SWITCHING EQUIPMENT 507 



with equalizing shields serve to evenly distribute the potential 

 gradient of the bushing. 



For each pole there is a separate oil tank provided with gas 

 vents and oil gauges. Drain-cocks may also be obtained if desired 

 and are to be recommended for all large floor-mounted switches. 



Fig. 311 shows a large-capacity tank-type oil circuit breaker for 

 indoor services at 135,000 volts. It is almost identical to the 

 switches previously described, the main points of construction 

 being apparent from the illustration. At the upper end of each 

 bushing is a combined expansion chamber and gauge glass which 

 affords opportunity to view at all times the insulating compound 

 with which the bushings are filled. The terminal on the upper 

 end of a bushing is of such shape that it can be used for attaching 

 a crane hook to lift the bushing out of or replace it in the breaker. 



High-grade mineral oil should be used for all oil circuit breakers. 

 It should have a high flash and ignition point as well as high 

 resistance to carbonization. 



Relays. Relays may be defined as protective devices used in 

 connection with circuit breakers to disconnect any part or section 

 of a system on which a fault occurs but leave the rest of the system 

 in operation without being further affected by the faulty section. 

 In general, a relay consists of, first, a coil or system of coils con- 

 nected either directly in series or in parallel with the circuit con- 

 trolled or to secondaries of current or potential transformers, the 

 current and potential coils then being wound for a low value, 

 usually five amperes for the current coil and 110 volts for the 

 potential coil, although other values might be used if desired. 

 In the former case it is termed a primary or series relay and in the 

 latter a secondary relay. Second, a relay consists of a movable 

 part such as a plunger or a revolving disk, etc., whose travel is 

 controlled by the relay coils, and third of a contact device which 

 is actuated by the movable part and which controls the operating 

 circuit, such, for instance, as the trip coil of the circuit breaker 

 to which it is connected. Although smaller circuit breakers 

 may be opened by the relay core striking the tripping latch 

 directly, larger breakers are usually provided with separate 

 tripping coils, the cores of which, when completing their travel 

 strike the latch and release the switch. 



The impedance of a relay coil is relatively small compared to 

 that of an oil circuit breaker trip coil, and if a number of instru- 



