OK, nevermind... That'll teach me to respond before I know the
question even after reading it
more than 3 times.
For some reason I read the Voc's as being close in voltage as you said
here, Nick.
But yes, 247V Vs. 437V would certainly cause a back feed !
Must be late.
boB
On 7/1/2011 12:12 AM, boB Gudgel wrote:
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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