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

Kent Osterberg kent at coveoregon.com
Tue Jul 17 21:26:56 PDT 2007


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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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