Patent GB-490600-A (GB)
Title: Improvements relating to protective arrangements for electric cable systems
IPC: H02H7/22
Filed: 1937-02-16
Abstract: 490,600. Protective cut-out systems. OKONITE CO. Feb. 16, 1937, No. 4652. Convention date, Feb. 28, 1936. [Class 38 (v)] Electric cable systems used in mining are protected by an arrangement comprising a grounded wire embodied in the cable structure, one relay adapted to cut off the supply in case of a fault between the wire and the cable conductors, and another relay adapted to cut off the supply in case of a break in the grounded wire. In Figs. 1, 2, the ground wires 6 .. 9 are arranged in a 3-core cable with a metal sheath 11, and are connected together at the point 18 and through an impedance 19 to the earth 20 of the shovel or other machine 1. A circuit for the closing coil 21 of the breaker 12 extends from the secondary of transformer 13 over contacts 24, 25 of a relay 14 and contacts 16, 27 of a relay 15, such that operation of either relay opens the holding circuit. Relay 15 is energized from transformer 13 over the wire 9 and paralleled wires 6 ..8 so as to release its contact 16 when either 9 or all of 6 .. 8 break. If a connection occurs between a power conductor and one of leads 6 .. 9, a large current flows to earth at 17 and operates relay 14 to open contacts 25, 24. In Fig. 5, the power conductors are provided with individual earthing sheaths 30 in parallel with one another 'and energized by transformer 13 over a relay 31. Earth wires 6<1>.. 8<1> are each connected to a relay 15a .. 15c providing a series of parallel return paths from sheaths 30 to transformer 13. A break in an earth wire de-energizes the appropriate relay 15a &c. to open the holding- coil circuit 21, whereas if all the shields 30 break, relay 31 will release its contact and de-energize 21. Relay 14<1> is earthed at 17 and connected over relay 31 to the sheaths 30 so as to open its contact when a fault occurs between a sheath and power conductor. In Fig. 3 (not shown), each earth wire has its own relay 15a &c., but no earthing shields 30 and relay 31 are provided. In a further form, Fig. 7 (not shown), a single earthing sheath surrounds the entire conductor and has a relay similar to 31 of Fig. 5, but in this form the relay 14<1> is connected to wires 6<1>... 81 so as to be responsive to a connection between one of those wires and a power conductor, and between the latter and the earthing sheath. In Fig. 9 (not shown), the arrangement of Fig. 7 is modified by dispensing with the earth wires and their relays the single overall sheathing being earthed as at 20 in Fig. 1, and connected in series with a transformer at the machine 1 and with windings such as 14, 15 of that Fig. In Fig. 11, instead of an open-circuit on the earth wires producing a trip, an indicator 40 is energized from a transformer 41 on the machine, the arrangement being such that if an open-circuit occurs on wire 9, relay 43 releases to operate the indicator, whereas an open-circuit on all the wires 6 .. 8 causes relay 42 50 actuate the indicator. The winding 14<1> is energized from transformer 41 over wire 9, winding 43, and the earth circuit 17, 20 but is normally insufficiently energized to open the circuit of coil 21.