SYNCHRONOUS GENERATORS 



309 



temperature of reference, that is, the cooling air surrounding 

 the machine is given as 40 C., and by deducting this ambient 

 temperature from the maximum permissible temperature given 

 above, the permissible temperature rise is obtained. As it is 

 usually impossible to determine the maximum temperature 

 (hottest spot) attained in insulated windings, a correction factor 

 must be applied to the observable temperature, so as to approx- 

 imate the difference between the actual maximum temperature 

 and the observable temperature by the method used. This 

 correction, or margin of security, is provided to cover the errors 

 due to fallibility in the location of the measuring devices, as well 

 as inherent inaccuracies in measurement and methods. 



TABLE XLIII 



PERMISSIBLE TEMPERATURES AND TEMPERATURE RISES FOR INSULATING 



MATERIALS 



* For cotton, silk, paper and similar materials, when neither impregnated nor 

 immersed in oil, the highest temperatures and temperature rises shall be 10 C. below 

 the limits fixed for Class A. 



