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ELECTRICAL MACHINERY 



nating currents are to be measured. An ammeter to carry 

 5000 amperes would be very cumbersome and expensive; 

 besides, a five-ampere meter in connection with a current 



transformer, answers just as 

 well. A current transformer is 

 used also when it is desired to 

 measure the current in a high 

 voltage transmission line. It is 

 not advisable to connect an 

 ammeter directly in series with 

 such a line, so a current trans- 

 former is used, and the ammeter, 

 connected in the secondary, is 

 thus insulated from the high 

 tension line. Such a transformer 

 is shown in Fig. 197. 



Fig. 198 shows a transformer 

 designed to be connected in series 

 with the station bus-bars to indi- 

 cate the station current. It has 

 a ratio of 1500 : 5 and a five- 

 ampere* ammeter is connected in 

 its secondary circuit. Of course 

 when the meter indicates the 

 full scale deflection (i.e., five 

 amperes on the usual calibra- 

 tion) it signifies that the bus- 

 bars are carrying 1500 amperes 

 and so the full scale of the meter 

 is marked 1500 amperes, etc. 



When a current transformer is 

 carrying current, its secondary 

 circuit must not be opened. Two 



bad effects may result, either the transformer may become 

 overheated or else a dangerously high voltage may be 

 generated in the open secondary. 



FIG. 197. Current Trans- 

 former for Use in High- 

 tension Transmission line. 

 General Electric Co. 



