224 



ALTERNATING CURRENTS 



secondary to primary turns causes the transformer to act as a 

 step-up transformer. Also, the counter ampere-turns of the sec- 

 ondary will no longer exist so that the flux in the core instead 

 of being due to the difference of the primary and secondary 

 ampere-turns will now be due to the total primary ampere- 

 turns acting alone. This means a very large increase in the 

 flux, causing excessive core losses and heating, as well as a high 

 voltage across the secondary terminals. 



FIG. 216. Typical connections of instrument transformers and instruments for 

 single-phase measurements. 



Therefore, a current transformer should always have its secondary 

 short-circuited. 



Figure 216 shows the method of connecting a typical instru- 

 ment load, through instrument transformers, to a high-voltage 

 line. The load on the instrument transformers includes an am- 

 meter, a voltmeter, a wattmeter and a watthour meter. Each 

 secondary is grounded at one point. Correction for ratio of 

 transformation must be applied to all the instrument readings, 

 the wattmeter and watthour meter involving the ratio of both 

 the current and the potential transformers. Usually in perma- 

 nent installations, as on switchboards, the instrument scales 

 themselves are so marked as to take into consideration these 

 ratios. Therefore, the primary power may be read directly. 



