More needs with grounding [RE-wrenches]

[Jason Fisher]

Kurt wrote:

"The sharing of a common bus for grounding and neutral in any main or
sub-panel would qualify for a ground bonded in multiple locations (bonding
screw/jumper installed)?  A separate grounding bus would be required from
that of the neutral with it bonded to the enclosure?"

Kurt, I'm not sure if I'm following this question completely so I'll just
layout what I know on the topic and perhaps there will be something useful
in it for you. Basically you can only bond the "grounded" current carrying
conductor (neutral or negative depending on what system) at one point in the
wiring system. This point, in an AC utility service, is restricted to the
main service disconnect, typically the main CB panel. In a DC system this is
either internally handled in the inverter (grid tied), or done in the main
battery disconnect enclosure like in the DC250 or PSDC. This is because once
you've made this connection with the "grounding" system, you have now
essentially connected all the current carrying neutral conductors in the
building with all the non-current carrying equipment grounding conductors in
the building. They have zero potential to each other but both now have
potential to the ungrounded "hot" conductor(s). If you were to then make an
additional connection between a neutral wire and a ground wire, like is
frequently done by unskilled individuals adding a circuit into a subpanel,
you have now setup a parallel path for current flow downstream of that
connection between the neutrals and grounding conductors. As an old
electrician once said to me, "the wires don't know what color they are"
meaning that any conductor will carry current once it is placed in the
circuit, an obvious but important point. There is an exception to this which
is section 250.32  "Two or More Buildings .. Supplied by a Common Service".
The logic here, as I understand it, is that the NEC folks can recognize that
the additional cost and complication of adding a separate grounding
conductor between two buildings separated by a long distance may not be
justified in this case. Since the NEC requires grounding electrodes at these
separate structures, and requires local service disconnects at the
structures, all the proper elements are already there to allow for this
exception. An example of how this is practically done is the utility supply
example I used earlier. Though there is now a potential parallel path for
neutral current flow on the grounding conductors, this path is dependent on
the earth to complete the circuit from the rods at one building to the rods
at the other. This is a very high impedance path however when compared to
the neutral wire. The path is also not within a building but is outside of
the buildings. This path needs to exist, however, to allow for the discharge
of fault currents to ground.

"And on the buried ground wire...  this time both the home run conductors
which are in 1-1/4 PVC conduit and the ground wire (wire tied to the outside
of the conduit) slither thru the
forest floor above ground... Is this above grade ground wire a long wire
antenna just waiting for a near-by hit to smoke all the nice stuff bolted to
the wall, or will the grounds, into clay, gonna keep it all to ground?"

Good question. I can't say for sure and hopefully others have an opinion on
this but my feeling is that whether the wire is on the ground, or 18" below
it may not really matter significantly in it's attractiveness to surges but
may have more to do with its ability to disperse surges. Multiple rods will
help disperse any surges and hopefully keep them from flowing to the home.
One thing I'd make sure of is that this wire is connected at the rods of the
house and stops there. There is no need or desire to bring it into the house
panel. Outside of that, there is a slight potential as I see it for arcs to
jump from the exposed ground wire to other things such as trees or metal etc
during a surge event. This could be considered a safety hazard to people or
creatures. Both for this reason and for physical protection is why the wire
should be buried. Your call on that one.
Hope this helps.
Cheers,
Jason Fisher

Aurora Energy LLC
P.O.Box 3315
Annapolis, MD 21403

voice / fax: 410-268-3684
email: jason at aurora-energy.com

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