[RE-wrenches] Module shorting to ground?

[Rebecca Lundberg]

I have an array that has been up and running with no problems for 6
months. The array includes 58 Sanyo HIT 220 watt N-Series modules, 13
of which are on Enphase M210 and the rest in strings to (2) Power One
transformerless (ungrounded) inverters. This week one of the Power One
inverters shows a RISO error, indicating a connection to ground on one
of the current carrying conductors.

I went out yesterday to look at the array, thinking these were the
three potential issues (in descending order of likelihood):
- animal damage to the wires (though there are no trees near the house at all)
- contractor damage to wires (no work has been done on roof or attic
since we were there)
- inverter problem

I found one inverter input that shows continuity to ground at the DC
disconnect (which indicates the inverter is doing just what it is
designed to do!). I opened the Soladeck on the roof (which is where
the PV output circuit/PV wire is transitioned to wire in conduit run
through the house), disconnected the connection, and found the PV
output circuit itself shows continuity to ground. I inspected the
array carefully, and all wires are strapped securely off of the roof
and out of sight, everything looks just as I left it 6 months ago.
I've dealt with significant animal damage before and know what to look
for.

My question: I can lift each of these 8 modules in the string up and
check the wires carefully, but outside of that being a hidden cause,
is it possible that one of the modules itself is shorting to ground?
If so, how do I check that and what else will I need to do to prove
that to the distributor for warranty replacement? So far I've used the
continuity tone on the volt-meter for testing the short to ground. I
have a megger but it is new, so, therefore, is my knowledge of how to
use it...

Thanks in advance.

Keep Shining!
Rebecca Lundberg
NABCEP Certified Solar PV Installer ®
Owner/CEO, Powerfully Green
Rebecca.Lundberg at PowerfullyGreen.com
763-438-1976