / 6mm on a 30/32A fuse /breaker) should be ignored, because you know better!!

Think about it, if your own parents had an electric oven/cooker, or anyone else that you have known since childhood. Have you ever known or heard of, in all your years, of cooking an Xmas dinner ever tripping a breaker or fuse?? .....I personally haven't, and i'm pretty sure i've been around a lot longer than you.

Not only what telectrix has told you, about all element being thermostatically controlled, but there are elements that cannot be operated together at the same time on virtually all ovens. One example is the grill and main oven elements, so that's at least 1.5/2KW that your not going to see. ....Is it making a little more sense to you now??