Cooker socket

[bernardgreen]

The oven cable won't be overloaded because the oven is a fixed load.

But isnt that a but like saying no appliance needs a fuse? As it's a fixed load?

So none of the elements in this "fixed load" will ever develop a fault and take more than it's fixed design load. I do not think so ! !

 

[ban-all-sheds]

Manufacturers should not rely on circuit protection devices to protect their appliances.

 

[JohnW2]

Manufacturers should not rely on circuit protection devices to protect their appliances.

That's not what's being discussed. What's being discussed is the protection of the oven's (flexible) cable - protection which is reliant on CPDs. The line of the discussion started with:

If the circuit is, say 6mm on a 30 amp fuse and a 13amp oven is wired directly to this...shouldn't it have a 13 amp fuse? Ie a fcu. As it's unlikey the oven cable will draw 30 amps safely.

... If it's a 13A oven, its flexible cable probably won't be rated at 30A, yet (with the arrangement being discussed) is being protected by a 30 OPD. EFLI suggested that this is OK because:

The oven cable won't be overloaded because the oven is a fixed load.

we've been over this one before, and I think there are varying opinions. It is true that in the absence of a fault, a fixed load like an oven cannot take more than its design current (i.e. an 'overload') - which is obviously goint to be OK for its cable. As far as faults are concerned, BS7671 seems only to consider 'bolted shorts' (i.e. zero impedance L-N or L-E) in which case the oven's cable will probably be adequately protected by the required disconnection time of the CPDs.

What those arguments don't cater for are a 'modest fault', resulting in excessive current for an appreciable period of time or faults in the oven's cable itself. Are you happy for a (maybe) '13A cable' to be protected in such situations only by a 30A CPD?

Kind Regards, John.

 

[JohnW2]

So none of the elements in this "fixed load" will ever develop a fault and take more than it's fixed design load. I do not think so ! !

This is the issue - see what I've just written to BAS. BS7671 seems to only consider two things - 'overload' and 'zero impedance L-N or L-E fault'.

I have previously expressed the opinion that sub-zero-impedance faults (e.g. shorting of part of a motor winding), which could result in 'overload' with a fixed appliance, are far from impossible - but others seem to think that so unlikely that it can be ignored.

Kind Regards, John.

 

[ban-all-sheds]

Any fuse upstream of the appliance is there to protect cables.

It behoves manufacturers to make their appliances in a way which prevent them from drawing more current than the rating plate specifies.

 

[hulahoop09]

Okay so I am going to leave this discussion to it I think. Lol. All I know is that my new oven arrives today between 2-6pm and is going to be connected to my current set up and hope for the best. I dont have the cash to have it any other way so just hoping it really works!!!

Just not sure if the new oven will come with a cable or not as some appliances requiring hardwiring need wire and some come with!!!

 

[EFLImpudence]

Just not sure if the new oven will come with a cable or not as some appliances requiring hardwiring need wire and some come with!!!

If it doesn't - use 4mm² then all will be well, fuse or not.

 

[hulahoop09]

Thanks. I will have to try get some from somewhere. Was going to use cable off my old one which is 2.5mm (looks a bit thicker than a bog standard appliance/electrical cable) approx as that is what my old oven is using which is 2.4kw new is 2.3kw. Not sure if that will make a difference?

Thanks for all the advice on here, you have been a great help.

 

[xr4x4]

So if the cable attached to a 13amp appliance, for example..... is rated the same or above the cpd (circuit protection device?) then it doesn't need a separate fuse.

Is that what we are saying?

So if I wired all my household appliances with 2.5mm (on RFC for example) then they don't needs plug top fuses?

Just trying to get it straight in my head.

 

[EFLImpudence]

So if the cable attached to a 13amp appliance, for example..... is rated the same or above the cpd (circuit protection device?) then it doesn't need a separate fuse. Is that what we are saying?

Correct.

So if I wired all my household appliances with 2.5mm (on RFC for example) then they don't needs plug top fuses?

It wouldn't be very sensible for portable appliances nor can you get plugs without fuses.
If you wanted to screw your kettle base to the worktop you could hide the cable (come to think about it, not a bad idea).
Everyone seems to favour FCUs for fixed appliances (so larger cable - just DP switch).

Are we on to something here? Probably not.

 

[xr4x4]

Thanks for that.

I wasn't suggesting the latter, just trying to get my point across.

 

[JohnW2]

So if the cable attached to a 13amp appliance, for example..... is rated the same or above the cpd (circuit protection device?) then it doesn't need a separate fuse. Is that what we are saying?

Correct.

So if I wired all my household appliances with 2.5mm (on RFC for example) then they don't needs plug top fuses?

.... Are we on to something here? Probably not.

I'm not so sure - maybe we are 'on to something'!

A 2.5mm² ring final circuit is obviously a somewhat strange animal that has been 'allowed' by the regs, in as much as even the fixed wiring of the the ring itself is rated below the In of the circuit's OPD; I would imagine that a 32A load concentrated near one end of the ring could bring the current of one leg of the ring above the CCC of the cable.

However, once we have a spur off the ring, we are expected to limit the current through the cable of that spur to less that the 'full rating' of the circuit, either by having only one socket (into which fused plugs will be inserted) or by having a (maximum) 13A fuse in a FCU. I'm not convinced that a 2.5mm² fixed-wired spur connection (without fuse) to an appliance would be within the same spirit (with a length of 2.5mm² cable protected by nothing other than a 32A MCB), would it?

