Double sockets

[JohnW2]

A matter of curiosity .....

I have often seen it written here, and elsewhere, that nearly all double sockets (I think probably all other than MK) are 'rated' only at 20A, not 26A.

For a start, I think it should go without saying that, if true, this is a daft (and potentially dangerous) situation; we can be sure that the vast majority of users are blissfully unaware of this, and therefore would have no reason to think that there was anything wrong in plugging two 3 KW loads into one double socket. It probably won't happen often but, by the same token, it is virtually bound to happen sometimes!

However, I cannot help but wonder what this 20A 'rating' actually means - or, rather, what it is that causes the manufacturers to say that the total load should not exceed 20A. If one looks at a double socket, virtually all of the components are totally separate for the two 'halves' - switch (if present), all the components of the actual socket (that which accepts pins of the plug) and even the connections from the terminals to the individual switch or socket - and no manufacturer is suggesting that one of the sockets of a pair, in itself, cannot carry a 13A load. In fact, the only components that seem common to the two halves of a double socket are the terminals. Not only do they usually look (to me) plenty meaty enough to carry 26A but, given that double sockets are very commonly installed in RFCs, the cables terminated in the double socket could be carrying up to 32A, anyway, regardless opf how much (or little) current is being drawn from the double socket in question.

So what is going on here? I strongly suspect that all double sockets are perfectly OK with a 2x13A load, but something is leading most manufacturers to want to wash their hands of responsibility for what happens if the total load exceeds 20A. Being 'cautious' is all very well, but if that's their approach, they might just as well sell 13A plugs 'rated' at 10A!

Kind Regards, John.

 

[DetlefSchmitz]

It's BS1363 which specifies the 20A figure. Manufacturers test against that spec.

To be honest most sockets in use will overheat on long term full current usage, because the pins of plugs and corresponding parts of sockets get oxidised.

Try measuring the R1+R2 with proper test equipment on old sockets, and you find that many sockets require pushing the plug in and out a few times to get a decent result.

 

[PrinceofDarkness]

So what's the worst that might happen in ether case?.......


A slightly fishy smell from the socket and slight discolouration of the plastic around the 13A plug pins.......


Lucia.

 

[sherlockholmes]

Five days ago, around midday, in Christchurch New Zealand, someone plugged two 3kW loads into one double outlet.

 

[JohnW2]

It's BS1363 which specifies the 20A figure. Manufacturers test against that spec.

I have to say that I find that quite extraordinary, given the apparent absence of a requirement for any sort of warning or, at least, 'publicity'. I'm sure that 99%+ of the general public assume that a 'double 13A socket' can be used to do what is apparently written on the tin! And if the 20A 'rating' is specified by BS1363, then what's this about MK's apparently 26A rated ones? - and how do they differ from everyone else's?

To be honest most sockets in use will overheat on long term full current usage, because the pins of plugs and corresponding parts of sockets get oxidised.

I don't doubt that, but that's obvioulsy not peculiar to double sockets - or are you going to tell me that BS1363 'rates' single 13A sockets at 10A?!!

Kind Regards, John

 

[RF Lighting]

MK twin sockets are only rated to 13A total.

It's not really a problem. It's rare that someone will plug two heavy loads into a twin socket and if they do, it might be ok anyway, and if it isn't all that happens is the socket burns out and needs replacing.

I see loads more sockets burnt out by a single 13A load.

 

[PrinceofDarkness]

"Fourscore and seven years ago our fathers brought forth, on this continent, a new nation"...........

 

[ban-all-sheds]

MK twin sockets are only rated to 13A total.

No - they would not be compliant with BS 1363 if that were the case.

 

[1john]

Full load can pose a problem, I have witnessed an Ashley double socket with a commercial style deep fat fryer welded into it, there were two same units plugged into the same socket and one of them became a permanent fixture! As stated though, this is generally a one off. Most sockets will have only one heavy load in it at any time and if they have two (kitchen for instance) the full loads are only on for a few mins (boil kettle, make toast).

 

[JohnW2]

Full load can pose a problem, I have witnessed an Ashley double socket with a commercial style deep fat fryer welded into it, there were two same units plugged into the same socket and one of them became a permanent fixture! As stated though, this is generally a one off. Most sockets will have only one heavy load in it at any time and if they have two (kitchen for instance) the full loads are only on for a few mins (boil kettle, make toast).

Yes, I think we have all agreed that full loads are iffy (increasingly so as sockets and plugs age) and that two persistent full loads on a double socket is rare, but nevertheless happens.

