Double oven supply -fuse blown, but only once!

[tpt]

Hi All,
Had a new kitchen about 8 weeks ago, part of that was a double bosch oven on the opposite site of the kitchen. Spark extended the existing 6mm dedicated cable by 7m (run was 6-7m already).

Tiler unscrewed the isolator and left it hanging loose on the screws - been like that for 2 weeks as we were intending to swap the sockets etc for brushed chrome ones. Oven has been working happily during all this time.

Anyway - today, sockets have been swapped for b\chrome ones, including the isolator. i got some longer screws and screwed the dangling sockets back to the wall, checked everything was working and forgot about it all.

came to use the oven, and it blew the fuse (oldish style CU - RCD but no MCBs - due for replacement in new year). Fuse is a 30amp and hasnt been changed since moving from a single oven to a bigger double one (as we are soon to change the whole CU). replaced with a new 30amp fuse and its working fine again.

so.....is the fuse too small and we have just been lucky it held so far, or could i have trapped\split a wire when screwing faceplate back? logic tells me the wire is damaged it would blow again and again and again - as soon as the isolator is switched on (as oven has a clock so is using power all the time)

oven is http://www.diy-kitchens.com/shopdisplayproducts.asp?id=5472&cat=BOSCH - Double Ovens

 

[PrenticeBoyofDerry]

The spec states 4.4kW.
So no reason why the fuse should break due to overload.
If you had trapped a wire causing the fuse to break straight away, you would expect that to happen again.
The best test to find this out, would be an insulation resistance test. This should have been made by your electrician and when the outlet plates are fixed. But seeing your tiler had removed it and then you have refitted, the reading could now be wrong.
But it could be that the fuse is old and time has took it's course.
Still get it checked out though!

 

[tpt]

Thanks for the reply.

its me that has replaced the faceplates (3 dbl sockets, light switches and the 45amp isolator switch). to be honest, i wasn't sure if this was notifiable, but as i'm just changing like for like (but prettier!) i didnt think it was - so i cant blame my spark.

 

[martinxxxxxx]

The spark should have fitted an RCD as this required for all new wiring.

Did he leave a certificate perchance?

Martin

 

[tpt]

hiya - the existing CU has an RCD. its about 10 yrs old.

The dedicated oven circuit is on the side not covered by the RCD. He only extended the existing 6mm cable, he didnt install it from scratch, there are no sockets connected to the 6mm, and the hob doesnt connect to it either, its just an oven connection with a 45amp isolator.

Yes i have a cert left by him.

He did suggest replacing the CU, and we were expecting to change it - hence the job in the new year.

 

[PrenticeBoyofDerry]

Thanks for the reply.

its me that has replaced the faceplates (3 dbl sockets, light switches and the 45amp isolator switch). to be honest, i wasn't sure if this was notifiable, but as i'm just changing like for like (but prettier!) i didnt think it was - so i cant blame my spark.

It's not notifiable, but you should run through the tests to be sure it is safe to energise the circuit.

 

[PrenticeBoyofDerry]

He only extended the existing 6mm cable, he didnt install it from scratch

How did he extend it, where is the the joint located?
It is still notifiable work, did you get the correct documentation after the alteration?

 

[tpt]

hi there. as far as i could see..

removed existing isolator - extended using 60amp junction box (which is behind the kitchen units in the service gap, fitted to the wall), extended the existing 6mm with 6mm - i saw the cable, and it has metal trunking type stuff over it - this was surface fit as the kitchen units are over it.

he has fitted a new isolator to the right of the new oven.
all this work i have paperwork for.

isolator - which is what i have just replaced with a b/steel one and screwed back with longer screws due to the tiling (double pole 45amp isolator). i've double/treble checked l/n/e connections, they are tight and correctly in the in\out side. - this bit i dit myself, and obviously have no paperwork for

 

[PrenticeBoyofDerry]

Do you not have an EIC (electrical installation cert) with schedules of inspection and tests? Issued by your electrician.

 

[tpt]

i have paperwork...not too sure of the content! he's niceic etc. just had a quick look and yes there is an EIC.

its been a few weeks since works work completed (apart from be doing the faceplates - today)

 

[Taylortwocities]

So,

lets get this straight


an electrician did the installation job and issued an installation certificate about 8 weeks ago.

all has been working just fine

until

YOU change the accessory fronts and screw them into place

then the fuse blows

and you are asking how it could happen.....

I think YOU know what the cause is. Why ask us?

 

[tpt]

YEP - exactly what im asking - my question revolved around two things is a 30amp fuse adequate for a 4.4kw oven (thanks prentice) and if i've trapped\damaged a wire wouldnt it just blow the fuse all the time?
it didnt blow straight away, it was working fine after my changes too, for half a day, and its working fine now after changing the fuse...

im not stupid, i know its likely to be connected to my changes and if you'd read my question you might even be able to be helpful....

 

[ban-all-sheds]

YEP - exactly what im asking - my question revolved around two things is a 30amp fuse adequate for a 4.4kw oven (thanks prentice) and if i've trapped\damaged a wire wouldnt it just blow the fuse all the time?

Maybe the fault blew itself away, and has left you with a damaged cable.

it didnt blow straight away, it was working fine after my changes too, for half a day,

No - it was working fine until you turned the oven on - I suspect the time interval was irrelevant.

im not stupid

Good - you'll be able to test the circuit then.

What are the readings for insulation resistance and continuity?

 

[tpt]

the oven\grill was used several times between the change of isolator and the fault.

but anyway, i had a good look today, took it back off and inspected all the cables. the live cable had been pushed back onto the earth connector when screwed back and the insulation had been damaged, the earth connection edge was like a razor blade! - just as ban all sheds says, the cable had been damaged and i guess this had stopped the contact.
i cut it back and reseated the cables - all fine now.

the reason i had a go in my last post was because i wasnt looking to blame the spark, i was looking for help in diagnosing the fault - i fully expected it to be something i had done...

thanks for your help, all of you who did.