124 THE DESIGN OF STATIC TRANSFORMERS 



eight hours, as determined in the hottest part of the oil, shall 

 not be more than some 40 to 45 above the temperature of 

 the room. It would be much better to stipulate that tin- 

 ultimate temperature rise as thus determined should not be 

 more than 50 (or preferably 45) above room temperature. 



The Engineering Standards Committee's Report No. !>(' of 

 August, 1907, contains (on pp. 13 and 14) the following clauses 

 dealing with the temperature rise of transformers : 



" (vn.) For alternating- curren^ transformers, whether oil-cooled or 

 otherwise, in which cotton, paper and its preparations, linen, imV;mit<-, 

 or similar insulating materials are employed, 80 C. shall be the hi^ln-t 

 mean temperature permissible. 



" (vin.) They shall be run on load for a period sufficiently long to 

 enable the transformer to attain such a temperature that the increase of 

 temperature does not exceed 1 C. per hour. Under these conditions the 

 observed temperature rise at the hottest part, as determined in Clause (x.), 

 shall not exceed 50 C., based on the assumption that the air temperature 

 of the room in which the transformer is to be used in actual service will 

 not exceed 25 C. 



" (ix.) If the air temperature of the room in which the transformer is 

 to be used in actual service may exceed 25 C., then the temperature rise 

 specified above is to be decreased by one degree for each degree of 

 difference between the room-temperature possible and 25 C. 



" (x.) In the case of air-cooled transformers the temperature shall be 

 ascertained by rise in electrical resistance. 



"(xi.) In the case of oil-cooled transformers the temperature shall be 

 ascertained by rise in electrical resistance, in addition to which the 

 temperature at the surface of the oil shall be ascertained by means of 

 a thermometer, and should the temperature obtained by the two methods 

 differ, the lower figure shall be discarded." 



The following clause in the Standardisation Rules of the 

 American Institute of Electrical Engineers relates to the 

 permissible temperature rise of transformers for intermittent 

 service, and consequently to most transformers supplying 

 lighting loads : 



" In the case of transformers intended for intermittent service, or not 

 operating continuously at full load, but continuously in circuit, as in 

 the ordinary case of lighting transformers, the temperature elevation 

 above the surrounding air temperature should not exceed 50 C. by 

 resistance in electric circuits, and 40 C. by thermometer in other parts 



