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Topic Title: Help in calculation
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Created On: 22 March 2013 07:11 PM
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 22 March 2013 07:11 PM
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frspikeyhead

Posts: 804
Joined: 27 December 2004

I have just put in a sub main for a customer which is 15 metres from the main intake (TN-S, 80A BS1361, 25 mm tails). Ive run 16mm 3 core swa to second CU, swa protected via BS EN 60269 63A fuse. First thing, I am pretty sure the fuses will not discriminate therefore I ve asked DNO to upgrade main fuse to 100A. Second, I was recently browsing over calculations concerning protection trying to revamp my fading knowledge. Normally I'd have no concerns over a sub run in 16mm 3 core swa and protected via a 63/80A fuse. However I am not so sure now having concerned myself with fusing factors etc that say the current carrying capacity of the cable needs to be 1.45 times the nominal rating of the fuse. I haven't got the green book with me but that would mean that 16mm is undersized for 63A fuse as that comes to around 92A. In fact it would generally mean that most domestic premises with 100A main fuse have undersized 25mm tails. Am I missing something here??????
 22 March 2013 08:28 PM
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Avatar for UKPN.
UKPN

Posts: 502
Joined: 17 January 2012

put that green book away-
100amp service with 25sqmm + 63amp sub main with 16sqmm is spot on.
worry no more.

Regards
 22 March 2013 08:35 PM
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frspikeyhead

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Joined: 27 December 2004

I know its ok as its what I have usually done but now I have opened the can I need to understand the worms.
 22 March 2013 09:20 PM
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AJJewsbury

Posts: 11379
Joined: 13 August 2003

that say the current carrying capacity of the cable needs to be 1.45 times the nominal rating of the fuse

Am I missing something here??????

You are indeed - the requirement is that 1.45x the cable rating >= current causing effective operation of the fuse (I2 ) - not its rating (In).

For fuses to BS 88-2, BS 88-3 etc selecting In <= Iz automatically means that I2 <= 1.45 Iz. - so you've no need to worry - the Green book says so!

- Andy.
 22 March 2013 09:29 PM
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slittle

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Originally posted by: frspikeyhead

I have just put in a sub main for a customer which is 15 metres from the main intake (TN-S, 80A BS1361, 25 mm tails). Ive run 16mm 3 core swa to second CU, swa protected via BS EN 60269 63A fuse. First thing, I am pretty sure the fuses will not discriminate therefore I ve asked DNO to upgrade main fuse to 100A. Second, I was recently browsing over calculations concerning protection trying to revamp my fading knowledge. Normally I'd have no concerns over a sub run in 16mm 3 core swa and protected via a 63/80A fuse. However I am not so sure now having concerned myself with fusing factors etc that say the current carrying capacity of the cable needs to be 1.45 times the nominal rating of the fuse. I haven't got the green book with me but that would mean that 16mm is undersized for 63A fuse as that comes to around 92A. In fact it would generally mean that most domestic premises with 100A main fuse have undersized 25mm tails. Am I missing something here??????


Nothing wrong with the 80A fuses in the cutout. The chances of the 100 A fuses grading with the 63A ones in the event of a close short circuit are minimal.
We've got loads of sites with 100A in the cutout and 100A in our switch fuse. It's 50/50 who's lets go first but as long as one does it's done the job. Ok if it's the cutout fuse you've got to wait for those nice people from the DNO to come out but chances are if you've managed to do that you're going to be onsite for a few hours fixing a nasty fault anyway.

Stu
 22 March 2013 09:49 PM
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Parsley

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Joined: 04 November 2004

From app 4

Where overload protection is required, the type of protection does not affect the current-carrying capacity of a cable for continuous service (Iz) but it may affect the choice of conductor size. The operating conditions of a cable are influenced not only by the limiting conductor temperature for continuous service, but also by the conductor temperature which might be attained during the conventional operating time of the overload protective device, in the event of an overload.

This means that the operating current of the protective device must not exceed 1.45 Iz. Where the protective device is a fuse to BS 88 series, a circuit-breaker to BS EN 60898 or BS EN 60947-2 or a residual current circuit-breaker with integral overcurrent protection to BS EN 61009-1 (RCBO), this requirement is satisfied by selecting a value of Iz not less than In.

In practice, because of the standard steps in ratings of fuses and circuit-breakers, it is often necessary to select a value of In exceeding Ib. In that case, because it is also necessary for Iz in turn to be not less than the selected value of In, the choice of conductor cross-sectional area may be dictated by the overload conditions and the current-carrying capacity (Iz) of the conductors will not always be fully utilised.

The size needed for a conductor protected against overload by a BS 3036 semi-enclosed fuse can be obtained by the use of a rating factor, 1.45/2 = 0.725, which results in the same degree of protection as that afforded by other overload protective devices. This factor is to be applied to the nominal rating of the fuse as a divisor, thus indicating the minimum value of It required of the conductor to be protected. In this case also, the choice of conductor size is dictated by the overload conditions and the current-carrying capacity (Iz) of the conductors cannot be fully utilised.

Hope that helps, I really must try to get out more!

Regards
 22 March 2013 10:03 PM
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frspikeyhead

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Joined: 27 December 2004

I think I am just getting older and forgetting all this stuff. This is no joke but you've got to keep going.
 22 March 2013 10:11 PM
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UKPN

Posts: 502
Joined: 17 January 2012

only remember the important stuff! the detail is in the books.

Regards
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