472 ELECTRICAL EQUIPMENT 



ported at only frequent intervals rather than continuously, as is a 

 cable, the force on any support may become excessive. 



The above refers to a three-phase short circuit. If, however, 

 the short is between two of the conductors instead of between 

 all three, the force will only be 86.6 per cent of the three-phase 

 value, based on the same current. 



If the bus-bars are installed in the same plane, the force acting 

 on the outside bars is only 86.6 per cent of what it would be if the 

 bars were spaced at the vertices of an equilateral triangle. 



The heating of cables may, on the other hand, be the limiting 

 feature as far as the permissible short-circuit current is concerned, 

 since it is quite possible for the temperature of the conductor to 

 rise to such a point as to endanger the insulation of the cable even 

 in the short time that it takes an oil switch to open, especially if it 

 is non-automatic or provided with a definite time-limit relay. 

 The calculations involved in determining the temperature rise are 

 intricate and the reader is referred to a paper by I. W. Gross in 

 A..I.E.E. Proceedings for January, 1915. 



In calculating the short-circuit current let us, as an example, 

 first assume a system consisting of four 10,000 Kv.A. generators, 

 with 10 per cent inherent reactance, operating in parallel on a 

 bus. With a short circuit in one of the step-up transformers, 

 what would be the required instantaneous rupturing capacity of 

 the low-tension transformer circuit breaker? 



Since the four generators are connected in parallel, the com- 

 bined reactance will be equal to = 2.5 per cent and the total 



10 000 



short-circuit current, expressed in Kv.A., equal to ' X100 



Z.o 



= 400,000 Kv.A. The bus-bars must then be designed to with- 

 stand the mechanical stresses due to twice this current on account 

 of the possible unsymmetrical nature of the current wave, while 

 the rupturing capacity of the switch would have to be about 60 

 per cent of the above, or 240,000 Kv.A. 



As far as the generators themselves are concerned, it has pre- 

 viously been stated that those of modern design are now being 

 designed to safely withstand short-circuits. The generator 

 switches under the worst condition, i.e., with a short in one 

 of the generators, would be called upon to break the combined 

 current of only three generators, and as these switches as a rule 



