2.5mm2 cable protected by 13A BS1362 fuse...

[MattChan]

On further googling I found these fuse factors:
BS88 1.6
BS1361 1.5
BS1362 1.9
Type 1/2/3/4 <= 10A 1.5
Type 1/2/3/4 > 10A 1.35
Type B/C/D 1.45
BS 3036 2.0

The 1.35 factor would be caught by clause (ii) and would be limited to a 1.45 factor.

All other types will have a derating to fully comply I guess.

433.1.202 does seem to be superfluous. I assume this is to draw attention to this one needing a substantial correction where as the others would be "within tolerances". I assume they don't draw attention to BS1362 as they hadn't thought of the FCU situation.

 

[JohnW2]

... The 1.35 factor would be caught by clause (ii) and would be limited to a 1.45 factor. ... All other types will have a derating to fully comply I guess.
433.1.202 does seem to be superfluous. I assume this is to draw attention to this one needing a substantial correction where as the others would be "within tolerances". I assume they don't draw attention to BS1362 as they hadn't thought of the FCU situation.

Actually, on further reflection, when I wrote ...

... what is the point of 433.1.1(ii)? Is it simply there in case one has a device with a fusing factor less than 1.45 (if they exist) - since, if it is 1.45 or more, it's surely inevitable that anything which satisfies (iii) will also satisfy (ii)?

... given that the tabulated figures for Iz presumably assume a 'fusing factor' of 1.45, with a 'fusing factor' <1.45 (and you have now found one!), 433.1.1(ii) would actually be unnecessarily onerous, wouldn't it?

Kind Regards, John

 

[plugwash]

I doubt that any installer would in any remotely normal circumstance run cable through the middle of insulation, they would run it either under or over the insulation so it is only surrounded on one side.

What I suspect all too often happens is that cables are run on top of insulation, and then later more insulation is added, so the cables are now completely enclosed by the insulation.

 

[JohnW2]

I doubt that any installer would in any remotely normal circumstance run cable through the middle of insulation, they would run it either under or over the insulation so it is only surrounded on one side.

Agreed - with the caveat that "any installer" is perhaps going too far, since some have been known to do totally bizarre things!.

What I suspect all too often happens is that cables are run on top of insulation, and then later more insulation is added, so the cables are now completely enclosed by the insulation.

Indeed - and is is presumably the changes in recommendations/practices in relation to insulation which most often causes that 'more insulation' to be added.

Returning to the figures, the realisation (thanks to nusku) that 433.1.1(iii) is written in terms of I2 has made me think more widely.

Given that it seems that the fusing factor of a BS1362 fuse may be as high as 1.9, the required cable CCC (as tabulated, assuming a fusing factor of 1.45) to be adequately protected by a 13A BS1362 fuse (and assuming no other de-rating factors) presumably could be as high as 17A.

In terms of fixed T+E wiring, that will probably rarely be an issue - since even 1.5mm² (and certainly 2.5mm²) T+E will, under most circumstances and with the most common/sensible installation methods have a tabulated CCC >17A. However, when it comes to flexible cables (per Table 4F3A), 1.5mm² would be inadequate, and at least 2.5mm² would be needed (and I would have thought that the wiring of, say, immersion heaters, is probably 'fixed' enough to be within the scope of BS7671).

However, whilst I realise that it is nearly always going to be beyond the scope of BS7671, if one utilised the same calculation [in particularly, per 433.1.1(iii)] for flexible cables connected to BS1363 plugs, one would presumably come to the conclusion that (unless one were able to invoke an argument that overload protection was not required) when the plug contains a 13A fuse, the connected cable should be at least 2.5mm².

Is any of that reasoning incorrect?

