Cable Over Current Protection

[seajayess]

OK guys need some advice.

Scenario 400 Amp 3 Phase head supplying 3 Phase isolator, Supplying a 3 Phase buzz bar. All this is connected with 150mm Cable so no problems here, The issue is there is a 24 way 3 phase consumer unit connected directly to the buzz bar with 25mm Tails, Does the consumer unit need to have a 3 Phase MCB as the main switch I know it will only be 63 Amps, or a 100 Amp 3 Phase Fused Isolator to protect the 25 mm Tails from over current, or is it OK to connect to the Consumer Unit just with a 3 Phase Isolator as a main switch with no over current protection. The Consumer Unit is within 2 meter's of the head.

I think it needs over current protection but another electrician thinks an isolator would be OK.

Thanks in advance CJS

 

[revjames]

25mm tails protected by 400A fuses?

 

[seajayess]

This is my point , which is why I want another opinion , as I said if we were to put a 63 Amp 3 Phase MCB instead of main switch or a 100 Amp Fused Isolator before the Consumer Unit it would be OK.

 

[Sidewinder]

There is a reg to allow this I think something to do with close coupling type stuff.

I would like to see a downstream MCCB at least as the incomer as it may not be possible to fit protection upstream.

This is common with bus bar chamber type installs, as ususal no book around, but IIRC there is something like this allowed!

 

[steptoe]

I have done similar, but usually a SW Fuse is between chamber and DB,

I think it is permissable to do without a S/F but it would be dependant on loading of DB,

Im really not too clued up so maybe take sidewinders or someone elses advice on this.

 

[revjames]

On the overhead cranes in the Al smelter the pick up rails were protected by 1600A CB Pick up shoes fed power to the meain isolator on the bridge. On the supply side there was a piece of 1.5mm that fed a 6A MCB for the auxilliaries, RC etc. So technically, 1.5mm cable protected by a 1600A breaker? I never liked it but the engineer didnt see a problem.

 

[steptoe]

ps, depending on earthing system as to whether you need a 3p isolator/S.F/MCCB or 4 pole .

 

[Sidewinder]

Peeps,

This is not a criticism of anyone, we are getting more and more complex questions as well as the more & more simple questions it seems on here too!

BTW this is NOT a reflection on the OP or any other posts in this thread, it just popped into my head, as this is not an easy one to justify.

I may take my brb tomorrow when being driven to the job and look it up.

We can discuss it then mate!

 

[Sidewinder]

Try 433.3.1 & 434.2, there may be more or others not allowing it IIRC these were the ones that popped into my mind immediately even though the numbers escaped me!

 

[Badger]

Peeps,This is not a criticism of anyone, we are getting more and more complex questions as well as the more & more simple questions it seems on here too!

BTW this is NOT a reflection on the OP or any other posts in this thread, it just popped into my head, as this is not an easy one to justify.

I may take my brb tomorrow when being driven to the job and look it up.

We can discuss it then mate!

aww sidewinder have you been banned from driving again :slap

 

[sparkytim]

BRB Pg 74 & 75

434.2.1

433.3.1

 

[kme]

Now I may be missing summat here; but surely the MD of the 24 way board will have an effect on the answer; i.e.

If the load through the DB approaches or exceeds the capacity of the 25mm tails, what is going to protect them?

Similarly on the DB main switch - Unless it is absolute and definite that the board cannot be asked to deliver >100A, I`d want a linked MCB (3 or 4 pole, as Albert noted).

KME

 

[Chr!s]

Now I may be missing summat here; but surely the MD of the 24 way board will have an effect on the answer; i.e.If the load through the DB approaches or exceeds the capacity of the 25mm tails, what is going to protect them?

Similarly on the DB main switch - Unless it is absolute and definite that the board cannot be asked to deliver >100A, I`d want a linked MCB (3 or 4 pole, as Albert noted).

The problem would be on short circuit if you used a mcb you would have no short circuit discrimination, it would need to be fused.

 

[seajayess]

Now I may be missing summat here; but surely the MD of the 24 way board will have an effect on the answer; i.e.If the load through the DB approaches or exceeds the capacity of the 25mm tails, what is going to protect them?

Similarly on the DB main switch - Unless it is absolute and definite that the board cannot be asked to deliver >100A, I`d want a linked MCB (3 or 4 pole, as Albert noted).

KME

This is my point exactly, but this morning was told that the BRB as said in a previous post does allow it , and I have been over ruled but thanks for the replies CJS

 

[binky]

Think loading is the issue as alredy pointed out - bit like old houses with seperate DB for shower in 6mm tails protected by DNO fuse - not an issue as fuse in DB prevents overloading the tails.

 

[Chr!s]

This is my point exactly, but this morning was told that the BRB as said in a previous post does allow it , and I have been over ruled but thanks for the replies CJS

The only reg applicable that i can see would be 433.3.1, is it possible for a 24 way board to be overloaded?

and it still needs to be protected against fault current reg 434

 

[Andy™]

id change the main switch to MCB as minimum

 

[Sidewinder]

Andy,

Doubtful you will get an mcb to fit in place of the main fuse it will almost certainly be an mccb.

This will need the thermal & magnetic settings adjusting to meet the requirements of the load.

It can be allowable to supply from reduced size wiring, I left my brb in the hosue this am, and spent the journey chatting with my chauffer!

Only an hour anyway.

Is it possible to overload the "tails"? I'll use the term tails from now on to mean the reduced size links to the bus bars.

Can they be damaged by short circuit?

OK, can any individual circuit fault on the outgoing db overload the tails?

What are the chances of 2 simultaneous faults overloading the tails?

What is the total load capability of the board, does this overload the tails, please remember that many 125A 3ph boards have diversity of 0.8 or 0.9.

Thus what is the maximum steady state current that can be drawn from this board?

Will this overload the tails.

As Plumber says, discrimination is not "automatic" with mcb/mcb cascading, it is possible, but difficult to work out without suitable software to assist.

Next... ; )

 

[Chr!s]

As Plumber says, discrimination is not "automatic" with mcb/mcb cascading, it is possible, but difficult to work out without suitable software to assist.

It all depends upon the pfc, if its high, you will require fused protection.

I would agree with kme fused protection, unless 433.3.3 is applicable. Dont agree with using 433.3.1 as there is a risk of overload.