498 ELECTRICAL EQUIPMENT 



current that occurs during the few first cycles, but it has to be 

 strong 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 second, and the power which has to be broken under 

 such conditions averages under the worst conditions approxi- 

 mately 60 per cent of the maximum instantaneous value. For 

 non-automatic switches or switches equipped with definite time- 

 limit relays with a setting over 0.8 second, the rupturing capacity 

 corresponds to the sustained short-circuit current, while for 

 switches with inverse time action the condition approximating 

 " instantaneous," as above, must be assumed. When speaking 

 of the maximum instantaneous value, the root-mean-square value 

 is meant. 



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, the added react- 

 ance of transformers and lines serve to reduce the value of the 

 short-circuit current. (See also section on " Current-limiting 

 Reactors.") 



Unfortunately there is some difference in rating oil circuit 

 breakers, and it is very important, in any oil circuit breaker nego- 

 tiation, that the actual meaning of the guarantee is fully under- 

 stood. So, for example, the term " rupturing capacity " has 

 been given two meanings; one, as indicating the rated Kv.A. 

 capacity in generators which may be short-circuited and under 

 such conditions opened by the breaker in question; the other, as 

 indicating the actual current which the breaker opens at the time 

 of short-circuit, this capacity generally being expressed in Kv.A. 

 equivalent to the actual current opened at the normal circuit volt- 

 age. Furthermore, the term " ultimate breaking capacity " has 

 been used to indicate either of the above conditions, and it can be 

 seen immediately to what confusion this difference in the meaning 

 of the guaranteed rating can lead. The importance of a clear 

 understanding of just what is meant cannot be over-emphasized. 



Fig. 306 represents a type of circuit breaker which is intended 

 for use in small and moderate-capacity stations for voltages up 

 to 22,000. It can be mounted on the pipe frame supporting the 

 switchboard panels, on framework remote from the panel, or in 



