2P 63A mcb Rating help?

[bk81]

So your trying to get a 125A supply from a DB by using 2 x 63A MCBs?

Wouldn't recommend it! Can see all sorts of issues with it!

Hi yellowvanman,

Can you tell some of the issues?

I'm trying to find out

A. If it will work
B. what the current rating would be

 

[yellowvanman]

1st thoughts:

Safe isolation and ensuring that both MCBs are somehow interlinked
Just the fact that you have to strip the conductor back far enough to split the wires into two seperate breakers
Would the overcurrent and fault protection characteristics of 2 x 63A really be equal to 1 x 125A?
Just seems rough generally?

I assume you're doing this because you can't get a 125A MCB.

The technique of using parallel conductors are covered in the regs: 433.4 & 434.4 - maybe better if you did it that way - in which case you are working within the regs. Although some of that doesn't get over my issue with safe isolation where >1 protective device protects one circuit.

 

[bk81]

You've become too cynical yellowvanman. lol There's also no parallel conductor, 1 x 50mm.

A 2P mcb is interlinked not just with the bar at the front but also internally so safe isolation procedures can still be followed with the appropriate locking off kit etc.

The 50mm single cable is connected to the 2P mcb with a parallel connector like the one below (3 pole version), so there will definately be no stripping back and splitting the cable to get in to the breaker. Hadn't even crossed my mind to do that, you must have seen too many cowboys! lol

 

[Spoon]

Why not just use a single pole 125A? Talking about cowboys... if they do use a 2P 63A mcb it will look like a really poor design. (thats putting it politely... not wanting to swear after that thread about people swearing)

 

[bk81]

The reason is the supply busbars would have to be altered as the type of mcb's used only come in 1.5P 125A, so in order to keep modifications to a minimum we're trying to use a 2P mcb.

On all the spec sheets from the manufacturer they confirm that the 2P mcb can be used in this way. The system is a dc power supply, this is quite a standard item to see in this industry.

 

[Darkwood]

Its extremely difficult to answer your questions when we are drip fed the info bk81.... you have now told us its DC that you have which has its own criteria to be met..... i see you are probably just out off your comfort zone and getting all confused, the DC set-up is required to be know i.e-

A-Source has 1 polarity earthed
B-Source has middle point earthed
C-source is insulated from earth

Each method requires different set-ups of isolation also you need to consult the manufactuers of the mcb's to refer to any adjustments needed of calculations to be made to ensure the mcb to get the correct rating under its particular use... and is the negative isolated also through part of a DP device that isolates both positive and negative.

What is the voltage?
Is this a supply as you suggest or part of a control circuit/system?

In your position id be weary going in all guns blazing i have a suspicion these guys are clued up and i can't see issue creating a parallel path through the protective device to double its rating as long as all other adjustments to device rating have been calculated with regard to DC use etc. I would be keen for anyone to explain why the set-up is cowboy and where any issues lie.... as goes this thread we dont have half the info needed to make such an accusation.

 

[Darkwood]

Why not just use a single pole 125A? Talking about cowboys... if they do use a 2P 63A mcb it will look like a really poor design. (thats putting it politely... not wanting to swear after that thread about people swearing)

Explain why?..... just because something is out of your experience area it is assumed to be poor design or of a cowboy nature.... the OP has expressed limitations on other methods so why in your opinion is the suggested method poor?

 

[bk81]

Darkwood,

Fair point, here's the details.

-48Vdc telecom power supply, so positive side is grounded (A)

The data sheets say this type of GE G60 Type C 2P 63A is paralleled you can use up to 110Vdc.

I think a lot of the confusion has been caused by assumptions of it being live on 1P and N on the other.

but regardless if the MCB is for AC or DC use (assuming I have already got confirmation that it's ok for its intended use), the current rating is the thing I'm trying to figure out.

I should have asked;

If a 2P 63A mcb is to be used in parallel with 1 load cable connected over the 2 poles what would the current rating of the device be?

It's not about trying to prove someone wrong or proving a point, just would like to know for future reference...

 

[Darkwood]

Like anything on here if not specified we assume normal 230/400v ACdepending on nature of question and most of the info supplied in the later posts is vital to the set-up you have and changes the whole scope of the thread ....im off back to work now and will have a gander later if of course you haven't already received the info you require by then il see if i can help you.

If you send the current through a DP mcb it will share its load down both side in the same way parallel cables work... the exact rating of the mcb needs calculation but in essence is 63amps each side, effectively 126amps across the device minus any grouping, dc use adjustments etc etc that you need to apply this also my vary from band to brand of mcb.

 

[Spoon]

Explain why?..... just because something is out of your experience area it is assumed to be poor design or of a cowboy nature.... the OP has expressed limitations on other methods so why in your opinion is the suggested method poor?

It was only after my post that the OP expresses the limitations....