Gridtie, breaking grounded conductor [RE-wrenches]

[Bill Brooks]

Mr. Bob,

This is a classic situation of over-interpretation of the code. Article 690,
for some crazy reason, has a fine print note in 690.42 that says,
"Locating the grounding connection point as close as practicable (I love
these made up words) to the photovoltaic source will better protect the
system from voltage surges due to lightning."

How this fine print note made it through the last 3 revisions baffles
me--particularly since we were instructed to remove as many of these types
of references as possible.

The nearest "practicable" point for an inverter with GFP (most inverters) is
at the inverter. Only when GFP is not required would we ever think of
grounding the negative at the array.

The inspector got a shock because all PV systems have a voltage to ground
when they are ungrounded. That is because the resistivity of glass is not
infinity, it is very high, however. Depending the quality of the glass, you
will get a stronger or a weaker shock. Sometimes the shock is enough to make
you do a dance--but it is not leathal (unless you in turn fall off the
roof).

Upshot is to disregard the fine print note--unless it is not a rooftop
system, then either method is okay, but the GFP will have to be
defeated--some inverters may not allow this so don't do it on those.

Bill.


-----Original Message-----
From: Bob Maynard, Energy Outfitters [mailto:bob at energyoutfitters.com]
Sent: Tuesday, February 24, 2004 11:01 AM
To: RE-wrenches at topica.com
Subject: Gridtie, breaking grounded conductor [RE-wrenches]


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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