INSTRUMENT TRANSFORMERS 567 



nection with wattmeters in all sorts of acceptance tests of alter- 

 nating-current apparatus, as well as in connection with watt-hour 

 meters on all high-capacity alternating-current circuits. 



General Considerations. In all instrument transformers, the 

 primary must be thoroughly insulated from the secondary and 

 from the core and case, so that there is little chance of punctur- 

 ing the insulation. In addition, the secondary circuit should be 

 grounded so that the operator is protected even though the in- 

 sulation between the primary and secondary breaks down. 

 Grounding the secondary circuit prevents errors due to accumu- 

 lation of electrostatic charges on the instruments. The coils must 

 be held in place so firmly that there is no chance of mechanical 

 injury when short-circuits occur. The primary and secondary 

 terminals must be so far apart that there is no liability of an arc 

 forming between the two circuits when the line circuit is violently 

 disturbed. 



The ordinary vector diagram for a transformer is shown in 

 Fig. 349. It is not drawn to scale, however, and gives no idea 

 of relative numerical magnitudes. 



FIG. 349. Vector diagram for transformer. 



By means of the diagram, a general explanation of the phe- 

 nomena occurring in instrument transformers may be obtained. 

 For the potential transformer, the ratio which is used is 



Yl 

 V* 



It differs in magnitude and in the phase of its components from 

 r,- 1 ' which is the ratio of the number of turns, or the true "ratio of 



P2 



