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

[Bill Brooks]

Kent and Jim,

I'm intrigued but not convinced. Does anyone have any evidence out there of
galvanic corrosion in such a situation? I think there is plenty of evidence
of corrosive soils, but galvanic corrosion between two rods at 1 Volt
potential difference being far apart is difficult to believe--guess I need a
Myth-Busters episode to help me out. I understand Ohm's law, just wondering
how much resistance and how long it would take to create damage. I'll grant
a problem with salty wet ground on the beach.

As far as grounding in two places, we are treating the array as a separate
structure and a separate service at that structure. It may not be the best
way to do it anyway. The 2008 NEC is likely to allow a frame ground at the
array without requiring bonding of the electrode to the main building
grounding system for lightning protection. It is not expressly allowed in
the code currently. Now we are required to tie the two grounding systems
together with a grounding electrode conductor.

Bill.

Bill Brooks, PE
Brooks Engineering

> -----Original Message-----
> From: Kent Osterberg [mailto:kent at coveoregon.com]
> Sent: Wednesday, July 18, 2007 5:24 PM
> To: Wrenches
> Subject: RE: Inverter location, ground loops, ... [RE-wrenches]
> 
> 
> 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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