may be the deciding factor (along with resistivity) in an anode-type 

 choice. 



As discussed in an earlier section, galvanic anodes installed in soil 

 environment should normally be completely surrounded by chemical backfill. 

 This is to provide uniform low resistance to the soil. Galvanic anodes 

 installed in water do not require any backfill as the water will provide 

 uniform contact with the anode. Impressed current anodes in soil also 

 require backfill but for different reasons. Here the backfill material is 

 carbonaceous in nature. The anode makes electrical contact with the back- 

 fill. Ion transfer, with resulting loss of material, occurs at the backfill 

 interface with the soil. This results in longer anode life and lower 

 contact resistance as the backfill is of low resistivity and much greater 

 surface contact area with the soil than would be possible with the anode 

 alone. As with galvanic anodes in water, impressed current anodes in water 

 do not use backfill. 



(3) Calculating the Resistance of a Single Anode . Anode installa- 

 tion design requires that the effective resistance of the anode (or anode 

 bed) to its environment be known or be calculated. Dwight's equation is 

 generally used for single galvanic or impressed current anode resistance 

 determinations in either water or soil. The equation is 



where 



R = resistance of vertical anode or backfill to ground 

 or water in ohms 



L = length of anode in centimeters 



a = radius of anode in centimeters 



p = electrolyte resistivity in ohm-centimeters 



This can be simplified to: 



0.012p . ,35. 3L, ,„. 



Rv = — j— ■£■ log (-g ) (2) 



where 



Rv = resistance of vertical anode or backfill column 

 to ground (soil or water) in ohms 



L = length of anode or backfill column in feet 



D = diameter of anode or backfill columns in inches 



p = electrolyte resistivity (backfill, soil, or water) 

 in ohm-centimeters 



(a) Anode Installed in Backfill Soil . If a single anode is to 

 be used, in soil, the internal resistance (anode to backfill) should be 

 considered. Internal resistance is not a factor when the anode is installed 



362 



