Fusing down

[JohnW2]

I suppose it is. So is it the switches and lampholders which may be damaged by this current even if the cable would not?

You're asking the wrong person, but I suppose that is what some people think/believe. Personally, I would lose no sleep whatsoever if I had, say, '6A rated' switches roses, pendants or whatever in a 16A lighting circuit.

I think one might struggle to come with reasons which sounded particularly 'advantageous', since it's generally a matter of convenience/laziness/cost (i.e. avoiding installing overload protection or fatter cable), mightn't one?

Well - if you like but if it complies with the omission of overload regulations it could also be considered advantageous.

Advantageous to whom, and for what reason? - so one didn't have to install an OPD or use fatter cable? As you know, I'm on your side as regards omission of overload protection in many circumstances, when fault protection is adequate (which, as I've just written to eric, it virtually always will be), but I'm not sure that I could claim that it offered any tangible 'advantages' (other than saving a little time, cost and maybe a bit of copper - none of which would be of much concern/interest to the regs!).

Kind Regards, John

 

[EFLImpudence]

At least, that's how I read the regs - do you disagree?

I do not disagree.

Thee three metres is only relevant where it is NOT protected against fault current but is installed in such a manner to reduce the risk of fault to a minimum etc. regarding minimum fire and danger risk.




Plus, I meant advantageous for the method of installation - not the circuit.

 

[JohnW2]

I do not disagree. ... Thee three metres is only relevant where it is NOT protected against fault current ...

Quite so - and, as I said, I would have thought that it would be very rare for the 'reduced CSA' cable not to have adequate fault protection.

Kind Regards, John

 

[ericmark]

As to 3 meter rule clearly it's where not protected up-stream. The question is does a 32A MCB protect a 2.5mm² cable.

I have only once and oddly in my dads house found a cable buried in the wall damaged by overload. It was a 2.5mm² spur from a 7/029in² spur near to a fire place which had been used for an extended time to supply a 4 bar electric fire.

The 7/029in² was fine it was only the last bit of 2.5mm² which showed signs of damage. Just the last meter.

But in real terms that last meter was protected by the 13A fuse in the plug and in all but that case the 30A fuse or 32A MCB has opened before cable has been damaged so although officially the 2.5mm² can be overloaded with a 32A MCB in real terms I think the MCB provides enough protection. There will be as above always an exception.

Lighting is an odd one as with tungsten bulbs the bulb its self had overload protection or at least it should have been included so switches and bulb holders were protected. As to the ceiling rose this is another story as it is a junction box feeding other lamps although in real terms it can take more than 6A in the same way as 2.5mm² can also take quite an overload because it feeds many other devices no just one lamp then one should really keep to the 6A limit.

But where I really see a problem is with a 45A cooker supply and a oven over the 3kW limit to use a FCU and from the twin cooker outlet to oven having 2.5mm² cable. With a 32A MCB it is still overloaded under fault conditions but unlikely the 2.5mm² will fail. But with a 45A MCB remembering as a fixed appliance it may not have RCD protection there is a good chance either the internal wiring in the oven or the 2.5mm² feed cable will fail before the MCB trips.

I see no great design problems in either having a 16A fuse built into ovens over 3kW or having a 16A FCU the blue 16A cartridge fuse as used in the old fuse boxes is not that big and it could be easy housed in a special FCU but to the best of my knowledge no such unit exists.

 

[JohnW2]

As to 3 meter rule clearly it's where not protected up-stream.

Do you mean downstream overload protection, which I was talking about, or are you talking about upstream (fault) protection - which, as I said, will nearly always be adequate.

It sounds as if I was not clear enough. There is a "3 metre rule", but, as I said, the one I know is in 433.2.2, and relates to the situation in which there is downstream overload protection - but, as I said, only in the (I think very rare) situation of the cable not having adequate fault protection.

The regulation which allows overload protection to be omitted (potentially completely) if it is considered unlikely that the load will create an overload current (AND if there is also adequate fault protection) (i.e. the reg which EFLI and others would like to sometimes invoke) is 433.3.1(ii), and that regulation says nothing about "3 metres" (indeed, nothing about cable length or installation method at all).

So, per my reading of the regs, if the characteristics of a load (or supply) are such than an overload current is considered unlikely, and if the cable has adequate fault protection (which I think it nearly always will have), then it seems that you can have a cable without 'adequate' overload protection (in relation to its CSA) which is as long as you like.

... unless, of course, there is some other "3 metre rule" which I have missed.

Kind Regards, John

 

[JohnW2]

But where I really see a problem is with a 45A cooker supply and a oven over the 3kW limit to use a FCU and from the twin cooker outlet to oven having 2.5mm² cable. With a 32A MCB it is still overloaded under fault conditions but unlikely the 2.5mm² will fail. But with a 45A MCB remembering as a fixed appliance it may not have RCD protection there is a good chance either the internal wiring in the oven or the 2.5mm² feed cable will fail before the MCB trips.

Even if present, an RCD would obviously not operate under overload (as opposed to L-E fault) conditions, and, as I've said, I would think (without doing the sums) that a B45 would probably give adequate fault protection to a 2.5mm² cable. The important point to understand is that those who seek to invoke 433.3.1(ii) in relation to an oven or cooker do so on the basis that they do not believe it is possible/likely for such an appliance to create an overload (as opposed to fault) current.

That's how I understand the situation, anyway.

Kind Regards, John

 

[JohnW2]

I see no great design problems in either having a 16A fuse built into ovens over 3kW ...

True, but if you were one of those people who doesn't believe that an oven is ever going to ('unlikley to', per regs) create an overload, and given that there is always going to be a requirement for upstream fault protection, anyway, I imagine you might ask 'what's the point?'. ... we are, after all, generally only too quick to point out to people that 'there is no point' in adding extra protection to a circuit that is already adequately protected (e.g. RCDs or 20A MCBs at both ends of an outhouse supply, or an FCU feeding a socket for an appliance with a fused plug).

Kind Regards, John