[RE-wrenches] Unisolar Intermittent Ground Fault

[Robert Nuese]

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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