Misleading fault indications [RE-wrenches]

[William Miller]

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Mark and Christopher:

Thanks for looking at this.  My concerns are:

1. The fault is in the inverter yet the PV breaker trips.  This does not 
seem right.
2. There is a fault in the inverter yet the inverter still has power after 
the fault.  This also troubles me.
3. The fault is incorrectly indicated as a fault in the PV charging system, 
making it difficult to troubleshoot.
4.  Any accidental connection between the ground and neutral busses will 
draw a 175 amp arc.
5. The above would not occur if the installer had not removed the 
neutral-ground bond.  In all North American power systems, the "star" 
grounding system is an essential element of the safety of these 
systems.  The two references below establish the necessity for maintaining 
the bonding jumper in these systems.  Is it wise to remove this jumper in a 
PV wiring system?

The ground fault systems developed to satisfy section 690 requirements may 
create other lesser known problems, like those cited above.

I would point out that the GFCI receptacle in your bathroom works on an 
entirely different principle that does not require that the main bonding 
jumper be removed from the service panel.  Furthermore, the garden variety 
GFCI receptacle is sensitive enough to protect against fatal fault currents 
while the 0.5 amp that it takes to trip an Outback breaker is fatal.  I 
wonder if the principle used for ground fault detection in solar electric 
systems may need a little more thought and research?

This situation is not the most dangerous scenario we face in the industry, 
but it is one in which I have  strong curiosity and one which may have 
broader implications as PV systems become more commonplace.

Thanks again for your thoughts on the subject.


William Miller

Citations:

250.28 Main Bonding Jumper.
For a grounded system, an unspliced main bonding jumper shall be used to 
connect the equipment grounding conductor(s) and the service-disconnect 
enclosure to the grounded conductor of the system within the enclosure for 
each service disconnect.

250.4 (4) Path for Fault Current. Electrical equipment, wiring, and other 
electrically conductive material likely to become energized shall be 
installed in a manner that creates a permanent, low-impedance circuit from 
any point on the wiring system to the electrical supply source to 
facilitate the operation of overcurrent devices should a second fault occur 
on the wiring system. The earth shall not be used as the sole equipment 
grounding conductor or effective fault-current path.


At 09:08 AM 2/29/2008, you wrote:

>William,
>
>The fault condition is a ground fault. Why is it that the tripped GFI
>breaker must only indicate a PV ground fault?  The ground - negative bond is
>the bond that serves the PV and the battery/inverter circuits, so why should
>the tripped breaker not indicate a ground fault in either circuit?
>
>Mark Frye
>Berkeley Solar Electric Systems
>271 Vistamont Dr
>Grass Valley CA 95945
>(530) 401-8024
>www.berkeleysolar.com


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