[RE-wrenches] EGC ground impedance

Wed Jan 15 11:03:19 PST 2014

Andy,

Larger cable tray systems have expansion joints (125 feet or more). If
bonding jumpers are not placed around these joints, you will get a high
reading.

However, it may just be that your meter is flaking out with the long path to
earth. I would try another meter before I would believe your high readings.

Bill.

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Andrew
Truitt
Sent: Tuesday, January 14, 2014 8:55 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] EGC ground impedance

Bill - I'm using an Amprobe DGC-1000A.  I let it self-calibrate and then
simply clamp it on to a conductor, and it reads the impedance to ground
(http://content.amprobe.com/manualsA/DGC1000.pdf).  I used it on a well
grounded RBI system (driven piles) a few months ago and it measured ground
impedances of less than 25 Ohms for all EGCs, as expected for a system with
so much contact between the metal structure and earth.

Ray - The tray does make a good conductor.  I did some more testing and any
point along the tray itself gives less than 10 Ohms; its only the #10
conductors between the array and the tray where the impedance spikes.  The
conductors are bonded to the tray with torqued, star-washered,
copper/stainless lay-in lugs.  There are no ground rods in the system due to
the requirements of the capped landfill, but there is a UFER ground at the
equipment pad that gives readings of < 1 Ohm for the GEC.

For a brighter energy future,

Andrew Truitt 

Principal
Truitt Renewable Energy Consulting, LLC

(202) 486-7507 <tel:%28202%29%20486-7507> 

LinkedIn Profile <http://www.linkedin.com/pub/andrew-truitt/8/622/713> 

Company Website <http://truittreconsulting.weebly.com/> 

NABCEP Certified PV Installation Professional ID: 032407-66

Colorado Journeyman Electrician License No.: 600132

"Don't get me wrong: I love nuclear energy! It's just that I prefer fusion
to fission. And it just so happens that there's an enormous fusion reactor
safely banked a few million miles from us. It delivers more than we could
ever use in just about 8 minutes. And it's wireless!"

~William McDonough

On Tue, Jan 14, 2014 at 10:47 PM, Bill Brooks <billbrooks7 at yahoo.com
<mailto:billbrooks7 at yahoo.com> > wrote:

Andy,

How are you testing the ground impedance of the EGC?

Bill.

From: re-wrenches-bounces at lists.re-wrenches.org
<mailto:re-wrenches-bounces at lists.re-wrenches.org>
[mailto:re-wrenches-bounces at lists.re-wrenches.org
<mailto:re-wrenches-bounces at lists.re-wrenches.org> ] On Behalf Of Andrew
Truitt
Sent: Tuesday, January 14, 2014 7:25 AM
To: RE-wrenches
Subject: [RE-wrenches] EGC ground impedance

Wrenches - What do you consider to be a safe ground impedance for a PV array
equipment grounding conductor?  NEC2011 250.53(A) says that if the impedance
between the grounding electrode and earth is less than 25 ohms then no
supplementary grounding electrode is needed, but I'm not clear on what that
means for the ground impedance measurement within the array.  For example, I
am inspecting a ballasted landfill system (i.e. no penetrating the earth
allowed - just a UFER at the equipment pad) where the ground impedance at
the GEC is very low (<1 Ohm), but the array EGCs are showing ground
impedances of between 500 and 1000 Ohms.  The EGCs (#4) are connected to
Cope cable trays, which are in turn connected to the inverter grounding bar
via #4 bonding jumpers.  The cable tray is grounded per manufacturer specs
(large bonding jumpers at each joint), but the runs can be quite long
(300'+) with many connections, though nothing appears to be loose.  I intend
to recommend that continuous conductors be installed to reduce array ground
impedance, but any input on the issue would be very much appreciated.

For a brighter energy future,

Andrew Truitt 

Principal
Truitt Renewable Energy Consulting, LLC

(202) 486-7507 <tel:%28202%29%20486-7507> 

LinkedIn Profile <http://www.linkedin.com/pub/andrew-truitt/8/622/713> 

Company Website <http://truittreconsulting.weebly.com/> 

NABCEP Certified PV Installation Professional ID: 032407-66

Colorado Journeyman Electrician License No.: 600132

"Don't get me wrong: I love nuclear energy! It's just that I prefer fusion
to fission. And it just so happens that there's an enormous fusion reactor
safely banked a few million miles from us. It delivers more than we could
ever use in just about 8 minutes. And it's wireless!"

~William McDonough

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