Kind Regards, John

 

[JohnW2]

duplicate deleted

 

[JohnW2]

In the loft:
Ambient temp assumed to be 50degC worst case:
Therefore tabulated cable rating: 17.03 / 0.71 = 23.99A (is 0.71 correct for the fuse type, I see some records point to 0.87 for non 1.45 fuse factor devices)
Cable laid over 100mm of rockwool with 200mm of rockwool on top of it for 3m length. Reference method 103 or clipped direct x 0.5 assumed.
10mm2 = 32A is the smallest cable with a valid rating at 50degC

No-one has so far commented on the impracticality/impossibility of what calculations based on your assumptions would theoretically require ...

... some will argue (strictly correctly) that no BS1363 sockets are 'rated' to accommodate 10mm² conductors. However, many are 'rated' for 2 x 6mm² and/or 3 x 4mm² (12mm² total in either case) and, in practice, such sockets will usually accommodate (single) 10mm² conductors without difficulty.

However, what I'm pretty certain you will not find is a socket with terminals which could accommodate two 10mm² conductors - so you would not be able to wire a radial supplying 4 sockets with 10mm² cable.

Having said that, although strictly non-compliant, 6mm² T+E (two conductors of which size would fit into many/most sockets) has a tabulated Iz of 23.5A (for Method 103), which is incredibly close to your calculated 23.99A requirement.

However, we keep coming back to the problem of your installation method - since it does seem to be pretty 'ridiculous' to have to contemplate even 6mm² for a "13A circuit"!

Kind Regards, John

 

[EFLImpudence]

I keep wanting to say that what you are discussing is 'just not done' - because it isn't, yet I can find nothing wrong with the figures and so no reason why it should not be done.

There are 'guides' (and you know my views on them) that say all the common OPDs, except BS3036 fuses, may be treated the same as MCBs, having a fusing factor of 1.45, but obviously they are not the same.


Would the same calculations not apply to a cable from a plug with a 13A fuse? Where does that leave us?

 

[EFLImpudence]

Re: the 10mm² surrounded by thermal insulation and socket terminals:

It only has to be 10mm² in the insulation; not the whole circuit.

 

[JohnW2]

I keep wanting to say that what you are discussing is 'just not done' - because it isn't, yet I can find nothing wrong with the figures and so no reason why it should not be done.

Exactly the same here.

There are 'guides' (and you know my views on them) that say all the common OPDs, except BS3036 fuses, may be treated the same as MCBs, having a fusing factor of 1.45...

Indeed, that's what I've always 'thought' - and, indeed, is what I was saying in the earlier part of this thread. However, now that I have been made to consider what 433.1.1(iii) 'actually says', that is not the case. In ≤ Iz (Iz being derived from Tables which already assume a FF of 1.45) is only correct (assuming no other de-rating factoirs) if the FF is 1.45 (or less). For a a FF >1.45, the criterion becomes [ In x (FF/1.45) ] ≤ Iz (as tabulated)

Would the same calculations not apply to a cable from a plug with a 13A fuse? Where does that leave us?

Quite so - see what I wrote in msg #19 above. If the conditions of omission of overload protection were not satisfied, such a calculation would require a flex of at least 2.5mm² if the plug had a 13A fuse. Of course, that is probably not usually within the scope of BS7671.

Kind Regards, John

 

[JohnW2]

Re: the 10mm² surrounded by thermal insulation and socket terminals: It only has to be 10mm² in the insulation; not the whole circuit.

Very true, but 'the alternative' would require one or more 'unnecessary joints' in the wiring, which is never a desirable thing.

Kind Regards, John

 

[MattChan]

Cable rating on an extension lead is likely covered in EN61242 section 11 preview here: https://www.evs.ee/products/evs-en-61242-2001

I would assume a plug top fuse is not providing overload protection on anything but an extension lead?

 

[JohnW2]

Cable rating on an extension lead is likely covered in EN61242 section 11 preview here: https://www.evs.ee/products/evs-en-61242-2001

I wasn't actually thinking/talking about extension leads ...

I would assume a plug top fuse is not providing overload protection on anything but an extension lead?

