310 



ELECTRICAL EQUIPMENT 



There are three different methods provided for determining 

 the temperature of different parts of a machine. These will be 

 briefly described in the following and the respective permissible 

 temperature rises given, based on class A insulation. 



I. Thermometer Method. This consists in applying a thermom- 

 eter to the hottest accessible part of the completed machine. 

 With this method a correction of 15 C. must be made, 

 that is, the permissible observable temperature rise as 

 read by the thermometer cannot exceed 50 C. 



An exception to this rule is the case, when thermometers are 

 applied directly to the surfaces of bare windings, as the field coils. 

 Then only a 5 C. correction has to be made, so that the permis- 

 sible observable temperature rise is limited to 60 C. 



II. Resistance Method. This consists in the measurement of the 

 temperature of windings by their increase in resistance, cor- 

 rected to the instant of shut-down when necessary. In the 

 application of this method, careful thermometer measure- 

 ments should also be made, whenever practicable, in order 

 to increase the probability of revealing the highest observ- 

 ble temperature. Whichever measurement yields the 

 higher temperature that temperature shall be taken as 

 the " highest observable " temperature and a hottest- 

 spot correction of 10 C. added thereto. The permissible 

 temperature rise with this method is, therefore, 55 C. 



TABLE XLIV 



TEMPERATURE COEFFICIENTS OF COPPER RESISTANCE 



