184 COMMERCIAL ELECTRICAL TESTING 



connecting up the secondary neutral, the double and high 

 potential. tests should be omitted until everything is com- 

 pleted. If double potential can not be obtained from a 

 three-phase alternator on account of the high magnetizing 

 current, the test can be made on one phase at a time. 

 Double potential for one minute should always be fol- 

 lowed by one and one-half times normal potential for 

 five minutes. 



High Potential 



The high potential should always be applied after 

 all changes have been made, such as tightening loose 

 iron, connecting primary delta or Y. The polarity should 

 always be tested after the delta or Y connection has been 

 made. 



Other tests, such as overload heat run, hot resistance, 

 and air readings, are made in a manner similar to that 

 followed for single-phase transformers. 



