RATINGS AND GUARANTEES. l6/ 



instance, the test load should not be applied until the transformer has been in circuit 

 for a sufficient time to attain the temperature elevation due to core loss. With trans- 

 formers for commercial lighting, the duration of the full-load test may be taken as 

 three hours, unless otherwise specified. In the case of railway, crane and elevator 

 motors, the conditions of service are necessarily so varied that no specific period cor- 

 respoding to the full -term load can be stated. 



35. The commercial rating of a railway motor should be the h.p. output, giving 

 75 C rise of temperature, above a room temperature of 25 C , after one hour's con- 

 tinuous run at 500 volts terminal pressure on a stand, with the motor covers removed. 



For determining the service temperature of a railway motor, the temperature rise 

 should be determined by operating the motor on a straight and level track and under 

 specified conditions : 



1 I ) As to the load carried in tons per motor. 



(2) The schedule speed in miles per hour. 



(3) The number of stops per mile. 



(4) The duration in seconds of the stops. 



(5) The acceleration to be developed in miles per hour per second. 



(6) The braking retardation to be developed in miles per hour per second. 



These specifications should be determined or agreed upon, as equivalent to the act- 

 ual service, and the motors to be closed or open, according to the way in which they 

 are to be operated in service. 



The tests should be made in both directions over the same track. 



By a "level track" should be understood a track in which the gradient does not 

 exceed one- half per cent, at any point. 



By a "straight track" should be understood a track in which the radius of curva- 

 ture is nowhere less than the distance travelled by the car in 30 seconds, at the maxi- 

 mum speed reached during the run. 



The wind velocity during a test should not exceed 10 miles per hour in any direction. 



INSULATION. 



36. The ohmic resistance of the insulation is of secondary importance only, as com- 

 pared with the dielectric strength, or resistance to rupture by high voltage. 



Since the ohmic resistance of the insulation can be very greatly increased by baking, 

 but the dielectric strength is liable to be weakened thereby, it is preferable to specify 

 a high dielectric strength rather than a high insulation resistance. The high-voltage 

 test for dielectric strength should always be applied. 

 INSULATION RESISTANCE. 



37. Insulation resistance tests should, if possible, be made at the pressure for which 

 the apparatus is designed. 



The insulation resistance of the complete apparatus must be such that the rated 

 voltage of the apparatus will not send more than 1/1,000,000 of the full load current, 

 at the rated terminal voltage, through the insulation. Where the value found in this 

 way exceeds I megohm, I megohm is sufficient. 



DIELECTRIC STRENGTH. 



38. The dielectric strength or resistance to rupture should be determined by a con- 

 tinued application of an alternating e.m.t. for one minute The source of alternating 



