AUXILIARY APPARATUS 407 



is opened and so hold over and burn from one post of the 

 switch to the other. To prevent this disastrous arcing 

 quick-break switches are sometimes used. These are so 

 designed that the switch is opened by a spring, snapping 

 the blade back very quickly. 



" Cutting " of a Switch. Switches sometimes begin to 

 " cut," especially in the hinge. This is caused by grit 

 getting into the joint and wearing off little pieces of copper 

 as the switch is opened and closed. Then the little bits 

 of copper help this rubbing process until the blade is worn 

 very rough in the hinge and so makes poor contact and 

 is caused to overheat. As soon as "cutting " is detected, 

 a switch should be taken apart and the rubbing surfaces 

 smoothed down with a file and emery. A little grease 

 in the joint will generally keep the rubbing surface in good 

 condition. 



Oil-break Switch. For voltages higher than 600 an 

 oil-break switch is generally used. This type of switch 

 opens in oil and the oil prevents the formation of a bad 

 arc as the circuit is broken. Oil-break switches are generally 

 used only on alternating current circuits, because the voltage 

 of a continuous-current system is seldom greater than 600 

 volts while the voltage of an a-c. system is seldom less than 

 2300 volts. A high voltage a-c. circuit opens quietly with 

 an oil switch, but an oil switch would not operate satis- 

 factorily on a high voltage c-c. circuit. 



By means of an oil-break switch a high-voltage a-c. 

 circuit may be opened even when a high current is flowing 

 through the switch. The arc which forms when the switch 

 is opened is smothered by the oil and serious arcing or 

 burning of the switch contacts does not occur. 



The oil-break switch is generally located in an oil tank 

 directly behind the switch board, and the handle by meanfl 

 of which the switch is opened is located on the front of the 

 board. A link connects the handle and switch proper. 

 A small oil-break switch is shown in Fig. 266. 



