174 COMMERCIAL ELECTRICAL TESTING 



the cubic feet per minute can be easily calculated. A 

 large box with an opening in the bottom should be held 

 against the transformer, using a small piece of felt as 

 packing and being careful to allow no air to escape. 

 The size of the orifice should be noted, and the time 

 that the air meter is allowed to run. Always record the 

 reading in cubic feet per minute. The air readings are to 

 be taken with the dampers in the same position that they 

 occupied during the heat run, and at the same air pres- 

 sure. 



High Potential Test 



The application of a high potential to the insulation of 

 a transformer is the only method for determining whether 

 the dielectric strength is sufficient for continuous opera- 

 tion. Mechanical examination amounts to little and 

 measurement of insulation resistance is equally valueless , 

 since insulation may show high resistance when measured 

 by a voltmeter with low voltage, but offer comparatively 

 little resistance to the passage of a high tension current. 



The insulation test which should be applied to the 

 windings of a transformer depends upon the voltage 

 for which the transformer is designed. In testing between 

 the primary and the core or the secondary, the secondary 

 should be grounded for the following reasons : In testing 

 between one winding and the core, a potential strain 

 is induced between the core and the other winding which 

 may be much greater than the strain to which the in- 

 sulation is subjected under normal operation, and 

 therefore greater than it is designed to withstand. In 

 testing between the primary and the core, the induced 

 potential between the secondary and core may be 

 several thousand volts, and the secondary may thus be 

 broken down by an insulation test applied to the pri- 

 mary under conditions which would not exist in normal 

 operation. During the test, all primary leads, as well 

 as all secondary leads, must be connected together. 

 If only one terminal of the transformer winding is con- 

 nected to the high potential transformer, the potential 

 strain may vary throughout the winding, and at some 

 point may even be greater than at the terminal of which 



