Les Desser
wibbled on Friday 13 November 2009 15:33

I am following this thread with some interest (and amusement).

I am now confused.

I understood that a radial circuit to have a single outlet, fused
appropriately for the job.

It could, but it might be clearer to refer to that as a dedicated circuit
rather than a radial. This would commonly be the circuit feeding a storage
heater or immersion heater for example.

A standard radial 13A socket circuit (ie IEE definition of standard) may
include:

20A radial with many sockets;
32A radial with many sockets amongst other options.

I then understood that there was no need for a fuse in the appliance
plug.

That could be the case. eg I will be using a 45A RCBO to feed my heating
system (mostly gas when it exists). This will feed a panel consisting of:

6A MCB for boiler and pumps and control. I might drop that to 3A if I think
I can, then, without argument, I can dispense with the 3A fuse on the
boiler.

Then, 3 lots of:

16A MCB for each back up 3kW immersion heater. These will then not have a
switch or a fuse in the flex connection unit as there is no point. I hate
having random fuses in odd places. There are times it's necessary, like an
FCU feeding some bit of equipment from a ring, but for a properly designed
system like a heating system I'd rather dispense with them and bring all
protection into a single panel.

Now that I have seen mention of a 32A radial circuit with daisy-chain
sockets, how can it be considered safe to use un-fused plugs?

It isn't. You would never install (eg) a 15A round pin socket on a circuit
other than one fused at =16A. You can daisy chain 16A commandos (unfused)
on a 20A circuit which is a slight exception, but 20/16A isn't
disasterously out of sorts.

If such a daisy-chain is in fact used in real life, I do not see how it
differs from a ring - other than it will need heavier cable.

It does. That's why a 32A radial is usually impractical. In many real life
installations you might need 6mm2 cable due to derating. And the max
terminal capacity of a 13A socket is generally 1 x 6mm2, 2x 4mm2 or 3+ x
2.5mm2 (slightly manufacturer dependent, unless you use the BS as the
lowest denominator). So a 32A radial would be hard without using junction
boxes.

I will be using one 32A radial: I have a dedicated cooker circuit, 32A.
However, I will be using gas (but it would be stupid not to put conduit in
for an electric cooker). So I decided, after a little conflab, to stick a
13A socket in the 47mm backbox behind the cooker. This is good because:

a) The cooker is logically isolated by using the cooker switch, whether gas
or electric;

b) Don't need to install another socket behind the cooker and fused switched
spur point above -or- have the gas cooker plugging into a socket at worktop
height;

c) The gas cooker will have an electric fan oven, so that sheds some fixed
load from the ring;

d) Installing an electric cooker - change the 13A socket to a cooker outlet.
Job done...

cheers

--
Tim Watts

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