For general pipeline use where two of more rectifiers are used, synchro- 

 nous interrupters are required. Usually two such interrupters for adjacent 

 rectifiers will provide sufficiently accurate OFF potential data. Inter- 

 rupters using quartz timing are available, providing for accurate synchro- 

 nization. A timing interval of 25 seconds ON and 5 seconds OFF usually 

 will be satisfactory for pipeline work. 



A possible source of error when 

 measuring substructure potentials 

 could be IR drops caused by presence 

 of stray currents of unknown magnitude 

 in the contacting electrolyte. The 

 best protection against such error is 

 to place the reference electrode as 

 close as possible to the structure 

 under test. 



To summarize briefly, a 

 cathodically protected substructure 

 potential to copper sulfate electrode 

 of -0.85 volt or more indicates that 

 protection exists under the following 

 conditions: 



(1) If substructure is 

 immersed in seawater or similar 

 electrolyte, with rectifier on 

 and reference electrode contact- 

 ing electrolyte within 1.5 

 meters of substructure; or 



TO RECTIFIER OUTPUT 



NORMALLY CLOSED 

 -1 



TO PROTECTED SUBSTRUCTURE 



MICROSWITCH 



^ 



I I 



NORMALLY OPEN 



POTENTIOMETER 

 VOLTMETER 



Figure 99. 



Circuit for off potential 

 measurement . 



(2) if substructure is buried in soil with rectifier output 

 momentarily interrupted and reference electrode contacting soil 

 above or immediately adjacent to substructure. 



c. Design . 



(1) General . Ideally, design for cathodic protection should be a 

 part of the original design of the substructure. Placing an existing 

 substructure under cathodic protection, if installed without cathodic 

 protection considerations, can be expensive. Some measures to ensure low 

 cathodic protection current requirements that should be included at the 

 design stage of subsurface structures are: 



(a) If coating is feasible, use a high quality coating and 

 apply properly with minimum bare areas or discontinuities (holidays); 



(b) provide electrical isolation for large bare metal areas, 

 if not to be protected, such as steel bulkheads, tank bottoms, 

 electrical grounding systems, and local utility piping; and 



(c) be sure that there are no metallic contacts with other 

 subsurface structures such as pipelines, pipeline casings, or 

 cables . 



360 



