Ring extension load

[AndyPRK]

I would have thought that the OP should replace the FCU with a good quality one which will blow the fuse before its switch contacts fail.

And ensure the double socket is a high quality item. E.g an MK one (as per datasheet above).

I'm not saying you should run 2 items together, but from what we have heard about the current setup it would sound like a good idea in case a new owner plugs two high load items in together.

 

[Risteard]

the Standard requires 20A rating and they must not be used to supply more than this.

Please show us where the standard says that.

A quote from an MK catalogue:

"All MK socket-outlets are manufactured to comply with BS1363 part 2: 1995 and are rated at 13A per unit. Double socket-outlets have been manufactured and tested to exceed this rating by margin that allows electrical safety and reduces the risk of heat and mechanical damage to components due to overloading. It should be noted that BS1363 part 2: 1995 does not allow double sockets to operate at twice the permissible maximum loading and it should be remembered that double socket-outlets are not manufactured to be able to withstand a 26A load for sustained periods of time.

"Research by ourselves and third party organisations has shown that all MK double sockets can safely withstand a continuous load of 19.5A for an indefinite period. Increasing the load slightly will begin to cause heat and mechanical stresses on the components in a relatively short period. Testing showed that a load of 22.3A was sufficient to cause heat stress that would cause a browning of the faceplates and sufficient heat to cause insulation damage to cable cores. A load of 24A for 43 hours was sufficient to cause significant heat damage to the material in which the socket-outlet was situated and within 75 hours sufficient to cause significant damage that would lead to the very real potential of fire.

"MK recommend that users of their sockets consult professional design Engineers when designing installations to avoid the possibility of heat and mechanical stress to components and installations caused by overloading of MK socket-outlets."

 

[ericmark]

As I said I was surprised when I started to read MK spec on their sockets. To me it does not matter is the socket has USB outlets as well, or has filters on it, or has a RCD combined, or just a simple double socket it is used the same. Be is a grid switch face plate with two single sockets or a double socket again used the same way.

In real terms there should be no or at least very little heat in the socket, but the plug will clearly generate quite a bit of heat. That's how a fuse works. The heat from the plug can escape in two ways, one into the air, and two into the socket, the socket can again get rid of heat in two ways, into the air or into the wall. As a result the colour, finish, conducting properties of the wall will all affect the heat dissipation from a plug.

The point is of course the plug should be in free air. However we have a tendency to fit them where the appliance restricts air flow. In some cases the appliance has fans this is often the case with an oven.

So because it works OK or does not work OK in one house does not mean in another house it will be the same. There are too many variables.

However a normal washing machine only uses the heater for a short time, it also the tumble drier can't normally keep up with the washer. So in real terms the load on a double socket is mainly that of the tumble drier the washer even if it runs for same time will hardly heat the socket up at all. So in real terms a double socket is likely to be able to dissipate more heat than a single socket so even with the washing machine as well will likely run cooler than a single socket running just the tumble drier.

To my mind if a FCU suffers damage due to heat before the fuse blows either the FCU or fuse must be faulty. Although again a FCU in a plaster board back box will likely not dissipate heat as well as one in a metal box fitted in a brick wall. The exception is where the fuse is swapped without allowing the unit to cool first.

Reports seem to say the fuse does not blow? the FCU is damaged without the fuse blowing. Although a 13A fuse can take over 13A at 20A in the 1.5 hours or more the two appliances are running should rupture the fuse, so either fuse is faulty or the average draw of both washer and tumble drier is less than 20A.

So it should not matter if using a FCU or simple spur un-fused there should not be a problem. Personally I would use a FCU with a 13A fuse but really it should not matter.

As to the quote from an MK catalogue do you really think any woman is going to have both washer and drier running continually for 43 hours. Would be doing well to hit 8 hours.

 

[ban-all-sheds]

A quote from an MK catalogue:

So I ask where a standard says something and you quote a manufacturers catalogue.


And a manufacturer who is also wrong.

It should be noted that BS1363 part 2: 1995 does not allow double sockets to operate at twice the permissible maximum loading

It's not that it's not allowed, it is not required.

 

[Risteard]

It's not that it's not allowed, it is not required.

Why don't you prove your assertion with reference to BS1363.

 

[JohnW2]

It's not that it's not allowed, it is not required.

Why don't you prove your assertion with reference to BS1363.

AFAIAA, BS1363 says nothing about what usage of accessories is (or is not) "allowed" - it merely indicates what tests of 'minimum performance' must be passed in order for an accessory to be compliant with the Standard.

Why they chose to require a minimum specification which did not guarantee that a socket remained 'safe' if an 'unsuspecting' member of the general public plugged two 13A (long-term) loads into a double socket is anyone's guess. I suppose they must have thought that the probability of this happening was, in practice, very low - but it still seems rather odd in a world in which regulatory requirements often relate to incredibly improbable situations.

