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

[boB Gudgel]

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'd probably use a DC clamp on ammeter.  Clamp on and measure the top 
and bottom current of each string...   What comes in ~should~ come out.

If it's much different on one end vs. the other end of one string, maybe 
that string is conducting around the path somehow.  Remember that
sometimes just turning the clamp around and measuring the opposite 
polarity may measure slightly differently than the opposite polarity and
that zero calibration is usually necessary.

boB
??
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