180 



COMMERCIAL ELECTRICAL TESTING 



this, open line No. 1 and if the reading for wattmeter 

 No. 3 is positive, the needle will drop to some value 

 above zero. If the needle drops off the scale below zero, 

 the reading must be recorded as negative. If the first 

 meter tested reads positive, the sign of the other meter 

 should be determined by this method. 



An impedance curve should be taken from 50 per cent 

 to 125 per cent full load. 



In making the short circuit test on ABH (Fig. 57-) 

 transformers, each phase must be short circuited 

 independently of the other. In taking readings, hold the 

 current constant in one line from the testing table. 



Three-phase Supply 



Fig. 60 

 Connections for Impedance Test 



Read the three-phase volts and the two wattmeters. 

 Hold the voltage constant across one phase and read the 

 current in the other phases. 



Core Loss 



In the core loss test, wattmeter readings are taken in 

 the same manner as for the impedance test. The same 

 precautions must be observed in regard to fastening up 

 the primary leads so that there is no chance of danger. 

 Take core loss curve from 50 per cent to 125 per cent 

 normal voltage. 



In ABH transformers, with the middle points of each 

 phase connected together to form a neutral for a three- 

 wire direct current system, some confusion may result 



