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Difference in AC and DC disconnects #200885 04/25/11 02:36 PM
Joined: Sep 2003
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bwise121 Offline OP
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I just put in my first DC disconnect and for the life of me I can't see what the physical difference is. Besides the fuses is there a difference? Is there such a thing as a non-fused disconnect?

Thanks,
Byron

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Re: Difference in AC and DC disconnects [Re: bwise121] #200886 04/25/11 02:56 PM
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Vindiceptor Offline
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Is this a PV DC disconnect?

Some manufacturer's have different arc chutes and most will require the use of multiple poles being used to facilitate the breaking of the DC arc.

Square D and Siemens both have wiring diagrams, have not used a Cutler-Hammer Disconnect yet.

Yes, you can get them as non-fused.

Last edited by Vindiceptor; 04/25/11 02:58 PM.
Re: Difference in AC and DC disconnects [Re: bwise121] #200887 04/25/11 03:47 PM
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bwise121 Offline OP
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Yes, it is a 30 amp disconnect for a PV system.

Re: Difference in AC and DC disconnects [Re: bwise121] #200888 04/25/11 03:58 PM
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gfretwell Online Content
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You can usually put a big diode across a DC contact to put out the fire.


Greg Fretwell
Re: Difference in AC and DC disconnects [Re: bwise121] #200889 04/25/11 07:50 PM
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Vindiceptor Offline
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Originally Posted by bwise121
Yes, it is a 30 amp disconnect for a PV system.


2 or 3 pole and how did you route the circuit through the disconnect?

What's the Isc for the circuit? That would determine whether you can run the circuit through a single pole, 2 or three (when 2 is required and you have a fuse, some manufacturer's want the fuse in the center pole 'B phase').

You can NOT route a circuit through a single pole and maintain the UL listing of the switch, Square D used to imply that you could, but they have since reworded their documentation.

I'm only mentioning this because I've seen a LOT of botched/non-code compliant designs and installs using 30, 60, 100 and even 200 amp 600 VDC disconnects with 3 circuits going through them.

We have been using combiner boxes with integral DC disconnects since mid 2009 because of the confusion with AHJ's, unfortunately, most don't even know what they're looking at when they're inspecting them, but they are definitely getting better and more informed since the PV market blew wide open.

Re: Difference in AC and DC disconnects [Re: gfretwell] #200894 04/26/11 12:38 PM
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LarryC Offline
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You can usually put a big diode across a DC contact to put out the fire.

How will that help if the load is not inductive? The diode shorts out the inductive kick from a collapsing magnetic field because the induced voltage is the opposite polarity. A diode or even a RC network across contacts won't do much if the DC load is not inductive.

Re: Difference in AC and DC disconnects [Re: bwise121] #200895 04/26/11 12:57 PM
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gfretwell Online Content
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Non inductive loads break cleaner.


Greg Fretwell
Re: Difference in AC and DC disconnects [Re: bwise121] #200897 04/26/11 04:37 PM
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Tesla Offline
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The DC from PV arrays is NOT an infinite bus.

It breaks MUCH cleaner for that reason.

There is only so much energy flowing into the PV cells.

So even shorted PV DC does not produce the astounding arcs expected.

Not that I'd recommend trying it.


Tesla

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