Patent GB-691831-A (GB)
Title: Improvements relating to apparatus for the detection of earth faults in two-wire electric supply systems
IPC: G01R27/18
Filed: 1951-05-08
Abstract: 691,831. Detecting faults. WESTING- HOUSE BRAKE & SIGNAL CO., Ltd., KING, K. G., and DUCKITT, H. March 11, 1952 [May 8, 1951], No. 10769/51. Class 37. Apparatus for the detection of earth faults in two-wire supply systems in which neither line wire is normally earthed comprises two electromagnetically-operated devices adapted to operate indicating or control means and each having a control or operating winding connected between one of the line wires and an earth connection in circuit with a polarizing rectifier, so that an earth fault causes current to flow in the control or operating winding of one or the other device, thereby giving an indication of the line wire on which the fault occurs. An arrangement is shown for detecting earth faults on a D.C. supply system. Each of two magnetic amplifiers 1, 2 has two separate cores on which are wound A.C. primary windings 3, 4, secondary windings 5, 6 and D.C. control windings 7, 8. The primary windings 3, 4 are connected to an A.C. supply 9 in series with resistors 10, 11. The secondary windings 5 are connected across the A.C. terminals 12 of a bridge rectifier 13, the D.C. terminals 14 of which are connected in series with a resistor 15 across the operating winding 16 of a relay 17. The secondary windings 6 are similarly connected to a bridge rectifier 19 and the operating winding. 22 of a relay 23. The resistors 15, 21 are shunted by front contacts 24, 25 on the relays 17, 23 so that if a relay has released it will not pick up again until the resistor 11 has been momentarily short-circuited by the front contact 26 of a reset push-button 27. The control windings 7, 8 in series with polarizing rectifiers 28, 29 are connected between two equal resistors 32, 33 connected in series between the line wires 34, 35. The circuit includes the back contact 36 on the reset pushbutton 27 and front contacts 37, 38 on the relays 17, 23. Contacts 39, 40 on the relays 17, 23 connect signal lamps 41, 42 across a source of power 43. Should an earth fault occur on one of the line wires 34, 35, the leakage current flows through the control windings of one amplifier or the other, determined by the polarizing rectifiers 28, 29 and the line on which the fault occurs. The corresponding relay is thereby released and closes the circuit of its signal lamp. In a modification for testing A.C. lines the resistors 32, 33 are omitted and the control windings 7, 8 are connected directly to the line wires 34, 35 through polarizing rectifiers. Rectifiers are also connected across the windings 7, 8 and are so poled as to maintain the flow of current through the windings during the half-cycle when the associated polarizing rectifier does not conduct. These rectifiers may be replaced by condensers. In another modification the two electro-mechanically-operated devices are D.C. relays, the operating winding of one relay being connected in series with a polarizing rectifier between one line wire and the earth connection, whilst the operating winding of the other relay is connected in series with the other polarizing rectifier between the other line wire and the earth connection, the two rectifiers being poled the same way. Each relay may be provided with a holding winding which is short-circuited by a normally closed contact of the relays and adapted to maintain the relay energized once it has been energized as a result of a fault until the relay is released by a manually-operated contact.