A new or improved method of and apparatus for earth resistance testing

摘要

748,458. Electric tests. FOSTER TRANSFORMERS, Ltd., and JENNINGS, R. E. June 4, 1954 [June 9, 1953], No. 15809/53. Class 37. Apparatus for determining the resistance between the neutral line of an A.C. supply and earth comprises a high impedance transformer for supplying an alternating voltage to known resistance measuring circuits. In one form (Fig. 2) the A.C. supply at L, N is fed through a first reversing switch 5, with a neon lamp 6 on the output side to confirm which line is the neutral, and a second reversing switch 7, to a high internal reactance transformer T whose secondary voltage is fed to a bridge circuit comprising a potentiometer 12, a standard resistor 13, and the earth resistance, a meter A in the diagonal of the bridge enabling the balance position of the potentiometer slider to be located. The potentiometer 12 may be calibrated in resistance values, and the earth resistance may be replaced by standard resistors by operating switch 10, to check the calibration. To allow for standing voltages between N and E, the bridge polarity is reversed by operating switch 7, and the average of the two readings obtained. In a modification (Fig. 3) several standard resistors 131, 132, 133 are available, and two potentiometers 121, 122. are used to permit additional voltages to be added at one end of each potentiometer. In a simplified arrangement the transformer output is fed via a changeover switch to a direct short-circuit or through the earth resistance, and the currents compared.