SWITCHING EQUIPMENT 563 



spring and both throws (closing and opening), are interlocked. It 

 is provided with a mechanical device to indicate which throw was 

 last closed and, in addition, with red and green bull's-eye lamps to 

 indicate the actual position of the circuit breaker. The necessary 

 auxiliary switches for these lamps are provided with the breaker. 



Mimic Buses. It is sometimes customary to place on the 

 switchboard copper connections, known as mimic buses, rep- 

 resenting the main connections of the station. These are often 

 desirable as they keep before the operator the whole arrange- 

 ment of the circuits, enabling him to see at a glance what is the 

 proper switch to open or close. On the other hand, their use may 

 sometimes cause either a crowded or unsymmetrical arrange- 

 ment. 



Figs. 337, 342, or 344, illustrate the use of such mimic buses. 



Bus and Switch Structures. As previously stated, bus-bars or 

 electrically operated oil circuit breakers are not necessarily placed 

 near the controlling switchboard, but should be placed with con- 

 venience to connections and safety from fire and in handling. 



Isolating barriers or compartments are recommended for 

 voltages up to 15,000 where the capacity is above, say, 5000 Kw. 

 in order to prevent any destructive effects of short-circuits from 

 spreading and involving the entire bus structure. 



Furthermore, the compartments act as a guard against anyone 

 touching the exposed parts of the buses and breakers and gives a 

 certain amount of finish and completeness to the station. The 

 cost of the cell structure is not of great consideration and is only a 

 small percentage of the total cost of the station. 



For higher voltages the currents naturally become corre- 

 spondingly less, minimizing the destructive effects of short-circuits, 

 and, on the other hand, the spacings required are greater so that 

 open work generally becomes preferable. 



Various materials have been used for bus and oil circuit 

 breaker compartments, namely, brick, concrete, soapstone and 

 slate, and sometimes a combination of brick with one of the other 

 materials. Brick compartments are the cheapest and if properly 

 made give the best appearance. The use of common brick is, 

 however, not recommended because most of the walls are four 

 inches thick and the sizes of the brick vary so, while, on the other 

 hand, the bonds are so large that a neat job cannot generally be 

 obtained. Inasmuch as the cost of laying the brick is about 75 



