Gridtie, breaking grounded conductor [RE-wrenches]

[Bill Brooks]

Robert,

You are correct about a-Si arrays. This is due primarily to the lower
quality glass used in a-Si modules. That is why I said it can make you
dance. At PVUSA we had the largest a-Si array in the world. It was very
scary stuff--you were much more concerned about ground faults than the
capacitive charge. As John B. pointed out, you will measure a voltage to
ground, but depending on the size of the array, that voltage may go to zero
due to the meter, or it may not-even if there are no ground faults with a
large a-Si array. With a ground fault, it will never go to zero unless you
hard ground the wire. I'm sure you could come up with a situation where you
could get severely hurt on a large a-Si array, but the probability of
getting killed because of a ground-fault is FAR higher.

NEVER touch wires that are connected to PV modules, because, as Joel says,
that is begging for a Darwin award--and it is a bad habit to get in to even
when it is relative benign.

You are also correct about the listing of the SB inverter. Connecting the
ground in a way other than according to the manufacturers directions is a
violation of the listing--the listing is REQUIRED by the code. The Fine
Print Note is merely a SUGGESTION and does not carry the weight of the
listing requirement (690.60). These misunderstandings come from well-meaning
inspectors that have not received enough education about PV systems.

Bill.



-----Original Message-----
From: Robert Warren [mailto:robertwarren at mail.com]
Sent: Tuesday, February 24, 2004 11:01 PM
To: RE-wrenches at topica.com
Subject: RE: Gridtie, breaking grounded conductor [RE-wrenches]


If the PV system was one with amorphous modules instead of crystaline,
that capacitive charge that John B. and Bill Brooks speak of is not such
a small shock: it can pack quite a wallop! Apparently the nature of
amorphous cells is conducive to building up and holding a huge capactive
charge. Sure, it will zero out through your DVM, but it it happens to
zero out through your fingers, it can knock you on your butt, as friend
or mine in Scotland found out while testing a 12 kW Unisolar amorphous
system while standing on a metal roof.
 The lesson here is to always treat all wires as live. I don't think
this means that we need a disconnect on the roof, though.
 As for the location of the connection of the negative to ground (or the
suggestion that it be possible to break it), doesn't the recommended
practice of a UL listed device such as the Sunnyboy inverters
automatically supercede any new and fuzzy installation concept that
inspectors might come up with? I remember a clause in the NEC code
saying something to the effect that the manufacturer recommendations may
superceed the NEC code in terms of being more safe than Code. Let the
GFI do its job.
Robert Warren


John Berdner wrote:
>
> The shock the inspector experienced form the negative conductor is a
> capacitive discharge due to the small capacitive coupling between the
> array and earth (as Bill Brooks correctly pointed out).
> You can verify this with a DVM measurement from PV + to ground and PV -
> to ground with the GFDI fuse removed.
> Unless you have a ground fault the system is ungrounded at that point
> and you should see about 0.5 * Voc on each measurement.
> This voltage should decline towards zero as the capacitive coupling is
> discharged through the DVM.
> A fixed voltage that does not decline normally indicates a ground
> fault.
>



Check out my website on making your own fuel:
http://running_on_alcohol.tripod.com
robertwarren at mail.com

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