SWITCHING EQUIPMENT 561 



Throw-over Switches. A sudden failure of the source of 

 power for the lighting system in the power station is a more or 

 less frequent and troublesome occurrence. To take care of such 

 an emergency and facilitate the re-establishment of normal con- 

 ditions where apparatus may have been shut down due to the fail- 

 ure of power, a switch for automatically throwing the lights to an 

 auxiliary or reserve source becomes very handy. The switch 

 shown in Fig. 350 accomplishes this result. The device consists 

 of a special double-throw switch held closed by a latch on one 

 throw against a pair of springs. 



To close the lighting circuit with the normal source of power in 

 operation, the switch is thrown in the lower set of contacts and 

 latched in the closed position by hand. When a failure of the 

 source occurs, a low-voltage release is caused to drop its armature, 

 tripping the latch free from the crossbar above it. The springs 

 on the hinge clips of the switch then quickly force the switch into 

 the upper set of contacts, which are connected to *the reserve 

 source of power. At the same time an auxiliary switch at the 

 top is thrown into contact, causing a bell or other indicator to 

 operate to attract the station attendant's notice. After the 

 resumption of normal conditions, the switch must be thrown by 

 hand into the lower contacts and latched. 



Calibrating Terminals. A quick and convenient method of 

 making connections for calibrating instruments, etc., is very 

 desirable, and this has led to a very general use of providing 

 calibrating terminals on all important switchboards. These may 

 be mounted either on the front or back of the panels, the choice 

 being governed by the conditions. For example, where it is 

 difficult to carry on such tests on the back of a board, the ter- 

 minals may readily be mounted on the front, while if there is 

 plenty of room in the rear, it may be advantageous to locate the 

 calibrating terminals there in order to utilize the space on the 

 front otherwise. 



The terminals for the current transformer connections should 

 be such, that the testing instrument can be connected in the cir- 

 cuit without breaking the continuity of the circuit, as explained 

 under " Current Transformers." 



Control Switches. Remote electrically operated oil or air 

 circuit breakers are controlled by small double-throw control 

 switches, usually mounted on the main switchboard. How- 