Kind Regards, John

 

[PBC_1966]

but it still seems rather odd in a world in which regulatory requirements often relate to incredibly improbable situations.

Indeed, whatever the chances of it happening, to design something which is intended to be connected to a circuit fused at up to 30/32A, provides two outlets for plugs each of which can be fused at 13A maximum, yet allow the device to overheat dangerously if more than 20A is drawn for an extended period of time seems a rather cavalier approach. But then as has been discussed before, there are many aspects of the whole BS1363 & ring concept which are questionable.

 

[JohnW2]

Indeed, whatever the chances of it happening, to design something which is intended to be connected to a circuit fused at up to 30/32A, provides two outlets for plugs each of which can be fused at 13A maximum, yet allow the device to overheat dangerously if more than 20A is drawn for an extended period of time seems a rather cavalier approach. But then as has been discussed before, there are many aspects of the whole BS1363 & ring concept which are questionable.

Indeed, but this particular issue has got nothing to do with ring finals. A BS1363 double socket is no more required (by the Standard) to be able to safely tolerate 2 x 13A loads for an extended period of time when fed by a radial circuit than when fed by a ring circuit.

Kind Regards, John

 

[PBC_1966]

A BS1363 double socket is no more required (by the Standard) to be able to safely tolerate 2 x 13A loads for an extended period of time when fed by a radial circuit than when fed by a ring circuit.

Agreed, and obviously if it's fed by a 30A ring or a 30A radial makes no difference to the issue at hand. But the BS1363 system was introduced with the ring concept.

 

[AndyPRK]

Oh hello John. Had a nice break. I was missing you.

Extended period is the interesting term. I don't think many people would want it to run for an extended period, as defined by MK's testing

 

[JohnW2]

Extended period is the interesting term. I don't think many people would want it to run for an extended period, as defined by MK's testing

I agree totally - it would be an extremely improbable (i.e. rare) situation. However, as I said, I'm nevertheless surprised, given that many of the regulatory requirements we work with relate to protection related to 'extremely improbable situations'.

Kind Regards, John

 

[stillp]

It's not that it's not allowed, it is not required.

Why don't you prove your assertion with reference to BS1363.

AFAIAA, BS1363 says nothing about what usage of accessories is (or is not) "allowed" - it merely indicates what tests of 'minimum performance' must be passed in order for an accessory to be compliant with the Standard.

That's correct.

Why they chose to require a minimum specification which did not guarantee that a socket remained 'safe' if an 'unsuspecting' member of the general public plugged two 13A (long-term) loads into a double socket is anyone's guess. I suppose they must have thought that the probability of this happening was, in practice, very low - but it still seems rather odd in a world in which regulatory requirements often relate to incredibly improbable situations.

No different from allowing the concept of diversity in other aspects of circuit design.

 

[DetlefSchmitz]

I think it must be pretty clear, if you think about it, that when they introduced the test in the standard, it turned out that 20A was the largest continuous current that the best available samples could cope with.

So a pragmatic decision was made that this level of current would be acceptable as a likely practical maximum.

From what I understand of modern connector theory (e.g. 5A through a tiny USB C-Type connector) there is no reason that those massive 13A plug pins couldn't have coped with the current, but maybe metallurgy science at the time was not up to the job.

Logically, if they had thought that a double socket could cope with 26A they would have made that the standard. I guess we will never find out which manufacturer was most involved with the standard development. But they have a lot to answer for.

 

[stillp]

I think it must be pretty clear, if you think about it, that when they introduced the test in the standard, it turned out that 20A was the largest continuous current that the best available samples could cope with.

I suspect that the drafting group thought that 20A was the greatest current likely to be required in any one place. Remember that in the 40s and 50s any load over 1 kW was quite unusual in a domestic situation.

 

[JohnW2]

No different from allowing the concept of diversity in other aspects of circuit design.[/QUOTE]I think it actually is rather different from almost all other domestic applications of the concept of diversity. Application of diversity does not usually compromise 'safety' in any way. It allows one to use cable of smaller CSA than would be possible without application of diversity, but that cable still has to be protected by an OPD appropriate to its CSA. Hence, the worse that happens 'if the concept of diversity fails' (which, being a statistical concept, it sometimes will) is that the OPD operates.

As above, diversity is a statistical concept, which implicitly accepts that there is a finite probability that the actual current drawn may exceed the current calculated by application of diversity. There is thus a finite (albeit very small) probability that a double socket could be loaded to a level not required by the BS1363 tests.

Don't forget that, in the absence of any required tests at currents >20A, a double socket could theoretically be BS1363-compliant and yet 'fail catastrophically' at currents above 20A carried for relatively short periods of time. Very unlikely - but not impossible.

Kind Regards, John