Fuses and cable CCC

[JohnW2]

Compliance with 433.1.1(iii) is, indeed,not 'optional', but that does not mean that every circuit complies. My point is simply that IF (iii) is satisfied (which, as said, is a non-negotiable requirement of the regs), then (ii) will also inevitably be satisfied - hence my recent suggestion that 433.1.1(ii) is actually redundant

 

[JohnW2]

One interesting (and, to me, a little surprising) thing that I am reminded of by the table BAS posted is that, whilst BS88s are the same as MCBs in this respect, BS3036 and BS1362 fuses are 'tighter' than MCBs, in the sense of their I2/I1 ratios - that ratio being 1.28, 1.28, 1.11 and 1.1875 for MCBs, BS88s, BS3036s and BS1362s respectively.

Kind Regards, John

 

[ban-all-sheds]

My point is simply that IF (iii) is satisfied (which, as said, is a non-negotiable requirement of the regs), then (ii) will also inevitably be satisfied - hence my recent suggestion that 433.1.1(ii) is actually redundant

Except in the case of BS 3871 MCBs > 10A

 

[JohnW2]

I doubt that many circuits are being designed (or will be designed in the future) with BS 3871 MCBs as their OPD! However, I agree that they are a (rare) example of a type of OPD which has a 'fusing factor' less than 1.45 - so I suppose EFLI would say that, if using one, the cable CCC should (or, at least, could) be 'up-rated'.

 

[ban-all-sheds]

No.

(ii) is more onerous than (iii) for a change, that's all.

 

[JohnW2]

Well, when (as is the usual situation for devices other than 60898 MCBs), (iii) is more onerous than (ii), EFLI sometimes expresses that as "the CCC of the cable has to be de-rated" - so, when (unusually, quite probably uniquely in terms of what devices are around), (ii) is more onerous than (iii) ...... !

 

[ban-all-sheds]

But then he is being as perverse as the people who wrote Appendix 4.

 

[EFLImpudence]

Well, when (as is the usual situation for devices other than 60898 MCBs), (iii) is more onerous than (ii), EFLI sometimes expresses that as "the CCC of the cable has to be de-rated"

Is that not the case?

so, when (unusually, quite probably uniquely in terms of what devices are around), (ii) is more onerous than (iii) ...... !

I don't suppose there would be a cable with a CCC of Iz(1.35/1.45) of the cable chosen but, as said, (ii) would not then be met.
I suppose it could be if there were an infinite number of cable c.s.as.



I'm not sure where perversity comes in to it other than your seeming inability to accept what seems obvious.

 

[ban-all-sheds]

I just find that the way Appendix 4 expresses the calculations is perversely AAF.

 

[securespark]

I'm not sure where perversity comes in to it

Hello, everybody!!!

 

[JohnW2]

I don't suppose there would be a cable with a CCC of Iz(1.35/1.45) of the cable chosen ....

Probably not, but it's very unlikley that 'the cable chosen' would have only "just" (i.e. by the skin of it's teeth) a high enough Iz to satisfy 433.1. That being the case, it's far from impossible that 'the next available cable size down' would be adequate by virtue of the 1.35/1.45 factor.

I'm not sure where perversity comes in to it other than your seeming inability to accept what seems obvious.

I presume that must be directed at BAS, since he is the one who brought perversity into the discussion. However, I do have some sympathy with what he is saying about Appendix 4 being "AAF", and the same is really also true of 433.1.....

... 433.1(i) is fair enough, since one first determines the Ib, and then is required by that reg to select an OPD with an In which is not less than Ib. However, (ii) and (iii) say that In or I2 must not exceed Iz or 1.45Iz respectively. As I think you have implied, that is, indeed, rather 'AAF'. One does not select a cable, find its Iz and then determine what In and I2 the OPD has to have in order to satisfy the regulations. It is the necessary cable size (based on characteristics of the OPD) that one determines, not the characteristics of the OPD (based on the cable size and installation method).

Kind Regards, John

 

[EFLImpudence]

Probably not, but it's very unlikley that 'the cable chosen' would have only "just" (i.e. by the skin of it's teeth) a high enough Iz to satisfy 433.1.

Well, if it is the case and the cable chosen has plenty of tolerance between In and Iz then presumably the next cable size lower will be unsuitable or it would have been chosen in the first place.

That being the case, it's far from impossible that 'the next available cable size down' would be adequate by virtue of the 1.35/1.45 factor.

I'm not sure that is relevant. If it is possible to use that figure for cable CCC (1.45/1.35) 107.5% then the converse is that MCBs are rated at 93% In.

