98 ALTERNATING CURRENTS 



Measurement of Power in Two-phase Systems. In a two-phase, 

 four-wire system, connected as shown in Fig. 97, the total power 

 may be measured by two wattmeters. If the system is inter-con- 

 nected, the loads must be balanced or this method is incorrect. 

 If, however, the two phases are insulated from each other, two 

 wattmeters measure the power correctly regardless of unbalance, 

 power-factor, etc. 



If the loads of a four-wire inter-connected system are not 

 balanced, three wattmeters must be used, as shown in Fig. 98. 

 The power is the algebraic sum of their readings. The power 

 in a two-phase, three-wire system may be measured by two watt- 

 meters connected as shown in Fig. 99. 



