Current ratings of accessories

[JohnW2]

We know (or assume!) that the tabulated maximum current-carrying-capacities of cables we work with have been compiled with a built-in allowance for the characteristics of OPDs. For example, a tabulated maximum CCC of, say, 10A appears to indicate that a current of 14.5A for about an hour (which is what a B10 MCB could allow to flow) would not be harmful to the cable.

However, is the same true of accessories? In other words, does a 10A 'rating' of an accessory mean that it is considered able to safely carry 14.5A for an hour? Although I don't know, I suspect not! Is this perhaps why, for example, most light switches are rated at 10A, even though the usual OPD is a B6 MCB (which should operate within about 10 minutes at 10A)?

Kind Regards, John

 

[Porque223]

The life of insulation halves for each 10C increase in temperature.

Temperature rise above ambient is roughly proportional to I^2 but see the Neher Mcgrath formula.

The 'life' of Romex is 50 years but no one has been able to tell me how the end-of-life is determined. Possibly this could be an insulation resistance of less than 1 megohm or an insulation resistance of less than half (or 10%?) of the original value.
I don't know the shelf life of Romex.

I imagine these current ratings are determined such that the consumer is not alarmed by the short life he/she got from their electrical hardware. And abrupt violent failures of equipment tend to upset people.

 

[stillp]

John, a rating is simply a value assigned by the manufacturer. There are, for most products, several different "ratings", hence there are several possible answers to your question. Go to your local library and you can usually browse British Standards online, which will probably inspire you to ask several more questions!

 

[JohnW2]

John, a rating is simply a value assigned by the manufacturer. There are, for most products, several different "ratings", hence there are several possible answers to your question.

I realise that, but these manufacturer-created 'ratings' seem to acquire a fair degree of 'power'. We are forever seeing discussions in this forum about whether an accessory with a certain 'current rating' is 'allowed' in a circuit protected by an OPD with a particular In. There have been examples in the last few days, which is what moved me to start this thread. It is therefore not just an academic question.

Kind Regards, John

 

[Porque223]

The NEC does not publish the premises and logic behind their rules and some of them violate the laws of physics and logic as near as I can tell.
I don't like this but if I were on an NEC Code Panel I might think differently, possibly due to peer pressure.
The IEEE helps the NEC with the physics but I've never correlated the IEEE findings with the end result.

My concern is that the end user gets 1$ worth of safety for each dollar spent. This may be true for circuit breakers but I doubt it's true for AFCIs.

What helps organizations like this is that the public has a 'zero risk bias'.

 

[Adam_151]

I have seen on a few occasions a 100A rated RCD melt and fail when subjected to continious overload of a level which did not cause the 100A fuse upstream to operate straight away

 

[stillp]

The NEC does not publish the premises and logic behind their rules and some of them violate the laws of physics and logic as near as I can tell.
I don't like this but if I were on an NEC Code Panel I might think differently, possibly due to peer pressure.
The IEEE helps the NEC with the physics but I've never correlated the IEEE findings with the end result.

My concern is that the end user gets 1$ worth of safety for each dollar spent. This may be true for circuit breakers but I doubt it's true for AFCIs.

What helps organizations like this is that the public has a 'zero risk bias'.

Porque, this is a UK forum, and references to the NEC, IEEE and AFCIs are irrelevant to us.

 

[ericmark]

I do think in domestic there is a problem with protection. But not sure if this is really a risk? I look at a hob as an example. An induction hob would really need semi-conductor fuses to protect it the B32 MCB would be far too slow. However it only needs protecting when it goes wrong so although the B32 will not stop further damage when it goes wrong it will protect the user from fire. When it goes wrong we replace the whole unit.

Items like ovens is another where reading the installation instructions there is often nothing to indicate the max size MCB which should feed it. I am sure with a standard 32A supply there is not a problem but with a 45A supply would the internal cabling take the current?

With lights 5A ceiling roses with 10A switches and 2A BA22d bulb holders is a common mix. In theory the bulb should have a built in fuse to allow for ionisation when the bulb blows so the 2A bulb holder is not really a problem but I have seen many times where 10 and 16 amp MCB's have been fitted with the electrician claiming BS7671 allows up to 16A which it does but not when 5A ceiling roses are used or 10A switches.

It was not that long ago that the house basher was considered the lowest branch of the electrical trade but that is no longer the case. However we still have some old school electricians working who have not moved with the times. I still find electricians who have not got test sets and don't test before adding new sockets and the like. I am not talking about DIY but how can we in the trade tell the DIY man he can't do the work within the requirements of the regulations when the guy he employs does exactly what he would have done. The worrying bit it's not the sole trader taking short cuts but the larger firms.

 

[ban-all-sheds]

Items like ovens is another where reading the installation instructions there is often nothing to indicate the max size MCB which should feed it. I am sure with a standard 32A supply there is not a problem but with a 45A supply would the internal cabling take the current?

