Misleading fault indications [RE-wrenches]

Ray Walters walters at taosnet.com
Wed Mar 5 10:23:22 PST 2008 

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Thanks John, I was wondering if there had been some "event" to prompt  
this code change.
I'm not as experienced as many on this list with high voltage, high  
amp arrays (we do off grid primarily) but if the DCGFI just unbonds   
DC when it trips, wouldn't this be another argument for unbonded  
arrays in the first place, and just skip the DCGFI all together?
ALso, as I think many others are alluding to, if there is a fault and  
the DCGFI does its job, there is still a lot of energized wiring  
running from the array to the DCGFI that is possibly  dangerous.  
Should there be a requirement for the DCGFI to be located outside,  
near the array?
I'm sure this is a reasonably important issue, but meanwhile, the NEC  
is ignoring other much more dangerous issues such as identifying DC  
polarity, and preventing battery short circuits.

ray at solarray.com


On Mar 4, 2008, at 6:19 PM, John Berdner wrote:

>
> Mark/Mark/Wrenches:
>
> I don't really have anything to add to the original question but
> perhaps some background would help frame the issue - or at least muddy
> the water further ;^)
>
> The issue of a low impedance fault in the grounded PV conductor is an
> interesting one.
> It is hard to detect this type of fault since a very low impedance
> fault looks remarkably like an intact fuse or closed breaker  
> between the
> grounded conductor and ground.
> During the development of UL 1741 we had some spirited discussions on
> the topic and it is clear that the intent of the GFDI requirement (per
> NEC and UL1741) was to cover all types of ground faults including  
> those
> in the grounded conductor.  Not all implementations of GFDI circuits I
> have seen will do this and, because the NEC or 1741 are not  
> explicit on
> this point these devices continue to be Listed.  I am not saying  
> anyone
> is wrong or right here only that the Standards do not explicitly  
> address
> the issue.
>
> If we look for guidance it is sometime useful to go back to the
> original purpose of the GFDI.   The GFDI is there to eliminate the
> possibility of ground fault currents flowing in "unintended  
> conductors".
>   The idea is that if you have a ground fault you will see currents
> flowing in support structures, conduits, hangers, and all kinds of  
> metal
> bits that might be connected between the fault and ground.   
> Conduits for
> example are supposed to carry the fault current long enough to open  
> the
> over current device feeding the circuit.  This is part of the  
> reason we
> use conduit, it is normal, and the fault current is usually only very
> short term.
>
> The big BUT here we size our normal PV over current devices at 1.56  
> Isc
> so it is possible for these ground faults currents to flow  
> continuously
> for indefinite periods of time without tripping the normal PV over
> current devices.  None of this "metal stuff" is intended or  
> evaluated to
> carry current for long periods of time and so it can possibly over  
> heat
> and thereby create a fire hazard.  Originally the NEC only required  
> GFDI
> for rooftop mounted arrays on dwellings (which includes more than just
> homes by the way).  A couple of years ago there was a ground fault  
> on a
> ground mounted medium sized commercial system and the fault melted
> through the side of the metal conduit. This was the seminal event  
> which
> prompted the recent NEC requirement for GFDI in all grounded PV  
> systems
> (which I support and believe is a resonable requirement).
>
> If you have a low impedance fault in the grounded conductor you can
> clearly get fault currents to flow in the "unintended conductors" and
> therefore the GFDI should detect it and interrupt the fault current.
> Nicked cables in the grounded conductor are certainly not unheard  
> of and
> this would normally blow the GFDI fuse or trip the GFDI breaker.  This
> type of fault normally has a high enough impedance to be detected and
> trip the GFDI circuit as well.  It is possible, however, (although  
> some
> would argue only theoretically) to have a low impedance fault that  
> will
> trip the GFDI over current device but remain undetected.  In that case
> there is a possibility, albeit remote, for currents to be flowing  
> in the
> unintended conductors.  Will it be detected ?  This all depends on the
> nature of the ground fault and how the GFDI detection circuitry was
> implemented.
>
> Best Regards,
>
> John Berdner
>
>
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>>>> mark.edmunds at xantrex.com 2/29/2008 06:05:18 PM >>>
>
> Hi Mark,
>
> For most faults there will be an indication, but like on a lot of the
> Grid Tie Inverters in the unlikely case of a very low impedance fault
> connection directly from the PV negative to ground back to the XW
> charge
> controller chassis the fuse could blow, protecting against the fault
> but
> not showing an indication. We have never seen this type of fault
> before,
> but the fuse certainly would protect against any continued fault
> current
> flowing.
>
> Mark
>
>
> Mark Edmunds
> Director Engineering
> Residential Renewable Energy Group
> Xantrex Technology Inc.
>
> -----Original Message-----
> From: Mark Frye [mailto:markf at berkeleysolar.com]
> Sent: Friday, February 29, 2008 5:18 PM
> To: RE-wrenches at topica.com
> Subject: RE: Misleading fault indications [RE-wrenches]
>
>
> Mark,
>
> Perhaps you could respond directly to issues raised, will the DC
> negative to ground fault persist and go un-indicated?
>
> Mark Frye
> Berkeley Solar Electric Systems
> 271 Vistamont Dr
> Grass Valley CA 95945
> (530) 401-8024
> www.berkeleysolar.com
>
>
>
>
>
> --
> [Non-text portions of this message have been removed]
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>

R. Walters
Solarray.com
NABCEP # 04170442	




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