Current ratings of accessories

[JohnW2]

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?....

Indeed - and that's one of the reasons why a one-size-fits-all 'rating' is probably never going to be very meaningful. It also works at both ends of the spectrum, since the switch which is virtually never operated, or the plug which is never removed from the socket may also introduce problems. In an industrial situation, certainly for safety-critical components, one will often find different 'ratings', different maintenance schedules and different predicted life-expectancies according to various different 'patterns of usage', but that's unlikely to happen with everyday electrical components. Indeed, even if it did, it wouldn't help that much, since (in domestic, and many commercial, situations) once the designer and electrician have done their work and gone away, there's no telling what sort of 'usage' the components will be subjected to. In an industrial environment, things will often be different, and more predicatable.

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'.

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

True, but your code people probably face the same tradeoffs as our code people. Cost vs. safety vs. profit for makers vs. what will the public go along with.

And if you don't use AFCIs, good. They seem prone to false alarms.

Who do your code people turn to when they have a problem that stumps them? Can you post links to your electrical safety assurance agencies?
Some of the stuff that is allowed in the US would not even be discussed in other countries.

 

[ban-all-sheds]

That's true - given how litigious a lot of your countrymen are, it beggars belief how unsafe your electrical standards are.

 

[Porque223]

That's true - given how litigious a lot of your countrymen are, it beggars belief how unsafe your electrical standards are.

With many of our agencies I suspect this \/

Regulatory capture occurs when a regulatory agency, created to act in the public interest, instead advances the commercial or special concerns of interest groups that dominate the industry or sector it is charged with regulating. Regulatory capture is a form of government failure, as it can act as an encouragement for firms to produce negative externalities. The agencies are called "captured agencies".

I only found out about this term after I was illegally forced out of my government job. I was too naive for too long.

 

[SimonH2]

Indeed, and it would have been better in hindsight if I'd written "to protect the circuit supplying the equipment" - where that circuit may include switches etc.
Which of course doesn't alter your question.

But I seem to recall discussions where people do think it's OK to have <things> in the circuit which are rated for less current than the CPD - on the basis that the downstream load will be limited in the abnormal current it can draw, and a fault is unlikely to draw enough current to overload the <thing> without drawing enough to trip the CPD quickly.

Examples of <things> that come to mind would include :
Light circuits fed from a 10A breaker. The argument goes that a light bulb is highly unlikely to draw significantly more than it's rated current without fusing quickly - so overload of the switches etc is unlikely. A fault on the other hand is unlikely to draw less that <some large figure> which will trip the breaker very quickly.
Shuko socket (or any of the various Euro national sockets) fed with a 16A breaker - but feeding equipment with a thin flex. The flex may be well under rated for 16A+, but a fault is highly unlikely to draw something that will melt the cable but not trip the breaker. If the equipment if properly designed (internal fusing), then it won't permit an overload to persist.

I can see the logic in these arguments, but as you point out, it seems to contradict "accepted wisdom" and the regs which generally require anything downstream of a fuse or breaker to be able to carry whatever that fuse/breaker will allow the circuit to take.

 

[bernardgreen]

The regs ( when followed ) do ( should ) prevent cables glowing cherry red when they ae overloaded by too much load on them. OK that is extreme. maybe not cherry red but hot enough to damage the insulation.

 

[stillp]

Indeed - and that's one of the reasons why a one-size-fits-all 'rating' is probably never going to be very meaningful.

Hence my earlier post John. There isn't just a single rating, which is why you need to read the product standards and ask a more precise question(s).

 

[JohnW2]

Indeed, and it would have been better in hindsight if I'd written "to protect the circuit supplying the equipment" - where that circuit may include switches etc. Which of course doesn't alter your question.

I'm not sure that would have satisfied everyone. Some seem to adamantly assert that the OPD is there only to protect the cable.

But I seem to recall discussions where people do think it's OK to have <things> in the circuit which are rated for less current than the CPD - on the basis that the downstream load will be limited in the abnormal current it can draw, and a fault is unlikely to draw enough current to overload the <thing> without drawing enough to trip the CPD quickly.

Indeed. In fact, one can attempt to apply that argument to a high proportion of hard-wired loads, arguing that they 'cannot cause an overload'. To a large extent it's probably true (the most obvious exceptions being electromechanical things - like motors which can jam/stall and relays/contactors which can fail to close properly). However, there are always going to some cases (even if 'freak' situations) in which it does happen, so I can't really see it as justifying the omission of 'adequate' protection against possible overload. Having said that, I agree that there are some loads (like 'lamp bulbs') where sub-fault 'overload' seems next-to-impossible. In any event, if they so wish, people can obviously argue that any equipment should have built-in overload protection.

