Fuse rating required for fluorescent fittings?

[Robgers]

John I don't know if this is relevant and I'm quite happy to be corrected. Maybe the reason for the 6A mcb is to help prevent the lighting circuit being tapped into for the purpose of supplying sockets.

I only suggest this after seeing a friend wire an extension lead into a ceiling rose when his flat was being plastered. A few trips later he gave up on the idea.

 

[Taylortwocities]

You may also notice that light switches are usually rated 6X and 10X.

This relates to the maximum current that the switch contacts are rated at.

 

[JohnW2]

You may also notice that light switches are usually rated 6X and 10X.
This relates to the maximum current that the switch contacts are rated at.

Indeed. That was the very suggestion which EFLImpudence made, and I would think it is the most likely reason for the 5A/6A protection. However, as I pointed out to him, it rather undermines the assertion (which he also made) that the protective device is there only to protect the cable and not 'what is connected to the end of it'.

Kind Regards, John.

 

[JohnW2]

John I don't know if this is relevant and I'm quite happy to be corrected. Maybe the reason for the 6A mcb is to help prevent the lighting circuit being tapped into for the purpose of supplying sockets.

Yes, it would also have that effect, but I suppose that's just another way of saying that it is there 'to protect the cable' (and,even then, a 10A MCB would still be OK).

Kind Regards, John.

 

[JohnW2]

The ceiling rose is rated 5/6A so in the main lighting circuits are limited to 6A as somewhere there is likely a ceiling rose. .... etc.

Quite so. It seems very likely that the reason for the 6A devices is to protect things connected to the circuit (ceiling roses, switches etc.) and not just the cable. That is the whole point of this tangential discussion, since it flies in the face of the oft-seen assertion (such as we saw here yesterday) that the protective devices are there only to protect the cable, and not 'what is on the end of it'.

Kind Regards, John.

 

[EFLImpudence]

So how are you going to get out of that one, then? You appear to be saying that the rating of the protective device is being determined by 'what is on the end of the cable', not the cable itself which you suggested is the only thing that it is protecting?

Fair point, but I was stating why, possibly, 6A protection is afforded to lighting circuits but perhaps it need not be. After all what would happen to your lamps if the circuit was on a higher rated fuse? 6A is far to high for each lamp: even a 100W lamp is less than half an amp. Why aren't there FCUs before each lamp with 0.5A fuses?

However, switches and pendants are part of the circuit. Are the wires in pendants (and other light fittings) less than 1mm²? I shall have a look later. I concede they are unlikely to be overloaded but you wouldn't join a higher current wire with a 5A terminal block.

It has been stated before that a cooker on a 32A circuit consists of components of vastly different ratings with no further protection. None of the internal wiring remotely approaches 4mm² - and there is a 20W lamp.

 

[JohnW2]

So how are you going to get out of that one, then? You appear to be saying that the rating of the protective device is being determined by 'what is on the end of the cable', not the cable itself which you suggested is the only thing that it is protecting?

Fair point, but I was stating why, possibly, 6A protection is afforded to lighting circuits but perhaps it need not be.

Yes, you were, and it seemed a credible explanation. However, in terms of the point I was making, I don't think the reason/explanation really matters. The one point which is clear is that one does not need protection as low as 6A for 1mm² cable, let alone 1.5mm² - so, whatever the reason for the 6A protection (even if only habit/tradition/convention), it cannot really be for protection of the lighting circuit cable - although, as below, it might conceivably be to protect pendant cables.

After all what would happen to your lamps if the circuit was on a higher rated fuse? 6A is far to high for each lamp: even a 100W lamp is less than half an amp. Why aren't there FCUs before each lamp with 0.5A fuses?

I realise you're playing Devil's Advocate, but (apart from 'common sense') I suppose one answer to that question is that attempts to 'protect' a lamp with a small fuse would be futile and unnecessary; a lamp effectively acts as its own fuse, and does not present a fire risk which would be reduced by the presence of a 0.5A (or smaller) fuse!

