DC CB Wiring

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DC CB Wiring

Postby swampdweller » Fri Aug 01, 2014 2:25 pm

Hello out there. I just bought a non-polarised 2 Pole DC circuit breaker to hook up my PV panels to the regulator and aso act as an isolator for the panels. I was surprised to see the wiring diagram included with the CB shows the 2 poles be linked at the bottom of the CB and both the supply and load connected to the top terminals. In other words, the 2 poles of the CB are hooked up in series and only one leg from the panel is switched, the other leg would be connected direct to the regulator or wherever.

Does this make sense? I had assumed that the positive was connected through one pole and the negative through the other, so that if tripped or turned off, both leads to the panels would be isolated.
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Re: DC CB Wiring

Postby Tracker » Fri Aug 01, 2014 2:42 pm

..
So they are suggesting that a double pole CB is just series'ed, to give give more isolation..
I prefer just cross-coupled as the contacts are moving in a different way for the current (ie. POS)..

Either way - The advantage is that you have two contacts opening together, hence reducing any arcing..

This is a pretty common practice, and I have done it with quad CB's, to disconnect both POS and NEG..

Te big issue is the VOLTage of the system.. If you are running ELV for off grid type applications, then you are not likely to get into too much trouble, just switching the POS lead..

So - more detail please so we can better picture the setup..

PS - you suggest POS and NEG being connected.. That could only happen if you switch both the POS and NEG with one pole each..
In a way you are justified to wonder just what the difference will be.. Either way you are switching the same voltage and current.
Again - two poles in series will give better DC arc suppression :)
..
.
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Re: DC CB Wiring

Postby Cherokee Solar » Fri Aug 01, 2014 4:47 pm

swampdweller wrote:I had assumed that the positive was connected through one pole and the negative through the other, so that if tripped or turned off, both leads to the panels would be isolated.


That's how I've wired up the DC circuit breakers here and they seem to work fine. I assume the circuit breaker rating in Amps is what you require? They often wire them in series so that you can double the current which will pass through the circuit breaker. I have a triple pole circuit breaker on my inverter for example.
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Re: DC CB Wiring

Postby swampdweller » Fri Aug 01, 2014 5:03 pm

Thanks Tracker,

Its a 12 Volt off grid system with 2 x 200 W 12V PV panels, so it will be lucky to switch more than about 18 Amps or draw much of an arc.

Until I saw the diagram on the CB, my plan was to connect both panels in parallel, +ve through one pole of the CB and -ve through the other, then into the regulator input terminals. From there, the -ve would go straight to the battery -ve terminal, +ve thorugh a Solar/Charger selector switch and a 40A fuse to the battery +ve. The battery is 700Ah 12V LiFeP04 (4 x 3.2V cells in series).

Is this enough info? If not, I may have to get out my felt tips and fire up the scanner to do a circiut diagram.

Nevetheless, I think you're telling me that I can either connect one leg only through both poles in series, or both +ve and -ve legs through one pole each?
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Re: DC CB Wiring

Postby swampdweller » Fri Aug 01, 2014 5:05 pm

Also thanks to Cheroke Solar - yes, the CB has the required current rating in straight through configuration
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Re: DC CB Wiring

Postby Gordon-Loomberah » Fri Aug 01, 2014 5:32 pm

Cherokee Solar wrote: They often wire them in series so that you can double the current which will pass through the circuit breaker.


I suspect you meant to say parallel there Chris. In this 12V case wiring them in series is somewhat of a waste of a CB, since they both are performing exactly the same function, and they don't increase the current carrying capacity at all- they will both trip at the same point, typically about 1 minute at twice their rating, although that can vary depending on the characteristic curves of the particular CB.


I have a triple pole circuit breaker on my inverter for example.


The Latronics inverters have a triple pole CB wired in parallel to increase the current rating.
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Re: DC CB Wiring

Postby Cherokee Solar » Fri Aug 01, 2014 5:43 pm

Gordon-Loomberah wrote:I suspect you meant to say parallel there Chris.


Hi Gordon. Thanks for the - yet again! :) - terminology correction. The cold weather must have frozen my brain. :lol:

Cheers. Chris
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Re: DC CB Wiring

Postby Tracker » Fri Aug 01, 2014 7:57 pm

..
OK ... pretty much sewn up..
The first thought is to wonder if that CB is genuine DC rated or just AC.
20A and 12V... no great dramas here... I would be thinking that the CB is simply a switch..
What else can it be as we know everything about the circuit.. the CB serves NO FUNCTION other than a switch..
There cannot be more than 20A. (Isc), and because it connects PV panels to a regulator, there really can't be any reverse current... ie.. it will NEVER function as anything other than an isolator switch.. it will never trip..

Be happy and connect PV Pos and Neg to the two switches...

Oh and PS... the real protection that is needed is between the battery and the load, and ideally between the battery and the charger (just in case it blows up and goes short)
..
.
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Re: DC CB Wiring

Postby swampdweller » Fri Aug 01, 2014 9:44 pm

Yes, got a big fat HRC fuse for that!

Agree, pretty much wrapped up, thanks everyone. BTW it is definitely a DC CB and I've just realised, it's rated to 1,000 V DC, so I think I've got a bit up my sleeve there! Correct I mainly need it for an isolation switch, also a very handy way to parallel the two incoming leads from the panels, as they are separately wired to reduce Voltage drop and the CB terminals will easily take 2 off 4mm2 cables.
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Re: DC CB Wiring

Postby Tracker » Sat Aug 02, 2014 7:22 am

..
Adding a general thought...
I would look around for say, a 32A double CB, and just use that.. ie.. the 1000V DC CB is a bit of a waste as a simple isolator at 12V... that can never "trip"
No harm to use it, but of NO practical system value.. IMHO
..
.
Retired Engineer and keen PV experimenter - Always ready to learn and share.
2 x CMS2000 (fan cooled) GCI and SE 170W panels
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Governments won't save the world :-) They will just TAX it :-(
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