That is: MCBs
6A - 5.58
10A - 9.3
16A - 14.88
20A - 18.6
25A - 23.25
32A - 29.76
40A - 37.2
45A - 41.85
63A - 58.59

I don't think any of those allow the use of a cable that would not have been chosen in the first place.

I presume that must be directed at BAS, since he is the one who brought perversity into the discussion. However, I do have some sympathy with what he is saying about Appendix 4 being "AAF", and the same is really also true of 433.1.....

... 433.1(i) is fair enough, since one first determines the Ib, and then is required by that reg to select an OPD with an In which is not less than Ib. However, (ii) and (iii) say that In or I2 must not exceed Iz or 1.45Iz respectively. As I think you have implied, that is, indeed, rather 'AAF'. One does not select a cable, find its Iz and then determine what In and I2 the OPD has to have in order to satisfy the regulations. It is the necessary cable size (based on characteristics of the OPD) that one determines, not the characteristics of the OPD (based on the cable size and installation method).

I think I said the equivalent a while ago and stated that the lower rated OPD was not an option - but he said I must have the same perversion as the authors of Appendix 4.

 

[JohnW2]

Well, if it is the case and the cable chosen has plenty of tolerance between In and Iz then presumably the next cable size lower will be unsuitable or it would have been chosen in the first place.

Maybe I wasn't clear.

What I was trying to say that if, without any 'up-rating', the cable size chosen has 'plenty of tolerance', but the next cable size down would not be adequate, if one were using a BS 3871 MCB and did apply the 1.45/1.35 factor (what you might call 'up-rating'), the 'next cable size down' might then be adequate.

I think I said the equivalent a while ago and stated that the lower rated OPD was not an option ...

Indeed. I think we're all agreed that to state requirements for In & I2 on the basis of an 'already chosen' cable size/method (hence Iz) is 'back-to front' in terms of how one designs. That being the case, 433.1.1(ii) and (iii) surely should say, "Iz must not be less than In" and "1.45Iz must not be less than I2", respectively, shouldn't they?

Kind Regards, John

 

[EFLImpudence]

What I was trying to say that if, without any 'up-rating', the cable size chosen has 'plenty of tolerance', but the next cable size down would not be adequate, if one were using a BS 3871 MCB and did apply the 1.45/1.35 factor (what you might call 'up-rating'), the 'next cable size down' might then be adequate.

Do my figures not show that that is not the case with appropriately selected cables?
For it to be so would surely mean that you had chosen an oversized cable in the first place.

Here is the other way round for cables- method C:
0.75mm² flex - 6A CCC - 6A MCB..........6.45A after 'uprating' the CCC
1.0mm² flex - 10A CCC - 10A MCB........10.75
1.0mm² T&E - 16A CCC - 16A MCB........17.2
1.5mm² T&E -20A CCC - 20A MCB.........21.5
2.5mm² T&E - 27A CCC - 25A MCB........29.05
4.0mm² T&E - 37A CCC - 32A MCB........39.775
6.0mm² T&E - 47A CCC - 40/45A MCB...50.525
10.mm² T&E - 64A CCC - 63A MCB........67.725
All make no difference - except one - you could have 6mm² with a 50A MCB - but for a shower or oven you can anyway.

I suppose there may be one example for special cables and/or the percentage of derating for other reasons but I'll leave you to find the exact permutation.

 

[JohnW2]

Do my figures not show that that is not the case with appropriately selected cables?

I agree that, because 1.45/1.35 is such a small ratio (only about 1.07), it is unlikely that 'uprating' a cable (because a BS 3871 was being used) would often (if ever - other than the one case you mention) allow one to use a smaller cable.

Thinking back to the converse, the 'de-rating' of CCC for BS 3036s (or, indeed, any fuses) most often leads to the need for a larger cable with Method C installation - since, more-or less by pure coincidence, Method C CCCs for standard CSA cables are mostly identical to, or just slightly above, the In of an available size of MCB. If one is 'sitting on that borderline' (e.g. 6A CCC and 6A MCB, 10A CCC with 10A MCB, 16A CCC with 16A MCB etc.), any device with a 'fusing factor' of 1.45 (even 1.45001 !) would push one over the line into needing a larger cable. If one looks at other installation methods, that 'co-incidence' is much less common, so one is less likely to need a larger cable for a 3036 than for an MCB (since, with an MCB, the CCC of the cable will often necessarily be appreciably greater than the 'next lower' standard MCB).

Kind Regards, John