[RE-wrenches] how would you measure a partial short circuit

[boB Gudgel]

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



-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20110701/7acf1517/attachment-0003.html>