For safety margin use 60 anodes, spaced evenly under the wharf, about 

 5 meters (16 feetl apart. Using equation (31 the anode resistance of each 

 component of the example project is calculated to determine the total anode 

 resistence: 



60 anodes, averaging 20 ohm-centimeters resistivity = 2 milliohms. 



Rectifier positive to center of header cable under wharf: 



30 meters No. 4/0 copper cable in conduit = 5 milliohms 



Rectifier negative to bulkhead: 



30 meters No. 4/0 copper cable in conduit = 5 milliohms 



Header cable under wharf, center connection, 



effective length, 155 meters No. 1/0 = 51 milliohms 



Bulkhead resistance is negligible. 



Total resistance = 63 milliohms 



(4) Voltage Requirement . Required rectifier voltage is obtained 

 using Ohm's Law: E = IR or required voltage is equal to total current 

 multiplied by total circuit resistance or E = 215 x 63 = 13 500 millivolts 

 (13.5 volts). For best results, a current and voltage requirement test 

 should be made after the anode installation is complete, including the 

 cables to the rectifier location. A direct current welding generator 

 capable of furnishing the above current and voltage should be used. For 

 test purposes, complete polorization is not required. However, current 

 should be applied for several days, with potential measurements made from 

 bulkhead to a reference electrode in the water within 1.5 meters of the 

 bulkhead (to minimize IR drop in the readings) at several locations and at 

 several depths, from surface to bottom. Readings of -0.85 volt to a copper 

 sulphate reference electrode or -0.84 volt to silver-silver chloride 

 reference electrode indicate adequate protection. 



(5) General . Some comments should be made regarding the example. A 

 continuous 25-millimeter-diameter steel rod should be welded in place in two 

 locations to each sheet pile for the full length of the bulkhead. This con- 

 nection rod should be well above the splash zone and should be well coated to 

 ensure permanent connections. Woven grounding straps bolted to the sheet pile 

 may also be used, if they are protected against the environment. The point 

 must be made that the connections must be flexible. A sheet-pile bulkhead may 

 deflect enough to break welds to a rigid 25-millimeter (1 inch) steel rod. 

 Electrical continuity to each and every sheet pile is essential for the success 

 of the cathodic protection system. 



Care should be taken to ensure that the steel bulkhead is electrically 

 isolated (insulated) from all other structures or piping. Again, this is 

 essential if the protected installation is to remain reasonably trouble 

 free. 



4. Cathodic Protection and Coatings in Combination . 



A combination of cathodic protection and coatings provides the advantages 

 of both. Protective coatings are known to be the primary considerations 



370 



