Misleading fault indications [RE-wrenches]

[William Miller]

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John:

Thanks for the detailed background on the fault scenario and the 
code-making process.

I try to differentiate between a fault in which the negative current 
returns on the equipment grounding conductor, the positive current returns 
on the grounding conductor or a plain old pos. to neg. short circuit.  I 
don't know if it is important to differentiate these faults here but I 
suspect it is.

I have some questions:

1. John, do you have any concerns about the scenario I presented?  It just 
does not seem right that a fault in an inverter indicates as a fault in the 
PV and that the inverter continues to have power supplied to it.

2. Also, do you share my gut reaction that un-bonding a neutral just to 
accomplish ground fault detection is somehow not the straightforwards 
approach?  The drawing that indicates this scenario is located at:
http://mpandc.com/case_studies/ground_fauilt/ground_fault.html  It is the 
second study from the top.

3. Has anyone attempted to detect ground faults by monitoring the 
difference in current on the positive lead versus the negative?

BTW, the top study on that page indicates a scenario as you mentioned in 
your reply and illustrates the importance of ground fault detection.

Sincerely,

William Miller



At 05:19 PM 3/4/2008, you 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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