Max current on a two plug socket - appliances

[JohnW2]

... so a twin or dual has to be capable of carrying 13A in each outlet, bearing in mind the limitations due to temperature rise will limit the time for which a dual outlet can carry 26A.

I have asked many times why that is the case (and no one knows). ... The only part in a socket which would be vulnerable to loads of over 13A (or 20A) are the rather thick pins and the rivets which join the pins to the prong assembly - nothing else would be subject to both 13A loads.

I'm not sure it's necessarily related to the (very few) parts which have to carry the combined current of both sockets but, rather, to the fact that two socket-outlets are separately generating heat which contributes to the temperature rise of the whole accessory. In other words, I'm suggesting that, whilst the amount of heat generated by an outlet supplying 13A (or 14A) may be sufficiently low for it to easily pass a temperature rise test, when there is another outlet supplying the same current (and generating the same amount of heat) in close proximity, and thermal contact, then the combined heat generation of the two results in a temperature rise of the whole accessory which would not pass the test.

Kind Regards, John

 

[JohnW2]

But they are only officially tested to 13 amps aren't they? I know the ones I have fitted have always said 13 amps max on it.

I don't know which make you have fitted, but dual socket-outlets are tested for temperature rise at a total of 20A. Each individual socket-outlet has to be capable of carrying 14A. ... They are 13A socket-outlets, and however many are included in a 'device', they are still 13A outlets, so a twin or dual has to be capable of carrying 13A in each outlet, bearing in mind the limitations due to temperature rise will limit the time for which a dual outlet can carry 26A.

Indeed - that's much the same as I wrote just before you. However, as joydivision indicated, one of the things that contributes to the confusion is that the moulding of the works of a double socket almost invariably bears a "13A" (admittedly, not "13A max" or "13A total") marking.

Kind Regards, John

 

[stillp]

They are 13A socket-outlets, and however many are included in a 'device', they are still 13A outlets, so a twin or dual has to be capable of carrying 13A in each outlet, bearing in mind the limitations due to temperature rise will limit the time for which a dual outlet can carry 26A.

I have asked many times why that is the case (and no one knows).
The only part in a socket which would be vulnerable to loads of over 13A (or 20A) are the rather thick pins and the rivets which join the pins to the prong assembly - nothing else would be subject to both 13A loads.

So, it must be the rivets.
Why do they not use rivets capable of handling 26A - or do they?

The reason for the 14A/6A test is a combination of the 14A test that a singe outlet has to withstand, with a load on the 2nd outlet that is derived from the expected use of the socket (diversity) combined with the ability of designs to withstand elevated temperatures at the time the standard was written. With today's plastics, 26A would be acheivable, although the plug would probably be too hot to remove.
Regarding the rivets, the best manufacturers do use rivets that can withstand in excess of 26A. That's not actually the problem - the problem is repeated heating and cooloing, which can loosen the rivets over time.

 

[JohnW2]

I'm a little confused by all this talk of rivets and parts of the double docket which carry combined current from the two outlets since, as I said, I would have thought that there would be a potential issue even without those considerations ... The physical size and thermal mass of one double sockets is less than double that of a single socket, and the number of cables available to conduct heat away is the same in the two cases. If each socket outlet generates the same amount of heat as would a single socket carrying the same current, then it surely follows that if one gets a certain temp rise in a certain time with a single socket carrying 13A or 14A), the temperature rise is likely to be appreciably greater for a double socket supplying 2 x 13A/14A.

Or is my basic physics too basic?

Kind Regards, John

 

[stillp]

Your physics looks good to me John, but EFLI had asked why 'they' don't use rivets that can withstand 26A.

 

[JohnW2]

Your physics looks good to me John, but EFLI had asked why 'they' don't use rivets that can withstand 26A.

Thanks. Yes, I understand what EFLI asked but, as I've suggested (with my 'physics') I think he's probably got hold of a pretty unimportant end of the stick, since I don't think the difference in temperature rise between single and double sockets is significantly to do with the few parts of a double that carry the combined current.

Kind Regards, John

 

[stillp]

I don't think he implied it was John, but he seemed concerned about the parts of a dual socket-outlet that carry the total load current, rather than the heat rise.

 

[EFLImpudence]

Thanks. Yes, I understand what EFLI asked but, as I've suggested (with my 'physics') I think he's probably got hold of a pretty unimportant end of the stick, since I don't think the difference in temperature rise between single and double sockets is significantly to do with the few parts of a double that carry the combined current.

I think that is even worse, then.

So, you are saying that, electrically, the parts (notwithstanding Bernard's post) are totally satisfactory but the product cannot dissipate the heat generated by that use.
Their solution is to apparently limit the usage but leave the users (general public) totally unaware of it.

 

[stillp]

So, you are saying that, electrically, the parts (notwithstanding Bernard's post) are totally satisfactory but the product cannot dissipate the heat generated by that use.
Their solution is to apparently limit the usage but leave the users (general public) totally unaware of it.

Yes, I think that's right, although the standard probably reflects the performance of the best products available at the time it was written.

