Misleading fault indications [RE-wrenches]

boB Gudgel boB at midnitesolar.com
Wed Mar 5 10:53:28 PST 2008 


If the impedance from PV- or PV+ to GND is REALLY low, I would think the 
chance of heating and starting a fire would be very low.  If the 
resistance to GND is just enough for high dissipation, then you should 
worry.  I like the idea of detecting when the electrons guzinta does not 
equal the electrons guzouta, like in the more expensive GND Fault 
detection method.

boB



R. Walters wrote:
> 
> 
> 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	
> 
> 
> 
> 
> --
> [Non-text portions of this message have been removed]
> 



boB Gudgel K7IQ
MidNite/Magnum  Solar/Energy
Washington AC


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