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Topic Title: Newcomer to IET forum
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Created On: 26 September 2017 12:07 PM
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 26 September 2017 12:07 PM
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StevieR

Posts: 4
Joined: 26 September 2017

Student of electrical regs wanting to join in with the forums and ask questions.
 26 September 2017 12:50 PM
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StevieR

Posts: 4
Joined: 26 September 2017

I may as well ask my first question here.

When removing an old oven I noticed that the cable connecting the oven to the cooker isolator switch was only 2.5mm2 with no fuse anywhere. The cooker circuit is on a 32A breaker so I thought the cable was too small for this and I'm thinking whoever installed it made a mistake here. The conductor size is enough for normal use of the oven but in the event of a fault the cable could over heat before the breaker trips.

The new oven I installed had a fixed 2.5mm2 cable with a plug and 13A fuse inside but it would've been tres difficile to get it to a socket outlet so I cut off the plug and connected it to a FCU with a 13A fuse inside which was then connected to the cooker isolator switch.

Did I need to do this or was the way the old oven was installed fine?
How likely is it that an oven or other appliance will develop a fault that makes it pull a few extra amps e.g. 31A for a prolonged period causing the 2.5 cable to over heat?
 26 September 2017 02:06 PM
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mapj1

Posts: 9346
Joined: 22 July 2004

Ah - you are not the first one to ask this.
If (only if ) the cooker is of a design that cannot overload, but will fail either open or short circuit - and simple heating elements arguably are like this, and any cooker with an internal fuse certainly is; then the 32 A breaker is not needed to handle overloads, only to handle dead shorts at the far end of the cable.
And that it almost certainly will, as a few hundred amps will pass for for the time for a flash and a bang to occur and you to say 'goodness gracious me' in your best Peter Sellers voice (or similar equivalent local phrase), and the MCB will do its thing.
So the 32A breaker and thin cable is fine, in most cases, but you need to think if the dangerous 'mild overload' case is credible, and if it happened, how bad could it be.
The other risk pushing you in the other way is that in normal operation a 13A fuse at 10A or so will run warm, and if the space behind is tight and airflow is bad, you may find it slowly toasts the FCU. Not very likely, but not totally unknown, immersion heater cupboards are a classic for this.
For what its worth I've got an oven (2.5kW) on a 32A breaker at home, but it is a delightfully simple model inside.

-------------------------
regards Mike
 26 September 2017 03:35 PM
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StevieR

Posts: 4
Joined: 26 September 2017

Thanks for your reply Mike. So the chances are the way the old oven was done was fine. The back of the new oven is very close to if not touching the FCU I installed. There I was thinking I was being clever and over the top safe lol. The trouble was there was no technicial installer information in the leaflets that came with the new oven so I didn't know if there was an internal fuse or not. I should've looked online for the info before I went ahead.
 26 September 2017 11:44 PM
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mapj1

Posts: 9346
Joined: 22 July 2004

Probably, but given the new one came with 13A fused plug, not bare ends, you have done no worse than the maker intended or expected .
The other feel better option some folk like, if it is only the oven on that circuit, is to exchange the breaker at the consumer unit, for one of the same make, but lower rating, and safely within the cable rating, such as a 16 or 20A for 2.5mmsq cable.
And as a PS, if you want more opinions than mine, you may want to change the title of the post, as most folk will skip over 'newcomer' or 'hi' if the can see it has already been answered. A title nearer the topic will attract attention by the right folk.

-------------------------
regards Mike
 27 September 2017 11:46 AM
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StevieR

Posts: 4
Joined: 26 September 2017

Yes I will next time. It was my first post and originally in the newcomer section and got moved here.

Thanks for your input Mike.
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