On the contrary, given that the cable connected to a plug will usually have too small a CSA to be adequately protected by a socket circuit's OPD, any such cable requires overload protection to be provided by the fuse in the plug - the only exception being when it can be argued that the nature of the load (connected to the other end of the cable) is such that it 'cannot' (or, at least, 'is very unlikely to') result in an 'overload' (as opposed to 'fault') current - that is probably true of loads which consist of just resistive heating elements etc. (immersions, ovens etc.) but not, for example, in the case of a load which includes a significant motor (which can result in an overload current if stalled).

Kind Regards, John

 

[MattChan]

I have done a little more reading of actual product design standards to try and look in to this and have pulled out some of the definitions used within them.

EN60335-1: Household and similar electrical appliances - Safety - Part 1: General requirements.
3.1.6 rated current
current assigned to the appliance by the manufacturer
NOTE If no current is assigned to the appliance, the rated current is
– for heating appliances, the current calculated from the rated power input and the rated voltage;
– for motor-operated appliances and combined appliances, the current measured when the appliance is supplied at rated voltage and operated under normal operation.

So rated current excludes fault conditions such as a stalled motor.

Table 11:
Rated current >10A <=16A -> Nominal cross section of the supply cord is 1.5mm2 or can be reduced to 1.0mm2 for portable appliances with a cable length of 2m or less.



EN62368-1: Audio/video, information and communication technology equipment - Part 1: Safety requirements
3.3.10.1 rated current
input current of the equipment as declared by the manufacturer at normal operating conditions

Table G.7.2:
Rated current >10A <=16A -> Minimum cross section of the supply cord is 1.5mm2 or can be reduced to 1.0mm2 for detachable power supply cords fitted with the connectors rated 16 A in accordance with IEC 60320-1 (types C19, C21 and C23) provided that the length of the cord does not exceed 2 m.



Non-adiabatic calculations
On further investigation I found some references to using a non-adiabatic approach to calculating ratings for flexible cables and cords. There is a walk through in the link below. The page I have directly linked to shows a BS1362 13A fuse characteristic v 1.0mm2 cable characteristic using the non-adiabatic method. Take a look and see what you think

https://books.google.co.uk/books?id...iabatic for flex&pg=PA140#v=onepage&q&f=false

 

[JohnW2]

Table 11: Rated current >10A <=16A -> Nominal cross section of the supply cord is 1.5mm2 or can be reduced to 1.0mm2 for portable appliances with a cable length of 2m or less.
....
Table G.7.2: Rated current >10A <=16A -> Minimum cross section of the supply cord is 1.5mm2 or can be reduced to 1.0mm2 for detachable power supply cords fitted with the connectors rated 16 A in accordance with IEC 60320-1 (types C19, C21 and C23) provided that the length of the cord does not exceed 2 m.

Yes, we discussed that here a num ber of times before.

However, interesting though this is, it represents a pretty major shift of the goalposts in relation to the primary topic of the thread. In terms of electrical installations, there is no doubt that BS7671 requires any cable (including that downstream of a 13A BS1362 fuse in an FCU) to have adequate overload (as well as fault) protection, the only exception being if the nature of the load is such that it is unlikley to result in overload currents. As I said before, a stalled motor could result in an 'overload' (as opposed to 'fault') current.

Non-adiabatic calculations
On further investigation I found some references to using a non-adiabatic approach to calculating ratings for flexible cables and cords. There is a walk through in the link below. The page I have directly linked to shows a BS1362 13A fuse characteristic v 1.0mm2 cable characteristic using the non-adiabatic method. Take a look and see what you think

Thanks. I'll have a look when I have a moment.

However, as an initial thought (before I see the article) .... "Overload" currents which are insufficient (in magnitude and/or duration) to cause an OPD to operate rapidly will not, by definition, represent adiabatic processes, anyway, so adiabatic calculation were never going to be appropriate in relation to 'overload' (which could be of such a magnitude/duration). Such calculations are appropriate for 'fault' currents since the requirement is for devices to clear such faults very rapidly, such that the process can be considered to be adiabatic.

Kind Regards, John