However, I think I'm starting to understand what is probably going on here. Although it is very common to see people asserting that most dual sockets are 'rated' at 20A, I suspect that this doesn't necessarily mean what many may think it implies (i.e. that it is known that 20A is the maximum safe total current). Rather, I'm coming to suspect that it probably merely means that it has been shown to be compliant with (i.e. 'no worse than required by') BS1363, which apparently only requires testing at 20A. However, if that is the case, it does preclude the possibility that the product is actually appreciably 'better' than that standard requires (i.e. would pass the standard's temperature rise test at loads appreciably greater than 20A). After all, if I'm told that my tap water is compliant with a standard that requires is to contain no more than X parts per million of cyanide, that does not mean that my tap water does contain X parts per million of cyanide

I guess BS1363 is just being pragmatic (i.e. similar to what we're all saying) in assuming that two persistent full loads on a double socket is unlikley to arise - although I remain surprised that it takes that approach, given that no-one tells the users that they should not fully load both sockets. From a theoretical point of view, I'd actually be quite surprised if problems with two fully loaded sockets on a dual socket are any more likely than a problem with a fully loaded single socket.

Kind Regards, John

 

[bernardgreen]

quoting myself from the Heavy duty welder thread

Odd that you mention that because back in the 1970's or 80's the GPO Telephone engineering department issued an edict that no 13 amp plug was to be used for more than 9 amps within the GPO .

It came about following problems with burnt plugs and sockets. This effectively limited the load on a double socket to 18 amps.

There was one exception for two makes of plug (MK was one of them ) when used in a single socket ( again the socket had to be an approved make ) but the acception had to be applied for and was not automatically given. Other organisations had similar restrictions.

Many of the burnt sockets I have replaced have been caused by poor contact between socket and plug causing heating. Or the pin to fuse overheating in a cheapo plug.

 

[mikhailfaradayski]

MK twin sockets are only rated to 13A total.

No - they would not be compliant with BS 1363 if that were the case.

This subject crops up from time to time.

I know i have asked this fairly simple, straight forward question before, but IIRC, not one person has been ever able to answer it.

Can anyone post a photo of a standard UK socket outlet showing the rating marked on the socket which is something other than 13A?

I'm not saying such an animal doesn't exist, but i have yet to see one for myself

The closest anyone has got is guiding me towards a technical spec sheet for MK sockets, thats not is whats embossed onto the socket itself

 

[JohnW2]

MK twin sockets are only rated to 13A total.

No - they would not be compliant with BS 1363 if that were the case.

This subject crops up from time to time.
I know i have asked this fairly simple, straight forward question before, but IIRC, not one person has been ever able to answer it.
Can anyone post a photo of a standard UK socket outlet showing the rating marked on the socket which is something other than 13A?
I'm not saying such an animal doesn't exist, but i have yet to see one for myself

I certainly have never seen such an animal, either, and I've just looked through a motley assortment of sockets, new and old, single and double, that I have in my cellar, just to be sure.

However, there is an interesting point here. As mikhailfaradayski implies, just like the single sockets, the double ones simple have (as well as a reference to BS1363) "13A 250V" embossed on the business part of the socket - not "2 x 13A" or anything like that. This is therefore at the least ambiguous, since the most obvious interpretation is perhaps that the entire assembly (not half of it) is 'rated' at 13A. After all, if I saw "1000W" embossed on a two-element fan heater, I think that would assume that 1000W was the total power, not the power of half of it! So, depending upon what one means by 'rated', what RF Lighting says is not necessarily all that far off the mark - given that the double sockets do have "13A" written on them.

As for BAS's comment, I don't think that there would be a failure to comply with BS1363 if a manufacturer chose to 'rate' (recommend) maximum currents less than the test current used by BS1363, provided that the product did pass the BS1363 test (at the BS1363 test current). Does BS1363 say anything about recommended maximum currents and/or the marking of sockets? BAS has indicated the BS1363 test conditions, but that is not the same as saying that those same conditions should be regarded as maximums for normal usage. In fact, in many fields it would probably be the exception, rather than the rule, for a product to be recommended for usage under conditions 'as extreme' as the test conditions.

Given the ambiguity of what is embossed on the products themselves, I guess that the decisive question relates to what the manufacturer's specification/data sheets say.

Kind Regards, John

 

[JohnW2]

Odd that you mention that because back in the 1970's or 80's the GPO Telephone engineering department issued an edict that no 13 amp plug was to be used for more than 9 amps within the GPO .
It came about following problems with burnt plugs and sockets. This effectively limited the load on a double socket to 18 amps.

That seems very reasonable, since we all agree that overheating/burning is an issue with fully-loaded sockets, particular after some ageing has occurred.

As I just wrote in response to BAS's comments, I don't think it would be a problem as regards BS1363 compliance even if the manufacturers made similar recommendations - provided that the socket actually did pass the standard's test. As I said, in my experience it's actually quite unusual for it to be recommended that a product is used right up to its testing conditions.

Kind Regards, John

 

[ricicle]

My take on this is that although MK have type tested their sockets to 20A (which IMO is sensible), the standard only relates to 13A outlets and as such other manufacturers have designed to this.
Perhaps MK only emboss 13A on the back to show it has met the standard.
If the standard was changed to cover 20A then all manufacturers would (begrudgingly ?) have to change their designs.