TRANSFORMERS 445 



Where the rise is 50 C. and higher, oil with the following char- 

 acteristics is used: 



Flash-point 160 C. 



Burning-point 175 C. 



Freezing-point - 10 C. 



Viscosity at 40 C 60 sec. 



Transformers which may be operated under severe weather 

 conditions, such as outdoor types, may also be supplied with an 

 oil having a freezing-point of 30 C. 



The necessary puncture strength of oils is: 40,000 volts punc- 

 ture with J-inch discs spaced 0.2 inch apart; or, 22,000 volts 

 puncture with 1-inch discs spaced 0.1 inch apart. 



In order to ascertain the temperature at which a transformer 

 is operating, it is advisable to equip them with thermometers and 

 these should be located in such a place that they can easily be 

 read. Different thermometers are in use, some being of the 

 ordinary mercury type, this being mostly supplied with self-cooled 

 transformers and may be equipped with electrical contacts for 

 connecting to an alarm circuit. 



A thermometer which is very extensively used in connection 

 with water-cooled transformers is illustrated in Fig. 281. It 

 depends for its operation upon the expansion of mercury in a 

 sensitive steel tube. The bulb is connected to the indicating 

 instrument by a small capillary steel tube, this tube being con- 

 nected to a spring to which the indicating pointer is attached 

 through a rack and pinion. The capillary tube is of such length 

 that the bulb may be placed in the oil at the hottest part of the 

 transformer. Variations of temperature at the bulb cause cor- 

 responding contraction or expansion of the liquid confined in 

 this bulb, and this is transmitted to the capillary tube connecting 

 to the indicating mechanism. The instrument can readily be 

 equipped with contact points for connection to an alarm circuit. 



Drying Transformers. Transformers shipped assembled, but 

 not filled with oil, should be very thoroughly and intelligently 

 inspected before deciding that the drying may be omitted. In 

 every case a thorough inspection is necessary, and if there is any 

 evidence of mildew or moisture, a drying-out run is necessary. 

 Recent improvements in design and method of shipping make it 

 practicable where conditions demand it, to ship transformers with 



