Oven connection

[johnny2007]

Hi everyone,
I came across the following wiring in a kitchen and the owner said that he doesn't want an electrician around before I fit the new kitchen as everything has always worked fine and I would like your opinion.
From cooker switch above worktop 6mm cable to cooker outlet plate.
From there hob is wired with 6mm cable and From the same cooker outlet plate there's a 2.5mm twin & earth cable going 4 inches down to a 13A rcd socket where the oven is plugged in with its original 13A plug.
It's this ok?
Also the rcd socket is cracked and the owner wants to change it to a standard one (I'm not touching it, he will do it himself) because he said the oven plug is fused.
I would appreciate your expert opinion.
Thanks

 

[ericmark]

I think we have a major problem with kitchens in general and in the main it is down to RCD protection and the 2kW limit for fixed appliances. To follow the regulations to the letter is hard and one does some times need to bend the rules.

But a RCD offers a very different protection to a fuse and clearly the guy has no idea.

The description you give does not seem to break any rules. One is allowed up to 3 meters of cable if protected for likely damage before the fuse so coming of a cooker supply to a 13A socket with 2.5mm is OK as fuse in the plug.

As to dedicated circuits supplying more than one item this could be seen as wrong but then double connection units are sold so oven and hob on same supply so clearly this is a common practice.

My daughter has a RCD socket by the back door and if this was to be damaged I would not hesitate from replacing it with a standard socket. After the RCD socket was fitted the consumer unit was changed so all sockets now RCD protected. So one does need to be careful with statements it could be an A1 installation it could also be a death trap can't tell from what you say.

 

[johnny2007]

Well, I try to simplify.
The cooker switch is on a separate 32A rcd from the main, all wired with 6mm t&e, so no extensions of any kind from main fuse board to cooker switch.
Then, from this cooker switch a 6mm t&e cable powers the cooker outlet plate.
And here it splits: one 6mm t&e cable powers the hob and a 2.5mm t&e powers the 13A rcd socket where the oven is plugged in.
Is this acceptable?
Second question is: can the 13A rcd socket be replaced with a standard socket to plug the oven in? I mean, would it be safe?
Thanks

 

[JohnD]

The cooker switch is on a separate 32A rcd from the main

do you know the difference between an RCD and an MCB?

 

[johnny2007]

In principle yes, but I'm not an electrician that's why I don't touch wires.
You probably pointing out that I mentioned the 32A consumer unit 'rcd'.
It should read mcb instead.
Sorry for the confusion.

 

[ericmark]

Nothing of what you report seems wrong as it stands.
However general rule is you leave things the same of safer than when starting so removing the RCD protection on sockets only permitted if RCD fitted else where.

 

[johnny2007]

Nothing of what you report seems wrong as it stands.
However general rule is you leave things the same of safer than when starting so removing the RCD protection on sockets only permitted if RCD fitted else where.

Ok, thanks.
I will try to convince the landlord to replace the rcd socket with one of the same, even if he's convinced that a standard socket will do because oven plug is fused.

 

[EFLImpudence]

Whilst people can install safety device if they wish, there is no need for an rcd socket for and behind an oven.

Dedicated does not mean individual.

 

[winston1]

I think we have a major problem with kitchens in general and in the main it is down to RCD protection and the 2kW limit for fixed appliances.

There is no such limit, it's a vague guidance and a very silly one at that considering such appliances always come with a fitted BS1362 plug.

 

[ldoodle]

Is the 2.5mm cable from the cooker outlet plate to the 13a RCD socket not under-rated?

32a MCB > 6mm > 32/45a cooker switch > 6mm > cooker outlet plate > 6mm to hob & 2.5mm to socket

? As it's before the 13a RCD socket, shouldn't it also be 6mm as there's nothing prior to de-rate to something 2.5mm is suitable for?

 

[EFLImpudence]

Is the 2.5mm cable from the cooker outlet plate to the 13a RCD socket not under-rated?

Under-rated for what?
Because of the plug it will not have to carry more than 13A (plus fusing factor).

It should be rated for fault current (but it will be).

32a MCB > 6mm > 32/45a cooker switch > 6mm > cooker outlet plate > 6mm to hob & 2.5mm to socket

? As it's before the 13a RCD socket, shouldn't it also be 6mm as there's nothing prior to de-rate to something 2.5mm is suitable for?

Read 433 and 434 regarding omission of overload (which does not apply because of the 13A fuse but this could be omitted) and positioning of protective device.

 

[ldoodle]

Under-rated for what?

A 32a MCB?

In other words, what would happen if I ran some 2.5mm from a 32a MCB to an FCU, then from the FCU to an appliance.

The 2.5mm between the MCB and FCU is not appropriate, right?

 

[EFLImpudence]

Under-rated for what?

A 32a MCB?

The 32A MCB will protect the cable from fault current should a short occur before the plug fuse subject to the CPC being able to withstand this.

2.5 has a max CCC of 27A but any current flowing through it cannot exceed 13A because of the plug fuse.
Actually the plug fuse is not really required because the oven cannot cause an overload ( obviously rated <13A).

You are only thinking the 2.5 cable should have protection from that which cannot happen.

 

[ldoodle]

2.5 has a max CCC of 27A but any current flowing through it cannot exceed 13A because of the plug fuse.

Even though the plug fuse is after the 2.5mm cable? The way I imagine this circuit is the 2.5mm cable has nothing before it down-rating the 32a circuit. In other words it is being 'fed' by a 32a MCB.

MCB's aren't self negotiating, are they? So take a simple circuit of MCB > cable > FCU > appliance, even though you might have a 3a Fuse in the FCU, the circuit is delivering 32a up to the FCU? After that it's 3a so can use 1mm cable.

Not sure if you saw my edit before posting?

 

[JohnW2]

MCB's aren't self negotiating, are they? So take a simple circuit of MCB > cable > FCU > appliance, even though you might have a 3a Fuse in the FCU, the circuit is delivering 32a up to the FCU? After that it's 3a so can use 1mm cable.

Where do you think the other 29A would be going? If it is a very simple circuit such as you describe, if the maximum current that can flow downstream of the FCU is (say) 3A, then that is the maximum current that can flow anywhere in the circuit, including upstream of the FCU.

As has been implied, I think you are probably confusing 'fault' and 'overload'. In the above example, the cable can't be overloaded, since the maximum current that can flow through it, in the absence of a 'fault', is 3A (or whatever). As far as 'faults' are concerned (e.g. shorts between conductors, because someone has put a nail through the cable), the regs only consider faults of negligible impedance (which is probably what most real-world faults approximate to), hence fault currents usually of hundreds of amps - plenty to make the 32A MCB operate.

Kind Regards, John