INSTRUMENT TRANSFORMERS 



565 



two parts which are hinged together so that the magnetic circuit 

 can be opened when the screw clamp is loosened. This allows the 

 transformer to be placed around a cable and permits the current 

 in a single conductor cable to be measured without interrupting 

 the service. 



Fig. 347 indicates the connections for a simple current measure- 

 ment. For obvious reasons this device is sometimes called a 

 series transformer in distinction from a potential transformer, 

 which is frequently called a shunt transformer. 



Load 



FIG. 347. Showing manner of using current transformer. 



For a current measurement, 



/ = (Ratio) X (Reading of ammeter). 



Convenience dictates that the ratio be a constant. This involves 

 a difficulty, for, as the load changes, the transformer must work 

 under widely varying conditions. Experiment shows that the 

 ratio is not constant, being to a certain extent dependent on the 

 strength of the current which is being measured, and also on the 

 number and the character of the instruments in the secondary 

 circuit. 



Power Measurements. In power measurements on high- 

 voltage circuits, it is necessary to use both current and potential 

 transformers. As shown in Fig. 348, the connections are such 

 that the current and voltage as well as the power are measured. 



With the connections as shown, the power, to a fair degree of 

 approximation, is given by 



P = (Ratio of current transformer) X (Ratio of potential trans- 

 former) X (Reading of wattmeter). 



Another difficulty is here encountered. In the discussion of 

 power measurement, it was repeatedly emphasized that for ac- 



