CURRENT-LIMITING REACTORS 471 



corresponds to the sustained short-circuit current, while, for 

 switches with inverse time action, the condition approximating 

 " instantaneous," as above, must be assumed. The maximum 

 instantaneous value means the root-mean-square value of a sym- 

 metrical wave. Similarly for the rupturing capacity of oil circuit 

 breakers, as tests have shown that the wave becomes practically 

 symmetrical in the minimum time in which a breaker can open. 



There is a great variety of oil-circuit breakers in the market 

 with rupturing capacities of several hundred thousand Kv.A. 

 As a rule, switches with the higher rating will be required near 

 the generating station, while under some conditions smaller 

 switches may be used, for instance, in substations, where the 

 added reactance of transformers and lines serve to reduce the 

 value of the short-circuit current. 



The mechanical forces acting between the conductors of a 

 three-phase cable may be obtained from the following formula. 

 It is assumed that all three conductors are equally spaced and 

 simultaneously short-circuited, the r.m.s. current being equal in 

 each phase. Then the force, Fo, tending to repel any conductor in 

 a direction at right angles to a plane passing through the other 

 two is: 



T ir- /max. 



where 7 = r.m.s. value of sine wave = -=-\ 



V2 



a = Distance between conductors in inches. 



Thus, in a paper insulated, lead-covered cable, the force is exerted 

 on the over-all wrapping around all three conductors and also on 

 the lead sheath, and the tensile strength of the paper and lead 

 must be sufficient to withstand the stress thus placed upon them. 

 On bus-bars this force tends to throw the bars away from the 

 center of the equilateral triangle of which each bus is assumed to 

 form one apex, and produces a tension or compression on the bus- 

 bar clamps, depending on the location of the insulators. The 

 bus-bars, due to their spacing being inherently greater than the 

 conductors of a cable, are subject to a much lower disruptive 

 force per unit length, but, on the other hand, since they are sup- 



1 Gross, A.I.E.E., Jan., 1915. 



