444 ELECTRICAL EQUIPMENT 



other words, until the burning-point is reached. It is, therefore, 

 obvious that high flash and burning-points are desirable in in- 

 sulating oils in order that the fire risk attendant on their use 

 may be reduced to a minimum. 



Of extreme importance is also the percentage of deposit, which 

 may be thrown down from an oil in service. Most organic sub- 

 stances, when exposed to even moderate temperatures, are sub- 

 ject to slow changes, which, in case of oils, are probably due to 

 chemical change, such as oxidation of some of the constituents, 

 and when this deposit is excessive efficient cooling is very much 

 restricted. Somewhat similar to this deposit, some oils produce a 

 jelly-like substance, which forms after continuous operation, and 

 in general, the higher the temperature the more rapid these changes 

 take place. A very slight trace of the deposit is in no degree 

 harmful and will ordinarily only be found under the most severe 

 conditions following a long period of service. 



Transformer oils must also be watched for presence of injurious 

 impurities such as acids, alkalis and free sulphur. An access of 

 acid particularly would result in deterioration of insulation and 

 other materials of which the transformer is constructed. Free 

 sulphur, even in extremely minute quantities, is seriously detri- 

 mental to the windings, the chemical action on exposed copper 

 causing the conductors themselves to be gradually eaten through. 

 These characteristics are, however, very carefully watched by the 

 transformer manufacturers, so that the oils furnished are ordinarity 

 free from such injurious impurities. 



The characteristics of oils in general use vary somewhat, 

 depending on the type of transformer as well as on the practice 

 of the transformer manufacturer. One of the largest of these sup- 

 plies oil of the following characteristics for its water-cooled trans- 

 formers: 



Flash-point 130 C. 



Burning-point 145 C. 



Freezing-point 15 C. 



Viscosity at 40 C 40 sec. 



This kind of oil is also supplied with oil-cooled transformers 

 and combination self- and water-cooled transformers where the 

 guaranteed normal load temperature rise is less that 50 C. 



