RATINGS AND GUARANTEES. 165 



RISE OF TEMPERATURE. 

 GENERAL PRINCIPLES. 



26. Under regular service conditions, the temperature of electrical machinery should 

 never be allowed to remain at a point at which permanent deterioration of its insulat- 

 ing material takes place. 



27. The rise of temperature should be referred to the standard conditions of a 

 room-temperature of 25 C., a barometric pressure of 760 mm. and normal conditions 

 of ventilation ; that is, the apparatus under test should neither be exposed to draught 

 nor enclosed, except where expressly specified. 



28. If the room temperature during the test differs from 25 C., the observed rise 

 of temperature should be corrected by J^ per cent, for each degree C. Thus with a 

 room temperature of 35 C., the observed rise of temperature has to be decreased by 

 5 per cent., and with a room temperature of 15 C., the observed rise of temperature 

 has to be increased by 5 per cent. The thermometer indicating the room temperature 

 should be screened from thermal radiation emitted by heated bodies, and from draughts 

 of air. When it is impracticable to secure normal conditions of ventilation on account 

 of an adjacent engine, or other sources of heat, the thermometer for measuring the air 

 temperature should be placed so as fairly to indicate the temperature which the 

 machine would have if it were idle, in order that the rise of temperature determined 

 shall be that caused by the operation of the machine. 



29. The temperature should be measured after a run of sufficient duration to reach 

 practical constancy. This is usually from 6 to 18 hours, according to the size and 

 construction of the apparatus. It is permissible, however, to shorten the time of the 

 test by running a lesser time on an overload in current and voltage, then reducing 

 the load to normal, and maintaining it thus until the temperature has become con- 

 stant. 



In apparatus intended for intermittent service, as railway motors, starting rheostats, 

 etc., the rise of temperature should be measured after operation under as nearly as 

 possible the conditions of service for which the apparatus is intended, and the con- 

 ditions of the test should be specified. 



In apparatus which by the nature of their service may be exposed to overload, as 

 railway converters, and in very high voltage circuits, a smaller rise of temperature 

 should be specified than in apparatus not liable to overloads or in low voltage appara- 

 tus. In apparatus built for conditions of limited space, as railway motors, a higher 

 rise of temperature must be allowed. 



30. In electrical conductors, the rise of temperature should be determined * by their 

 increase of resistance where practicable. For this purpose the resistance may be 

 measured either by galvanometer test, or by drop-of-potential method. A temperature 

 coefficient of 0.42 per cent, per degree C., from and at o C., may be assumed for 

 copper. Temperature elevations measured in this way are usually in excess of tem- 

 perature elevations measured by thermometers. 



* By the formulas 



Rt Q (i -f 0.0042;) and R t +o = ^oL 1 + O.OO42(/ + 0)] 



