Discrimination

[atm84]

I am using Amtech software and I am having trouble trying to achieve discrimination between a 100 amp SPN MCCB (Schneider TM-D) and a 200Amp BS88 in a TPN Switch Fuse.

Anyone else had this problem and know how to sort it. I can only achieve discrimination if I change the mccb for a 63 Amp one but that isn't what I want really. It's serving a submain circuit.

 

[Darkwood]

Limited size front end fusing will stop you achieving full discrimination, unless you are at a design stage and can increase the front-end fuse rating then partial discrimination is sometimes the best that can be achieved, take domestic for example, a S/C can sometimes take out the DNO fuse even against a 32amp mcb, although the do counter this effect by fitting type III or IV, although if we did that we wouldn't meet Zs permitted values but the DNO are not under the BS7671.

I can't remember the MCCB - BS88 discrim' values but Schneider NS100 would require a NS250 to meet full discrimination.

If you think about it, a DB with approx 80amp load + 20% spare would need to be supplied with a supply cable large enough to meet Zs from a 250amp MCCB


PS - Edit .... why not uprate the BS88's and take steps to reduce Zs values, not sure of your set-up but as you have a 100 MCCB then you can treat it as a fixed load as it can only draw the 100amp of the SP MCCB protecting it.

 

[atm84]

It's a refurb project and the dno cutout is rated at 200 amp. Downstream of that is a S/F unit with 200A BS88 fuses. That subsequently serves an mccb panel board where I want to fit a 100amp or even a 125amp mccb. However my discrimination calcs don't allow and biggest I can get away with is a 63amp

 

[atm84]

The reason I want to fit a 100amp or 125amp mccb is because of discrimination further down with 32amp mcbs

 

[Leesparkykent]

Darkwoods solution in the edited part of his post is a good solution. I would ignore the DNO fuse as that's only there to protect their equipment.

 

[atm84]

Ok thanks for the advice

 

[Darkwood]

A 100amp mccb would only achieve full discrimination with up to a 16amp breaker, in order to give full discrimination you would be looking at a 160amp front end, this should give you discrimination to the full range of 1 - 63amp mcbs.

To design a install correctly it can mean cables are many orders larger than the load they will ever carry just to achieve discrim', this is why building with small supplies <200amp are limiting with achieving full disrimination.

What I find is this is a subject often mis-interpreted and often misunderstood, many sparks believe that say a 63amp mcb will discrim' against a 10amp mcb, that fact is that the 1000+ amps that flow during a dead short is well beyond the partial discrim' ampage that is covered by the size difference, this results in either or both devices been just as likely to trip.

Discrimination does vary between brands and fuse types, I was referencing schneider as an example with typical Type (B) mcb's.

 

[PEG]

It is commonplace,to encounter experienced sparks,who do not grasp the theory behind these facts,and assume the function of some discrimination ideas,as acceptable,purely because the magnitude of fault events,has not caused an issue.

On many occasions,arriving long after the design/install stages,one just has to accept,the possibility of a large fault magnitude,causing more than the original OCPD to let go.

The rectification of such possible problems,being a step too far,in terms of cost and upheaval.

 

[atm84]

Hi Darkwood, that is very true. The largest mcb for a final circuit on one of the sub boards is 32amp and to achieve full discrimination with that I require a 125amp mccb however this would then not achieve discrimination with the 200amp bs88 fuses installed in the ukpn service head let alone the 200amp bs88 fuses installed in the existing switch fuse unit which will be located upstream of the panel board

 

[Darkwood]

You need to rememeber here what is protecting what, if you have a SWA supply cable to a submain then in the scope of things and the limitations of the supply size, it not really an issue to contemplate a S/C on the sub- supply cable. Given that the service fuses are not typically type II but higher means there is also scope for movement too (finding comparison tables or time/current charts for DNO equipment is another thing).

You really need to concentrate on the final circuits against the submains distribution fuse, its on these final circuits that it is more than likely to experience a S/C as oppose to the to the submain cable itself which would look at the difference between the submains fuse and the cutout fuse, as long as your well clear of your final circuits against the cutout fuse then your OK.

This is a subject that IMHO isn't covered enough by any coursework and leads to many a bad design out there, also a company who achieves discrimation in design and tenders a quote can fall foul of another company not taking into account dicrim' and under cutting costs by a good margin, this may not come to light until many years down the line where there is little that can be done about it.

 

[atm84]

You have hit the nail on the head Darkwood. I agree, this subject is not taught enough. Many thanks for your advice. I agree, the main concern is discrim against my final circuit device and sub main device, which I have managed to achieve.

In regards to tenders, I agree. I really do wonder how many contractors do actually carry out full discrimination checks and then 'just hope for the best'

 

[GEOFF S]

I am using Amtech software and I am having trouble trying to achieve discrimination between a 100 amp SPN MCCB (Schneider TM-D) and a 200Amp BS88 in a TPN Switch Fuse.

Anyone else had this problem and know how to sort it. I can only achieve discrimination if I change the mccb for a 63 Amp one but that isn't what I want really. It's serving a submain circuit.

Hi

What is the max fault level at the MCCB, id be surprised that discrimination can not be achieved.

Cheers

 

[atm84]

It's not just energy based discrimination but time based too

 

[GEOFF S]

It's not just energy based discrimination but time based too

Hi

So you are using time cascade and current based discrimination? Anyhow what are the fault current levels?

Cheers

 

[atm84]

Yes both discrimination. Fault current levels (based on the generic 16kA at the incomer) there is a fault current of 12kA at the mccb