Inverter location, ground loops, ... [RE-wrenches]

[Kent Osterberg]

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Bill,

The current that flows through the earth when the ac neutral is grounded
at two locations is an ac current, not dc.  As such it contributes
little if any to galvanic corrosion.  Grounding a dc current carrying
wire in two locations will cause a dc current in the earth which will,
if only slowly, cause galvanic corrosion.  Lets assume the grounding
resistance is 50-ohms at each end and there is a voltage drop of one
volt in the current carrying conductor, that's a 10 mA current.  I don't
know how long it'll take, but eventually it'll take all the copper or
galvanizing off of a ground rod.

Increasing the distance between the two grounds does not help. The
incremental resistance is an r-squared function and quickly approaches
zero.  If you look at the data for a fall-of-potential test, nearly all
of the resistance is near the ground rod.  The fall-of-potential test,
is determining the resistance of a ground to a point that is
hypothetically infinitely far away.  The resistance between any two
grounds is less than the sum of the individual resistance values, but
it's only significantly less when the two grounds are close together.

In regards to grounding dc systems, take a look at NEC 250.164.  It
makes it clear that a dc system is to be grounded at only "one"
location, even if that location is off premises (remote).  Given that
only one dc ground is allowed and that the earth shall not be used as a
grounding conductor, there is no way to install a remote PV array
without a ground wire being run with the conductors.  No matter how far
away it is.  And the farther away it is the more important it is to have
surge arrestors at both ends.

Kent Osterberg, PE
Blue Mountain Energy, LLC


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