Selling tactic for MPPT [RE-wrenches]

[Bob-O Schultze, Electron Connection]

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William,
I can't believe that I now find myself on the OTHER side of this 
thing! I fought this battle in Home Power years ago with J.W. He was 
maintaining that you couldn't use QOs in a 24V PV system because the 
Voc adjusted for edge of cloud effect, temperature, and the standard 
25% cushion put it above the 48VDC rating. I STILL think that's 
ridiculous, but this is different. I'm not talking Voc. I'm talking 
the working voltage that the CB is subjected to. By your explanation, 
you could use a QO at 120,480, whatever VDC. If you did that, I 
suspect that the contacts might arc and weld if you actually needed 
them to break. As I said, it will probably work just fine in your 
case and no big deal. On the other hand, an inspector could make the 
valid argument that you are routinely exceeding the UL tested, 
working voltage of the breaker, especially on the PV side of the CC. 
Who needs that? Better to use a different CB or RK-5 fuses. The QO 
has a Aic of 10K Amps, BTW.
Best, bob-O

>
>Isn't the DC rating on the QO circuit breakers at something
>like 5,000 or 10,000 amps interrupting current. Just because
>the OCV of the panels is 80 or 120 VDC, I don't see what
>relevance that has to the interrupting capacity of the breaker.
>At OCV, by definition, there is NO current. The battery
>would be the principal source of significant fault current.
>At 5,000 amps current out of the battery pack, the source
>voltage at fault is obviously going to be lower... well below
>array voltage, probably well below 52 or 48 volts. With
>battery resistance and wire losses considered, I suspect
>that the max fault current at an array combiner for most
>PV/battery systems would be below 5,000 amps.
>
>Shouldn't that be a factor?
>Am I missing something?
>
>/wk
>>
>>Gang,
>>Four 12V PVs in series-especially running into a MPPT controller, 
>>will pretty easily push the incoming voltage over the 48VDC rating 
>>of the QOs. Probably still work fine, but....
>>Bob-O
>>
>>>  Assuming they're Siemens M50 modules, the
>>>manufacturer's  literature
>>>>clearly shows daisy chaining up to 4 modules. Then I would bring  the 2
>>>circuits into a 2 pole QO
>>>  >box indoors with 2 15 A breakers.

-- 
Bob-O Schultze, Electron Connection
PO Box 203, Hornbrook, CA 96044
800.945.7587 or 530.475.3402
fax 530-475-3401
www.electronconnection.com

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