Is an RCBO an adequate replacement for an isolator?

In article , Andy Wade Tue, 17 Nov 2009 01:17:19 writes

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.

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.

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.

In article , John Rumm Wed, 18 Nov 2009 18:44:29 writes

I get it. Thank you.

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

In article , Doctor Drivel Sat, 21 Nov 2009 15:38:35 writes

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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Owain

Normality is putting in a bulb. Irrespective if the situation arises the flex can act as a resistance heater.

The bulb holder will limit the amount of current you can take. It's more likely to burn out than the flex.

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.

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.

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.

In message , "Dave Plowman (News)" writes

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

In article , Steve Firth Mon, 23 Nov 2009 00:31:42 writes

I remember it well.... though we ran it off the wall socket of course.

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.