Ground Loop Prevention [RE-wrenches]

[Kent Osterberg]

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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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