24 STANDARDIZATION RULES Ol I III: ,!././ 



an auxiliary ratio transformer. It is permissible, however, to measure the 

 voltage at other places, for example, on the primary of the transf< 

 provided the ratio of transformation does not materially vary during the 

 test; or that proper account is taken thereof. 



249 SPARK GAP tND VOLTMETER. The spark gap may be employed as a 

 check upon the voltmeter used in high-tension tests in order to determine 

 the transformation ratio of the transformer, the variation from th 

 wave form and the like. It is also useful in conjunction with voltmeter 

 measurements to limit the stress applied to the insulating n 



(D) APPARATUS FOR SUPPLYING TEST VOLTAGE. 



260 The GENERATOR OR CIRCUIT supplying voltage for the test should 

 have ample current carrying capa* hat the current which 



be taken for charging the apparatus to be tested will not mau-ri.-ilh 

 the wave form nor otherwise materially change the voltage. 



The TESTING TRANSFORMER should be such that its ratio of trans- 

 formation does not vary more than 10 per cent when delivering the charg- 

 ing current required by the apparatus under test. (This may be deter- 

 mined by short-circuiting the secondary or high voltage winding of the test- 

 ing transformer and supplying 1/10 of the primary voltage to the pr 

 under this condition. The primary current that flows under this condition 

 is the maximum which should be permitted in regular dielectric test.) 



261 The VOLTAGE CONTROL may be secured in either of several inch 

 in order of preference, are as follows: 



262 1. By generator field circuit. 



263 2. By magnetic commutation. 



264 3. By change in transformer ratio. 

 266 4. By resistance or choke coils. 



266 In GENERATOR VOLTAGE CONTROL, the voltage of the generator si 

 preferably be about its approximate normal rated load value when the 

 lull testing voltage is attained, which requires that the ratio of the r 

 transformer be such that the full testing voltage is reached when the gen- 

 erator voltage is normal. This avoids the instability in the generator 

 which may occur if a considerable leading current is taken from it when 

 it has low voltage and low field current. 



267 In MAGNETIC COMMUTATION, the control is effected by shunting the mag- 

 metic flux through a secondary coil so as to vary the induction through 

 the coil and the voltage induced in it. The shunting should be effected 

 smoothly, thus avoiding sudden changes in the induced voltage. 



268 In TRANSFORMER VOLTAGE CONTROL, by change of ratio, it is neces- 



that the transition from one step to another be made without inter- 

 ruption of the test voltage, and by steps sufficiently small to prevent 

 surges in the testing circuit. The necessity of this precaution is greater 

 as the inductance or the static capacity of the apparatus in the t 

 circuit under test is greater. 



269 When RESISTANCE COILS OR REACTORS are used for voltage control, it 

 is desirable that the testing voltage should be secured when the controlling 

 resistance or reactance is very nearly or entirely out of circuit in order 

 that the disturbing effect upon the wave form which results may be negli- 

 gible at the highest voltage. 



F. CONDUCTIVITY 



260 COPPER. The conductivity of copper in annealed wires and in electric 

 cables should not be less than 98 per cent of the Annealed Copper Stand- 

 ard, and the conductivity of hard-drawn copper wires should not be less 

 than 95 per cent of the Annealed Copper Standard. The Annealed 

 Copper Standard represents a mass-resistivitv of 0.153022 ohm per meter- 

 gram at 20 deg. cent, or 873.75 ohms per mile-pound at 20 deg. cent.; or 

 using a density of 8.89, a volume-resistivity of 1.72128 microhm-cm., or 

 microhms in a cm. cube, at 20 deg. cent, or 0.67767microhm-inch at 20 

 dee. cent. 



