I believe he is saying that if you are using a non fused plug, then the
onus is on you the designer to ensure that it is adequately protected.
"Protected" in this case being both from fault and overload.
That does not necessarily equate to the flex having to be able to carry
the full circuit load (in much the same was as the cable for a spur
from 32A ring does not need to be able to carry the full circuit load).
Indeed. It may be that you have a 6A circuit protected by a type B MCB,
and you have a plug in lamp that has provision for a single BC bulb. The
flex between lamp and plug may be 0.5mm^2 3A rated flex - which on the
surface at least would appear to be unprotected.
However you can make a good argument that this is not the case by
splitting the responsibility for fault and overload protection rather
than assuming that the main MCB does both. So the fault protection is
provided by the MCB (i.e. damage and short the flex, and enough fault
current will flow to open the MCB, but the let through energy will not
be enough to damage the flex (can be shown by application of the
adiabatic equation)). The overload protection is delegated to the design
of the lamp fitting. By virtue of having only one BC socket, there is
very little chance you will find anything you could plug into it that
would create a long term overload on the 3A flex.
The flex is still not protected from high current which could melt it as it
gets hot. He is assuming you can't put into the lamp holder anything that
can fail and draw high current to make the table lamp flex act as a
resistance heater. If the situation arises where the current is high
through the flex there is no protection and a fire situation arises. It can
have 32A before the mcb cuts out. That is simple and obvious
Simple, obvious, and daft really. The largest bulb you could
realistically get in there may draw 2A tops (and that is pushing it). If
it fails in some way and goes faulty, it will draw 10s or 100s of amps
which will open the MCB before any damage is done to the flex. The only
way you can sustain flex damage is by drawing a current well in excess
of the flex rating, but below the fault and overload trip thresholds of
I can't think of any way a bulb could realistically draw say 6A for
several minutes - where is it going to store the 1.4kWh worth of energy
In reality this is a non issue. The only way of sustaining an overload
through a BC socket is by using an old BC flying lead plug and hooking
it up to an inappropriate load.
You don't need to go back that far. I can remember my great aunt
plugging an electric heater, an iron and a 100W light bulb into a single
BC light fitting. The adapter that she used was fairly commonly
available in the early 60s. One BC plug to 3xBC sockets with one of the
sockets having a pull chain to turn it on/off.
I don't think the heater had a thermostat either. It looked like an old
paraffin stove but had a heating element that was a length of nichrome.
Not on a ceramic former like a barred electric fire. just a length that
looked like a spring strung between two ceramic blocks.
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
20A radial with many sockets;
32A radial with many sockets amongst other options.
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
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.
It isn't. You would never install (eg) a 15A round pin socket on a circuit
other than one fused at <A. 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.
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
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
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
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.
Yes - that's why I mentioned:
"a) The cooker is logically isolated by using the cooker switch, whether gas
But, yes, it is a general requirement that fixed appliances with concealed
sockets or hard wired in must have an accessible DP isolator nearby.
What I was trying to do is avoid having two different isolators for a high
power electric cooker and a low power gas cooker.
You have just answered a problem that has been nagging at the back of my
Recently one of my rings developed a short under a concrete slab so I
had to create a break in the ring.
I was wondering how to make it legal but then forgot about it.
So if I fuse each leg at 20A it should be OK.
In fact if I fuse both combined at 20A that should also be legal. (Not
sure if I still have a spare way in my old box)
Just got to go and find some of the old large brown Crabtree MCBs.
Anyone know off hand what I need to Google for?
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