406 ALTERNATING CURRENTS 



which may occur, but are not intended to withstand stress in the 

 direction of the line. When these towers are used, an anchor 

 tower about every mile is necessary, in order to take care of any 

 unbalanced longitudinal forces which occur when conductors 

 break. When suspension insulators are used, a steel ground 

 wire is necessary at the top of the structure to give longitudinal 

 support to the tower. The advantage of flexible tower construc- 

 tion lies in the fact that the towers are usually assembled complete 

 in the shop and are easily erected. 



SUB-STATIONS 



173. Transformer Sub-stations. The function of the sub- 

 station is to receive the electrical energy, usually at a voltage too 

 high for commercial purposes, and to deliver this energy at other 

 voltages and sometimes at other frequencies such as may be 

 required for the district served. 



The sub-station may be a transformer station only, receiving 

 energy at a voltage of 26,400 volts, for example, and transforming 

 it to 2,300 volts for general distribution. Figure 372 shows 

 the wiring diagram of such a station. Two distribution lines 

 leave the station at 2,300 volts, one for lighting and one for 

 power. Power loads and lighting loads should be kept separate, 

 if possible, in order to avoid the flickering of lamps when the 

 motor loads are thrown on and off the line. Usually 2,300 to 

 230-115 volt transformers are used to step down the voltage for 

 lighting purposes, a three-wire system being employed for the 

 secondary. (See page 379, Fig. 343.) Owing to the possibility 

 of the low-voltage wires coming in contact with high-voltage 

 wires, and so exposing the consumer to danger, one wire of the 

 secondary of lighting circuits, usually the neutral, should be 

 grounded at each consumer's premises. As motor loads are 

 usually three-phase, two V-connected transformers, three 

 single-phase transformers, or a single three-phase transformer 

 may be used for stepping down the voltage. In order to 

 save secondary copper, motors are often operated at 440 or 550 

 volts. Some large consumers, employing a few large motors, may 

 operate them at 2,300 volts and thus eliminate the step-down 

 transformers. 



