Galvanic Corrosion yet again [RE-wrenches]

[Bill Loesch]

Good Morning Guys and Gals,
Whose surge arrestor do you folks favor for this application?
TIA,
Bill Loesch, Solar 1 - Saint Louis Solar

----- Original Message ----- 
From: "Jim Easton" <jeaston at ucsd.edu>
To: <RE-wrenches at topica.com>
Sent: Friday, July 20, 2007 12:46 AM
Subject: Re: Galvanic Corrosion yet again [RE-wrenches]


>
> I have a feeling we are boring everybody on this list to tears, however I
am
> not entirely comfortable that your understanding of the situation
> corresponds to mine.
>
> When conductive paths are in parallel, the current is shared in inverse
> proportion to each path's resistance, and the total potential drop across
> each of the paths will be identical.
>
> In other words, when there is a ground loop, connecting a wire between two
> grounding electrodes DOES NOT eliminate the potential difference between
> them. The various current paths -- including the grounding electrode to
> earth to grounding electrode path -- will all share the current and there
> will be ground current induced electrolytic corrosion.
>
> If the copper parts of the system are connected to any non-noble (i.e.
iron,
> steel, zinc, etc.) hardware in contact with the earth, that hardware will
be
> preferentially attacked and may cause failure of the safety ground.
>
> I like, and Xantrex reccommends, connecting the array ground to the
inverter
> ground using a bare physically robust (AWG #4 or larger) wire run outside
> the conduit in contact with the earth between the PV array and the
inverter.
> However, the PV neutral should not be connected directly to the array
ground
> at the array. Connecting a lightning arrestor to the neutral and the hot
at
> the array to conduct overvoltages to ground is acceptable.
>
> Jim Easton, PE
>  (E 11974)
> 4364 Bonita Road, No. 166
> Bonita, CA 91902-1421
> Tel: 858-527-0240
> Fax: 619-470-8616
>
> ----- Original Message ----- 
> From: "Bill Brooks" <bill at brooksolar.com>
> To: <RE-wrenches at topica.com>
> Sent: Thursday, July 19, 2007 7:11 PM
> Subject: RE: Galvanic Corrosion [RE-wrenches]
>
>
> >
> > Jim,
> >
> > I understand the concept of galvanic corrosion and cathodic protection,
> > and
> > the difference between that and soil corrosion due to soil acidity. I
> > think
> > your discussion of copper electrodes answers the question. Use copper
> > electrodes and this is really not much of an issue and will save a lot
of
> > copper in the process.
> >
> > If lightning is an issue, which it is in many places, I will usually
bury
> > a
> > bare copper grounding electrode conductor connecting the two ground rods
> > together for maximum protection--no galvanic corrosion because no
> > potential
> > difference. Also no grounded conductor connection to ground at the
array.
> > Surge arrestors on all conductors (grounded and not grounded).
> >
> > Bill.
> >
> >> -----Original Message-----
> >> From: Jim Easton [mailto:jeaston at ucsd.edu]
> >> Sent: Thursday, July 19, 2007 8:24 AM
> >> To: RE-wrenches at topica.com
> >> Subject: Re: Galvanic Corrosion [RE-wrenches]
> >>
> >>
> >> Galvanic (Electrolytic) corrosion caused by DC current flow is
different
> >> from corrosion caused by soil composition.
> >>
> >> Galvanic corrosion such as that caused by a DC ground loop will remove
> >> metal
> >> from the side being corroded in virtually exact proportion to the
current
> >> flowing and the time it flows.
> >>
> >> Using copper electrodes can reduce the corrosion since copper has a
> >> higher
> >> electrochemical potential than hydrogen, and the current will
> >> preferentially
> >> decompose water rather than corrode the copper. However, any less
> >> refractory
> >> metal (i.e. iron, steel, zinc, etc.) exposed to the potential will be
> >> preferentially corroded leading sometimes to rapid failure. This
> >> typically
> >> applies to steel bolts, nuts, washers, and cores of copper clad ground
> >> rods
> >> exposed through abrasion.
> >>
> >> Jim Easton, PE
> >>  (E 11974)
> >> 4364 Bonita Road, No. 166
> >> Bonita, CA 91902-1421
> >> Tel: 858-527-0240
> >
> >
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