Mike:
Is there such a tool as a DC clamp-on that can reliably measure 10 mA? My
experience is any clamp meter is unreliable below 5A.
William
At 10:09 PM 5/23/2012, Mike Sullivan wrote:
Hi Bob,
Sorry if I am duplicating any of the replies you've already received
regarding the possible ground fault. I think the best way to determine
whether GFDI is tripping on 1 A ground current or 80 A circuit current is
to use a sensitive DC clamp-on to check the current on your system ground
under normal operation. If it is more than about 10 mA when things are
dry, you probably do have a ground fault issue to resolve. Assuming that's
true, I recommend that you buy, borrow or rent a megohm tester (like those
by Megger) to check out your circuit wiring resistance. Once you fully
isolate your strings at the combiner box, you can quickly zero in on where
the fault lies -- just divide and conquer. You may need to wet down the
array and wiring to make the fault more detectable. Often, the fault will
be in wiring that was damaged when pulled from a splice or combiner box
into conduit, esp. if there's no bushing on the conduit terminal adapter.
A word of warning -- if you have more than one ground fault, some parts of
your equipment grounding system will be carrying current, which may be
sizable. Use precautions when disconnecting any wiring or grounds. A
clamp-on is really useful here...
Hope that helps!
Cheers,
Mike Sullivan
Wrenches:
?
Sorry to bring up this situation again, but I could not find a previous
discussion thread that talked about step-by-step procedures for tracking
down ground faults (I believe it was John Berdner who provided the bulk of
the writing in that regard and it related to high voltage strings?not that
75-85 volts per string is all that low).
?
We have an Outback FLEXpower Two system with 8, 3-module strings (Silicon
Energy 190W, 25.3Vmp, 7.5 Imp) combined in a Midnite Solar MNPV12 combiner
with breakers.? The cables from the array are routed to two different
Wiley 4-string pass-through boxes (located under the two center strings)
and then off of the roof and into the PV combiner.
?
Up until this week the system has seen several weeks of awesome power
production.? Yesterday, the rains came and the GFDI breaker tripped.?
Every time it rains, the little green breaker on the GFDI trips off. ?That
tells me that the ground fault is occurring in a place where at least one
of the conductors on one of the strings (possibly more) is getting wet and
conducting current to ground.
?
It would seem to me that the most likely place for moisture to cause a
ground fault to occur in this system would be in the pass-through boxes.?
This would mean removing the central modules to access the pass-through boxes.
?
Any of you who have mounted the Silicon Energy modules know that the
cables are contained in trays along the side of the modules until they are
routed through conduit to the pass-through boxes.? So, if there is current
going to ground from any of these cables, it could even be occurring in
any of the 9 trays along side of the modules.? I do not like the idea of
tracking down a ground fault in one of these trays as they are all
interconnected.
?
How best (and safely) to track down where the ground fault is occurring??
Any advice as to the best and safest way to track down the ground fault(s)
would be greatly appreciated.
?
Bob Clark
SolarWind Energy Systems, LLC
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