AUXILIARY APPARATUS 409 



switch. In large stations the oil switch may be opened by a 

 small motor, the motor being located on the top of the 

 switch tank. The current supply for the motor is con- 

 trolled by a small switch placed conveniently for the opera- 

 tor; the oil switch itself may be far from the controlling 

 switchboard. Instead of using a small motor for operating 

 a remote-control switch, the action of an electromagnet 

 may be used; in some types of switches (e.g., such as is 

 used on many railway equipments) compressed air may be 

 used to open and close the switch. It is the remote-control 

 switchthat makes the compact, remote-control switchboard 

 of a large modern power plant possible. 



132. Fuses. When a machine or circuit is carrying 

 more current than that for which it was designed, serious 

 injury may result from overheating. The purpose of a 

 fuse is to prevent such a possibility. A fuse consists of a 

 piece of easily melted alloy, in the form of a wire or ribbon, 

 connected in series with the machine or circuit to be pro- 

 tected. The size of a fuse is so selected that when a dan- 

 gerous current is being carried by the circuit it is designed 

 to protect, the heat generated by the I 2 R loss in the fuse is 

 sufficient to melt it and so the circuit is automatically opened. 



Replacing a Fuse. When a fuse is blown, the circuit in 

 which it is connected should be opened by first opening the 

 proper switch and then a new fuse may be inserted. A new 

 fuse should not be inserted until it has been ascertained 

 that the circuit is dead; neglect of this point is likely to 

 prove dangerous to the operator putting in the fuse as the 

 new fuse may blow (melt) while he is inserting it and so 

 cause a dangerous burn. 



There are several types of fuses in common use. The 

 earlier type was the string fuse, which consisted merely 

 of a piece of fuse wire inserted in the circuit by suitable 

 clamps and screws. The disadvantage of this kind is that 

 there is some danger of starting a fire when the fuse blows 

 and throws melted lead around. 



