OVER-VOLTAGE PROTECTION 613 



to see that they are in proper condition, it is well to keep on file 

 exact plans of the location of ground plates, ground wires and 

 pipes, with a brief description, so that the data may be readily 

 referred to. From time to time the resistance of these ground 

 connections should be measured to determine their condition. 

 This is very easily done when pipe grounds are installed, as the 

 resistance of one pipe can be accurately determined when three or 

 more pipes are used. For example: If there are three pipes, 

 namely, X, F, and Z, and the resistance of X-\-Y = 20 ohms, as 

 measured by a voltmeter, the resistance of X+Z=15 ohms, and 

 the resistance of F+Z = 20 ohms, then, by solving the equations: 



Y Z= 5 subtracting; 



y-Z = 5 

 F+Z = 20 



2F =25 adding 



y=12 ohms 



Z = 20-12J = 7f ohms 



The resistance of a single pipe ground in good condition has an 

 average value of about 15 ohms. A more approximate method 

 of keeping account of the condition of the earth connections is to 

 divide the earth pipes into two groups and connect each group to 

 the 110-volt lighting circuit with an ammeter in series. If there is 

 a flow of about 20 amperes the conditions are satisfactory pro- 

 vided the earth pipes are properly distributed around the station. 

 Aluminum cell arresters for non-grounded as well as grounded 

 circuits above 7250 volts consist of four units, each containing a 

 single or a double stack of cells depending on the voltage. Three 

 of the units have one terminal connected to the circuit, the other 

 being connected together; the fourth unit is inserted between this 

 multiple connection and ground. This gives the same protection 

 between line and line as between line and ground. A transfer 

 device is provided for interchanging the ground unit with one of 

 the line units during the charging operation so that the films of all 

 the cells will be formed to the same value. 



