TRANSFORMERS 455 



Temperatures should be read daily (or more often), and if an 

 oil temperature of 80 or over for the self-cooled is indicated, or 65 

 or over for the water-cooled, the transformer must be cut out of 

 service at once and the cause of the excessive heating looked into. 

 These or higher temperatures of oil may indicate that the interior 

 temperature of the windings were exceeding the safe hottest spot 

 value, this being limited to 105 C. for self-cooled and 90 C. for 

 water-cooled transformers as previously stated. 



Regardless of oil temperature as indicated by thermometers, 

 transformers should not be operated at overloads not stipulated 

 by the specifications. When operating water-cooled transformers 

 at an overload the amount of water should be increased in propor- 

 tion to the load. On account of the increased amount of water 

 during overload, the temperature of the oil will not rise as fast as 

 the temperature of the windings and any of the causes leading 

 to excessive heating will have more pronounced effect under these 

 conditions. Therefore, transformers during overload should be 

 watched with especial care to see that the oil temperatures are 

 kept well below the temperature limits specified. 



Compartments in which oil-insulated self-cooled transformers 

 are installed should be thoroughly ventilated. Openings for cool 

 air should be provided at various points near the floor, and outlets 

 should be in or near the roof, which should not be closer than 6 to 



10 feet from the top of the transformer. The room temperature 

 in which transformers are installed should not exceed the tem- 

 perature of the air entering the room by more than 5, and pre- 

 sumably, the entering air will come from the outside, or, at least, 

 from a source not much warmer than the outside air. 



There is practically no danger of condensation of moisture in 

 transformers which have no chloride breathers if the oil at all 

 time is kept 10 or more above the room temperature. It is also 

 desirable, especially in moist climates, to keep the oil in idle 

 transformers (not equipped with breathers), slightly warm in 

 order to eliminate the chance of the oil becoming moist. This 

 may be accomplished by applying voltage alone for a few hours 

 each day Water-cooled transformers should be watched to see, 

 that the oil temperature does not drop below the limits specified; 

 and if it does, the amount of water must be decreased until the 



011 attains a temperature of at least 10 above the surround- 

 ing air. 



