Factory Ring Main Question

[RyanM]

I was looking through some old factory wiring schematics, where I have started a new job, and don't quite understand them (still an apprentice), and the guys here don't have an answer, so I'm hoping someone here might be able to?

There is an old ring main ran around most of the building in 50mm SWA with a 200A MCCB, which go to various tap off boxes. So SWA in, SWA out of the box, through 4 boxes, then back to the MCCB.

There is no fuse, or switch etc, just bus bar inside the tap off boxes.

From these boxes the cable changes to 25mm or 16mm SWA which then feeds distribution boards with various size switches/isolators on (125A, 100A and 160A etc).

My main query is, what is protecting the cable leaving the tap off boxes? As I know the 200A MCCB isn't?

From my limited knowledge, this surely isn't correct, although I am told they have been inspected and tested?



Thanks for any advice, as I don't really understand.

 

[RF Lighting]

I'm assuming the smaller SWA feeds into a fused device?

If so, the smaller SWA can not be over loaded as it feeds direct into an MCCB which is also the dist board main switch?

If the dist board tries to draw more current than the SWA can carry, the MCCB will open, but leave the rest of the factory on supply.

If there is a fault with the smaller cable before the fusing down, then it will still be of sufficient magnitude to take out the main 200A feeder.


You always need to protect cables against faults and overload. Normally this is achieved by a single device before the cable which has a rating smaller than the cable can safely carry, but in your factory it is achieved by two separate devices at different positions, but the protection is still there.

 

[JohnW2]

If there is a fault with the smaller cable before the fusing down, then it will still be of sufficient magnitude to take out the main 200A feeder.

That presumably relies on satisfactory design? I don't think that a 200A MCCB would 'inevitably' provide adequate fault protection for 16mm² cable, would it?

Kind Regards, John

 

[RF Lighting]

I haven't done the calculations, but at just over twice it's rated CCC I'd bet it'll be fine.

 

[JohnW2]

I haven't done the calculations, but at just over twice it's rated CCC I'd bet it'll be fine.

I haven't done any calculations, either, but I agree that the CCC of the cable would be very unlikely to be a problem - but it wasn't CCC that I was thinking about. I was thinking that, particularly if there were a long run of downstream 16mm², the EFLI (at the point of the fault) might not be low enough for the 200A MCCB to operate (rapidly) in response to an L-E fault that was a long way along that cable.

Kind Regards, John

 

[ban-all-sheds]

the guys here don't have an answer

That's worrying, as it means they don't understand either.

 

[SimonH2]

I was looking through some old factory wiring schematics, where I have started a new job, and don't quite understand them (still an apprentice), and the guys here don't have an answer ...

I'm assuming the smaller SWA feeds into a fused device?

If so, the smaller SWA can not be over loaded as it feeds direct into an MCCB which is also the dist board main switch?

the guys here don't have an answer

That's worrying, as it means they don't understand either.

Putting all that together, you have a recipe for someone adding a new board, and not realising they need overcurrent protection on the supply side of the new board.

But I would not be too surprised about people not knowing why it was done that way. It's really common, in fact I'd say it's the norm, that a lot of stuff is known by those who were involved and those who have learned from them. You can have all the documentation in teh world, and still not have the "why" of stuff.
And of course, when those individuals move on, then the knowledge goes with them.

 

[Adam_151]

I think we need to have pictures if we are to clarify whats going on, thats if you are allowed to take them at work, OP?

It wouldn't be the first time something was done very wrong in a factory by those who ought to know better.

In one place I found that a submain supplying a 100A rated three phase board in 35mm² had been doubled up onto a 400A MCCB supplying a sub panel for the welding bays. No further over current protection was provided. The board was 16way TPN loaded with a mix of 32A and 63A breakers with most ways in use.