186 COMMERCIAL ELECTRICAL TESTING; 



former has been filled so full of oil that it is likely to 

 overflow, draw off some oil. The leads coming from the 

 transformer must not siphon the oil. In locating ther- 

 mometers on the outside of the tank, place one at the 

 top, about the height of the oil line, and on very largo 

 transformers, one near the bottom of the tank, ahvuys 

 using the putty provided. As it is not possible to get the 

 temperature of the core, the oil temperature must be 

 carefully obtained. Whenever possible, place one ther- 

 mometer near the center of the transformer so as to 

 measure the temperature of the oil as it comes from the 

 coils. The bulb of the thermometer should be about 

 two inches under the oil. Place one thermometer in the 

 oil about three inches from the side of the tank. 



Oil-cooled transformers usually require a very long 

 heat run, varying from six to fifteen hours depending 

 on the size. The heat run should be continued until the 

 temperature rise is less than one degree in two hours. 

 Do not make a short circuited heat run on an oil-cooled 

 transformer if it can be avoided ; if unavoidable, make a 

 short circuited heat run on the coils to constant temper- 

 ature, then take double potential for one minute, one 

 and one-half potential for five minutes, and one and one- 

 quarter potential for three hours. 



High Potential or Insulation Test 



Many oil-cooled transformers are built for 50,000 to 

 100,000 volts and require a correspondingly high insula- 

 tion test. The wiring from the high potential trans- 

 former to the transformer to be tested should be arranged 

 so that no one can possibly come in contact with it. 

 It must be securely strung to prevent its falling on any 

 one. 



The voltage applied is controlled either by varying 

 the field of the alternator supplying power to the low 

 potential side of the testing transformer; or, if the power 

 is taken from the constant potential shop circuit, by a 

 single-phase potential regulator. The spark gap should 

 always be used and, if the power is supplied from a sep- 

 arate alternator or is controlled by a potential regulator, 

 a high resistance consisting of two or more glass tubes 



