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

[boB Gudgel]

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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