I do not recall but if they did (quote maximum rating) would they, surely, not have to also quote a minimum - or would they both be the same?
Fuse rating required for fluorescent fittings?
- Thread starter crj10
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My recollection is also hazy, but I've got a suspicion that it may have been sort-of both - e.g. a statement that "this appliance must be protected by a 32A MCB" (rather than 'at least 32A' or 'no more than 32A'). I'll see if I can find it.
Kind Regards, John.
Hmm. This is all looking more controversial than I expected!
I think my view (with no formal qualifications as an electrician but a certain amount of experience surrounding the design of appliances) is that some parts of the load cannot be protected by a separate fuse, so have to be covered by the circuit's protection. Ceiling roses and light switches, for example. Or, in my fluorescent lighting scenario, what about the connector block inside the fitting, if wiring more lights beyond the first? There are plenty of circumstances in which some components within the load will be handling more current than the load consumes itself.
Even on a ring main, the fuse is surely protecting the sockets as well as the cabling?
For that matter, a lighting circuit's protection is also there for the benefit of the person who sticks their fingers in a bayonet socket, just as the protection in a ring main is for the idiot who leaves the back off a rewirable plug.
I think my view (with no formal qualifications as an electrician but a certain amount of experience surrounding the design of appliances) is that some parts of the load cannot be protected by a separate fuse, so have to be covered by the circuit's protection. Ceiling roses and light switches, for example. Or, in my fluorescent lighting scenario, what about the connector block inside the fitting, if wiring more lights beyond the first? There are plenty of circumstances in which some components within the load will be handling more current than the load consumes itself.
Even on a ring main, the fuse is surely protecting the sockets as well as the cabling?
For that matter, a lighting circuit's protection is also there for the benefit of the person who sticks their fingers in a bayonet socket, just as the protection in a ring main is for the idiot who leaves the back off a rewirable plug.
Quite so. I agree entirely. The only reason I jumped into this discussion was to challenge an appearance of the often-seen assertion that "protective devices are there only to protect the cable, not what's connected to the end of the cable".
As is implicit in what you go on to say, one cannot tell a protective device what it's protecting and what it's not protecting - it will offer some degree of protection to everything downstream (and, in reality, will provide a substantial degree of upstream protection as well). Whether the degree of protection is adequate for everything other than the cable is obviously a different matter - e.g. a 32A MCB on a ring final circuit would not afford 'full protection' for a 13A socket on that circuit, let alone thin wiring within whatever is plugged into a socket. Nevertheless, it obviously still affords some degree of protection to the socket and whatever is plugged into it.
I don't think there is actually any significant difference of opinion amongst those involved in this discussion as regards what is going on electrically. It's just really an (arguably 'pedantic') matter of that assertion (which irritates me a little every time I see it!).
Kind Regards, John.
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Having just re-looked at this with a clearer mind, I thought that the max CCC of 0.75mm flex was 6A, not 12A. Am I wrong? If I'm right, that would obviously put 'protecting the pendant' back in the frame as a contender for the reason for 6A protection.
Kind Regards, John.
Not really, a fuse won't protect in either case. Even with an RCD you could still end up hurt, especially if you stick your fingers into a BC lampholder.
Also with a clearer mind, I was probably wrong but, surely, it should be more than six.
Have a look here but note the value for 1mm². 2.5mm² flex is the same rating as solid core: Is this just very poor quality?
If correct then that would appear to be the answer.
Intuition would suggest so, but I've always been surprised by the low official ratings of the smaller flexible cables.
Indeed, and those figures are close to the 'chapter and verse' in Table 4F3A of the regs. I wonder if there is perhaps a built-in safety factor with small flex sizes to take in to account the damage that might result from frequent flexing?
Provided you believe the appendices of BS7671, it's certainly correct - so, indeed, it could account for the 6A MCB (although my bones still tell me that it probably simply results from 'thoughless history'!).
Kind Regards, John.
Not really but I don't think you are differentiating between the two things the fuse/circuit breaker does or rather the two ways it does it.
Firstly it protects the cable from overload. This is when too much equipment is placed on a circuit. If more current is drawn than the fuse rating (albeit there is quite a lot of leeway built in) then the fuse will melt and cut the power - this could take tens of minutes. This cannot happen to the wiring inside an appliance because no more current than that used by the appliance will flow through that wiring.
Take your scenario of one hundred 36W lamps (3600W) on a 6A circuit (max.load 1380W). If installed in a line on a radial circuit then the wire between the fuse and first lamp will be carrying 3600W - progressively diminishing until the wire between the penultimate and last lamps will be carrying only 36W. However the wires in all of the lamps will only be carrying 36W so none of these is carrying more than intended.
Secondly it protects the cable when there is a fault . This could be a short-circuit when the line conductor, all by itself, becomes disconnected and touches the neutral or the metal parts of the fitting - earth. This is extremely unlikely to happen. More likely is that you may touch the line connector and earth (or neutral connector) with your screwdriver. This will cause a large flash and burn your screwdriver and the touched connectors BUT the fuse will blow almost instantaneously before any of the wires have had chance to get hot enough to cause any damage to them.
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