Should i Install FCU's?

[mgsinx]

I am extending my ring main into a new extension, which is a kitchen. I am considering putting in an FCU for the socket that i will plug a dishwasher into (it will be below the worktop), but wondered whethere this was absoluelty essential to comply with the regs - can anyone help?

Thanks,
Tom

 

[RMS]

A means of isolation would be a good idea, what if the machine needed to be isolated for service or in the event of an emergancy?

 

[JohnD]

A good way of doing it is to fit a 20A DP switch with neon, above the worktop, feeding an unswitched 13A socket below.

The switch will be in plain view and adjacent to the appliance, so it is a good means of isolation (if the use of the switch is not obvious, affix a durable label saying "dishwasher" or whatever).

If you are using a 13A plug below, a switch is better than an FCU, because an FCU would mean there are two identical fuses for the appliance. The ideal number of fuses is 1.

 

[RMS]

A good way of doing it is to fit a 20A DP switch with neon, above the worktop, feeding an unswitched 13A socket below.

The switch will be in plain view and adjacent to the appliance, so it is a good means of isolation (if the use of the switch is not obvious, affix a durable label saying "dishwasher" or whatever).

If you are using a 13A plug below, a switch is better than an FCU, because an FCU would mean there are two identical fuses for the appliance. The ideal number of fuses is 1.

What is the problem with having two fuses? I find that it is more common that appliances in kitchens are fed via a FCU rather than a 20A DP switch. I find it strange that there would not be 13A plug on the appliance as surely this would invalidate any warranty on the machine and most plugs on appliances are fixed anyway.

 

[JohnD]

...What is the problem with having two fuses..?

Or, as I say, the idea number of fuses is 1

1 fuse gives you the protection you need.

Although fuses on appliances blow approximately never, if you have two...

Which one has blown? You may have to take out both to find out (double the work) very likely both have blown (double the work and double the cost).

You might not even realise that there are two fuses; check or replace one, not know why the appliance still doesn't work; more than double the time and work trying to trace what is in fact a simple cause.

And what benefit do you get from having more than one fuse? None.

 

[RMS]

Just curious as to why a double pole switch was better.

The benefit although not required would be that the cable from the switch to the unswitched socket in protected by the 13amp fuse. As i say though, its not required by BS7671, as it would be classed as a spur from the ring.

I don't think any good electrician would complain at this whilst fault finding. Its not the hardest of faults to find. If the fuse in the FCU had gone then it would save pulling the appliance out although it would still not be an issue.

 

[JohnD]

I just believe it's simpler and more rational to have 1 fuse, than to have more than 1.

 

[RMS]

I agree, its just i seem to find that other electricians and companys always go for FCU's for some reason.

 

[securespark]

Personally, I think this way is best:

A DP switched fused spur above, with a 15A Plug/Socket below.

Why? Because there is only one fuse, and that is accessible.

The only thing is you need to check with the appliance manufacturer it is OK to replace their plug top, but in my experience, this has never been a problem.

 

[mgsinx]

Hey, thanks guys i think i will go for the single socket spur with the switch, rather than double up on fuses.
Thanks for the advice.

 

[The Jeep]

Here's a thought though...(and I've been mulling over this one a lot in connection with a job I'm doing):

While I agree that doubling up on fuses is not ideal, if you do use a 13A FCU and then, say, a single unswitched socket for a washing machine or similar with a 13A plugtop fuse, then the single length of 2.5mm² T&E will remain protected if someone puts an adapter on the single socket with two appliances totalling, say, 30A. This load would not be enough to trip the main MCB, but would still exceed the cable rating (23A, method 3) and potentially cause overheating. Admitedly, this would be an unlikely thing to happen, but still...

Interestingly, a single non-fused spur is allowed to feed a double 13A socket, giving a maximum theoretical load of 26A, which slightly exceeds the cable rating mentioned above.

What d'you reckon, securespark?

 

[The Jeep]

..sorry, John D et al, too....

 

[JohnD]

A double socket is not actually safe to run two 13a appliances at the same time. I believe the design current is about 20A.

The original thinking (going back to the 1940's) was that a person with a double socket might plug in one heavy load (e.g. an electric fire or a washing machine) and use the other for a light load (e.g. a table lamp or a wireless set). It was not thought that anyone would have two washing machines in the same room, or two electric fires next to each other. At the time people had far fewer electric appliances than today, and almost none of them were as high as 10Amps load.

The thought processes are still fairly correct; except in a kitchen, where people now have a multiplicity of heavy loads (dishwasher, washing machine, tumble drier, combination microwave) that can be about 13A each. So for your single large undercounter appliances, one switch and one single socket are preferable. For your overcounter appliances, like the kettle and toaster which are only on for short periods, or the food processor and juicer, double sockets are fine, and the more the better.

I did see a lab report (might have been by MK) on a double socket with two continuous 13A loads on it; it overheated quickly, soon started to smell and became permanently damaged. Fortunately, a lot of heavy load appliances (e.g. many single ovens) are only 10A each, and have intermittent loads, so you can use two of them in a double socket without it melting.

 

[securespark]

I've had this out with manufacturers before. A 13A accessory is designed to be constantly loaded with 13A. However, I have spoken to technical at MK and Tenby, MK have continuously loaded theirs with 20A for several hours without visible deterioration, and Tenby have run a similar test up to 16A.

Jeep:
About your question, all the adaptors I have seen are fused to 13A anyway, so your scenario will not materialise.

John: not familiar with double sockets prior to 1950's!! Guess they were around, but have only ever seen singles from that era.

 

[JohnD]

You're probably right, 1940's was when the designs and principles of the Socket Ring were being worked up. Towards the end of the war, in preparation for the anticipated post-war reconstruction boom, the industry started thinking about how to replace the multiplicity of sizes and styles of plugs and sockets. I came across some of the papers from this time and found them very interesting.