THE TRANSFORMER 



221 



By means of instrument transformers, instruments may be 

 entirely insulated from the high-voltage circuit and yet indicate 

 accurately the current, voltage, power, etc., in the high-voltage 

 circuit. Moreover, low-voltage instruments having standard 

 current and voltage ranges may be used for all high-voltage 

 circuits, irrespective of the voltage and current ratings of the 

 circuits. 



96. Potential Transformers. Potential transformers do not 

 differ materially from the constant-potential transformers already 

 discussed, except that their power rating is small. Below 5,000 

 volts they are usually air-cooled and above this they are usually 

 oil-cooled, the oil being used more for its dielectric qualities than 

 for cooling purposes. As only instruments and sometimes pilot 



Fio. 212. Use of potential transformer on a 13, 200- volt si 



circuit 



lights are connected to their secondaries, such transformers 

 ordinarily have ratings of from 40 to 200 watts. The low-tension 

 side is almost always wound for 110 volts and the ratio is then 

 determined by the rating ( ,f the high-voltage winding. For 

 example, a 13,200-volt potential transformer would have a 

 ratio of 13,200/110 = 120:1. The ratio of turns may vary a 

 per cent, or so from this value to allow for the transformer im- 

 ice drop under load. Figure MIL? -hows a simple connec- 

 tion for measuring voltage in a 13,200-volt circuit by means of 

 a potential t ra n.-t'ormer. The secondary should always be 

 grounded at one point to eliminate "static" from the instrument 

 and further to insure safety to the operator. Figure 'JUi shows 

 a potential transformer u-ed in conjunction with a current trans- 

 former for measuring power by means of a wattmeter. 



97. Current Transformers. To avoid connecting alternating- 

 current ammeiM- and the current coil- of other instruments 



iv in high-voltage ime-, current transformers are used. In 



