SWITCHING EQUIPMENT 505 



automatic or non-automatic operation, and may be closed by hand 

 or solenoids. 



The automatic breakers are tripped under overload by series 

 trip coils or secondary relays, the latter method being almost 

 entirely used in modern installations. The secondary tripping 

 mechanism consists of a system of toggles and latches so con- 

 structed that only a slight pressure is needed to open the breaker. 

 The tripping coils may be energized from standard current trans- 

 formers, from bushing-type current transformers or from a source 

 of constant potential, the current adjustment being accom- 

 plished by varying the position of the plunger in the trip coil and 

 the inverse time relay by a dash pot. (See also " Relays.") 



The operating mechanism is secured to the cast-iron cover of 

 the heavy welded sheet-steel tank. There are two fixed contacts 

 in each switch element between which one phase of the circuit is 

 made and broken by a horizontal contact blade. Each contact 

 blade is connected to the operating mechanism by a specially 

 treated, hard wooden rod which passes through the cover of the 

 switch in an insulating bushing. The stationary contacts con- 

 sist of widely flared fingers and long arcing tips which also act as a 

 guide to the entering blade. The movable contacts are wedge- 

 shaped, which confines the arc to the top edge of the blade and 

 the flared portion of the finger tips. The contacts are always 

 smooth and bright due to the sliding effect which they are sub- 

 jected to on opening and closing, and the arrangement of the 

 burning tips. 



The design of the bushings depends entirely on the voltage for 

 which the switch is intended. For the 35,000-volt size, they are 

 made in one piece of wet porcelain and extend from the terminal 

 to the contacts below the oil. For higher voltages each bushing 

 consists of two porcelain sections, an upper and a lower, joined 

 together by heavy supporting iron flanges, which also serve as a 

 means of attaching to the breaker or for housing the bushing trans- 

 formers, where such are required. For moderate voltages the 

 contact rod which passes through the bushing is simply insulated 

 by an insulating material and the bushing filled with an insulating 

 compound of high dielectric strength. For higher voltages, 70,000 

 and above, the bushings generally contain a number of cylinders 

 of insulating material concentric with the conducting tube, the 

 whole being filled with compound. These cylinders in connection 



