[RE-wrenches] EGC ground impedance

[Ray Walters]

I believe the cable tray will still be an excellent conductor, as long 
as the jumper connections are tight.  If you look at the cross sectional 
area of it, its probably the equivalent of some fairly burly cable.  
Also the jumpers are just part of the connection,  If you removed a 
jumper, there is still quite a bit of contact surface.  (I'm not 
recommending that though)  but you really have  parallel connections at 
each joint.
The high ground impedance measurements are par for the course for my 
measurements over the years.  A 5/8" ground rod in loose (land fill...) 
soil is going to be about that.  It might be possible to compact around 
the rods to improve that, and/ or add some mineral rich soil amendments 
around the rods to improve the impedance as well.
(an old phone tech that worked for me used to cure ground problems all 
the time by drinking lots of coffee, and then ahem, moisten the soil 
with mineral rich fluid, ahem.)
While it is a bit unnerving to measure ground impedances that high, the 
overall grounding system has low impedance.  Also, consider that the 
ballast trays will offer some conductivity to ground as well.  Any way 
to measure some of the ballasts' ground impedance?
If you did add continuous conductors, just make sure that they are 
connected in a radiating star pattern; don't create inductive loops, or 
you really will have some voltage differential across the grounding 
system in a lightning strike.

R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760

On 1/14/2014 8:24 AM, Andrew Truitt wrote:
> 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.
>
>
>
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