RATINGS AND GUARANTEES. 169 



Alternator field circuits should be tested under a breakdown test voltage corres- 

 ponding to the rated voltage of the exciter, and referred to an output equal to the 

 output of the alternator ; i. e., the exciter should be rated for this test as having an 

 output equal to that of the machine it excites. 



Condensers should be tested at twice their rated voltage and at their rated frequency. 



The values in the table above are effective values, or square roots of mean square, 

 reduced to a sine wave of e.m.f. 



44. In testing insulation between different electric circuits, as between primary and 

 secondary of transformers, the testing voltage must be chosen corresponding to the 

 high-voltage circuit. 



45. In transformers of 20,000 volts upwards, it should be sufficient to test the 

 transformer by operating it at 50 per cent, above its rated voltage ; if necessary, with 

 sufficiently higher frequency to induce this voltage. 



46. The test of the insulation of a transformer, if no testing transformer is avail- 

 able, may be made by connecting one terminal of the high voltage winding to the 

 core and low-voltage winding, and then repeating the test with the other terminal of 

 the high-voltage winding so connected. The test of dielectric resistance between the 

 low-voltage winding and the core should be in accordance with the recommendation 

 in Section 43, for similar voltages and capacities. 



47. High voltage tests on transformers or other apparatus should be based upon the 

 voltages between the conductors of the circuit to which they are connected. 



48. When machines or apparatus are to be operated in series, so as to employ the 

 sum of their separate e m.f.'s, the voltage should be referred to this sum, except 

 where the frames of the machines are separately insulated both from ground and from 

 each other. 



The insulation between machines and between each machine and ground should be 

 tested, the former referred to the voltage of one machine, and the latter to the total 

 voltage of the series. 



49. Underground cables, and line switches, should be tested by the application of 

 an alternating e.m.f. for one minute at twice the voltage at which the cable or switch 

 is to be operated. 



RATING. 



75- Both electrical and mechanical power should be expressed in kilowatts, except 

 when otherwise specified. Alternating-current apparatus should be rated in kilowatts 

 on the basis of non-inductive condition ; i. e., with the current in phase with the ter- 

 minal voltage. 



76. Thus the electric power generated by an alternating-current apparatus equals 

 its rating only at non-inductive load, that is, when the current is in phase with the 

 terminal voltage. 



77. Apparent power should be expressed in kilovolt-amperes as distinguished from 

 real power in kilowatts. 



78. If a power-factor other than 100 per cent, is specified, the rating should be 

 expressed in kilovolt-amperes and power-factor, at full load. 



79. The full-load current of an electric generator is that current which with the 



