160 COMMERCIAL ELECTRICAL TESTING 



is obtained which is as near 105 as possible, 

 unless a tap be brought out at a half turn, 

 which is seldom done. Changing to (3-9), six turns 

 are cut out and the primary voltage is decreased by 

 6 195 -0040 XI 55 volts. Now 6X24.2 = 149.2, which is 

 near enough to 155. The remainder of the taps should 

 be checked in the same manner. 



Great care should be taken in handling the voltmeter 

 connected to the taps, for while the voltmeter reading 

 is low, the circuit to which it is connected may be several 

 thousand volts above ground. If the opposite end of the 

 circuit be grounded, a severe shock may be obtained 

 from the meter. 



In checking 50 per cent taps, one meter should be 

 used as a check and another to read the voltage 



Fig. 45. Taps 



across each half of the winding, the readings being 

 taken first on one side and then on the other, 

 holding the same reading on the check meter. A 

 neat sketch showing the position of the taps should 

 always be made. On transformers with only one tap 

 on each end, it is often necessary to check its location 

 by polarity. (See Fig. 45.) This is done as follows: 

 with direct current flowing through the secondary, take 

 polarity of (1-4), (1-2) and (3-4); if all the deflections 

 are in the same direction, the taps are properly brought 

 out; if some are reversed, the tap and line lead are 

 interchanged. 



Impedance 



The expression C = ^ for continuous current circuits 

 K 



