On 7/1/2011 12:07 AM, boB Gudgel wrote:
On 6/30/2011 11:35 PM, boB Gudgel wrote:
On 6/30/2011 8:35 PM, Nick Vida wrote:
Hi wrenches.
I had an interesting case today that I have been thinking about
since it happened. I dont have all the details because the
homeownter had to leave before I was done with the service call, but
here is what I do know. SB5000 with 2 strings of 15 modules. String
one was 437 vdc, and string 2 was 247vdc. I decided to look for a
ground fault, and I turned off the inverter. I measured for ground
faults and found no evidence of anything wrong in that regard. As
the homeowner was out of time and wanted me to go, I re-landed the
wires in the dc disco, and when I replaced the fuses, I noticed a
minor arc when one of the fuses was going it. Although I had no time
to measure it, I think that there might be a partial short circuit,
possibly with one complete string and some panels of the other
string, and the remaining panels on the second string. My mind was
racing with how I could have measured for a partial short circuit
without seeing it as an arch. I can imagine such a thing on a bigger
system being a dramatic electrical event.
needless to say, I cant wait to get back there and go through the
signal path properly and find out what the electrician did. And of
course i realized that replacing the fuses in such a questionable
situation was probably not the best idea.
Do any of you have any thoughts on how one would measure for such a
thing?
nick vida
"I noticed a minor arc when one of the fuses was going it."
I see what you were saying. When the fuse was going "IN" ... Into
the fuse holder.
Could it be that the inverter was connected (but turned off otherwise)
and it arced as the inverter input capacitors were charging up ?
If so, then the current would die to nothing soon after the fuse was
in place. A clamp meter would show
that too.
ONE more thing on this... Because the strings were at slightly
different VOC's, it's possible that the
higher Voc string will back feed the lower voltage string and you may
very well see a small arc if the fuse
is connecting the two strings together. No ground fault needed for
that to happen.
boB
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