Kind Regards, John.

 

[EFLImpudence]

However, once we have a spur off the ring, we are expected to limit the current through the cable of that spur to less that the 'full rating' of the circuit, either by having only one socket (into which fused plugs will be inserted)

That would be the same, probably/definitely better, if limited to one 13A appliance.
Isn't a double socket, in effect, two spurs from one unfused point?
xr4x4 was just asking if the plug fuse would be needed if the appliance cable were large enough and obviously it would not.

or by having a (maximum) 13A fuse in a FCU. I'm not convinced that a 2.5mm² fixed-wired spur connection (without fuse) to an appliance would be within the same spirit (with a length of 2.5mm² cable protected by nothing other than a 32A MCB), would it?

Maybe not in the specific regulation specifying what a RingFC is but electrically I can't see a problem. It is no different than an unfused spur - and on which is permitted that double socket.

 

[JohnW2]

However, once we have a spur off the ring, we are expected to limit the current through the cable of that spur to less that the 'full rating' of the circuit, either by having only one socket (into which fused plugs will be inserted)

That would be the same, probably/definitely better, if limited to one 13A appliance.
Isn't a double socket, in effect, two spurs from one unfused point?

Just a few months ago, I would definitely have agreed. However, after a few protracted and 'robust' discussions here, I have learned that a fairly high proportion of people, maybe a majority, believe that a double socket is 'rated at' 13A total maximum. In this context, I suppose that the regs are consistent with that. Since they require that a spur supplyig more than one socket be protected by a 13A (max) fuse, if they had considered that a double socket could/should be loaded beyond 13A (whether to 20A, 26A or whatever), they presumably would have limited an unfused spur to one single socket.

xr4x4 was just asking if the plug fuse would be needed if the appliance cable were large enough and obviously it would not.

Yes, but 'large enough' for what? If we're talking at a 32A RFC and the appliance cable had a CCC of at least 32A, then your statement is obviously true. However, I think xr4x4 was talking about a 2.5mm² cable with a 32A RFC, in which case I would suggest that it's far from 'obvious' that protection other than the 32A MCB would not be required.

or by having a (maximum) 13A fuse in a FCU. I'm not convinced that a 2.5mm² fixed-wired spur connection (without fuse) to an appliance would be within the same spirit (with a length of 2.5mm² cable protected by nothing other than a 32A MCB), would it?

Maybe not in the specific regulation specifying what a RingFC is but electrically I can't see a problem. It is no different than an unfused spur - and on which is permitted that double socket.

As above, and despite my historical views on the topic, I suspect that we may have to think of a double socket as being a '13A max' load - in which case things would be 'different'.

Kind Regards, John.

 

[EFLImpudence]

However, once we have a spur off the ring, we are expected to limit the current through the cable of that spur to less that the 'full rating' of the circuit, either by having only one socket (into which fused plugs will be inserted)

That would be the same, probably/definitely better, if limited to one 13A appliance.
Isn't a double socket, in effect, two spurs from one unfused point?

Just a few months ago, I would definitely have agreed. However, after a few protracted and 'robust' discussions here, I have learned that a fairly high proportion of people, maybe a majority, believe that a double socket is 'rated at' 13A total maximum. In this context, I suppose that the regs are consistent with that. Since they require that a spur supplyig more than one socket be protected by a 13A (max) fuse, if they had considered that a double socket could/should be loaded beyond 13A (whether to 20A, 26A or whatever), they presumably would have limited an unfused spur to one single socket.

Yes, but an unfused spur to a double socket can be loaded to 26A by anyone who doesn't know this. Therefore the regulation is not very sensible allowing the possibility of socket overload so easily.
However, we weren't discussing sockets but whether if appliance cables were larger would we need a fuse in the plug.
This would make no difference to the socket as the load would be the same, albeit one kettle or two.

xr4x4 was just asking if the plug fuse would be needed if the appliance cable were large enough and obviously it would not.

Yes, but 'large enough' for what? If we're talking at a 32A RFC and the appliance cable had a CCC of at least 32A, then your statement is obviously true. However, I think xr4x4 was talking about a 2.5mm² cable with a 32A RFC, in which case I would suggest that it's far from 'obvious' that protection other than the 32A MCB would not be required.

Yes he and I were talking about 2.5mm² because this is allowed for a spur even directly from the MCB. I don't know why, apart from what you say above. It would surely be better and cost pennies more to insist on 4mm².

or by having a (maximum) 13A fuse in a FCU. I'm not convinced that a 2.5mm² fixed-wired spur connection (without fuse) to an appliance would be within the same spirit (with a length of 2.5mm² cable protected by nothing other than a 32A MCB), would it?

Maybe not in the specific regulation specifying what a RingFC is but electrically I can't see a problem. It is no different than an unfused spur - and on which is permitted that double socket.

As above, and despite my historical views on the topic, I suspect that we may have to think of a double socket as being a '13A max' load - in which case things would be 'different'.

We may, We do, but Mr. & Mrs. Public do not.

So our hypothetical hard wiring of a single 13A appliance would be safer and indeed no different than an under-worktop appliance so wired.