Copper-Conductor Ground Ring 2

[busbar]

I am retired from a weapons-test site where, for safety concerns, every building, bunker and magazine has a ground ring of 4/0-AWG copper conductor directly buried, encircling each—with ¾”x10’ copper-clad rods at corners. Early on, {1950s to 1970s} seven-strand conductor was used, but later contractors got away with installing more readily available nineteen strand.

The soil there is immensely corrosive, and concerns are that 19 strand has a greater surface area exposed to corrosive soil than 7 strand. Figure (7/19)^0.5 = 0.61 per-unit—thus 7 strand has 39% less surface area than 19 strand.

My question is, in this application, should 4/0-AWG 7-strand conductor perform appreciably longer than 19 strand?

 

[alehman]

That would seem to make sense. 7-strand (and its connectors or weld molds) may be hard to find. You may also want to consider the specific alloy...

http://www.copper.org/resources/properties/db/CDACorrosionSelectionServlet.jsp

 

[7anoter4]

Hi busbar
I don't know if the weapon-test site is provided with cathodic protection.
If no cathodic protection is provided the embedded [buried] structure reinforcing steel will be corroded and the entire underground structure will be in danger, namely if it is in a highly corrosive soil. So, I think that using copper bare cables and the copper-clad rods what concerns is the embedded steel corrosion and less the corrosion of the copper surface.
I think copper lead [or tin] covered cable and stainless steel grounding rod will solve both corrosion problems. A new problem will be the connections [better welded and covered also with a moisture resistant material as bitumen or hot shrinking polymer]
See

http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=5413554

As copper lead covered cable supplier may Bayka [Germany]

http://www.electricity.german-pavil...df?PHPSESSID=4e8f9de94391e92d1cc8f457076cc318

For welding material may be

http://www.erico.com/products.asp?folderid=41

Also for stainless steel electrodes see:

http://www.erico.com/public/library/fep/LT0359.pdf

As sealant for welded surfaces may be:
RAYCHEM S1061-2-500[hot shrinking]
See:

http://catalog.tycoelectronics.com/TE/bin/TE.Menu?M=MENU&ID=20519&BML=18022,,20855&LG=1&I=13

BTW:I think that the outer surface of 4/0 cable does not depend so on the number of strands. And if the surface exposed is bigger the current density will be smaller so the corrosion will stay in the same limits.
This is my opinion.
Best Regards

 

[busbar]

7anoter4, there’s no cathodic protection there—just grounding for lightning-air terminals, service-entrance, and diagnostic equipment. Most buildings are under 1500 square feet.

My view is that moisture and dissolved corrosive salts would affect all surfaces on the conductor stranding.

Thanks a lot for your comments.

 

[jghrist]

Copper is cathodic with respect to most other metals that are likely to be buried, so will be resistant to most corrosion. One concern often expressed in corrosion is the concentration of corrosion that occurs in coating voids. From the respect of concentration, increasing the surface area should be an advantage rather than a disadvantage.

19 strand conductor has a somewhat lower inductance than 7 strand and thus will be slightly more effective in dispersing high frequency lightning surges into the earth.

 

[bacon4life]

Jghrist, do you have more info on the corrosion that occurs in coating voids?

 

[7anoter4]

I agree with jghrist about the coating voids but the cable ground of copper lead [or tin] covered is not coated. The cable consists of bare copper stranded [nor compacted neither compressed] and jacketed with lead alloy [with bismuth and antimonium (stibium)].The jacket is about 2 mm thick [40 mils] and is applied by means of a lead press [or injection lead].The eventually cracks may be easy identified and fixed.
Best Regards

 

[jghrist]

IEEE Std 80-2000, IEEE Guide for Safety in AC Substation Grounding, discusses corrosion and coating voids in Section 11.2.

The disadvantage of using a tinned copper conductor is that it accelerates and concentrates the natural corrosion, caused by the chemicals in the soil, of the copper in any small bare area