CHAPTER XII 

 INSTRUMENT TRANSFORMERS 



In the course of development of high-voltage alternating-cur- 

 rent systems of transmission and distribution it has been found 

 necessary to remove the various instruments, as well as the de- 

 vices used to actuate the switching gear, from direct contact with 

 the line circuits and to operate them by means of properly con- 

 structed transformers, since direct connection between the high- 

 tension lines and the devices on the front of the switchboard must 

 be avoided. This method of operation through transformers re- 

 duces to a minimum the possibility of personal injury to the 

 station attendants and enables them, especially in emergencies, 

 to operate the apparatus with confidence, thus contributing to 

 maintaining continuity of the service. 



Again, it is frequently necessary to meter very large currents 

 in circuits of only moderate voltage ; and as it is highly desirable 

 to avoid the expense of carrying heavy leads to the switchboard, 

 current transformers are used. 



By properly choosing the transformers, it is possible to use in- 

 struments and switchboard devices wound for 5 amperes and 110 

 volts, for installations of all capacities. This reduces the instru- 

 ment cost and is now the accepted American practice. 



Potential Transformers. Where it is necessary to measure a 

 high voltage, a potential transformer is used to reduce this vol- 

 tage to a more convenient and safe value for measurement. Fig. 

 344 shows two such transformers; they are connected in the 

 circuit as shown in Fig. 345. 



The transformer in Fig. 345 is diagrammatic only; as con- 

 structed, the primary and secondary windings are superposed. 



As potential transformers are usually operated under practi- 

 cally fixed conditions of applied voltage, frequency, and number 

 and character of the instruments in the secondary circuit, one 

 would expect them to be instruments of precision, and experience 



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