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Topic Title: fuse discrimination
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Created On: 15 February 2013 08:22 AM
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 15 February 2013 08:22 AM
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misterben

Posts: 415
Joined: 11 June 2007

Hi all,

Can anyone tell me whether a 1361 type 2 100 amp fuse at the meter will discriminate with a BS 88 100amp on a submains?
If the customer has a 100amp cutout and the submains has a 100 fuse then you would want the submains to blow first, but surely downgrading it to say an 80amp could potentially leave the customer short of 20amps.
I agree that most domestic supplies will not use this much but just wanted to be sure, any help appreciated.

regards
Misterben
 15 February 2013 09:05 AM
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broadgage

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It is most unlikely that two somwhat similar fuses of the same nominal rating will discriminate.

I would not worry in most cases.

In a properly designed and installed job, the operation of the cut out fuse, or a similar fuse on the consumers installation should be an extremely rare event and having to replace either or both fuses is therefore of little consequence.
Are you worried that the 100 amp cut out is likely to blow ? is so then the load is too great or the service insufficient.
If you are satisfied that the service is ample, then your 100 amp fuse wont blow.

From a strictly electrical point of view there is little or no point in fusing submains at the same current as the service cut out. It is however a requirement of the DNO.
Whilst the DNO state that their cut out fuse is not to protect the consumers installation, in practice adding a second fuse of the same size does nothing.

Against what eventuality does the 100 amp BS88 fuse protect, that is not covered by the BS1361 fuse ?
 15 February 2013 09:07 AM
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zeeper

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BS88-2 80A, looks like it will hold a 100Amps forever anyway . 140ish Amps for 6000s
 15 February 2013 09:08 AM
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misterben

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Hi,

I wasnt worried about the situation with regards overloading, to be honest its paperwork that needs filling out and its been picked up that they dont discriminate.maybe i should just change it to an 80?

regards
Misterben
 15 February 2013 09:39 AM
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OMS

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Can anyone tell me whether a 1361 type 2 100 amp fuse at the meter will discriminate with a BS 88 100amp on a submains?


Not a hope in hell

maybe i should just change it to an 80?


Almost no point - to clear the 80A, you'll be way beyond the pre arcing I2t of the 100A anyway

First question, why do you (or whoever) think discrimination (or coordination) is required.

You have a 100A and an 80A fuse in series - assuming no branches, then it doesn't matter which fuse operates - you'll still be off supply.

The only possible circumstance you might want total discrimination is if that 100A fuse feeds an assembly that more than one submain leaving it - and in that case, your individual sub main protection would be far smaller than 100A - at worst 63A and probably smaller. For a close up fault, you might jus tget away with a 100A grading with a 63A depending on the prevailing fault level.

Regards

OMS

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Failure is always an option
 15 February 2013 09:58 AM
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misterben

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I have been told that my EIC is incorrect due to having these fuses installed and to change it and my certificate.
This is from an QS on an N***IC company, as far as I am aware as a rough rule approximately half the size of fuse would be required to correctly discriminate?

regards
Misterben
 15 February 2013 10:05 AM
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zeeper

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My Advice, slot in an 80 And move on.
 15 February 2013 10:15 AM
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John Peckham

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As OMS says why is selectivity (aka discrimination) required?

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John Peckham

http://www.astutetechnicalservices.co.uk/
 15 February 2013 10:53 AM
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OMS

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Originally posted by: misterben

I have been told that my EIC is incorrect due to having these fuses installed and to change it and my certificate.

This is from an QS on an N***IC company, as far as I am aware as a rough rule approximately half the size of fuse would be required to correctly discriminate?

regards

Misterben


If he's a NICE QS, as him for the regulation number - that'll shut him up for a few hours. If he eventually comes back with 536.2 ask him what danger will occur when either fuse operating causes "off supply" - ditto for proper functioning of the installation - try not to laugh at him when you do this of course

A rough rule for fuses would be to miss out a fuse size (also true for MCCB, where you would miss out a frame size)

So for 100A, drop to 80A, miss that out and try a 63A fuse. In most cases, that'll work OK

Regards

OMS

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Failure is always an option
 15 February 2013 11:08 AM
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misterben

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Thanks everyone,

I will do that , is there a table or chart that shows what fuses will discriminate or do I have to work it out via some other way?

thanks

Misterben
 15 February 2013 11:24 AM
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broadgage

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Which fuses will discriminate with which depends to an extent on the fault current.
But in most cases the advice given in a previous post is fine, if you drop two standard sizes from one fuse to the next they will discriminate in but unlikely circumstances.

