GB833339A - Improvements in and relating to electrical testing apparatus for protective earthing - Google Patents

Abstract

833,339. Electric tests. ROSCOE, E. March 15, 1957 [Jan. 6, 1956], No. 507/56. Class 37. Apparatus for testing phase-to-earth loop impedance of A.C. mains comprises a switch 18 to connect in series, between phase and earth, the contacts 16, 17' of a high-speed circuitbreaker device and a non-inductive selected resistor R 1 -R 4 ; voltage-sensitive display means 31 supplied with a voltage derived from that developed across the selected resistor and adapted to operate when this voltage exceeds a minimum value; and means 24 for tripping the circuit-breaker within one cycle of the supply frequency after operation of the display means. The apparatus is mounted in a container 11 with terminals 12, 13, 14 for connection to phase, neutral and earth respectively; and voltmeter 37, switchably connected between terminals 12, 13 or 12, 14, provides a polarity check for the correct connections to terminals 12 and 13. The phase conductor is earthed through switch 15, contacts 16, 17, and either a lamp 22 or one of the resistors R 1 -R 4 . The trip coils 161, 171 are connected to neutral through D.C. relay contact 23 having its winding 24 shunted by rectifier 26, and slugged to operate after 1 ¢ cycles of the supply frequency. A proportion of the voltage developed across the selected resistor is fed from potentiometer tapping 29 to the grid of thyratron 31 which is initially adjusted by applying 120 volts R.M.S. to test points 36 and setting slider 29 so that the thyratron just strikes. A voltage drop of 120 volts R.M.S. across any chosen one of resistors R 1 -R 4 causes the thyratron to strike, and thus indicate, for 240 v. mains, a maximum phase-to-earth impedance equal to that of the chosen resistor. The slider 29 is calibrated for various values of mains voltage.