Of course, one of the big 'holes' in the regs is that they don't seem to recognisie the existance of 'faults' of less than negligible impedence ('bolted faults'). Such faults can theoretically arise in any fault/load, even if very rarely.

In any event, as I've said, it's not really trying to discuss here. What I'm asking is what maximum current an accessory is deemed able to carry safely, and for how long - something which is not as obvious as one might hope from what is (maybe) written on the product and/or (maybe) in the documentation. I'll then leave it to others to argue about whether the accessory needs a CPD which is 'appropriately rated' to protect it from currents greater than that 'maximum'.

Kind Regards, John

 

[JohnW2]

Indeed - and that's one of the reasons why a one-size-fits-all 'rating' is probably never going to be very meaningful.

Hence my earlier post John. There isn't just a single rating, which is why you need to read the product standards and ask a more precise question(s).

If I ever find the time, I will, but I'm not sure that is necessarily the point - particularly given that, as you've said, the 'ratings' to which users are exposed are generally an invention of manufacturer's, not necessarily directly related to (or dictated by) a Standard or anything else. At least in terms of residential (and probably most commercial) installations, most practicising electricians (and certainly DIYers) are not going to go unting through libraies of Standards. So long as there are things being sold called (and labelled) '10A switches', '20A JBs' etc., people are going to want a to know what this actually means in terms of the normal/average installation - e.g. is it to be considered unsafe to subject a '10A switch' to more than 10A for an appreciable time (hence the need to protect it with, say, a B6 MCB) or, as is the case with cable 'ratings', does it mean that it's safe for it to be protected by a B10?

As things are, a lot of 'assuming' is going on. For example, give or take quibbling about the shortfall of 2A, people seem to accept that it's OK to have a '30A junction box' protected by a B32 MCB - thereby assuming that the 'rating' has been adjusted for the characteristics of a Type B MCB, in the same way that BS7671's cable CCC tables are 'adjusted'. Is that actually what the manufacturer's mean by their 'rating'?

This is neither hypothetical nor academic. We have people in this forum saying that it's 'not acceptable' to have accessories 'rated' at 6A on a lighting circuit protected by a B10 MCB. If they share my uncertainty about what the manufacturer's ratings mean, they might go on to question whether it's acceptable to have accessories 'rated' at 10A protected by a B10. I don't know the answers, but it's important that people know the answer (and without having to spend a morning in a library).

Kind Regards, John

 

[stillp]

No, they are not an 'invention' by manufacturers. The values are assigned by manufacturers, in accordance with the relevant standard. My point was that there are numerous ratings that can be assigned, with different meanings as well as different values, depending on the product and on the standard. If you ask about a specific rating for a specific accessory then it might be possible to provide an answer.
Of course the users of those accessories are not going to read through all the relevant product standards, which are written for manufacturers. They will just use the installation standards and MIs. However you seem to be trying to go further than that and understand some of the decisions made by the standards committees, but you need to be clearer about the exact question you're asking. For example, consider a "10a switch". What is the utilization category (nature of the load, frequency of operation, etc) for that rating? Is that installed, or in free air? Is it a switching rating, or the conditional current that can be interrupted... and so on.
I don't deal primarily with standards for accessories but I have access to some of them, so I might be able to help you, but the first thing to understand is that there are more than one 'rating' for any piece of electrical equipment.

 

[JohnW2]

Apologies for all the repetition in this reply - but I'm replying to repeated points

No, they are not an 'invention' by manufacturers. The values are assigned by manufacturers, in accordance with the relevant standard.

I agree it was a bad choice of word. What I meant was that the sort of accessories we are talking about generally come (at least as far as the average user is concerned) with a single 'rating', often even moulded onto the product (e.g. a "10A switch" or "20A JB"), which the manufacturer has assigned (in accordance with any relevant standard) on the basis of of assumptions about likely usage.

My point was that there are numerous ratings that can be assigned, with different meanings as well as different values, depending on the product and on the standard. If you ask about a specific rating for a specific accessory then it might be possible to provide an answer.

I understand your point. However, again, my point is that (for the sort of products we are talking about) the end-user is (usually) presented with a single 'rating', and needs to understand what it means.

Of course the users of those accessories are not going to read through all the relevant product standards, which are written for manufacturers. They will just use the installation standards and MIs.

Again, exactly - and if (as is usual) all they are presented with is a (single) 'rating', they need to understand which rating it is, hence what it means.

However you seem to be trying to go further than that and understand some of the decisions made by the standards committees, but you need to be clearer about the exact question you're asking. For example, consider a "10a switch". What is the utilization category (nature of the load, frequency of operation, etc) for that rating? Is that installed, or in free air? Is it a switching rating, or the conditional current that can be interrupted... and so on.

