208 



ELECTRICAL MEASUREMENTS 



electrification; from this result and the known length of the 

 sample the insulation resistance per mile or per 1000 ft. is 

 determined. 



Effect of Temperature. The substances classed as insulators 

 have very large negative temperature coefficients; that is, an 

 increase of temperature lowers their resistance. This is shown 

 in Fig. 117 which gives the result of tests on a sample of rubber- 

 covered wire. In this work it is customary to express the tem- 

 perature in degrees Fahrenheit. 



For purposes of comparison it is necessary to reduce the results 

 of insulation resistance measurements to some standard temper- 

 ature, 15.5C. This is usually done 

 by dividing the resistances at the tem- 

 peratures of observation by experi- 

 mentally determined factors, the 

 values of which will be different for 

 different insulating compounds. Con- 

 sequently, the various reduction fac- 

 tors quoted in electrical handbooks 

 should not be applied indiscriminately. 

 The great difficulty with tests for 

 insulation resistance as a guide to 

 the condition of underground feeders 

 after installation, is the uncertainty 



as to the temperature, due to the feeders having been in use, 

 or to the heating by currents in neighboring ducts. 



Insulation Testing by Voltmeter. Insulation resistances of 

 the magnitude of 1 or 2 megohms may be measured by aid of a 

 direct-current voltmeter of known resistance. Two readings are 

 taken, the first when the instrument is directly across the line, 

 the second when the line voltage is applied to the instrument 

 and the unknown resistance in series. The testing voltage must 

 remain constant. 



Call the reading when the voltmeter is across the line, D\, 

 and when it is in series with X, the unknown resistance D 2 . Then 

 if the resistance of the voltmeter be R v , and the constant of the 

 instrument considered as a current galvanometer be K, 



tf 50 CO 70 80 90 100 

 Temperature F. 



FIG. 117. Illustrating ef- 

 fect of temperature on insula- 

 tion resistance. 