However, switches and pendants are part of the circuit. Are the wires in pendants (and other light fittings) less than 1mm²? I shall have a look later. I concede they are unlikely to be overloaded but you wouldn't join a higher current wire with a 5A terminal block.

I thought that standard pendants used 0.75mm², or smaller, and I'm sure that I've seen wiring thinner than that inside some light fittings. I suppose this could be the answer to the 6A protection.

It has been stated before that a cooker on a 32A circuit consists of components of vastly different ratings with no further protection. None of the internal wiring remotely approaches 4mm² - and there is a 20W lamp.

That is very true - but to repeat the 'personal principle' I mentioned before, if one sizes the protection to be just large enough to sustain the load under normal operating conditions, one maximises the chances of the protective device operating in response to an internal fault before a fire arises (even if the current in question represents no risk to the final circuit cable). One can but assume/hope that the internal wiring within appliances is sized according to the 'normal' current carried by the wires in question - but almost anything can theoretically happen under fault conditions (depending on 'what ends up being connected to what').

I think that most of this comes down to common sense. Certainly in terms of my philosophy, I would not design a situation in which the rating of the protective device was the highest permissible in relation to the current-carrying capacity (CCC) of the cable if the situation was such that the actual current was never going to be anything like as great as that. With socket circuits one is generally forced to assume that the cables may be used right up to their CCC, but with dedicated circuits, it's often possible to use more sensibly tailored protection, far 'tighter' than would be needed to protect the cable alone.

Kind Regards, John.

 

[riveralt]

theorectical discussions are fine (and JohnW2 you are becoming the devils advocate) but in the real world - time is money.
Most of the red book's calculations are based on averages and seemingly with a bit added for extra safety!
MK says maximum load on ceiling rose is 6Amps so for lighting circuits it is 6 Amps - following manufacturer's instructions.
Designing the perfect circuit for every eventuality is fine - if you personally can control the environment for which you designed the circuit.

But this rarely happens.

 

[333rocky333]

Furthermore, if you really believe that protective devices exist only to protect the cables, why do we normally use a 6A MCB to protect a lighting circuit run in 1.5 mm² cable, which could happily cope with more than double that current?

Kind Regards, John.

Lighting accessories are mostly rated at 6 amp though

 

[JohnW2]

Furthermore, if you really believe that protective devices exist only to protect the cables, why do we normally use a 6A MCB to protect a lighting circuit run in 1.5 mm² cable, which could happily cope with more than double that current?

Lighting accessories are mostly rated at 6 amp though

Exactly. Some of the accessories are rated at 6A (and many at 10A), but clipped direct 1.5mm² T&E can carry 19.5A (and even 1mm² 15A) - so do you believe that the 6A MCB exists only to protect the final circuit cable and 'not what the cable is connected to'?

Kind Regards

 

[EFLImpudence]

I have looked at a pendant and it does look smaller than 1mm². I haven't measured it but assuming it is 0.75 then maximum CCC would be 12A but there is 6A imprinted on the rose - perhaps because of the connectors.

Excepting extractor fan manufacturers who seem to pick an arbitrary fuse rating for their products. They know it will, likely, be fitted on a 6A circuit so should make it to that rating - or fuse it themselves.

I think confusion arises due to manufacturer's instructions regarding stated fuse ratings.
For example an oven may come with requirement for a 13A fuse (ignoring foreign ones who state 16A because that is what they use), this is because it is supplied with a flex for which this is the maximum current that can be sustained without damage.
A cooker, on the other hand, is supplied without a cable. The manufacturer's instructions state that it should have a 32A supply. This is a minimum requirement because that is the current required for the cooker to operate properly. The actual fuse/circuit breaker can be selected dependent on the size of the cable. Admittedly if installing afresh you would choose a 32A device but if already installed with, say. 6mm² and 40A then there is no need to change it.

 

[333rocky333]

I would say the mcb is to protect the wiring mainly and the wiring accesories, like switches roses etc, but not neccesarily the fittings, commercial lights often have internal fuses i assume too allow for connection to larger mcb rated circuits or track/busbar systems etc.

