678 



ELECTRICAL MEASUREMENTS 



The plant where this method was first tried transmits power at 

 60,000 volts, 25 cycles, 3-phase, over lines 160 miles long. The 

 transformers are operated with a grounded neutral and the insu- 

 lator pins are grounded. Consequently if an insulator breaks 

 down a short-circuit is established and the trouble must be 

 rectified before service can be resumed. It is desirable that 

 special apparatus be avoided. 



The connections are shown in Fig. 420. The line is opened 

 at the far end by means of the three disconnecting switches. At 

 the near end, the disconnecting switches in A and B are opened 

 and jumpers are applied at both ends of the line as shown. 



FIG. 420. Diagram for Nicholson's application of the two-ammeter loop 

 test to high-voltage lines. 



At R is a rheostat arrangement to control the current after the 

 arc to ground has been established; F is an expulsion fuse which 

 short-circuits R. It permits the voltage at the fault to be raised 

 high enough to start the arc and then blows, throwing in the 

 controlling resistance R. For this current-limiting resistance four 

 concrete columns 1 foot square and 12 feet long with expanded 

 metal terminals have been used. Each has a resistance of about 

 2,000 ohms. They are used singly or in parallel as occasion re- 

 quires. From 50 to 100 amperes are required for the test. In 

 cases where the striking distance is several inches, the testing 

 current does not burn the aluminum conductors if it is kept on for 

 40 seconds, which is sufficient time to obtain the readings. 



Where a single size of conductor is employed, it is found that 

 the currents divide inversely as the resistances of the two paths, 

 so formula (6) is applicable. 



Murray Loop Test. In the Murray loop test the connections 

 are such that the resistances x and r form two arms of a Wheat- 



