3 phase distribution

[Tom Ashcroft]

Hi all,
Just an enquiry..
Our company have recently moved factory's. I am talking a very small unit.
At the moment there is one TPN distribution board which is at the top end of the factory..
Is there any problem running a 3 phase supply off a 63amp mcb which will feed the new d/b just to keep run of cables down to small sockets and lighting power.

 

[Leesparkykent]

There will be a lack of discrimination having MCB's in series. A switch fuse/fused switch to feed new distribution circuit would be a better design IMO.

 

[Tom Ashcroft]

I would still have to feed the fused switch off the existing d/b though off a 63amp MCB.

 

[Leesparkykent]

I would still have to feed the fused switch off the existing d/b though off a 63amp MCB.

That would be pointless though as your left with the same situation you were trying to eliminate by using a switch fuse. Why would you have to feed it from an MCB? What make is the existing DB?

 

[Tom Ashcroft]

The existing d/b is a Schneider ISO acti 9

 

[Leesparkykent]

The existing d/b is a Schneider ISO acti 9

3 of these.

100A 1 Pole Terminal Block | City Electrical Factors - http://www.cef.co.uk/catalogue/products/4080524-100a-1-pole-terminal-block

 

[westward10]

I see the discrimination point but in this situation I probably wouldn't consider it too much.

 

[i=p/u]

Either or providing you've got lights to see what u doing

 

[Darkwood]

Depends here on your supply incoming, if you only have a 3ph 100amp supply then discrimination is impossible and at best only partial which really means nothing, if the company would see great losses at a power failure due to lack of dscrimination if the wrong device tripped then it's a problem, if this is just an inconvenience then as long as it is made clear to the customer it is really not an issue, if it is then you need to split the supply if the option exists and fit a larger cable and have fully discriminated front end fusing, as long as the customer is made aware of all the options and get it in writing or email then your --- is covered.

 

[westward10]

Think this is being over whipped, most of us ignore discrimination everyday by having 13A fuses in our plug tops and FCUs.

 

[Darkwood]

On a factory level install, discrimination is important as it can be costly to get it wrong especially if production time is lost through bad discrimination, agree about domestic as the incoming is too small to achieve full discrimination and any circuit really so more often than not it not even factored in, these 5week short courses probably don't even bother teaching it although I stand corrected on that point.

 

[westward10]

I wonder what sort of "factory" it is with one TP board at the main intake.

 

[Leesparkykent]

Fuses compliant to BS EN / IEC 60269-1 or 60269-2-1 will provide total short circuit selectivity/discrimination when the rated current of the upstream fuse is 1.6 times the rated current of the downstream fuse. If the main fuse was 100A then ideally the fuse downstream ( in the switch fuse) would be rated at no more than 63A if full selectivity was/wanted to be achieved.

 

[Darkwood]

That's fine Lee until you start mixing ocpd types ..ie you need a 160amp mccb to give full discrimination on an mcb board and if I recall a 63amp mcb give full discrimination again similar mcbs only up to 10amp ....not sure of the standards you hi-light there but are you sure it's against short circuit with full discrimination when you cite those standards, sounds like overload discrimination to me.

 

[Leesparkykent]

That's fine Lee until you start mixing ocpd types ..ie you need a 160amp mccb to give full discrimination on an mcb board and if I recall a 63amp mcb

Yeah I agree that things become a little more tricky when mixing OCPD types.

60269-1 is the international standard for Low-voltage fuses - General requirements and 60269-2 is the international standard for Low-voltage fuses - Supplementary requirements for fuses for use by unskilled persons.

View attachment 35139

The OP says the new board is for small power and lighting. Looking through various charts and comparing the total operating I2t of a 32A B type MCB and pre-arching I2t of the 63A fuse he should be good up to a maximum fault current value for which full discrimination is ensured of around 2.4KA. If the Zdb at the new board was lower than 0.10 ohms he might have a problem.