 

[JohnW2]

theorectical discussions are fine (and JohnW2 you are becoming the devils advocate) but in the real world - time is money.
Most of the red book's calculations are based on averages and seemingly with a bit added for extra safety!
MK says maximum load on ceiling rose is 6Amps so for lighting circuits it is 6 Amps - following manufacturer's instructions.

I agree totally - so can we say goodbye to this theory that protective devices exist only to protect cable, not what the cable is connected to?

Designing the perfect circuit for every eventuality is fine - if you personally can control the environment for which you designed the circuit. But this rarely happens.

Indeed. In this thread and others I have made the point that 'proper design' is only possible with dedicated circuits supplying hard-wired loads.

I also agree that practicalities, pragmatism and expediency are all part of the equation - hence 'deemed-to-satisfy' provisions, rules of thumb and even just habit/convention, all of which save time and money. I'm not sure why, and don't really care why, 6A MCBs have become the norm for domestic lighting circuits, but it's obviously sensible that there is an established norm (even if a bit arbirary) that people can adopt 'without having to think'. Pragmatically, even if one knew for sure that the total load was way less than 6A, MCBs less than 6A are very hard to come by, so I wouldn't waste time even thinking about that. However, if I am connecting, say, a piece of continuously-running low-power electronic equipment, I would use a 1A fuse in an FCU or BS1363 plug, even though the cable supplying it did not need that degree of protection. Similarly, if for some reason I was running a dedicated circuit to a 3A load in 1.5mm ² cable, I would use a 6A MCB, not the 16A one which would be needed to protect the cable. Is any of that unreasonable?

Kind Regards, John.

 

[JohnW2]

I have looked at a pendant and it does look smaller than 1mm². I haven't measured it but assuming it is 0.75 then maximum CCC would be 12A but there is 6A imprinted on the rose - perhaps because of the connectors.

Yes, maybe.

Having been thinking about this, it would not surprise me at all if the reason for the 6A lighting circuits is simply historical and essentially arbitrary. 5A fuses for lighting circuits had become established for decades (in days when little engineering thought went into such decisions - so probably essentially arbitrary!) - so, when MCBs came along, they added on 20% on account of the different 'fusing factors' of fuses and MCBs and arrived at 6A.

I think confusion arises due to manufacturer's instructions regarding stated fuse ratings.
For example an oven may come with requirement for a 13A fuse (ignoring foreign ones who state 16A because that is what they use), this is because it is supplied with a flex for which this is the maximum current that can be sustained without damage.

Maybe, although 13A is sufficiently close to the consumption of the oven to make it difficult to distinquish between that and the cooker situation you go on to discuss...

A cooker, on the other hand, is supplied without a cable. The manufacturer's instructions state that it should have a 32A supply. This is a minimum requirement because that is the current required for the cooker to operate properly. The actual fuse/circuit breaker can be selected dependent on the size of the cable. Admittedly if installing afresh you would choose a 32A device but if already installed with, say. 6mm² and 40A then there is no need to change it.

I agree totally, but have I not read here recently of cookers for which the MI state a maximum rating of the protective device, rather than the minimum supply capacity?

Kind Regards, John.

 

[JohnW2]

I would say the mcb is to protect the wiring mainly and the wiring accesories, like switches roses etc, but not neccesarily the fittings, ....

That's fair enough. Certainly (IMO) not just the cable. Of course, the MCB will also protect the fittings (or whatever else is connected to the circuit) to some extent - the more so as the rating of the MCB reduces.

... the 'fittings'commercial lights often have internal fuses i assume too allow for connection to larger mcb rated circuits or track/busbar systems etc.

Indeed. In an ideal world, every piece of current-consuming equipment would contain a protective device with a rating tailored to its current consumption. In commercial and industrial situations, a fairly high proportion do, and even in the domestic situation quite a lot of things (e.g. electronic equipment) actually do (although they so rarely operate that we don't realise) and/or at least have thermal cutouts of one sort or another.

Kind Regards, John.