Interesting thread and a little "mention of twin socket rating" well sort of inference, if you like.

[ebee]

I saw this interesting thread on the IET Wiring and Regulations forum and a little comment by Chris Pearson mentioning the 14A + 6A twin socket test and stating "not 2 x 13A" too.
I thought the thread itself interesting and also that comment by dear old Chris too, I put it here as a Food for thought in ref to our discussion about the 13A rating of a twin socket recently.
I do not expect it to change anyone`s opinion either way but thought it might interest some that he made the comment.

In fact if we might get some additional observations as to why the sockets cracked too, so a win win kind of situation in the knowledge enhancement, I know that both forums have extremely excellent contributors from the ranks of both the trade and DIY, in fact some folk with deep understanding of apparently unrelated disciplines too.

 

[securespark]

Have you got a link?

 

[ericmark]

I have seen many times, it quoted that a twin socket is rated at 20 amp total, and also specials like filter sockets rated at 13 amp the pair, however you can get a pair of grid sockets in a single double socket plate.

We also are told sockets should be where any plug is in free air.

This raised the question when placed behind an appliance, is that considered as being in free air?

I am sure a socket in a cupboard supplying a waste disposal unit is no problem. And if we look at appliances which can draw near 13 amp for an extended time, there are not many, the immersion heater, rarely uses a 13 amp fuse, only other is the resistive clothes dryer.

We as a child had 3 kW electric fires, not seen them in years.

 

[JohnW2]

I saw this interesting thread on the IET Wiring and Regulations forum and a little comment by Chris Pearson mentioning the 14A + 6A twin socket test and stating "not 2 x 13A" too. .... I thought the thread itself interesting and also that comment by dear old Chris too, I put it here as a Food for thought in ref to our discussion about the 13A rating of a twin socket recently.

Thanks. Do you have a link to that discussion?

You know my view about the 'current rating'of double sockets, and I doubt that anything will ever change it.

As for that 'rating' of double sockets, I'm not sure that I have ever seen one for which the manufacturer said that the rating was "20 A total", but maybe there are some. Many people seems to assume that the 'rating' is 20A because of the 14A+6A 'minimum requirement test" of BS1363 - but, as often discussed, that tells us nothing about the 'rating' of an actual product, other than it must pass a temp rise test with 14A+6A (which really puts paid to those who believe that the rating of a double socket is '13A total').

Of more practical relevance, as I recently wrote, I am coming more and more to suspect that the stories we hear about thermally-damaged double sockets may be a complete 'red herring', the overheating nearly always originating in the plug, whether it is plugged into a single or double socket.

As I've said, that's certainly consistent with my personal (limited) experience. If I recall correctly, in the (very few) cases of thermal damage to sockets when high loads have been involved I have seen it has always been apparent that the overheating has originated in the plug (usually in the vicinity of the fuse), not the socket - and in most, maybe all, cases, it has been a single, not double, socket that has been affected.

The other thing to bear in mind is that there are pretty few 'plug-in' 3kW / 13A loads around in domestic premises these days, so there probably is not all that much opportunity for people to plug total loads >20A into a double socket, anyway.

Kind Regards, John

 

[JohnW2]

I have seen many times, it quoted that a twin socket is rated at 20 amp total

As I've just written, I'm not convinced that I have ever seen a manufacturer specifying a 20A rating. Can you indicatesome examples?

, and also specials like filter sockets rated at 13 amp the pair,

Indeed, but that is 'different', and understandable.

 

[ebee]

I did actually paste the whole of that IET thread, I wonder if a moderator has removed it (copyright etc) or its just done it automatically. I`ll have a look later and repost that bit or link to it as appropriate.

Yes it was about heat cracking sockets.

Personally I agree about the plug/fuse being the most likely scenario but am not totally discounting other suggestions including sockets that that not actually be to BS1363 or, in some cases, far from it even though marked up as such.
Chris mentioned the 14A + 6A test but alson mentioned Not 2 x 13A and I just wondered if he was little bit minded to the principles we were mentioning recently about actual total twin socket ratings, maybe but maybe not in my humble opinion. Either way I do not expect anyone on this forum to change their minds over this but it certainly struck me as of some interest, second to the other thoughts about overheating causes..

 

[ebee]

It looks to me like I might have actually posted it but on the wrong thread.
If that is my mistake it will not surprise many of you that I am slightly dafter than you already thought in the first place.

Dual master/slave outside lights​

have a look at that thread and see if you can see it?

 

[JohnW2]

Yes it was about heat cracking sockets. .... Personally I agree about the plug/fuse being the most likely scenario but am not totally discounting other suggestions including sockets that that not actually be to BS1363 or, in some cases, far from it even though marked up as such.

