without backfill, such as in seawater. This will depend on the type of 

 backfill used which will, in turn, depend on whether the anode is to be of 

 galvanic or impressed current type. For an impressed current anode with 

 carbonaceous backfill, a backfill resistivity of 50 ohm-centimeters may be 

 used. Assume a graphite anode 7.6 centimeters (3 inches) in diameter and 1.5 

 meters (5 feet) in length is to be centered in a vertical backfill column 

 20.3 centimeters (8 inches) in diameter and 2.1 meters (7 feet) in length (see 

 Fig. 100). Resistances are calculated for the anode and for the backfill 

 column using equation (2) with p = 50 ohm-centimeters. The difference between 

 the two values represents the internal resistance of the anode (0.213 - 0.128 

 = 0.085 ohm)_. For most conventional impressed current anodes used singly, a 

 figure of 0.1 ohm may be safely used. Where more than one anode is to be 

 connected in parallel, the internal anode resistance for the group becomes 

 the single anode internal resistance divided by the number of anodes in the 

 group. If the number of anodes to be parallel connected is more than three or 

 four, the internal resistance becomes negligible. The same method is used to 

 calculate the internal resistance of a single galvanic anode (see Fig. 101). 

 Here the backfill resistivity will be higher, with a resulting higher internal 

 resistance. 



(b) Anode Installed in Water . For suspended vertical anode 

 installations in water, the anode should be installed so that the top of the 

 anode is never less than 1.5 meter (5 feet) below the water surface. Refer 

 to tide data. The bottom of the anode should be 1.5 meters above the channel 

 or marine bottom. Header cables should be far enough above the water surface 

 to ensure no water contact. In protected areas this would be a minimum of 3 

 meters (10 feet). Header cables would have to be at much greater height for 

 open-sea areas. This will require anodes with leads large enough to permit 

 connection to the header cable with no underwater splicer required. In some 

 cases anodes may require installation in perforated nonmetallic pipe to 

 prevent damage by water movement. 



(4) Calculating the Resistance of Anode Groups . Usually anodes 

 will be used in groups, installed in a line, connected in parallel to a 

 header cable which in turn is connected to the substructure to be protected 

 (galvanic anodes) or to the positive output of the power source (impressed 

 current anodes). A calculation of the overall resistance of the parallel- 

 connected group (usually termed "anode bed" or "ground bed") will be required. 

 The effective resistance of the group, differing from normal parallel elec- 

 trical circuits, will not be equal to the resistance of one anode divided 

 by the number of anodes in the group. This applies to marine installation 

 also. Due to mutual interference between anodes the resistance of the 

 group will always be higher than that determined by parallel electrical 

 circuit calculations, varying with the number of anodes, anode spacing, and 

 electrolyte resistivity. Several methods have been used for the calculation 

 of parallel anode resistance. One method utilizes the following equation: 



n O.Q01566p ,, __._ , 2.94 L, 

 Rv = jjj- ^ {(2.303 log — g ) 



+ {—- 2.303 log Q.656 N) } (3) 



where 



Rv " resistance to electrolyte (soil or water) in ohms of 

 the vertical anodes in parallel 



363 



