kitchen wiring

You are having the same argument I have had several times. You risk upsetting some of the electricians and/or John if you continue.

You are like me, you think protection should always be at the start of what you are protecting not at the end. The regs appear to think its OK to put some of the protection at the end because nobody will ever get it wrong and if they do its their fault.

BTW the 13A fuse will allow >20A without blowing for a long time (hours/days) so it is possible to get 40A down a bit of 2.5 mm2 cable without doing any wiring mods if your house was wired prior to the ban on double socketed spurs.

A usual rule of thumb is 30 minutes for double the current on typical household fuses/breakers.

But equivalently, 40A through a "20A cable" does not destroy the cable instantly either. It heats the cable until at some point the cable's max operating temperature is exceeded (70C for most PVC) and the insulation starts to suffer degredation.

The fuse/breaker characteristics are designed to match or better the heating effect so that short term overloads are permitted but long term ones are cut before the protected components suffer damage.

What ban on double socket spurs? Is this in Amendment 1 which I do not have? Otherwise the 17th Fig 15A clearly allows double socket spurs.

Thank you. So many great replies; I may not be able to reply to them all.

At present each "switch" is connected to one single unswitched socket. I say switch in quotes because now that I have started all this controversy, I have had another look and not all of them are fused connection units, some are switches without fuses. Some look newer than the rest so I don't think a mixture was fitted originally. I guess it is just a case of what was to hand on the day. When I redecorate I may possibly change them all to match.

Thanks.

Well that would certainly save the hassle of a FCU; these must be a pain to wire as they have three cables in one box. But as they are already here, it would be even more hassle to remove them.

That said, like others, I thought that the switch was needed in case of fire or other emergency but as someone else has said, having the switch above the appliance probably is not very practical.

Thanks.

so the socket can be passed through and plugged in.

I suppose, playing devil's advocate, it is quicker to flip a switch than open a cupboard and reach inside. If I had brand new expensive cupboards, I don't know I would want to cut holes in them but I don't think there is a right or wrong answer, just personal choice.

I must have missed that one; what was it called?

They are only single sockets, though that said you can spur to a double. I couldn't see why there needed to be a fuse either. Many posts seem to suggest whoever did this used whatever they had at the time. I guess it is a circular thing: FCUs are cheaper because they are used more and they are used more because they are cheaper!

Thanks. I hadn't thought of that.

That's interesting. What do the regs require to have a switch: just fixed installations like cookers?

Do you generally fit switches, or does it depend what the customer asks for?

Thanks.

If the appliance is smoking etc, why not just switch it off on the front panel? That will isolate the problem in the vast majority of the cases.

As regards not cutting holes in new cupboards. how do you plumb in the sink? ;-)

Personally, I can't see the difference between changing a fuse in an FCU and plug. Both can be done without tools. (on many designs)

Not usually on trip, but they can suffer if closed onto a hard fault too many times.

He is not - quite...

Tim's assertion is basically sound - that a single socket fed from a 20A plate switch is safe. In reality its the design[1] that makes it safe - that there is also a 13A fuse downstream of it, is a bonus.

(noting that I don't accept your argument that unfused spurs are unsafe, so let's not do that again)

No need to be nasty ;-)

"Protection" is not one thing. In a circuit you need to consider fault protection separately from overload protection. They have different requirements, and effects, plus different causes.

Fault protection must always be at the circuit origin (and possibly again if there is a point of reduction in cable size). Overload protection may be at the origin, may be at some other place, or may be omitted altogether in some circumstances.

That appears to be your misunderstanding of the regulations and not what they actually say.

There is no ban on double socket spurs in the sense you mean. There may have been a time in the dim and distant past where two single sockets were allowed - raising the nominal maximum load from the 20A assumed for a double socket, to the 26A for a pair of singles.

However given this rule would have existed before the days of well insulated properties and building materials being in common use, they would have been a strong likelihood that they were being fed from a length of 2.5mm^2 T&E, installed using installation method C anyway, with its normal maximum rating of 27A.

How were you planning on introducing an overload upstream of the plug?

(you could introduce a fault upstream of it, but that is a different issue)

I normally use a 20A plate switch with neon for this application. Say something like:

I have read both yours and Tim's post on this one. Here is my view on it.

In conjunction with the rules that a 20A switch must only supply one single or one double socket when used as as part of an unfused spur then the 13A fuses in the plug are providing the overload protection.

The rule that states the 20A switch must only supply one single or one double socket for an unfused spur lays down the "law" and the 13A fuse(s) enforce the "law".

eg the 13A fuse stops you from using an electric cooker plugged into the socket controlled by the 20A switch. If you try then the 13A fuse in the plug will blow before the unfused spur cable is damaged due to overload.

Therefore I do believe that function of a 13A fuse is actually twofold

1. To protect the appliance
2. To ensure overload protection on a correctly installed unfused.

Cheers

But you are not a brain dead tenant demanding your landlords sends round an electrician.

But you're a member of uk.diy. Adam was talking about landlords and local authorities with tenants who are not diy minded.