If the sockets we hear about which have suffered thermal damage are 'sub-standard', then all bets are obviously off. The discussions we have are about sockets which do satisfy the minimum requirements of BS1363.

Chris mentioned the 14A + 6A test but alson mentioned Not 2 x 13A and I just wondered if he was little bit minded to the principles we were mentioning recently about actual total twin socket ratings, ...

Was he perhaps merely 'observing' that the BS1363 minimum requirement does not involve a 13A+13A test?

Standards (or regulations/laws) specifying minimumsafety standards usually have built-in 'margins of safety, not the least because of manufacturing tolerances. If the individual product(s) used for BS1363 tests only just scraped through that test by the skin of their teeth, then it is almost inevitable that some of the items coming off the production line would be 'sub-standard' - one might therefore expect that the test would have used, say 14A + 14A, maybe more.

As you are aware, it is only recently that I have really given 'deeper though' to this often-discussed issue and the more I do that thinking, the more compelling do I find the circumstantial evidence that the thermal damage people sometimes see when the total load on a double socket is high (but less than 26A !) has very probably got nothing to do with it being a double socket, and nothing to do with the 'rating' of double sockets:

1... At least in my (limited) personal experience, such thermal damage to sockets is at least as common with single sockets (carrying fairly high loads) as with double ones.​

​

2... When thermal damage to sockets occurs, it is usually apparent that the overheating has actually originated in the plug (usually in the vicinity of the fuse). I probably mentioned that the most recent example of (serious) thermal damage to socket+plug I saw was in relation to a 2 kW fan heater, hence less than 9A.​

​

3... The majority of thermal damage to sockets is nearly always in the vicinity of the holes for plug pins, usually the L pin. There are very few parts of a double socket which carry the combined current of both outlets, and none of them are particularly near the place where the thermal damage is usually concerned​

Do you not find that pretty 'compelling'? Have you ever seen thermal damage to a socket which was worst at some point distant from the plug pins? Have you often seen thermal damage to sockets which was not in the vicinity of plug pin receptacles and which seemingly originated in the plug?[

Kind Regards, John

 

[EFLImpudence]

There are very few parts of a double socket which carry the combined current of both outlets, and none of them are particularly near the place where the thermal damage is usually concerned

Good point.

So - when 'they' are doing this very strange 14A + 4A test, do they do it both ways round?
It would be very strange if they did not - so why not just test with 13A on both sides at once?


That must therefore mean that both sides can handle 14A; so is it likely that any heat would transfer from one side to the other and cause damage if both were 13A?

As you say, only a few parts carry the total current. It would not be difficult (or IS not difficult) to allow for this.

Why do we not know these things?



Another myth?

 

[ebee]

Good questions indeed EFLI.

John, yes I am actually pretty much in agreement in whole or in part of what you say, i never wasn`t.

This bit
"not the least because of manufacturing tolerances. If the individual product(s) used for BS1363 tests only just scraped through that test by the skin of their teeth, then it is almost inevitable that some of the items coming off the production line would be 'sub-standard' "
I am not so much in agreement with though.
if the tests are done meaningfully satisfactorily then that should be rare. With "Proper" makes of manufacturer using their own proper methods (indeed I would expect the BS for sockets to include this instruction either by refence or in detail) them such testing sould be done, in my opinion, by adopting proper meanings of "significance tests" it is a subject of its own I learned during my apprenticeship.
Of course, there could be some manufacturers letting the side down a little in this respect and especially with items "from other sources" shall we say. we can not do much to control them unfortunately.
I am not saying it could never happen though either.

 

[JohnW2]

John, yes I am actually pretty much in agreement in whole or in part of what you say, i never wasn`t.

That's good to hear.

This bit
"not the least because of manufacturing tolerances. If the individual product(s) used for BS1363 tests only just scraped through that test by the skin of their teeth, then it is almost inevitable that some of the items coming off the production line would be 'sub-standard' "
I am not so much in agreement with though.
if the tests are done meaningfully satisfactorily then that should be rare.

Given the nature of the real world, there has to be a 'safety margin' built in somewhere , so it depends upon where they have 'built it in'. IF the maximum acceptable temp rise (52 K) is less than would be considered unsatisfactory/unsafe' then, yes, I agree that the test would not necessarily need to be undertaken with a current (per outlet') greater than 13A.

However, having said that, safety margins' are commonly incorporated in relation to both the 'stress' and the 'strain'. For example, it is considered that, to be safe for intended use, a steel cable should only stretch by X% when subjected to a force of Y, then (a) X will be appreciably less than what would be considered unsatisfactory/'dangerous' and the test would often be done with a force at least a bit greater than y.

Kind Regards, John

 

[JohnW2]

Good point. So - when 'they' are doing this very strange 14A + 4A test, do they do it both ways round?

