Patent GB-649929-A (GB)
Title: Improvements in remote control electric circuits
IPC: H02H7/00
Filed: 1948-03-09
Abstract: 649,929. Protective cut-out systems ETABLISSEMENTS MERLIN & GERIN. March 9, 1948, Nos. 7257 and 7258. Convention dates, March 14, 1947 and Oct. 30, 1947. [Class 38(v)] A remote control circuit, especially suitable for use in mines, embodies a rectifier bridge, whereby the 3-phase controlled circuit (with or without an earthed neutral) is opened on occurence of fault either in the controlled or controlling circuits. Fig. 1 shows a 3-phase distribution circuit 1 with isolated neutral and controlled by a contactor 3 actuated by an electromagnet 4 in turn controlled by a relay 5 connected in the control circuit, which is supplied by a transformer 7. Also in the control circuit are an overload-actuated contact 8, an. " on " push-button 9 shunted by a resistance 9<SP>1</SP>, an " off " push-button 10, a rectifier 11, and the rectifier bridge 12 controlling a fault-detection relay 13. When not energized, the relay 13. completes a circuit through a warning lamp 14. The relay 5 is preferably shunted by a condenser 15 and one wire of the remote control circuit is earthed. To close the contactor 3, button 9 is pressed, the increased control current now flowing being sufficient to close the relay 5 and energize the electro-magnet 4 but insufficient to operate the relay 13. The reduced control current flowing on release of the button 9 suffices to keep the relay 5 closed, the contactor 3 being opened by depression of the button 10 or on breakage of a control wire. Should an earth fault develop in the non-earthed control wire, an alternating current flows through the control circuit, which, when added to the normal pulsed current, is adequate to operate the relay 13 thereby short-circuiting the relay 5 and opening the contactor 3. Opening of the relay 5 is additionally ensured by the condenser 15 which has a too small voltage created at its terminals, by the resultant asymmetrical alternating current, to keep the relay 5 closed. In the event of an accidental contact 16 between the unearthed control wire and one of the 3-phase lines an alternating current flows between the circuits due to the capacities 17 to earth of the power lines, and cause opening of the contactor 3. Fig. 2 shows an arrangement in which the neutral point of the 3-phase line 1 is earthed through a line 19 and a resistance 20 connected across the A.C. terminals of the rectifier bridge 12. Only a single control button 21 is provided which is held closed to maintain the contactor 3 closed. Should a control wire break or the non-earthed control wire go down to earth, the circuit functions as in Fig. 1. On accidental contact between the power line and the non-earthed control wire an alternating fault current flows through the relay 5 and condenser 15 back to the neutral point, thereby de-energizing the relay 5 and opening the contactor 3. Also a pulsed fault current flows back to the neutral point through the button 21, rectifier 11 and resistance 20. This fault current when added to the normal pulsed current is sufficient to operate the relay 13 and short-circuit the relay 5. On contact occuring between the earthed control wire and the power line 1, the relay 13 is operated by the increased current flowing therethrough. The resistance 20 has a value sufficiently high to prevent a dangerous current passing through a person touching a power line. To prevent automatic reclosure of the contactor 3 after opening under fault, the sensitivity of the relay 13 is such that it remains operated solely by the normal control current. In the arrangement shown in Fig. 3 and suitable for controlling circuits with or without an earthed neutral, an earthed relay 22 is energized by closure of the breaker 2 and controls the relay 5 so that on rupture of the earth wire or otherwise on undue increase of the resistance thereof the contactor 3 is tripped. When the " on button 9 is released the control current flows through the " ofl " button 10, a third control wire 41, a contact 42, which closes with the contactor 3, and a rectifier 43. This arrangement avoids current flowing through the control circuit when the contactor 3 is open, as in Fig. 1. The circuit of Fig. 4, especially suitable for an earthed 3-phase system 1 includes an additional pilot wire 25 permitting remote resetting of a relay 26 controlled by the overload device 8. On overload, the relay 26 is de-energized, the relay 5 thereby short-circuited, through wires 36, 37, and the main contactor (not shown) opened. The relay 26 is re-energized by pressing button 29 which completes a circuit through a resistance 30, rectifier 31 and relay 28, the rectifier 32 serving as a discharge path for the relay 28. Alternatively, the relay 26 may be reenergized by pressing button 27. Due to the conducting directions of rectifiers 11, 31, should a fault develop between the unearthed remote control wire and the wire 25, A.C. flows through the condenser 15 and de-energizes the relay 5 to open the contactor. If contact occurs between the wire 25 and one of the 3-phase lines, the fault current returns to earth through the relay 13 which operates as previously described to open the contactor. A core-balanced transformer 23 connected to the A.C. terminals of the rectifier bridge 12 similarly operates the relay 13 on occurrence of an earth fault in the power line.