See the other thread. IMO if a maker feels he needs his internal wiring to be protected then his equipment should provide that protection.

 

[JohnW2]

I have seen on a few occasions a 100A rated RCD melt and fail when subjected to continious overload of a level which did not cause the 100A fuse upstream to operate straight away

That moves the discussion a little bit away from 'accessories' in the sense that I meant, but it's another example of the same issue. Depending upon the type of fuse, a 100A fuse could allow ~200A to flow for a substantial period of time before operating. Hence the question - when a manufacturer says that an RCD is 'rated' at 100A, does this mean that it is safe when protected by a 100A fuse, or does it mean that 100A really is the maximum current it can safely cope with (in which case the maximum acceptable fuse protection may be ~50A)?

Worse, of course, if the overcurrent resulted from a fault, then it could possibly evolve (e.g. because of thermal effects in the load) into one involving an L-E leak - so the RCD could, theoretically, be called upon to actually break a current of up to 200A.

Kind Regards, John

 

[ricicle]

I would speculate by saying that accessories would have a similar threshold to wiring (within their relevant BS) in the fact that they could hold a certain small overload indefinitely, and then any increase after that will be inversely proportional to the time that it can take that overload without suffering permanant damage.
Perhaps a browse through BS1363 at bedtime John ?

 

[JohnW2]

I would speculate by saying that accessories would have a similar threshold to wiring (within their relevant BS) in the fact that they could hold a certain small overload indefinitely, and then any increase after that will be inversely proportional to the time that it can take that overload without suffering permanant damage.

You are really just repeating my OP. As you say (and I said) one can speculate what you say - but I was asking whether or not this speculation is correct.

However, as we've just seen in another thread, if that speculation is correct, it leads to another issue. Although people tend to think of the CCC tables in the regs as being 'the CCC' for the cables, that is only correct when the cables are protected by a Type B MCB (or something with similar characteristics) - if the protection is by, say a BS3036 fuse, then a substantial 'de-rating' of the CCC is necessary. So, if the same sort of margins had been built into 'ratings' for accessories, they would only apply to a certain type of OPD (probably again a Type B MCB) - i.e. a switch 'rated' at 10A would only have a 'rating' of about 7A if it were protected by a BS3036 fuse.

Perhaps a browse through BS1363 at bedtime John ?

I suspect it wouldn't help very much, since I don't think it deals very much with 'ratings' - which, as stillp has said, are essentially the invention of manufacturers. If you take the much-discussed issue of the 'rating' of a double socket, BS1363 gives detailed information about temperture rise tests at 20A total load, but that's about it. The accessories of more interest (like switches) do not come within the scope of BS1363.

Kind Regards, John

 

[SimonH2]

I look at a hob as an example. An induction hob would really need semi-conductor fuses to protect it the B32 MCB would be far too slow. However it only needs protecting when it goes wrong so although the B32 will not stop further damage when it goes wrong it will protect the user from fire. When it goes wrong we replace the whole unit.

As BAS says, if the internals of the device itself need protecting then either that protection should be part of the unit (a lot of equipment has a fuse at its inlet), or the MIs should specify what is needed. IMO in the case of something like a hob, then it's not really that practical to have specialist fuses in the supply, so if they are needed they should be part of the equipment.

This is the old "what is the fuse there to protect" discussion again. The simple answer bing that it's there to protect the wiring to the equipment.

 

[viewer]

Hope this isn't too much of a diversion, but it seems that there is an underlying issue here of how things are used. As mentioned, it is likely to be the switching load which is important to the life expectancy of a switch, however, how about the removal of a plug under load? What would one expect from the contacts? There is also the question of failed safety devices on equipment, e.g. most induction hobs will have a temperature sensing "probe" glued to the top surface; it can become unglued and so is not providing any over-temperature protection.
This will become more of a problem with "computer-controlled" systems; the IT side may have many safeguards built in, but it cannot possibly deal with the cooker covered by a flammable cloth.

 

[JohnW2]

This is the old "what is the fuse there to protect" discussion again. The simple answer bing that it's there to protect the wiring to the equipment.

It is, and, when I started this thread, it was not my intention that it should slide off in that direction!

However, there are some crossovers between that matter and the one I was discussing. It is obviously not sensible/practical for accessories such as switches, ceiling roses, JBs etc to come with their own 'internal protection' (fuses or whatever). That leads people to the conclusion that such things have to be protected by the circuit's CPD, hence all these discussions based on the fact that one cannot have a CPD of higher In than the 'rating' of any of the accessories connected to that circuit.

If one takes that latter view, then it leads directly to the question I asked - if, say, a switch is 'rated' at 10A, does that mean that it can only safely carry 10A (in which case the maximum permissible Type B MCB would be about 7A), or does it mean that it's OK when protected by a B10 (hence possibly 14.5A for an hour).

Kind Regards, John