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.
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?
If an accessory with a nominal rating of 13A, which in electrical engineering typically denotes its continuous operating rating, cannot carry 13A continuously and indefinitely, safely, in normal use (as suggested in the article, recommending limitation to 11.7A for continuous operation) then far from being over-engineered, I would argue it is concerningly under-engineered.
That in many applications the load current is below 13A and the accessory operates satisfactory is irrelevant - equipment should be capable of operating safely at its rated capability. If it cannot, then the standards which specify the design and testing of that equipment need improving to ensure it can function as specified, or the ratings within those standards need correcting the reflect the actual capability of the accessory.
The 13A plug or fused connection unit, should be re-named to reflect the reality that with the current standards and manufacturing practices, it is now actually an 11.7A device.
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?
I would argue that in engineering, specifications and ratings are very important and it is important that equipment is designed and testing so that operates safely at its rated values, so a device with a 13A rating should be capable of operating continuous and safely at 13A, without relying on introducing de-rating factors or other obscure caveats, whereby 13A actually means 13A for 30 mins and 11.7A continuously.
In this case, the better and safer approach should be to have the nominal rating changed to 11.7A with an overload rating of 13A for 30 mins.
The 13A plug or fused connection unit, should be re-named to reflect the reality that with the current standards and manufacturing practices, it is now actually an 11.7A device.
I disagree ... we are looking at currents likely to exceed 13 A for the most part with a 3 kW immersion heater, but also environmental conditions outside those in the product standard.
In addition, there's no problem with an appliance that intermittently uses 2990 W.
I would argue that in engineering, specifications and ratings are very important and it is important that equipment is designed and testing so that operates safely at its rated values, so a device with a 13A rating should be capable of operating continuous and safely at 13A, without relying on introducing de-rating factors or other obscure caveats,
That's interesting, when we accept OCPDs that don't operate until at least 1.4×In.
In reality, we are looking at definitions ... the fuse rating means it is rated to carry the current under specified conditions (not operate at that current), and similarly a BS 1363-4 FCU is rated to carry 13 A under specified conditions (and perhaps with a tolerance).
The mis-match is that there's the tolerance permitted by BS EN 60335-1, applied in addition to a tolerance that relates to utilization voltage range.
So, I agree that we need to be sure about what tolerances we are working with, in the same way that we know we are happy saying that protection against overload is generally provided by Iz≥In even though we know the OCPD won't operate until at least 1.4×In, AND knowing it's still necessary for the BS 7671 requirement that circuits shall be arranged so that small overloads for long duration do not occur ...
However, these points are really points for product standard committees to decide on ... noting that the general public won't get anywhere near the information we are looking at in this thread.
I disagree ... we are looking at currents likely to exceed 13 A for the most part with a 3 kW immersion heater, but also environmental conditions outside those in the product standard.
That is not what the article concludes, it refers to all domestic appliances, not just immersion heaters:
"In general, the maximum domestic appliance loads that use full power for periods above 30 minutes, connected to BS 1363 plugs and fused connection units, is limited to 2.7 kW (11.7 A)."
The implication of this statement is that the continuous current rating for safe operation of BS1363 plugs and fused connection units is only 11.7A, it is not 13A.
In addition, there's no problem with an appliance that intermittently uses 2990 W.
A higher, limited-duration rating for a piece of electrical equipment is an overload rating, the article is in-effect proposing that BS1363 plugs and fused connection units have a continuous rating of 11.7A and a 30 minute over-load rating of 13A.
The nominal ratings used to describe electrical equipment are normally the continuous ratings, we shouldn't be using the 13A overload rating as the nominal rating, if this should be limited to only 30 minutes for safe operation.
That's interesting, when we accept OCPDs that don't operate until at least 1.4×In.
In reality, we are looking at definitions ... the fuse rating means it is rated to carry the current under specified conditions (not operate at that current), and similarly a BS 1363-4 FCU is rated to carry 13 A under specified conditions (and perhaps with a tolerance).
Yes, for OCPDs and fuses we use the continuous rating of the device, we expect it to be able to operate continuously for an indefinite period at that nominal rating, we do not use a short-term overload rating for the fuse to describe its nominal capability.
If the 'specified conditions' for BS1363-4 FCU is that it can only carry 13A safely for a maximum of 30 minutes, then it should correctly be described as an FCU rated for 11.7A continuous and suitable for overload at 13A for upto 30 minutes.
An interesting debate between TurboGen and GK, but I am leaning towards the former.
Suppose that your car tyres are rated at 70 mph (in reality, perhaps 130 mph, or more). Would you assume that you can travel for hours at that speed (e.g. from the Channel coast to Côté d'Azure), or could you do it only intermittently?
An interesting debate between TurboGen and GK, but I am leaning towards the former.
Suppose that your car tyres are rated at 70 mph (in reality, perhaps 130 mph, or more). Would you assume that you can travel for hours at that speed (e.g. from the Channel coast to Côté d'Azure), or could you do it only intermittently?
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