 

[JohnW2]

Thanks. Yes, I understand what EFLI asked but, as I've suggested (with my 'physics') I think he's probably got hold of a pretty unimportant end of the stick, since I don't think the difference in temperature rise between single and double sockets is significantly to do with the few parts of a double that carry the combined current.

I think that is even worse, then. ... So, you are saying that, electrically, the parts (notwithstanding Bernard's post) are totally satisfactory but the product cannot dissipate the heat generated by that use. ... Their solution is to apparently limit the usage but leave the users (general public) totally unaware of it.

Yep, that's how I see it. In comparison with a single socket carrying the same current 'per outlet' a double socket presumably generates double the amount of heat, but dissipated into less than double the thermal mass - hence more temperature rise. If single sockets are, say, only just able to pass the temp rise test at 13A/14A, then one therefore wouldn't really expect a double one carrying 2 x 13A/14A to pass the test.

However, as you imply, and as I've always said, it seems unfortunate that the general public have never been made aware of this (or, looked at in a different way, unfortunate that this was ever 'allowed'). I suspect that it probably happened primarily because 'they' wanted double sockets to be narrower than a pair of single ones, and to have just a single moulding for their 'works'.

Kind Regards, John

 

[JohnW2]

Yes, I think that's right, although the standard probably reflects the performance of the best products available at the time it was written.

Yes, but as I've just written to EFLI, no matter what materials were available, a double socket could presumably have been designed (and required) to be able to carry double the total load of a single one had there not been a 'desire' to make the double sockets smaller (and cheaper to make) than two singles. Do I take it that it would have been those who intended to manufacture the sockets who would have 'told' the BSI what sort of design they wanted to use?

Kind Regards, John

 

[stillp]

Now your physics is getting shaky John! The issue is not the thermal mass - that will affect the rate of rise of temperature but not the dissipation. The heat dissipation will depend more on the total surface area and its thermal coupling to the environment. The principle is the same though, a dual socket-outlet will have less than double the surface area of a single.
Do the general public need to be aware? Probably not. A far bigger risk comes from the cheap far eastern circuit-breakers that many people fit.

 

[JohnW2]

Now your physics is getting shaky John! The issue is not the thermal mass - that will affect the rate of rise of temperature but not the dissipation. The heat dissipation will depend more on the total surface area and its thermal coupling to the environment. The principle is the same though, a dual socket-outlet will have less than double the surface area of a single.

Indeed, but it's not really my physics getting shaky, but rather my writing about it getting very sloppy! However, as you say, the bottom line would have been the same had I expressed myself correctly and clearly! Mind you, isn't the temp-rise test time-limited? If it is, then the higher rate of temp-rise due to the proportionately smaller thermal mass presumably has the potential to result in a higher temp rise by the end of the test period?

Do the general public need to be aware? Probably not. A far bigger risk comes from the cheap far eastern circuit-breakers that many people fit.

Whilst there may be some pragmatic truth in that, it's not really the attitude that I would expect a Standards organisation or engineer to adopt! In fact, I'm not even sure that the pragmatism is all that 'safe' - it's not that uncommon for double sockets to eventually end up pretty ill (or worse) when a dryer and washing machine are (as will often seem very logical and reasonable to the 'uninformed' general public) plugged into them.

Kind Regards, John

 

[stillp]

From memory, the test period is limited to 8 hours, far longer than the thermal time constant of the device under test.

A dryer and washing machine would probably be OK, since neither will take their full current for long periods. A bigger problem would be two 3kW heaters plugged into a dual socket-outlet, which does happen - I know, I've done it myself to dry out after a burst pipe!
That's the point though - in practice it is very unusual for both sockets of a dual outlet to be fully loaded concurrently, so the 14A/6A is not unrealistic.

 

[JohnW2]

From memory, the test period is limited to 8 hours, far longer than the thermal time constant of the device under test.

Fair enough.

A dryer and washing machine would probably be OK, since neither will take their full current for long periods. A bigger problem would be two 3kW heaters plugged into a dual socket-outlet, which does happen ...

Sure, there are even more extreme cases than the one I mentioned but, as you go on to say, they are probably pretty rare, I mentioned WM+dryer because it is far more common and, although one usually 'gets away with it', one also hears reports (and sometimes sees pictures, even in this forum) of it not working out so happily. AFAIAA, a dryer will draw full power more-or-less continuously, so if it has already heated the socket up when the WM does its 30 mins or whatever of 'high power', then things might get fairly hot.

That's the point though - in practice it is very unusual for both sockets of a dual outlet to be fully loaded concurrently, so the 14A/6A is not unrealistic.

I suppose that's true up to a point. However, whilst some things are necessarily probabilistic (aka 'gambles'), when one is talking about potential issues of safety, it can be argued that if there are ways in which one can minimise the degree of gambling, that is probably a desirable course. No matter 'who says what', and how many times they say it, I don't think I'll ever cease to be surprised that double sockets that cannot safely carry 2 x 13A indefinitely were 'ever allowed' - given that it never was 'necessary' that they should be.

Kind Regards, John