2-pole DC CB wiring

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2-pole DC CB wiring

Postby dRdoS7 » Tue Mar 07, 2017 2:48 pm

Hi,

I have a 2-pole 20A DC CB, I want to use it as a CB / isolation for 2 separate 12v circuits.

It has (+|-) at the top and (-|+) at the bottom, but that would be for Pos / Neg.

All wiring diagrams I have seen are for Pos / Neg, but no Pos / Pos.

If I put Pos in at the top of each, would that be OK?

I just want to make sure my thinking is correct.

Thanks,

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Re: 2-pole DC CB wiring

Postby APR » Tue Mar 07, 2017 11:17 pm

dRdoS7 wrote:I have a 2-pole 20A DC CB, I want to use it as a CB / isolation for 2 separate 12v circuit.
Thanks,
dRdoS7


Hi dRdo57, what you most likely have is a polarised DC CB. I expect the voltage rating of this CB is well over 12v... most likely over 60v.

When switching higher DC voltages it is possible to have an arc jump the open contacts, and it has been common for DC CBs to have mechanisms designed into them to extinguish this arcing. Polarised breakers are designed to extinguish this arc when they are connected polarity correct. You can also get unpolarised DC breakers which are designed to extinguish an arc no matter the polarity of the wiring to the breaker.

At 12v I wouldn't expect arcing to be an issue unless you are, say, switching the supply to a device that generates a lot of back EMF when the circuit is broken, and outputs a voltage on the breaker that is well over 12v.

What are you switching?
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Re: 2-pole DC CB wiring

Postby dRdoS7 » Wed Mar 08, 2017 9:26 am

Hi,

Yes, I would have said it was polarised.

It has DC800V, and 6000A, so well inside what I want it for.

The question is whether it is OK to put pos into the top of both, it makes sense to me.

But, if I look at this:

2-pole DC CB wiring.jpg
2-pole DC CB wiring.jpg (34.36 KiB) Viewed 562 times


It implies if I have 2 pos. I would put one in at the top, and the other in at the bottom.

I have never seen it drawn that way, so thought I should ask. Rather than lash out and by 2 new CBs, I could use the 2-pole I already have.

As you say if it's low voltage, and the amps are low (> 20A), arcing isn't going to be a problem.

Using it for 12v pump & lights.

Thanks,

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Re: 2-pole DC CB wiring

Postby APR » Wed Mar 08, 2017 10:35 am

I'd be using it in the application you are describing. At 12v the amount of arcing will be minimal, and I wouldn't expect to see any premature failure because of polarity reversal. It may be, depending on the design, that the amps at which it breaks open may become less accurate.

It may be worthwhile to incorporate a fuse in the cable from the energy source instead of relying on the CB for protection. I have a few 63A 250v DC Schneider Electric polarised circuit breakers that I have used on 12v and 48v in an application not dissimilar to what you are proposing. I have had no issues, although they have not been switched under load very often. No issues with arcing when switching 48volt at a few hundred watt when the polarity is reversed.
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Re: 2-pole DC CB wiring

Postby Gordon-Loomberah » Wed Mar 08, 2017 11:01 am

At 12V I dont think it will make much difference which way around you wire them, as I think 16 or 17V DC is required to maintain an arc.
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Re: 2-pole DC CB wiring

Postby dRdoS7 » Wed Mar 08, 2017 11:25 am

Hi,

OK. Yes to fuses.

Thanks,

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Re: 2-pole DC CB wiring

Postby davidg » Thu Mar 09, 2017 10:32 pm

dRdoS7 wrote:But, if I look at this:

2-pole DC CB wiring.jpg


It implies if I have 2 pos. I would put one in at the top, and the other in at the bottom.


That is the correct way, think of it this way the input connections are more positive or more negative compared to the output connection/s.
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Re: 2-pole DC CB wiring

Postby dRdoS7 » Fri Mar 10, 2017 12:05 pm

Hi,

davidg wrote:
dRdoS7 wrote:It implies if I have 2 pos. I would put one in at the top, and the other in at the bottom.

That is the correct way, think of it this way the input connections are more positive or more negative compared to the output connection/s.


OK. :shock: (couldn't resist given it's electricity!)

Thanks,

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Re: 2-pole DC CB wiring

Postby davidg » Fri Mar 10, 2017 1:41 pm

This is the reason why polarity sensitive DC CB's were done away with for installations, a amazing amount of these were incorrectly wired. It is not really a problem for voltage of approx 48V or less as there is not really an serious issue given that you are dealing with substantially lower voltages compared to grid connected systems.
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