Kelly,
It is my understanding that all types of grounding electrodes, whether
supplementary or not, for lightning protection or PV array grounding
shall be bonded together to form the grounding electrode system.
(250.106, 690.47(A) which sends you to 250.50, NFPA 780-2008 4.14).
If you do not do this and you do get a lightning strike or for that
matter any high frequency signal injected onto a grounding electrode
you will get what they call a ground potential rise onto one part of
the system. With the other separate grounding electrode not having
this same signal because of them not being common, you will have a
potential difference between the two separate electrodes even though
they are both in the ground. The ground (earth) is actually a very
poor conductor when it comes to high levels of high frequency current
Hopefully this helps,
Brian
NABCEP Certified PV Installer
To contact Brian L Crise:
Address:
16021 NE Airport Way
Portland OR, 97230
Office Phone: (503) 262-9991 x.5054
e-mail: bcr...@nietc.org
On Jun 5, 2009, at 12:10 PM, Kelly Keilwitz, Whidbey Sun & Wind wrote:
Mark,
I don't think that the 690.47D "PV GE" is part of the premises AC & DC
ground electrode system.
IMO, in order to function to properly as “enhanced protection from
lightning
induced surges” as described in the 690.47(D) section note in the
2008 NEC
Handbook, the “optional supplementary grounding electrode” should
not be
connected to the premises system AC and DC grounding electrodes, as
shown in
Exhibit 690.6 of the Handbook. If an additional #8 (per 250.166) is
bonded
to the premises AC/DC system GE, it's path will be parallel with, and
redundant to, the existing EGC. This could set up the potential for an
inductive ground loop, which might cause more damage to the system
during a
lightning strike than it would prevent.
If the “additional electrode for array grounding” (PV GE) of 690.47D
is not
required, then the PV GEC of that section should not be required.
Of course, these opinions are academic. What I really want to know
is what
the AHJ's think! Any of you have to deal with this, yet?
Thanks,
-Kelly
Kelly Keilwitz, P.E.
Whidbey Sun & Wind, LLC
Renewable Energy Systems
NABCEP Certified PV Installer
WA Electrical Administrator #KEILWKM923RB
987 Wanamaker Rd.
Coupeville, WA, 98239
360.678.7131
sunw...@whidbeysunwind.com
WA Electrical Contractor #WHIDBSW920MS
WA General Contractor #WHIDBSW946M1
On 6/5/09 10:01 AM, "Mark Frye" <ma...@berkeleysolar.com> wrote:
Kelly,
The question is, if the AJH had required you to run the new array
DC GEC
directly to an additional DC array GE, would that new DC GE system
need to
be bonded with the exisitng AC GE system with a bonding wire other
than the
new EGC running from the array to new DC GE?
Unfortunately, NEC 2008 is not clear about this with respect to the
new DC
array GE. That leaves us with only the general NEC requirement
that all
premise GE's be bonded into a single system.
So the total effect of the new 690.47D, assuming that the
"prcaticable"
location for the new DC array GE is not within 6 ft of the
exisiting AC GE,
is this:
A new DC array GEC must run directly from the array to a new DC GE
system
and and a new bonding jumper must be run from the new DC GE system
to the
exisitng AC GE system.
It the new DC GE system can be the same as the existing AC GE by
virtue of
it proximity (less than 6 ft) to the practiable location, then the
ne DC
array GEC must still be run directly to exist GE system which is
now the
AC/DC GE system.
Mark Frye
Berkeley Solar Electric Systems
303 Redbud Way
Nevada City, CA 95959
(530) 401-8024
www.berkeleysolar.com
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