[RE-wrenches] Unisolar Intermittent Ground Fault

[Chris Mason]

http://www.amazon.com/Extech-380260-Autoranging-Digital-Insulation/dp/B000EWW2UA/ref=sr_1_1?ie=UTF8&qid=1354065758&sr=8-1&keywords=extech+insulation

$239


On Tue, Nov 27, 2012 at 7:32 PM, Robert Nuese <robertnuese at sonic.net> wrote:

> Thanks to the Wrenches who have offered good advice.
>
> In answer to several of you who suggested megger testing,
> No, I'm afraid I don't own a megger. It had become clear to me that
> this might well be a useful tool in my case, but the things are expensive.
> I've managed to install pv systems for 10 years without one, so I can't
> justify buying one just for this one issue. I also had the impression
> that megger testing may be a bit hard to interpret, and perhaps incon-
> clusive, especially for someone with no previous experience.
>
> Now Larry Crutcher has drawn my attention to a Unisolar installation
> guide (newer than versions I've got) that gives instructions on how to
> use the megger to test a Unisolar installation. This is just what I need.
> I'm a bit surprised that the Unisolar techs I'd spoken to on several
> occasions about my problems had never talked about megger testing,
> or these instructions.
>
> So, I need a megger, at least temporarily.
>
> The Unisolar instructions call for the use of the BM21 or other quality
> megger. The BM21 is no longer in production. There are a confusing
> array of different models available. Can anyone offer suggestions
> about other models that would or would not be suitable for my use?
> According to the Megger co. website the new model that replaces the
> BM21 is the MIT525, which I can buy online for $3,576. Whew!
> On the other hand, I gather I can buy a Fluke 1507 insulation tester
> for $500. Would that be adequate? Anyway, even at that much
> cheaper, for solving one problem in 10 years, it would make more
> sense to rent…
>
> I called a few of the equipment rental agencies in my neighborhood,
> but none had ever heard of a megger.
> Does anyone have any idea where I could rent one - or is there a
> wrench in my neck of the woods (70 miles north of San Francisco)
> who would like to rent me theirs?
> Or does anybody have a suggestion about a particularly good deal on
> buying one?
>
> Or maybe there is a nearby wrench who I could hire to help, someone
> with a megger, who already knows how to use it.?
>
> Here is a big question. I had assumed that such testing would have to
> be done on de-energized circuits, which for the solar panel part would
> mean at night. The Unisolar instructions don't mention this. They say
> to disconnect each string and then short each out before the test, using
> a jumper. This strikes me as odd, and a bit scary. Would it not be
> better to do this on a de-energized circuit at night?
>
> Thanks very much for any further information,
> Robert
>
>
>
> Robert,
>
> Having installed hundreds of Uni-Solar modules, the only failure I every
> experienced was a framed US64 that had the PV negative short to frame.
> Flexing the module would create intermittent failure. You have PV laminates
> so your ground fault may be to the metal roof.  I might be stating the
> obvious but here's my list of tests:
>
>
>    - I would disconnect and hi-pot test each laminate (ref. sec. 4.6, pg
>    48, Installation Guide, AA4-3670-05, 10/05).
>    - If that does not produce results, apply pressure with a heavy roller
>    up and down the edges where the diodes and cell interconnect traces are
>    while testing.
>    - Check all wire penetrations into the channel.
>    - Make sure all wire expansion rules were followed, look for stressed
>    wire.
>    - Check for any screws installed in other than the two approved areas.
>    There are two versions of laminates. One has a VERY narrow area. 0.65" by
>    2.75",  for screw penetration (ref. sec. 3, pg 17, Installation Guide,
>    AA4-3670-05, 10/05). A misplaced screw may have allowed water to migrate to
>    the trace areas and be providing a path through the screw.
>
>
> RIP Stan and Uni-Solar, A brilliant mind and the best performing PV's ever.
>
> Larry Crutcher
> Starlight Solar Power Systems
>
>
>
>
>
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-- 
Chris Mason
President, Comet Systems Ltd
www.cometenergysystems.com
Cell: 264.235.5670
Skype: netconcepts
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