Misleading fault indications [RE-wrenches]

[John Berdner]

Mark/William /Wrenches:
 
Sorry for the delayed response - I was out sick with the flu and it was
nasty.
 
@Mark: The resistor is in parallel with the GFDI breaker and so the
system is still bonded, albeit with a resistive element to ground, when
the breaker opens.  This may or may not be part of the detection
circuitry - I don't know.  In past discussions with J. WIles he has
suggested that a resistive element to ground needs to be included to
prevent static buildup in the PV circuit conductors.  I don't buy it and
never have since most of the world runs ungrounded PV arrays and static
is not a big issue.  In our inverters we have high impedance resistive
elements to measure leakage current following a ground fault fuse trip. 
Also, UL requires bleed down resistors to bleed off energy in the caps
so I think the GFDI resistor is probably redundant (if used to dissipate
static potential).  
 
As an aside, the NEC does not specify a maximum allowable impedance for
grounds.  UL's criteria for a "ground" is 0.1 Ohms at 60 Hz but
alternative grounding methods are allowed as long as they have been
evaluated for that purpose.
 
@William:  You are correct (you get misleading fault indications) in
your scenario as drawn but...  As drawn the normal inverter current must
pass through the 0.5 Amp breaker so I think this set up would not work. 
I think the inverter negative needs to be moved to the Gray isolated
neutral bus bar in your diagram.  This allows normal currents to flow
from PV to battery to inverter with no problems.  If wired this new way
a fault in the inverter would cause the 250 Amp breaker to trip and an
ground fault would cause  the GFDI breaker to trip.  This change would
provide over current protection and GFDI protection for the system
without misleading indicators.  This does not violate 690.41 since there
is some exception type language in 690.41 -  or shall use other methods
that accomplish the equivalent system protection and that utilize
equipment Listed and identified for the use.  Listed GFDI devices are
deemed to meet this requirement. 
 
Your top scenario is an excellent example of why GFDI is needed and is
in the new Code.  It does not matter who's equipment was involved since
PV systems, as we design them with 1.56 Isc, need some type of GFDI
protection unless, of course, they are ungrounded as now allowed by
Code.
 
Other ways to detect ground faults: Sure there are other ways to do it.
 You can do a differential current measurement but it can get a bit
tricky in higher power circuits especially when you have a large
magnitude AC component or high levels of switching noise and you are
trying to resolve a small, e.g. less than 1 Amp, dc ground fault
currents.  Also, the differential current measurement handles the
sensing requirement but does not handle the interruption of fault
current.  Remember the GFDI circuit has to be fail safe so you have to
use a Listed something that will fail open.  The small over current
element (fuse or breaker) to ground is a  Listed, fail safe device that,
from a cost and complexity stand point, is hard to beat.    
 
Best Regards,
 
John Berdner

>>> markf at berkeleysolar.com 3/4/2008 10:13:51 PM >>>

William et. Al.,

In the diagram you post there is a resister across the 0.5A circuit
breaker.
Is it correct to say that the grounded DC conductor becomes "un-bonded"
when
the circuit breaker is open?

Mark Frye
Berkeley Solar Electric Systems
271 Vistamont Dr
Grass Valley CA 95945
(530) 401-8024
www.berkeleysolar.com 


-----Original Message-----
From: William Miller [mailto:wrmiller at charter.net] 
Sent: Tuesday, March 04, 2008 6:47 PM
To: RE-wrenches at topica.com 
Subject: RE: Misleading fault indications [RE-wrenches]


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




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