Double Pole switches and FCUs for kitchen applicances

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As part of renovating my kitchen, I intend to have 2 applicances (washing machine and dishwasher) plugged in to separate single unswitched sockets under the work surface, with those sockets controlled by switches on the wall above the work surface.
The switches will be on a 30amp ring circuit with a spur running to below the work surface.
Looking at various old posts in google, people have recommended using Double Pole Switches or FCUs for this task.
My undertanding is that both of these are double pole switches, the difference being that the FCU has a fuse protecting the live wire on the spur. Is this correct?
If so, which should I use?
Finally, I assume that both of these switches have 6 terminals - L,N,E for the mains and L,N,E for the load. I then wire both ring cables into the mains part and the cable for the spur into the load part. Is this correct?
Thanks in advance
Will
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Yes.
You may use either, provided they run to a single accessory. If you run to multiple accessories, you must use the FCU.

Yes, although the earth terminals might be shared, or not marked for load and supply, in which case you wire to the most convenient earth terminal. I used 20A DP grid switches, where you can get 1-2 DP switches in a single box, or 3-4 in a double box. It gave a neater appearance than having 3 adjacent FCUs.
Christian.
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I would add that the above is true so long as the cable size does not reduce from that of the ring. If you reduce the cable size (eg to 1.5mm sq), then a fused-switch unit is needed.
John
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John Watson wrote:

I was under the impression that so long as the cable used could safely take the full 13A load then it was ok to reduce cable size on the spur. 1.5mm sq ought to be fine I would have thought.
The time you need the FCU is when the potential load from the socket(s) could potentially overrate the cable conecting back to the ring - e.g. you have two or more sockets on the spur.
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John.

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wrote in message?

Correct.
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The cable also has to be able to take the fault current (short circuit to neutral or earth) for the time it takes the 30A fuse/MCB to trip without overheating -- that's where a 1.5mm cable is likely to come unstuck (maybe literally;-) Also, it must still pass enough current to blow the fuse within 400ms (IIRC).
Unless you are prepared to sit down and do the calculations to prove that it's OK, stick with 2.5mm. And what are you saving anyway? peanuts.
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Andrew Gabriel

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Andrew Gabriel wrote:

Yup - quiet agree! I was only commenting that there is not actually a requirement that you must not reduce cable sizes on the spur. Must admit I tend to find very little use for 1.5mm sq cable!
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John.

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For a couple of meters or less, as you say, there's not much point skimping... but just to be pedantic, surely the fuse you have to worry about on a (single) socket is the 13A in the plug, not the 30/32 in the board? Now, a double...
Hmmm...
Hwyl!
M.
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And for longer lengths, you're likely to find the impedance is too high to meet the trip times.

If the short happens in the connections to the back of the socket... ?
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Martin Angove wrote:

I think the difficult thing with a reduced size cable on the spur is coping with a wiring fault (or damage) to the spur itself.
The fuse in the plug will protect the flex to the appliance, and in the case of a fault in the appliance it will also protect the spur wiring. However a short in the socket itself for example, would necessitate the spur wire being able to take enough fault current to blow the main fuse on the ring circuit.
So basically if you are confident the reduced size cable will perform well enough under fault conditions then you can use it. I suppose the argument for "skimping" is strongest when you have a contract to wire five hundred flats all to the same wiring design - the saving in cable costs may then be noticeable. From a DIY point of view its a wast of time IMHO.
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John.

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Yes.
Yes.
And Yes.
The fused switches are not really necessary if your using remote sockets as the plug on the appliance should be properly fused, but they are used if you intend to make connections into permanent flex outlets.
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Hi. Why would you need to us either a DP switch or a fuse to connect a socket to a ring? A double socket would need IIRC a 20A switch, and a single one... presumably 13A one. (Not certain bout those figures, but think so)
Regards, NT
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A 20 amp DP switch is needed because the socket is unswitched and takes a 13 amp load, but you don't get 13 amp DP switches off the shelf. A double socket take two 13 amp loads so would need one 30 amp DP Switch to control it remotely.
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Hi. ISTR being told a 20A rating was sufficient for feeding one double socket, since regs take into account a limited amount of diversity. 6kW draw is not expected often IOW. Is that not so?
Why DP?
Regards, NT
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Would you work on a remote unswitched socket if it still had a neutral attached ? I wouldn't. That's why we use Double Pole switches to isolate the socket from both live and neutral supply.
If you want to run a double gang socket, then you have to take into consideration that both sockets will be used at the same time, so you need the correct switch gear to control it. Diversity is only considered in the whole installation, where there is an unlikely event that all appliances and every socket will be on and used at the one time.
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The normal practice would be to switch off power at the CU, surely.

Sounds like good sense to me, yet I've been told that cable feeding one double socket only needs to be rated at 20A, not 30, so a single piece of 2.5T&E clipped direct will do. Is that not so?
Regards, NT
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If you have two appliances connecting to a two gang socket, and both are known to contain heating elements, then would you trust a single piece of 2.5 mm T&E PVC ?
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So do you wire all your spurs in 4mm, just to be safe? If you have this kind of (expected) loading then you need to think again about the design of your circuits. On the other hand, IIRC 2.5mmT&E clipped direct is rated at 27A...
Hwyl!
M.
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Only to a radial two gang socket, Yes. That way, at least I know that it is going to take the same sort of load as the rest of the ring, which is literally wired with 5mm cable. I don't pull teeth on the extra 1mm strand. For a single gang radial socket, then I'd use 2.5.
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Thinking of a ring that way will give you a very misleading impression of how it works.
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