5i8 SWITCHES, FUSES, CIRCUIT BREAKERS [Cn. XIII 



jaw end of the switch as in fig. 277, then when the switch is open 

 the hinges and blades are "dead" and can be put in order with 

 safety. 



714. Snap Switches. These are sometimes used for turning 

 on and off the current at the operating table. They are mounted 

 on insulating material like porcelain, and are enclosed by a metal 

 covering which is lined with insulating material. The key or 

 button for turning on and off the current is also of insulating 

 material. This form of a switch around the work table is con- 

 venient, and avoids any danger of accidentally short-circuiting the 

 line. It should turn on the current and turn it off with a snap. It 

 is also desirable that there should be a sign indicating when the 

 current is on or off, as one cannot see directly as with the knife 

 switch. If such a switch is used, make sure that it is of the right 

 capacity for the maximum current and that it conforms in every 

 way with the standard requirements. It will be plainly marked so 

 that after it is installed one can see at any time the current and 

 voltage for which it is designed. Snap switches are better adapted 

 for small currents, than for large ones. Knife switches are to be 

 used on lines with large currents. 



FUSES AND CIRCUIT BREAKERS 



715. Fuses and circuit breakers are devices for opening or 

 breaking the circuit whenever the current in any particular situa- 

 tion becomes too great. For example, if a part of the line should 

 be short-circuited. 



The devices used are of two kinds ; fuses, and magnetic cut-outs 

 or circuit breakers. 



716. Circuit breakers. The circuit breaker is a device by 

 which a magnetic trip releases a catch which allows a large switch 

 to open, thus breaking the circuit. 



The great advantage of a circuit breaker is that nothing is 

 burned out or melted. It is only necessary to close the switch 

 again and the current will be on. It acts instantly whenever the 

 current rises above the amperage for which it is adjusted. 



