386 



ELECTRICAL EQUIPMENT 



for example, governed by the generator voltage and may be any- 

 thing up to 13,200 volts. The secondary of the step-up trans- 

 formers and the primary of the step-down transformers is deter- 

 mined by the most economical transmission voltage, which may 

 be as high as 150,000 volts. The secondary of the step-down 

 transformers is finally governed by the potential of the distrib- 

 uting system. Where this is extensive its voltage may be com- 

 paratively high, may be 33,000 or even higher, while, for smaller 

 systems it may only be 2300 volts and even lower. The voltages 

 generally used for power transformers are as follows: 



The test voltage which shall be applied to determine the 

 dielectric strength of the insulation is specified by the A.I.E.E. 

 rules as twice the normal voltage of the circuit to which the 

 transformer is connected plus 1000 volts. The test shall be made 

 at the temperature assumed under normal operation, and the 

 frequency of the test circuit shall not be less than the rated fre- 

 quency of the apparatus tested. The duration of the application 

 of the voltage shall be one minute, and it shall be successively 

 applied between each electric circuit and all other electric cir- 

 cuits and metal parts grounded. Inter-connected polyphase 

 windings are considered as one circuit and all windings except 

 that under test shall be connected to ground. Transformers 

 which may be used in Y-connection on three-phase circuits shall 

 have the test based on the delta or line voltage. 



The following exceptions to the above rule are given: 



(1) Alternating current apparatus connected to permanently 

 grounded single-phase systems, for use on permanently grounded 

 circuits of more than 300 volts, shall be tested with 2.73 times the 

 voltage of the circuit to ground plus 1000 volts. This does not, 

 however, refer to three-phase apparatus with grounded neutral. 



(2) Distributing "transformers for primary pressures from 

 550 to 5000 volts, the secondaries of which are directly connected 



