SWITCHING EQUIPMENT 497 



very high voltage work, each pole is in a separate steel tank of 

 such substantial construction as to be proof against any explosion 

 due to the effect of short circuit. 



The circuit-breaker rating should be based on the maximum 

 current which it is to carry continuously without overheating, and 

 a breaker should therefore be selected which has a capacity at 

 least equal to the maximum rating or the one or two-hour overload 

 rating of the circuit. At the normal rated- load, current-carrying 

 parts should not heat more than 30 C., above an ambient tem- 

 perature of 40 C., providing the connections to the breaker do 

 hot heat to a greater extent. The rise on tripping solenoids and 

 accessory parts shall not exceed 50 C. The dielectric test should 

 be 2J times rated voltage plus 2000. 



In selecting the proper type of breaker to use for a certain 

 case, it is not enough that the breaker has a sufficient current- 

 carrying capacity or that it is capable to withstand the operating 

 voltage. The amount of energy or kilovolt-amperes which the 

 switch may be called upon to rupture under abnormal conditions, 

 such as a short-circuit, is a very important matter and deserves 

 the most careful attention. 



Based on its rupturing capacity, the rating of an oil circuit 

 breaker is necessarily more or less empirical, and is generally 

 determined by exhaustive short-circuit tests. It depends prin- 

 cipally on the amount of oil over the break at the starting of the 

 arc, the amount of space above the oil for gas expansion, the shape 

 and strength of the oil tank and its fastenings and on the length 

 and rapidity of the contact movement. 



There are many different ways of rating oil circuit breakers, 

 but it appears that the most logical way would be to base the rup- 

 turing capacity on the maximum " instantaneous " kilovolt- 

 amperes which the switch would be capable of rupturing. By 

 " instantaneous " is here meant the elimination of time-limit 

 relays in tripping. The problem of choosing an oil circuit breaker 

 for a given location would then resolve itself in determining the 

 kilovolt-amperes that can be delivered on short-circuit through the 

 breaker. This value depends naturally on how quickly the oil 

 circuit breaker opens and also on the rate at which the short- 

 circuit current dies down. Due to inertia, it is, of course, impos- 

 sible for a breaker to open instantaneously, and consequently no 

 breaker is ever called on to open the momentary short-circuit 



