470 ELECTRICAL EQUIPMENT 



circuit, the synchronous apparatus connected to the system in the 

 form of load must, of course, also be taken into account, as on a 

 short-circuit there is a tendency for them to feed back into the 

 system, due to the inertia of their rotating elements. It is, of 

 course, also evident that strictly " spare " equipments need not 

 be included in the calculations. 



In dealing with the effects of short-circuits we must consider 

 the damages which they may cause to generators, transformers, 

 circuit breakers, cables or bus-bars and against which protection 

 must be provided in the form of reactors for limiting the excessive 

 currents to values which may be safely withstood by the apparatus. 



Generators and transformers are, as previously stated, now 

 designed with such mechanical rigidity that they can safely with- 

 stand the mechanical forces arising from dead short-circuits across 

 their own terminals. 



As far as oil circuit breakers are concerned, the problem is 

 much more difficult and their rupturing capacity is, as a rule, the 

 limiting feature in determining the value of the permissible short- 

 circuit current. The power which has to be broken on a short 

 circuit depends naturally on how quickly the circuit breaker opens 

 and also on the rate at which the short-circuit current dies down. 

 Due to inertia, it is, of course, impossible for a breaker to open 

 instantaneously and consequently no breaker is ever called on to 

 open the momentary short-circuit current that occurs during the 

 first few cycles, but it has to be strong enough mechanically to 

 resist the magnetic stresses set up during such a short-circuit. 

 Large capacity breakers equipped with " instantaneous " acting 

 relays can be made to open in about one-quarter of a second and 

 if the short-circuit occurs close to the generating station the 

 power which has to be broken averages approximately 60 per 

 cent of the maximum instantaneous value. If the trouble should 

 occur at a considerable distance from the power-house, the rate 

 at which the short-circuit current dies down would be much 

 slower, so that the power which would have to be broken might 

 be nearly equal to the instantaneous value, but due to the addi- 

 tional reactance of the line this value will, as a rule, be less than 

 the above, which, therefore, should be used in governing the cur- 

 rent which must be broken under the worst conditions. For non- 

 automatic switches or switches equipped with definite time limit 

 relays with a setting over 0.8 second, the rupturing capacity 



