Misleading fault indications [RE-wrenches]

[John Berdner]

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 





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