Ground Loop Prevention [RE-wrenches]

[Ray Walters]

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I  just finished a system where I grounded DC to AC as you described  
here, and then I reread the code. I believe now that it is a  
violation, and I will run a separate continuous ground wire to the  
ground rod, electrode, etc. with its own clamp. Its a detail the AHJ  
never busted me on, but I'd rather stay a jump ahead of him. Between  
690.47, 250.166, and 250.64, etc. its pretty clear what size the  
grounding conductor should be, that it should be separate, and that  
it should be continuous. I don't have much problem with that bit of  
code, but don't get me going on battery cables, boxes, venting, etc.  
You could drive a truck through the holes in the code there.

Ray


On May 30, 2007, at 11:07 PM, Kent Osterberg wrote:

>
> Drake,
>
> In Oregon concrete encased electrodes are required in all new
> construction so the RE panel is often grounded to the neutral/ 
> ground bus
> in the main panel because that is the most accessible point of the
> grounding system.  A small grid connected system may end up with #10
> conductors with a #6 ground running back to the main panel.  This  
> may be
> a violation of the 2005 NEC 250.53(C) requirement that the grounding
> electrode conductor is continuous.  But does it really matter if we  
> tap
> the common grounding electrode conductor by using the bus bar rather
> than using an irreversible splice a few inches away?  Is there any
> greater risk that someone will disconnect the RE ground from the  
> bus bar
> in the main panel than from the bus bar in the RE panel?
>
> The loop made by having parallel paths (equipment ground wires and
> separate grounding system bonds) between the ground bus of the main
> panel and the RE panel ground bus will not attract lightning.  The
> parallel paths serve to lower the impedance between the two ground
> electrodes, both resistance and inductance are reduced, which helps  
> keep
> the two ends of the grounding system closer to the same potential
> whenever there is lightning strike.  If an electric utility company  
> were
> to build a new 230-kV switch yard adjacent to an existing 500-kV yard,
> each system would have its own ground mat and those would be bonded
> together with MULTIPLE connections.
>
> With a remote PV array, more than 25 feet away should be sufficient to
> be remote, the PV array is a separate structure and its ground
> electrode(s) should not have to be bonded back to the grounding system
> for the house.  The equipment grounding conductor from the RE panel or
> house panel to the PV array should be sufficient.  However, it would
> improve lightning protection to bond the two ground systems separately
> from the equipment grounding conductor.  Doing so will reduce the  
> amount
> of current that flows though the RE panel whenever there is a  
> lightning
> strike.
>
>
>
>
> ===============================================
> Hello Wrenches,
>
> Article 690.47 (C) of the NEC requires that the DC grounding
> electrode conductor connect to the grounding electrode of the AC
> system, where DC and AC systems are present. Or, a separate DC
> grounding electrode may be installed, which is then bonded to the AC
> system grounding electrode.
>
> If an inverter is feeding the grid (such as an Outback with backup
> system) it will have an equipment grounding conductor that connects
> to the neutral bar of the service. It will also have a grounding
> electrode conductor that makes a loop to the ground rods, separate
> from the equipment grounding conductor, and also connects to the
> neutral bar. The equipment grounds and the grounding electrode
> conductor connect to the equipment grounding bar in the DC disconnect.
>
> Doesn't this create an induction loop that could bring lightning
> surges into the RE equipment and the building electrical system?
>
> A similar case, that would seem especially problematic, is a ground
> wire run to a ground rod from an array. It is often required to bond
> this rod to the existing system ground rods.
>
> The array is required to have an equipment grounding conductor run
> back to the DC equipment ground?   This will ultimately connect to
> the utility service neutral bar. This creates a loop.
>
> Is this really the best way, technically, to ground systems? With
> this requirement, how do we both meet code and avoid creating a
> grounding loop that will attract lightning surges?
>
> Thanks,
>
> Drake
>
>
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>

R. Walters
Solarray.com
NABCEP # 04170442	




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