[RE-wrenches] Finding ground faults

[Mike Sullivan]

Hi Bob,

Sorry if I am duplicating any of the replies you've already received regarding the possible ground fault. I think the best way to determine whether GFDI is tripping on 1 A ground current or 80 A circuit current is to use a sensitive DC clamp-on to check the current on your system ground under normal operation. If it is more than about 10 mA when things are dry, you probably do have a ground fault issue to resolve. Assuming that's true, I recommend that you buy, borrow or rent a megohm tester (like those by Megger) to check out your circuit wiring resistance. Once you fully isolate your strings at the combiner box, you can quickly zero in on where the fault lies -- just divide and conquer. You may need to wet down the array and wiring to make the fault more detectable. Often, the fault will be in wiring that was damaged when pulled from a splice or combiner box into conduit, esp. if there's no bushing on the conduit terminal adapter.

A word of warning -- if you have more than one ground fault, some parts of your equipment grounding system will be carrying current, which may be sizable. Use precautions when disconnecting any wiring or grounds. A clamp-on is really useful here...

Hope that helps!

Cheers,

Mike Sullivan



Wrenches: 
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Sorry to bring up this situation again, but I could not find a previous discussion thread that talked about step-by-step procedures for tracking down ground faults (I believe it was John Berdner who provided the bulk of the writing in that regard and it related to high voltage strings?not that 75-85 volts per string is all that low). 
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We have an Outback FLEXpower Two system with 8, 3-module strings (Silicon Energy 190W, 25.3Vmp, 7.5 Imp) combined in a Midnite Solar MNPV12 combiner with breakers.? The cables from the array are routed to two different Wiley 4-string pass-through boxes (located under the two center strings) and then off of the roof and into the PV combiner. 
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Up until this week the system has seen several weeks of awesome power production.? Yesterday, the rains came and the GFDI breaker tripped.? Every time it rains, the little green breaker on the GFDI trips off. ?That tells me that the ground fault is occurring in a place where at least one of the conductors on one of the strings (possibly more) is getting wet and conducting current to ground. 
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It would seem to me that the most likely place for moisture to cause a ground fault to occur in this system would be in the pass-through boxes.? This would mean removing the central modules to access the pass-through boxes. 
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Any of you who have mounted the Silicon Energy modules know that the cables are contained in trays along the side of the modules until they are routed through conduit to the pass-through boxes.? So, if there is current going to ground from any of these cables, it could even be occurring in any of the 9 trays along side of the modules.? I do not like the idea of tracking down a ground fault in one of these trays as they are all interconnected. 
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How best (and safely) to track down where the ground fault is occurring?? Any advice as to the best and safest way to track down the ground fault(s) would be greatly appreciated. 
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Bob Clark 
SolarWind Energy Systems, LLC