[RE-wrenches] Finding ground faults

[William Miller]

Mike:

Is there such a tool as a DC clamp-on that can reliably measure 10 mA?  My 
experience is any clamp meter is unreliable below 5A.

William


At 10:09 PM 5/23/2012, Mike Sullivan wrote:
>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:
>?
>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).
>?
>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.
>?
>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.
>?
>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.
>?
>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.
>?
>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.
>?
>Bob Clark
>SolarWind Energy Systems, LLC
>
>
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Miller Solar
Voice :805-438-5600
email: william at millersolar.com
http://millersolar.com
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