6-B] MEASUREMENT OF POWER. 229 



system* and is independent of any assumptions as to wave form 

 or the nature of the load. 



24. The arrangement of instruments is shown in Fig. i. The 

 wattnietersf are inserted in any two lines, as X and Y, the third 

 wire Z being considered as a common return. 



FIG. i. Two-wattmeter method for measuring power in any 3-phase or 

 other 3-wire circuit. 



The total power is the algebraic sum of the readings of the 

 two wattmeters. For high power factors (more than 0.5) this 

 will be the arithmetical sum, both wattmeter readings being posi- 

 tive. For low power factors (less than 0.5), the reading of one 

 wattmeter is to be considered negative, the total power in this 

 case being the arithmetical difference of the two readings, as 

 shown later in 31. 



25. There are several ways for telling whether one reading is 

 negative or not, the principal ones being as follows : 



(a) From the sense of the connections, 17. 



* (23a). If each end of a 3-phase line has its neutral well grounded, it 

 becomes virtually a 4-wire system ; the ground circuit can not be neglected 

 unless the load .is practically balanced. 



f(24a). Polyphase Wattmeter. Instead of two single-phase watt- 

 meters, a single instrument combining the two is commonly used. This 

 consists of two wattmeters, one above the other, with the moving elements 

 mounted upon a common shaft. The reading of such an instrument gives 

 the total power. The electrical connections are the same as for two 

 separate instruments. 



