ryefield board confusion

[apprentice123]

i am only a second year apprentice so please bear this in mind if this question comes across as stupid..

i am currently working on a site where one property (old pub) is being converted into 8 seperate flats, and at the minute we are working on the power distrobution to each individual flat, and this is where my question arises. we are installing a 9 way ryefield board in order to split the supply and to fuse each flat at 60A. my question is that instead of installing this board (which has cost nearly £800) why not just install a standard 3 phase board and put the supply to each flat on a 63A mcb or rcbo? I have already asked the sparky who im working with but his only answer is that thats the way weve been told to do it so thats the way we do it.

thanks in advance for any answers and apologies if there is an obvious answer that im missing.

 

[Darkwood]

Welcome to the forum

If you are on an active course then it may benefit you to apply to join our Trainee section where we have selected mentors to help you and other trainee's at a similar level, its not a requirement but it would be a good opportunity for you.

 

[Leesparkykent]

i am only a second year apprentice so please bear this in mind if this question comes across as stupid..

i am currently working on a site where one property (old pub) is being converted into 8 seperate flats, and at the minute we are working on the power distrobution to each individual flat, and this is where my question arises. we are installing a 9 way ryefield board in order to split the supply and to fuse each flat at 60A. my question is that instead of installing this board (which has cost nearly £800) why not just install a standard 3 phase board and put the supply to each flat on a 63A mcb or rcbo? I have already asked the sparky who im working with but his only answer is that thats the way weve been told to do it so thats the way we do it.

thanks in advance for any answers and apologies if there is an obvious answer that im missing.

most DNOs now stipulate you install a fused switch/ isolator as close as possible to the head then on to a ryefield board or similar like lucy. your are becoming the BNO (building network operator).

Having a fuse protecting the distribution circuit to each flat will provide for better discrimination/selectivity with the mcbs in each flat.

The other issue would be exceeding the maximum current rating of a standard 3 phase board if it was supplying 8 flats.

~https://www.ukpowernetworks.co.uk/internet/en/help-and-advice/documents/BNO-guide.pdf

 

[davesparks]

i am only a second year apprentice so please bear this in mind if this question comes across as stupid..

i am currently working on a site where one property (old pub) is being converted into 8 seperate flats, and at the minute we are working on the power distrobution to each individual flat, and this is where my question arises. we are installing a 9 way ryefield board in order to split the supply and to fuse each flat at 60A. my question is that instead of installing this board (which has cost nearly £800) why not just install a standard 3 phase board and put the supply to each flat on a 63A mcb or rcbo? I have already asked the sparky who im working with but his only answer is that thats the way weve been told to do it so thats the way we do it.

thanks in advance for any answers and apologies if there is an obvious answer that im missing.

I don't think I have ever answered a trainee or apprentice's question with such a hopeless answer, it does rather suggest that this guy may be a little short on knowledge.

I'm not sure how far the college work will have got for a second year apprentice yet, so apologies if you already know this!
Distribution circuits need a little more additional consideration when being designed, and there are a couple of reasons why HRC fuses may be a better choice than mcb's.

Firstly we have the issue of discrimination, which in simple terms means that you don't want a fault on a final circuit causing the distribution circuit to 'trip'. For example if a short circuit occurs on a 32A ring final circuit you can easily get a current high enough to trip a 63A mcb flowing in the circuit, in this case it is pure chance whether the 32A final circuit mcb or the 63A distribution circuit mcb will be the first one to trip.
However if you have an HRC fuse protecting the distribution circuit then it is more likely that the final circuit mcb will operate first.
In actual fact it is not always quite so clear cut as this, and often you need to use manufacturers data sheets to be able to check exactly what will or won't discriminate fully, but should give you a good idea of the basic theory.

Also you may need to consider that the prospective fault current at the intake position may be higher than an mcb can safely break. For example an mcb may only be able to break a 6kA fault current but an HRC fuse will be able to break a much much higher current.

The Zs of a circuit may also have an effect on the decision, as an HRC fuse may allow a higher circuit Zs than an mcb does.

My final comment, which may be what your electrician was alluding to, is that it could simply be that the DNO have specified that you must fit a ryefield for them to connect to. This may have something to do with the board being tested and approved for connection to their network whereas a standard mcb board may not be, or it could be any number of other reasons including them just liking to stamp,their authority over things.