8 COMMERCIAL ELECTRICAL TESTING 



one side of the measuring line to the terminal and the 

 other side to ground; third, measure the resistance 

 from the other terminal to ground, by shifting the 

 measuring line from the first terminal to the second. 

 If all measurements have been accurately made, the 

 sum of the second and third will be equal to the first, 

 and the location of the ground will be as far from one 

 terminal, expressed in terms of the total length of the 

 winding, as the measured resistance from that terminal 

 to ground is to the total resistance of the circuit. 



This test is outlined in Fig. 3, which represents a 

 single circuit, or phase, of an alternating current machine, 



Fig. 3. Test for Grounds on an A-C. Armature 



with a ground at the point X. If the resistance between 

 A and B is one ohm, between A and G 0.35 ohm, and 

 between B and G 0.65 ohm, the location of the ground 

 is 35/100 of the distance between A and B, from A. 

 As 10 coils are in the circuit, the measurements show 

 that the fourth coil is grounded, counting from A. 



In the case of an alternating current winding the 

 ohmic resistance measurement will not always detect 

 a wrong connection, such as a reversed coil, pole section, 

 or phase; the copper resistance measurement would 

 be correct, although the total winding might be partly 

 reversed and therefore, inoperative. Such faults may 

 be discovered by a polarity or impedance test with 

 alternating current. For this purpose a single-phase 

 current can be used, since a reading may be taken on 

 the different circuits, or between pairs of terminals 

 successively, by shifting the testing lines until the whole 

 winding has been tested. 



Short circuited coils on moderate size machines can 

 be readily detected by using a wound electro-magnetic 

 yoke excited with alternating current. This yoke is 



