BRIDGE METHODS FOR LOCATING RESISTANCE FAULTS 385 



temperature differs widely for different cable sections and even for the 

 individual pairs in a particular section. The average change per 

 degree Fahrenheit is probably about four per cent, for the temperature 

 range encountered in the plant. 



15100 



5200 



250 



225 



; 200 



30 



35 



40' 



45 Si 



— 50 Q 



Fig. 3— Variation of insulation resistance, loop resistance and temperature over 

 24 hour period — -typical 50 mile aerial cable pair. 



The curves of Fig. 3 illustrate comparative variations of insulation 

 resistance between wires of a representative cable pair, conductor 

 resistance of the pair, and outside temperature, during a 24-hour 

 period which included a sunny summer day. The curves were plotted 

 from measurements made every half hour, one-minute electrification 

 periods being used when measuring insulation resistance. It is not 

 uncommon to find that the insulation resistances of particular pairs 

 vary by factors of three to one during the course of a day. 



Comparative variations of average insulation resistance between 

 wires of pairs and of mean outside temperature over the course of a 

 year are illustrated by the curves of Fig. 4. The points for the insu- 

 lation resistance curve were obtained by measuring the insulation 

 resistances of a number of pairs each working day during the year, 

 using one-minute electrification periods, and averaging the measured 

 values for each day. 



In general, average insulation resistance is likely to vary by a factor 

 of 15 to one during the course of a year. Individual pairs are, of 

 course, subject to much wider seasonal variations in insulation re- 

 sistance. During winter it is not uncommon to find particular pairs 

 in a 50-mile repeater section with insulation resistances between wires 