Not at all. As above, I'm merely suggesting that if a manufacturer sells a "10A switch" to Joe Electrician or Joe Public, that Joe needs to know what 'rating' (based on whatever assumptions about utilisation, installation etc.) they are being presenting with - hence what it actually means.

I don't deal primarily with standards for accessories but I have access to some of them, so I might be able to help you, but the first thing to understand is that there are more than one 'rating' for any piece of electrical equipment.

Yet again, exactly. Since there is more than one 'rating', if a manufacturer presents (explicitly or implicitly) just one 'rating', the user needs to know which one is being presented.

It really all comes down to very basic, and practically very important questions - e.g. as far as the average domestic electrician is concerned, is it, or is it not, acceptable to have a bog standard light switch with 10A written all over it 'protected' by a B10 MCB? That's not an unreasonable question to want to know the answer to (without the need for a trip to the library), is it?!

Kind Regards, John

 

[stillp]

It really all comes down to very basic, and practically very important questions - e.g. as far as the average domestic electrician is concerned, is it, or is it not, acceptable to have a bog standard light switch with 10A written all over it 'protected' by a B10 MCB? That's not an unreasonable question to want to know the answer to (without the need for a trip to the library), is it?!

Kind Regards, John

Right, that's a simple question! There isn't a simple answer though. What do the MIs say?
The end-user of a domestic wiring accessory doesn't really need to understand which of the many possible ratings is marked on the product, but it is not unreasonable to expect the manufacturer to specify the required type of overcurrent protective device.

 

[JohnW2]

It really all comes down to very basic, and practically very important questions - e.g. as far as the average domestic electrician is concerned, is it, or is it not, acceptable to have a bog standard light switch with 10A written all over it 'protected' by a B10 MCB? That's not an unreasonable question to want to know the answer to (without the need for a trip to the library), is it?!

Right, that's a simple question! There isn't a simple answer though. What do the MIs say? The end-user of a domestic wiring accessory doesn't really need to understand which of the many possible ratings is marked on the product, but it is not unreasonable to expect the manufacturer to specify the required type of overcurrent protective device.

Quite - although application of your argument (which is perfectly reasonable) would preseumably mean that there's no single answer to that, either, wouldn't it?

As for the MIs, that's the point - I've yet to see one which sheds any light on this question. All they ever tell one is 'the current rating', without any explanation as to what they mean by that. For example, the 'Technical Datasheet for MK Logic Plus switches merely says:

Current rating: 10 amps – no derating when used on fluorescent or inductive loads

Is it clear to you what that 'rating' means?

Kind Regards, John

 

[JohnW2]

P.S. ....

The end-user of a domestic wiring accessory doesn't really need to understand which of the many possible ratings is marked on the product ....

I'm not so sure about this. If they don't understand which of the possible ratings they're being presented with, they are likely to make an assumption, which may be incorrect.

... but it is not unreasonable to expect the manufacturer to specify the required type of overcurrent protective device.

... if it requires any overload protection, that is! As you'll be aware, there is an argument afoot (with which I'm not comforable) that lighting fittings cannot really present an overload (as opposed to fault) situation, so that overload (as opposed to fault) protection for switches may not be required at all! If they think that argument works for switches, I'm not sure why they don't try it with the cable as well - I would imagine than many a 1mm² lighting circuit would get adequate fault protection from a B32 MCB!!

Kind Regards, John

 

[stillp]

It really all comes down to very basic, and practically very important questions - e.g. as far as the average domestic electrician is concerned, is it, or is it not, acceptable to have a bog standard light switch with 10A written all over it 'protected' by a B10 MCB? That's not an unreasonable question to want to know the answer to (without the need for a trip to the library), is it?!

Right, that's a simple question! There isn't a simple answer though. What do the MIs say? The end-user of a domestic wiring accessory doesn't really need to understand which of the many possible ratings is marked on the product, but it is not unreasonable to expect the manufacturer to specify the required type of overcurrent protective device.

Quite - although application of your argument (which is perfectly reasonable) would preseumably mean that there's no single answer to that, either, wouldn't it?

As for the MIs, that's the point - I've yet to see one which sheds any light on this question. All they ever tell one is 'the current rating', without any explanation as to what they mean by that. For example, the 'Technical Datasheet for MK Logic Plus switches merely says:

Current rating: 10 amps – no derating when used on fluorescent or inductive loads

Is it clear to you what that 'rating' means?

Kind Regards, John

I see what you mean. They aren't good at updating their compliance statements either!
I'll delve into the murky world of EN 60669-1 and see what I can find.