M-C branch plugs: Legal? Safe? [RE-wrenches]

[William Miller]

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Mark:

I think the point here is that the manufacturers of grid-tie inverters have 
certified and tested that their inverters can not back feed power from the 
grid into the PV arrays, even in a catastrophic failure.  I belive the 
manufacturers of charge controllers have not provided this certification, 
ergo you must protect all battery strings with fuses.

Assuming your inspector is knowledgeable on such matters, to be allowed to 
install 2 or 3 strings without fusing, you need to find a document from the 
inverter manufacturer (and I believe, certified from a testing agency) that 
this back feed can never happen.  My web site includes such a declaration 
from Xantrex.  I can no longer find the Fronius or SMA declarations.  If 
the manufacturer's representatives e-mail them to me, I can post them at:
http://mpandc.com/resources/resources.html

Deleting a fused combiner from a grid tie installation is a substantial 
savings in equipment and labor, so this exception is valuable 
to  understand and apply.

I hope this information helps.

William Miller

PS:  A note to Ray Walters:

When calculating string currents, you are allowed to subtract one 
string.  I think this is because in a string failure, you don't need to 
account for the current from that string.  Therefore, you can combine THREE 
strings of Solar World modules without fusing.  Solar World modules are 
unique in this regard.

WM



At 08:33 PM 2/4/2008, you wrote:

>I am starting to be a bit interested in the discussion regarding
>battery-less vs. battery.
>
>What is the difference between having a hugh AC grid on the other side of a
>transistor in an inverter and having a big battery on the other side of a
>transistor in a charge controller.  Are the charge controllers so different
>from the inverters that they can be expected to put current into the source
>circuits? In a fault?
>
>I don't really see how the batteries matter here.
>
>Isn't this really about, generally, depending on the math, that when you
>have three or more strings you should have individual string fusing per the
>SMA white paper.
>
>Perhaps someone could help clarify this for me.
>
>Thanks,
>
>MPF


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