GB358333A - - Google Patents

Abstract

358,333. Current, pressure, and power meters ; discharge apparatus. BRITISH THOMSON-HOUSTON CO., Ltd., Crown House, Aldwych, London.-(Assignees of Rex, H. B. ; 42, Magnolia Avenue, Lynn, Massachusetts, U.S.A.) Nov. 14, 1930, No. 34344. Convention date, Nov. 12, 1929. [Class 37.] In an alternating-current system in which currents proportional to the current or pressure to be measured are rectified and transmitted to a distance for measurement on direct-current instruments, the pilot circuits are protected against excess voltages by connecting across the pilot wires a device such as a gas discharge tube which permits initial passage of current only when the circuit voltage is in excess of the normal rectified voltage. A system is shown in Fig. 1 for measuring current, pressure and power at a distant point. A current transformer 14 has its secondary winding connected to the primary winding of a transformer 15, the secondary winding of which is connected to the anodes of a full-wave rectifier 18, preferably of the hot-cathode mercuryvapour type having its cathode supplied from the secondary winding of a transformer 19. From the mid-points of the secondary windings of the transformers 15, 19 rectified current is supplied over pilot wires 21, which may be telephone lines, to a direct-current ammeter 11. The resistance of the pilot circuit, represented diagrammatically by resistances 25, is adjusted to a standard value by a calibrating resistance 24. The rectified current is smoothed by a condenser 26, for example of 2 microfarads capacity, across the pilot circuit. Connected across the pilot circuit at the transmitting end is a neon tube 22 which will not pass current unless the line voltage rises above normal. Contacts 28 on the socket co-operate with contacts 29, Fig. 2, on the base of the tube 22 so that the pilot circuit is not complete unless the glow discharge tube is inserted in its socket. As a further protection a potential film cut-out 27 is connected in parallel with the glow discharge tube 22. A protective resistance 23 limits the short-circuit current in case of short-circuiting of the line 21 or breakdown of the tube 22 or film cut-out 27. A second pilot line 21<1> is similarly supplied with rectified current proportional to the voltage, and is connected to a direct-current voltmeter 12. A direct-current wattmeter 13 may be connected by switches 30, 31, 32 to read the voltamperes of the circuit.