Patent GB-892468-A (GB)
Title: Improvements in electrical testing devices
IPC: G01R27/18
Filed: 1959-08-22
Abstract: 892,468. Impedance, resistance, current and voltage measurements; indicating faults. NICHOLSON, C. H., and DUKE, G. E. Nov. 21, 1960 [Aug. 22, 1959], No. 28762/59. Class 37. An instrument for indicating the impedance of the earthneutral loop of a three-terminal single-phase source, comprises a transformer T having its primary winding T1 connected between the line and neutral terminals L, N and its secondary adapted to be connected across the neutral and earth terminals N, E by a normally open test switch TS, the loop impedance being determined from observation of the transformer primary current and secondary voltage in both the open and closed positions of the test switch. The terminals L, N, E are connected by a cable 11, a plug 12 and socket 13 to the source. Further terminals 7, 9, 10 are provided, 7, 10 being supplementary test terminals and 9 the earth of a standard 3-terminal socket outlet 24. After calibration, and with a reversing switch RS in the position shown, an isolating switch TS3 closed and switches TS1, TS open, the full secondary voltage is applied across a variable calibrating resistor R5 and a red filament lamp R in series. Under this condition the lamp R will be lit up and the primary current being small will be insufficient to light the green lamp G. Upon closing the test switch TS the earthneutral loop EL is put into circuit across the transformer secondary and, if the impedance of this loop is below a predetermined datum value, the red lamp R will go out and the green lamp light up. If, however, the impedance is above this value the green and red lights will maintain their original conditions. The indication sensitivity of the lamps is adjusted by means of the calibrating resistances R1, R3, R5. To calibrate the instrument, an impedance, equal to the maximum permissible value of the earth-neutral impedance, is connected across the terminals 7, 10, the reversing switch RS being in its right-hand position, the switch TS2 being closed and the switches TS, TS3 open. The calibrating resistances are then adjusted to bring the lamps R, G to approximately equal brightness but not to full incandescence. Approximate evaluation of loop impedance.- If the transformer input voltage is constant then the primary current will vary with the impedance load across the transformer secondary and so measurement of the primary current will give an indication of this load. The primary current is measured in terms of the voltage drop across the resistance R2 by voltmeter V. Initially, the transformer secondary is short circuited and the resistances R9, R1 are varied to give a predetermined primary current, and upon connecting the earth-neutral loop into the secondary circuit the primary current value gives an indication of the impedance. The voltmeter may be calibrated to read the impedance directly in ohms. Two readings are taken with the switch RS in its two positions to eliminate errors due to the outof-balance voltages on the neutral lead. With switch RS in the right-hand position, switches TS, TS1 open and TS2 closed the continuity of any low-impedance circuit connected between terminals 7, 10 can be tested, and an approximate value of its impedance obtained using voltmeter V. The circuit may be for instance the connection between the earth terminal 21 and the casing 19 of some apparatus. The sum of the impedances of the earth-neutral loop and of the earth terminal-casing may be checked by connecting the apparatus by its plug to the socket 24, having test switches TS open and TS2 closed and using a probe on the lead 17 to contact the casing. Further by contacting a metal casing 25 which is bonded to earth, by the lead 17, the effectiveness of the bond can be established. A lamp GL lights up on transposition of source line and neutral or line and earth connections. A negative temperature coefficient resistance R7 may be incorporated to compensate for the temperature variations in the transformer.