392 ELECTRICAL EQUIPMENT 



which are provided with two coils. So, for example, in Fig. 231 

 the two low-voltage coils are connected in parallel to supply 100 

 volts. 



In many instances it is deemed advisable to operate a three- 

 wire circuit from the low-voltage side of transformers, and thereby 

 reduce the cost of copper for the feeders. Such a connection is 

 represented in Fig. 232, where the low-voltage coils are con- 

 nected in series and their junction connected to the neutral wire. 

 This method of connection is used very extensively and is known 

 as the Edison Three-wire System. When used for combined 

 power and lighting load, the motors are usually connected to the 

 two outside wires, and the lights between the outside and neutral. 



The neutral wire generally carries less current than the out- 

 side wires, except in the case where the entire load is on one side. 

 The neutral wire should, for this reason, be of sufficient cross- 



< 1000 A 



WVNAA/WWVW 



FIG. 232. FIG. 233. 



section to safely carry a current which will blow out the main fuses 

 in case of short circuit on one side of the system. 



Fig. 233 shows the three-wire distribution where a grounded 

 neutral wire is employed, this system also being widely used for 

 general distribution, lighting, small motors, etc. 



The four terminals of the low-voltage coils are, as a rule, brought 

 outside the case in such proximity that they can readily be con- 

 nected in any desired manner by joining adjacent terminals. 

 Connection blocks are seldom used for the low-voltage winding of 

 distributing transformers, because of the large current-carrying 

 capacity required. 



The voltage stress on the windings naturally depends on the 

 voltage of the mains to which they are connected, and also on 

 abnormal operating conditions such as accidental grounds, light- 

 ning surges, etc. For the arrangement shown in Fig. 231 it is 



