Galvanic Corrosion yet again [RE-wrenches]

[Jim Easton]

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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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