634 ELECTRICAL EQUIPMENT 



75 C. for cambric, and 60 C. for rubber. Second, the tem- 

 perature must not be high enough to decrease the puncturing 

 resistance of the insulation below safe limits. This temperature 

 varies with the normal working e.m.f. of the circuit. Based on 

 these two considerations, it is recommended that the maximum 

 operating temperatures of the conductors of insulated cables be 

 limited to the values given below: 



Heating and Temperature of Cables (Standardization Rules of 

 the A.I.E.E.) . The maximum safe-limiting temperature in degrees 

 C. at the surface of the conductor. in a cable shall be: 



For impregnated paper insulation (85-#) ; 



For varnished cambric insulation (75-E) ; 



For rubber compound insulation (60-0.25#) ; 

 where E represents the effective operating e.m.f. in kilovolts 

 between conductors. 



Thus, at a working pressure of 6.6 Kv., the maximum safe- 

 limiting temperature at the surface of the conductor or conduc- 

 tors in a cable would be: 



For impregnated paper insulation 78.4 C. 



For varnished cambric insulation 68.4 C. 



For rubber compound insulation 58.35 C. 



The actual maximum safe continuous-current load for any 

 given cable is determined primarily by the temperature of the 

 surrounding medium and the rate of radiation. This current 

 value is greater with direct than with alternating-currents, and 

 decreases with increasing frequency, being less for a frequency of 

 60 cycles than for 25 cycles. This difference in carrying capacity 

 for direct- and alternating-current is of slight practical importance 

 for conductors less than 500,000 cir. mils in area, at commercial 

 frequencies, i.e., 25 and 60 cycles. 



Furthermore, owing to the fact that alternating-current flowing 

 in large cables has greater density on the surface of the conductor 

 than in the center, so-called skin effect, an ordinary cable will not 

 carry as many amperes alternating-current with the same tem- 

 perature rise as it will direct-current. To overcome this, it has 

 in the past been common practice on single-conductor cables, 

 700,000 cir. mils and larger for 60 cycles and 1,000,000 cir. mils and 

 larger for 25 cycles, to make up the cable in annular form, using a 

 non-conducting core (usually fiber), and stranding the copper 



