Gridtie, breaking grounded conductor [RE-wrenches]

[John Berdner]

Wrenches:
 
I happen to agree with the inspector that the ungrounded conductor
should be opened for servicing.
Unfortunately this is not Code at the moment and inspector confusion
over this issue is a common problem.
The NEC is quite clear on the issue.
Article 690.3 says Articles 690 and 705 over rule all other sections of
the code in the event of a conflict.
This is one of the places there are conflicts.
 
Article 690.13 specifically says that you may NOT have a switch,
breaker, or fuse in any grounded conductor UNLESS that device is part of
a Listed GFDI required by 690.5.
The FPN following 690.13 allows the grounded conductor to be
interrupted by a bolted connection to allow maintenance or
troubleshooting by qualified personnel.  The removable GFDI fuse in the
Sunny Boys is a bolted connection for this purpose.  
 
Article 690.5 outlines the requirements for the GFDI.
UL, ETL, or CSA tests the GFDI and certifies that the Listed GFDI
device meets the requirements of the Code.
The location of a Listed GFDI (inside or outside the inverter) is
irrelevant.
(The note following 690.5 in the 2002 NEC Handbook specifically points
out that the normally grounded conductor may be at any voltage up to Voc
below ground, e.g. -400 Vdc when the GFDI has interrupted the bond)
 
Article 690.17 gives the requirements for switches and circuit breakers
used as disconnecting means for UNGROUNDED conductors.  690.17 follows
the NEC's long tradition of not allowing switches in grounded
conductors. 
 
The shock the inspector experienced form the negative conductor is a
capacitive discharge due to the small capacitive coupling between the
array and earth (as Bill Brooks correctly pointed out).  
You can verify this with a DVM measurement from PV + to ground and PV -
to ground with the GFDI fuse removed.
Unless you have a ground fault the system is ungrounded at that point
and you should see about 0.5 * Voc on each measurement.
This voltage should decline towards zero as the capacitive coupling is
discharged through the DVM.
A fixed voltage that does not decline normally indicates a ground
fault.
The value of the fixed voltage compared to Voc even tells you about
where the ground fault is!
 
In a  Sunny Boy the bonding of the PV negative to earth is not "on the
AC side" as the inspector indicated.   
The dc system is bonded through the Listed GFDI device that complies
with the requirements of 690.5 as noted above.
There still may be some valid debate about the routing of the equipment
grounding conductor compared to the DC bonding jumper.
Although these conductors are redundant (going from the same place to
the same place) they have differing requirements for the size and
routing of the conductor.
The latest opinion I have on the subject from J. Wiles is that a single
grounding conductor meeting the more restrictive of the two requirements
might be enough.
 
Best Regards,
 
John Berdner
 
 


>>> bob at energyoutfitters.com 2/24/2004 11:00:46 AM >>>
Wrenches,

Now that Oregon has a gridtie rebate program (severely under funded),
we are starting to feel many of the inspector related issues that have
been discussed before. Oregon's program in addition to inspection by the
AHJ, must pass inspection by the rebate program's own inspectors. The
contractors in certain Oregon jurisdictions are being told by the AHJ's
inspector that a dc disconnect is required at the array on roof mounted
arrays, any advice is appreciated on the best way to address this.

More importantly is a recent issue brought up by a program inspector.
What could be happening?

The inspector said that he received an electrical shock from the
negative conductor within the dc disconnect of a standard residential,
battery less, PV system. At first, this seemed very odd to me since the
unbroken conductor should be bonded to ground and thus possesses the
same potential as ground. Upon further discussion with him, he is
convinced the system is ungrounded since the grounding bond occurs on
the ac side and the GFI prevents the proper dc grounding. He contends
that the bond needs to occur at the source; the PV array. I am still
puzzled about this. Since the grounded bond occurs within the inverter,
does this not properly perform the bond required between the negative dc
conductor and ground. Could there possibly be a capacitive charge build
up. If so, how is it possible with a bonded connection to ground? His
recommendation now is that the dc disconnect must break the array
positive AND negative.

We've got some real pro's on this list and I look forward to their
input.

Regards,
Bob Maynard
Energy Outfitters




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