Is an RCBO an adequate replacement for an isolator?



Are you saying that the appliance flex must be able to carry 20/32A?

But then the flex is very light. I do not understand.
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Les Desser
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Needs to be able to 'blow the fuse' it sees quickly. With a ring main this would be the 13 amp one in the plug. With a radial using unfused plugs, the CU one.

Lighting circuits are usually protected at 6 amps.
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Les Desser wrote:

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.
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[....]
I get it. Thank you.
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Les Desser
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Wed, 18 Nov 2009 18:44:29 writes

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
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Point taken, but that is no worse than some one rigging up a double 13A extension socket from a ceiling light. One has to assume some level of normality.
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Sat, 21 Nov 2009 15:38:35 writes

Normality is putting in a bulb. Irrespective if the situation arises the flex can act as a resistance heater.
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The bulb holder will limit the amount of current you can take. It's more likely to burn out than the flex.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Doctor Drivel wrote:

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 the MCB.
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 each minute?
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.
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Perhaps that's what dribble does. I've seen pics dating from around WW2 of people doing the ironing etc with it plugged into the light fitting.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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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.
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I remember it well.... though we ran it off the wall socket of course.
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Les Desser
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Steve Firth wrote:

My parents had something similar but with one BC socket at the bottom and a couple of sockets for 2 pin 5A plugs on the sides.
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My Nan used to plug her vacuum cleaner into the light socket (when I was a kid visiting - later 1960's, maybe still into the '70s
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wibbled on Friday 13 November 2009 15:33

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.

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.

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 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
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Tim Watts

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On 13 Nov, 19:03, Tim W wrote:

... provided you remember to provide a DP isolating switch, accessible, within 2m of the cooker location, as well as the outlet behind the cooker.
Owain
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wibbled on Friday 13 November 2009 21:27

Yes - that's why I mentioned:
"a) The cooker is logically isolated by using the cooker switch, whether gas or electric;"
:-)
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.
Cheers
Tim
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You have just answered a problem that has been nagging at the back of my mind.
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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Les Desser
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Les Desser wrote:

Yup.
Also ok, assuming that 20A total is enough capacity for your requirement

eBay might also help
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John.

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That is where I got one a couple of years ago, but I no longer remember the Crabtree name/type. I suppose I can call Crabtree or whoever owns them now.
Its a dark brown unit about 4" high.
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Les Desser
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