electrical.theiet.org/.../
Personally I think Rotary Isolator would be far better than 20 amp Double-pole switch shown Fig 2 as this allows for isolation and LockOff/LockOut
The cost of the Rotary Isolator is still quite low see URL below. (Other brands and other Wholesalers are available).
www.superlecdirect.com/.../
As always please be polite and respectful in this purely academic debate.
Come on everybody let’s help inspire the future
In the case if a plug, if rated for 13A and your test current is 14A, so +7.7% in current, or +16% in terms of I^2R heating, that's not much of a safety margin to cover, as you mention, effects from degradation - fuse spring pressure, contact oxidation etc all of which will increase heating above that of a newly manufactured plug/socket/accessory.
A subtlety that must not be forgotten, is that there are TWO tolerances to take into account that affect current and hence temperature rise.
The first is the supply and therefore utilization voltage, which can increase to + 10 % of nominal. So, if the nominal current is 11.98 A for a 240 V rated 3 kW heating element, the current at 253 V is 13.18 A ... and I agree that this is covered for in the testing in BS 1363 series.
Further ... BS EN 60335 series permits the load current of a heating element at nominal voltage to be 10 % more than the rating plate rating ... this pushes the maximum up to 14.50 A !!
When we add the increased temperature in the airing cupboard, I think we are well above "reasonable tolerance" for the poor FCU.
Similarly, for a 230 V rated 3 kW element, we have 13.04 A at 230 V (arguably still OK really), 14.34 A with voltage tolerance (now getting a little worrying), and 15.78 A when considering the tolerance permitted by BS EN 60335 series, which again is now getting worryingly high.
In 2025 we should not be having to discuss topics like whether a 13A load can be used with a 13A accessory in a common household application, it should just work and work safely.
There is certainly a question why BS EN 60335 series permits 10 % more current (or power) than the rating plate value at nominal voltage, rather than having a requirement that the rating plate current (or power) at nominal voltage should not exceed the rating plate value.
Combine that with the rather generous (to the manufacturers) allowance of a +52.5 K temperature rise, so in normal 20 deg C ambient conditions allowing a touch temperature of 72.5 deg C, already at a level which causes burns to human skin. There's not a lot of margin to begin with and that's with newly manufactured devices.
If the surface temperature is too far above 70 deg C, the temperature of the terminals is likely to be above 70 deg C ... not good for PVC, and we do see conductor insulation damage at accessories that have been overloaded.
Further ... BS EN 60335 series permits the load current of a heating element at nominal voltage to be 10 % more than the rating plate rating ... this pushes the maximum up to 14.50 A !!
When we add the increased temperature in the airing cupboard, I think we are well above "reasonable tolerance" for the poor FCU.
14.5A is only 11.5% over the 13A nominal, or 24% in terms of I^2R heating, in engineering terms that is not that a big difference compared to nominal, particularly for a situation involving the combination of two pieces of equipment with the same nominal ratings!
Temperatures inside the airing cupboard are not untypical for parts of domestic properties - airing cupboards, boiler cupboards, attics in summer, particularly given that FCUs are used for supply of higher powered heat generating loads - immersion heaters, storage heaters, heat-producing equipment etc.
I rather think to problem lies with insufficient current carrying capability of the accessories, there should be a greater margin between the declared nominal current rating and its actual design and test ratings, a bigger safety margin, to cover these tolerances in voltage, load current, ambient conditions and age-related degradation.
It strikes me as poor practice to have standards with so little margin between the nominal rating and the design/test rating of the accessory such that equipment with the same nominal current rating in typical conditions can take the accessory to the limit or beyond its capability. This then requiring numerous complex caveats and adjustments, to handle relatively typical domestic scenarios, to address what is basically an inadequacy in the standards for the accessory itself.
For example, if an accessory with a nominal 13A rating had a 50% safety margin for I^2 heating, it would be designed and tested at 15.9A, for a 100% margin that would be 18.4A.
I rather think to problem lies with insufficient current carrying capability of the accessories, there should be a greater margin between the declared nominal current rating and its actual design and test ratings, a bigger safety margin, to cover these tolerances in voltage, load current, ambient conditions and age-related degradation.
Difficult to disagree with that.
The easiest means would be to reduce the maximum permitted temperature rise - does it really need to be as much as 52 K?
It strikes me as poor practice to have standards with so little margin between the nominal rating and the design/test rating of the accessory such that equipment with the same nominal current rating in typical conditions can take the accessory to the limit or beyond its capability. This then requiring numerous complex caveats and adjustments, to handle relatively typical domestic scenarios, to address what is basically an inadequacy in the standards for the accessory itself.
In terms of waste and sustainability, some would argue that it is poor practice to over-engineer products when in general their use causes no such problems, and products that utilize 3 kW for extended periods of time can be connected in other ways?
For example, if an accessory with a nominal 13A rating had a 50% safety margin for I^2 heating, it would be designed and tested at 15.9A, for a 100% margin that would be 18.4A.
It doesn't really matter whether you change the product standard to accommodate a higher rating, or select a connection product with a higher rating.
As we have heard in this thread, others just do the latter anyway, so what's the point?
Having said that, there's nothing to stop anyone suggesting this is addressed at the next revision of BS 1363 series.
Whether you reduce the maximum permitted temperature rise, or increase the test current for the same temperature rise, I suspect they would both drive the design towards a similar outcome - reducing heat generation within the accessory from use of bigger CSA conductors, better fuse clips, measures to increase heat dissipation, all generally increasing the safety margin in the design of the accessory.
Whether you reduce the maximum permitted temperature rise, or increase the test current for the same temperature rise, I suspect they would both drive the design towards a similar outcome - reducing heat generation within the accessory from use of bigger CSA conductors, better fuse clips, measures to increase heat dissipation, all generally increasing the safety margin in the design of the accessory.
all generally increasing the safety margin
Beware the fallacy that improving component safety improves system safety. All we do is move the failure somewhere else.
Major fires, it would appear, are roughly every 10 years before the next major 'headline' event, followed by awareness, crackdown, slow fade (regression to the mean;-), focus elsewhere; Oops another fire.
We should know where our 'bottle necks' are and manage them. Slowly burnt out plugs (observable) maybe a good place to see degradations that are then captured by either humans (or remote circuit disconnections).
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