CHAPTER XIV 



AUXILIARY APPARATUS USED WITH ELECTRICAL 

 MACHINERY 



131. Switches. Air-break Switch. A switch is a device 

 for easily opening and closing a circuit. The simplest 

 type consists of a copper blade, hinged at one end and 

 fitting tightly between two copper plates at the other. A 

 wooden handle is fastened to the end of the copper blade, 

 by which the switch is operated. It is styled a knife switch, 

 or an air-break switch, because the current is ruptured 

 in the air. 



These switches are made in various sizes, from 25 amperes 

 to several thousand amperes. The larger ones use com- 

 pound blades, as the contact surface available on one blade 

 would not be great enough to carry the large currents with- 

 out over heating. The switches used for generators supplying 

 power to lighting circuits are generally double pole, one 

 blade being in each side of the line. Railway generators, 

 however, have the negative side continually connected to 

 the negative bus, and so to ground, so that these machines 

 require only a single-blade switch in the positive line. 



Size of Switch. The length of the blade depends upon 

 the voltage for which the switch is designed; those to operate 

 on a 600-volt circuit being considerably longer than those 

 for a 250-volt circuit. This is due to danger of a short 

 switch not opening a 600-volt circuit even though the 

 blade has been pulled back as far as it will go. 



Arcing at a Switch. Upon opening the switch an arc is 

 formed and the arc may run down the blade as the switch 



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