BS1363 does not require that. About the closest it gets to talking about placement of plugs/loads is this very vague bit ...

In the case of socket-outlets having more than one outlet, the disposition of the plugs shall be so arranged as to give the most onerous test conditions with regard to the position of terminals and fuses.

It would be very strange if they did not - so why not just test with 13A on both sides at once?

That is the question we are discussing - and, if they required that, presumably that would put a stop to all these suggestions that such sockets were 'rated' at les than 26A total.

Given the (at least, my) suspicion that nearly all the problem of thermal damage to sockets is usually down to the plugs, rather than the sockets, the other odd thing is that the temperature rise test is undertaken using a plug seemingly very atypical of what will normally be plugged into the socket during normal use. ...

Test plug for temperature-rise test
B.1 General
The test plug consists of a body made from insulating material and pins of brass.

The pin dimensions and centres are as shown in Figure 4 of BS 1363-1:1995. Inside the test plug a ceramic-covered wire-wound resistor is clamped to the line pin making no electrical contact between the pin and the resistor element, though good thermal contact is essential. Thermal contact compound shall be used on the interface between the ceramic resistor body and the metal clamp. The M3.5 clamp screw shall be tightened to a torque of 0.2 N·m ± 0.02N·m. The resistor leads passthrough the sides or cover of the test plug. Approximately 1 000 mm of 3-core flexible cord as given in Table 27 of BS 6500:2000 with nominal cross-section 1.25 mm2 is connected to the plug by soldering the line and neutral cores to their respective plug-pins. The earth core of the cord is not fitted to the earth pin. Details are shown in Figure 30.

B.2 Calibration
The test plug is mounted in the dummy front plate (see Figure 17 in BS 1363-1:1995) and the test carried out in accordance with 16.2.1 with 14 A ± 0.2 A flowing through the flexible cord attached to the plug.At the same time a separate low voltage d.c. supply is connected to the resistor and the voltage adjusted until the temperature rise on the plug-pin spacer stabilizes at 35 K ± 1 K. The value of the voltage applied to the resistor is noted. The calibration voltage is applied to the resistor when checking the temperature rise of a socket-outlet.

That must therefore mean that both sides can handle 14A; so is it likely that any heat would transfer from one side to the other and cause damage if both were 13A?

I don't think that there is any doubt that, if compliant with the Standard, both sides can 'handle 13A or 14A' when the other outlet is not loaded- all the discussion and argument is about the situation in which both outlets are supplying 13A (or 14A).

As you say, only a few parts carry the total current. It would not be difficult (or IS not difficult) to allow for this.

I'm not sure what you mean by 'allow for it'- the test is obviously conducted with a socket which has those (few) parts. However, I, for one, have never seen thermal damage to a socket in the vicinity of thjose parts (which carry the 'total load' - have you?

Why do we not know these things? ... Another myth?

It may have something to do with the fact that BS1363 is very old, and no-one seems to have felt any need to re-visit it for nearly 30 years.

 

[DetlefSchmitz]

and no-one seems to have felt any need to re-visit it for nearly 30 years.

Well as I have pointed out before, it was re-released last year and no effort was made to fix the obvious problems. But people still need to fork out several hundred pounds for a new spec not fit for purpose (imho).

 

[JohnW2]

Well as I have pointed out before, it was re-released last year and no effort was made to fix the obvious problems. But people still need to fork out several hundred pounds for a new spec not fit for purpose (imho).

I'd forgotten that. What was "re-released" - was it materially (or at all!) different from the 1995 version (which is the only one I have seen)?

 

[ebee]

It may have something to do with the fact that BS1363 is very old, and no-one seems to have felt any need to re-visit it for nearly 30 years.

Indeed, it may be that it has, in practice, stood the test of time in the real world pretty much.

Ok theoretical point just for illustration then.
It may be (or may not be) that if most of us or even plenty of us but not all of us, take a stance on something being required in normal practice and in general thinks occur mostly one way insofar as we are aware.
Lets say something is believed to be about 90% successfully safe.
If we all change our attitude and over the next few years it appears to become 95% safe insofar as we can see then the attitude might have attributed to that to some degree.
If, on the other hand, things get relaxed and it becomes less safe, example only 85% safe, then we might conclude that relaxation has had the opposite effect.
We do not know for sure unless we can eliminate loads of other things that might also have effected it. Almost impossible.
But we might draw a reasonable suspicion.

I do not expect a pile of dead bodies to be lining the streets if we "upgrade" from 13A total to 26A total and increase the expected usage time from 4 hours to 8 hours but I do believe it might well cause some problems that are best avoided. Unless BS states the 13A rating is not for the total load but can be increased significantly the I still believe they should tell us that we have misunderstood.