Using a MCB as a main switch

[mickfred]

My boss has just took over a new site and I went to look the layout. Some boards are damaged some cables need replacing and am sure some other stuff. The site has a old busbar with isolators feed various boards and of her isolators. It also has a board connected directly on to the busbar . The main switch for this board is rated 125a the tails are 35mm ,but the fuse in the maintenance isolators are 250amp DD fuses. So the board and the tails have no protection. My question is (and I know it's late so be gentle) could I swap the main switch for a MCB , or will I have to relocate and feed it off a isolator with correct fuses (the board is a Hager tp and n Invicta3 ) . Now to go home and bed

 

[Deleted member 26818]

What is the distance from the 250A fuses to the 125A board?

 

[Ian1981]

Check out section 433 and 434 regarding placement of an OCPD at less than 3 meters Where there’s a change in the ccc of conductors and the following conditions it requires where overload and fault protection are placed on the load side.

 

[westward10]

Reduce the size of the fuses.

 

[telectrix]

Reduce the size of the fuses.

but then they'd fall out of the fuse holder..

 

[MDJ]

Very common to have tails connected to a 250 amp busbar and then straight into an Isolator, I have seen 6mm2 tails connected to a 400 amp busbar before with the 32 amp Isolator bolted to it, very common practice years ago and you will see Busbars now still all over the country with the same set ups. In factories for example everything was bolted to a busbar and the tails from them to the busbar no where near the size of the incommer, no problem at all under this set up.

 

[Bobster]

Very common to have tails connected to a 250 amp busbar and then straight into an Isolator, I have seen 6mm2 tails connected to a 400 amp busbar before with the 32 amp Isolator bolted to it, very common practice years ago and you will see Busbars now still all over the country with the same set ups. In factories for example everything was bolted to a busbar and the tails from them to the busbar no where near the size of the incommer, no problem at all under this set up.

It's still common practice among experienced electricians.

There are specific regs that allow this practice.

 

[Deleted member 26818]

It's still common practice among experienced electricians.

There are specific regs that allow this practice.

I believe you’ll find that there are.

 

[Dillb]

Unless I have read it incorrectly, the OP is saying the DB is fed direct from the busbar which feeds several circuits. So there is no overload protection for the tails feeding the DB.

 

[Andy78]

Unless I have read it incorrectly, the OP is saying the DB is fed direct from the busbar which feeds several circuits. So there is no overload protection for the tails feeding the DB.

There is also a 250A fused isolator in there somewhere but I can't decipher if it before the busbar chamber or betwixt chamber and DB.

 

[Deleted member 26818]

I’m reading it, that there are 3 x 250A fuses, then the bus bar and then the feed to the DB taken off the bus bar.
Regulations allow protective devices to be installed within 3m of where a reduction in CCC of a conductor occurs.
If the 250A fuses are correct for the bus bar and the tails between the bus bar and the DB are less than 3m, there’s no problem.

 

[Dillb]

There is also a 250A fused isolator in there somewhere but I can't decipher if it before the busbar chamber or betwixt chamber and DB.

I think thats whats is protecting the cables feeding the bus if the OP has expressed concern, also 35mm tails feeding the DB

 

[mickfred]

Yes the 250ampnduses are protecting the whole busbar other isolators are connected going off the. Feed other stuff (not important) but the board in question is bolted on to the busbar with no peotprotec . I could of course remove the board and fit a isolator and relocate the board (but am lazy).tha t is why am asking about the MCB for main switch. (sorry if the details in my post was confusing , just wanted to give as much detail as Possible

 

[Nigel]

I’m reading it, that there are 3 x 250A fuses, then the bus bar and then the feed to the DB taken off the bus bar.
Regulations allow protective devices to be installed within 3m of where a reduction in CCC of a conductor occurs.
If the 250A fuses are correct for the bus bar and the tails between the bus bar and the DB are less than 3m, there’s no problem.

There is a problem if the DB has the potential to take the current through those 35mm tails over what they can handle and lower than the 250A fuses.

Op - there needs to be some sort of OCPD between the bus bar and the DB. You need to ensure those tails can not be overloaded and if I have read the op correctly those tails, in there current state could be overloaded long before the 250A fuses blow.

 

[Nigel]

Yes the 250ampnduses are protecting the whole busbar other isolators are connected going off the. Feed other stuff (not important) but the board in question is bolted on to the busbar with no peotprotec . I could of course remove the board and fit a isolator and relocate the board (but am lazy).tha t is why am asking about the MCB for main switch. (sorry if the details in my post was confusing , just wanted to give as much detail as Possible

Are you talking about replacing the main switch with say a 63A MCB in the DB?