356 BELL SYSTEM TECHNICAL JOURNAL 



Diagram (b) the connection for ground leakage tests. Since the 

 leakage resistance measured is relatively high, the most accurate 

 results are obtained by using a high resistance voltmeter and a fairly 

 high test voltage. In practice, a l()(),0()0-ohm voltmeter is generally 

 used with a test battery of from 100 to 150 volts. A test voltage of 

 200 is also provided in circuit with a milli-ammeter and protecti\e 

 resistance for use in checking the strength of insulation of central 

 office wiring and subscriber's lines. 



00 2 ©0 



? 



fal Leakage Between Wires (b) Leakage to ground 



Fig. 2 



Considering the circuits shown in Fig. 2, the voltage of the battery 

 E is equal to the IR drop over the voltmeter plus the IR drop or the 

 drop due Xo leakage over the remainder of the circuit back to the 

 battery. Designating the insulation resistance being measured by X, 

 the voltage of the test battery b>- K, the deflection in volts of the 

 100,000-()hm \-oItmeter by D and the current flowing by /, tlien 



K = D + XI. 

 D= 100,000 /, 

 and „ ,,.„,.,.. //i-/^> 



A' = 100,000f -^V 



In practice, tables are provided from which the insulation resistance 

 or leakage can be read directly for various deflections of the voltmeter. 

 When expressed in terms of insulation resistance, the most conxenient 

 unit of measurement is the megohm. If expressed in terms of leakage, 

 the unit used is a reciprocal function of the megohm known as the milli- 

 micromhf). The results of measurements for complete circuits are 

 generally reduced to api)l\' lo a unit leiigih of circuit such as a mile 

 so that the testing resulis on (iniiiis of (lilTrmu lcnt;iiis will l)c com- 

 parable. 



The open win- lull riicuils in the lrlc|iliiiiie |)lant are tested period- 

 ically by the method just described. The leakage of circuits is ma- 

 terially increased by defective or broken insulators and by contact of 

 the wires with foreign objects such as trees, particularK- unfier damp 

 weather conditions. Troubles of this kind are detected \)\ careful 



