TRANSFORMERS 451 



grooves of the pyramid surface, to the lower right-hand corner 

 of the plate, and then through a series of small holes drilled from 

 the surface of each plate to a cored passageway, similar to the 

 inlet. A rotary gear or multi-stage centrifugal pump is used for 

 forcing the oil through the filters. 



One of the greatest advantages of this outfit is that the treat- 

 ment can be carried on while the transformer is in operation, and 

 without the use of separate tanks for the oil, as seen in the illus- 

 tration. 



Oil Testing. The sample bottles or cans should be thoroughly 

 cleaned and dried before using, and it is generally satisfactory 

 to rinse very thoroughly with clean, dry oil and allow the recep- 

 tacle to drain for a few minutes. The test samples should be taken 

 only after the oil has settled for some time, varying from eight 

 hours for a barrel to several days for a large transformer. Cold 

 oil is much slower in settling and may hardly settle at all. Oil 

 samples from barrels should be taken about J inch from the bot- 

 tom of the drum and a brass or glass " thief " can be conveniently 

 used for this purpose. The same method 'should be used for 

 cleaning this as is used for container. 



A compact oil-testing set by means of which the dielectric 

 strength of oil can easily be determined is illustrated in Fig. 284. 

 It consists of a testing transformer with an induction regulator 

 for voltage control and an oil-spark gap, all of which are assembled 

 as a unit. Before using, the spark gap should be cleaned by simply 

 rinsing with clean, dry oil. Its terminals, which are 1.0 inch in 

 diameter, should be adjusted 0.1 inch apart by means of a gauge. 

 The spark receptacle should be nearly filled with the oil and 

 allowed to stand for a moment to give bubbles time to escape, 

 especially if the oil is cold. The rate of increase of voltage should 

 be as fast as can be accurately read on the voltmeter, the total 

 time of application of voltage, from zero to breakdown valve, 

 usually being about five seconds The average voltage of five 

 tests is generally taken as the dielectric strength of the oil. 



When drawing samples of oil from the bottom of transformers, 

 or large tank, several quarts should be drawn off before taking 

 sample in order to eliminate dirt or water which may have accumu- 

 lated in the valve, connecting pipes, etc. The best way to clean 

 and dry oil drums is to rinse them very thoroughly with five or 

 ten gallons of gasolene, benzine, or dry transformer oil. The 