80 amp following 100, doubtful
63 amp following 100 amp, not absolutely reliable but in practice normally OK.
 15 February 2013 11:27 AM
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zeeper

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If he eventually comes back with 536.2


But the Cutout fuse would not come under the scope of 7671and does not provide protection for the consumers installation.

The whole argument is with out merit.
 15 February 2013 11:36 AM
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UKPN

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-zeeper is correct-leave it @ 100. 7671? (not again)
 15 February 2013 11:42 AM
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OMS

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Try looking for what we used to call a "bullrush chart" - it will show vertical ines for each fuse - usually a thin line for pre arcing I2t and a thicker section at the top showing the arcing I2t (and it looks like a bullrush) - the overall height is the total I2t

For the relevant fault level just run a ruler across and by observation you'll see what fuses discriminate - basically you need the total I2t of the down stream device to be less than the pre arcing I2t of the upstream device.

You can also find the same information in tables from fuse manufacturers

Regards

OMS

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Failure is always an option
 15 February 2013 02:08 PM
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misterben

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I have emailed lawson fuses hopefully for a chart as i cant find one elsewhere. In the mean time I will contact the QS armed with my new knowledge!

regards

Misterben
 15 February 2013 02:16 PM
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OMS

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Originally posted by: zeeper

If he eventually comes back with 536.2


But the Cutout fuse would not come under the scope of 7671and does not provide protection for the consumers installation.

The whole argument is with out merit.


Really - try looking at ESQCR and then read regulation 434.3 (iii) in relation to the tails from the meter to the consumer unit as an example - the cut out fuse most certainly does provide protection for the consumers installation - it's a statutory obligation that it do so.

The regulation is valid, but as I explained, not relevant in this case as it is simply two fuses in series - discrimination isn't an issue - and it's unlikely to cause danger.

Regards

OMS

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Failure is always an option
 15 February 2013 02:53 PM
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rogersmith7671

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Quote;
Can anyone tell me whether a 1361 type 2 100 amp fuse at the meter will discriminate with a BS 88 100amp on a sub-mains?
If the customer has a 100amp cutout and the submains has a 100 fuse then you would want the sub-mains to blow first, but surely downgrading it to say an 80amp could potentially leave the customer short of 20amps.
I agree that most domestic supplies will not use this much but just wanted to be sure, any help appreciated.

I think your QS is right to ask for discrimination in this case, as pointed out, the supply fuse is in series with your sub-main fuse . Your best approach should have been to provide adequate discrimination for the sub-main, at say, a junction between the supply fuse and any appropriate part of the main installation. At present the design cannot confirm that in the event of a fault the supply fuse will not disconnect before the sub-main, with all the obvious and possibly dangerous consequences for the main installation being plunged into darkness at a minimum.

Regards
 15 February 2013 03:03 PM
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OMS

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Originally posted by: rogersmith7671

Quote;

Can anyone tell me whether a 1361 type 2 100 amp fuse at the meter will discriminate with a BS 88 100amp on a sub-mains?

If the customer has a 100amp cutout and the submains has a 100 fuse then you would want the sub-mains to blow first, but surely downgrading it to say an 80amp could potentially leave the customer short of 20amps.

I agree that most domestic supplies will not use this much but just wanted to be sure, any help appreciated.



I think your QS is right to ask for discrimination in this case, as pointed out, the supply fuse is in series with your sub-main fuse . Your best approach should have been to provide adequate discrimination for the sub-main, at say, a junction between the supply fuse and any appropriate part of the main installation. At present the design cannot confirm that in the event of a fault the supply fuse will not disconnect before the sub-main, with all the obvious and possibly dangerous consequences for the main installation being plunged into darkness at a minimum.



Regards


I think we have a different picture of the installation Roger - cut out fuse to meter to consumers main switch to consumers installation (ie at the end of a sub main) is how I read it.

If that is the case, it doesn't make a blind bit of difference which fuse blows - the installation is plunged into darkness anyway regardless if it's the DNO fuse or the consumers sub main fuse operates.

There is no reason for discrimination - in fact to do so would deny the consumer approx 40% of available supply capacity and achieve nothing.

Regards

OMS

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Failure is always an option
 15 February 2013 03:10 PM
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misterben

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Just to clarify it is a 100amp BS1361- Meter - 5419 Wylex (BS88) then on to consumer unit

regards
Misterben
 15 February 2013 03:30 PM
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OMS

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That's what I had in mind - so just leave the 100A BS 88 where it is and tell the QS he's an idiot.

Regards

OMS

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Failure is always an option
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