Patent GB-648790-A (GB)
Title: Improvements in electrical remote control systems
IPC: H02J13/00
Filed: 1948-02-02
Abstract: 648,790. Electric control systems. ROSS, T. W., SMITH, R. M. A., and METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Feb. 2, 1948, No. 3031. [Class 40 (i)] A switching device responsive only to electric signals of a predetermined duration comprises a main stable two-position thermal relay 12 including heater elements 14, 15 for closing and opening, respectively, contacts 16, 17 for connecting a load across lines L1, L2 and an auxiliary thermal self-resetting relay 13 having a heater element 18, adapted to be energized with one or other of the windings 14, 15 over switch 10 in response to a signal, and including a movable contact 19 which short circuits one or other of the heater elements 14, 15 according to its position whereby a signal of predetermined duration causes the auxiliary relay to operate during the first part of the signal to condition the main relay 12 for operation in one direction during a later part of the signal, whereas a signal of predetermined shorter duration operates the main relay in the opposite direction before the auxiliary relay responds to the signal. The signals used are ripple currents of predetermined frequency sent over the power lines L1, L2 to actuate a resonance relay 1, e.g. as described in Specification 517,971, [Group XXXVII], and connected to line through a series resonant circuit 6, 7. The switch 10 is of the quick action hot wire type and is closed by the resonance relay 1 intermittently removing the short circuit from the heater coil 8. As shown, the auxiliary relay 13 short circuits the heater winding 14 of the main relay so that when switch 10 closes in response to the first impulse, the relay windings 18, 15 only are energized. The heating of winding 15 has no effect since the main switch 16, 17 is open, but after a delay of, say 35 seconds, the auxiliary relay 13 operates to short circuit heater 15 and remove the short circuit from heater 14. Provided the ripple signal continues for a further predetermined period, say 30 seconds the heater 14 operates to close the main switch. The auxiliary relay reverts to normal when the ripple signal ceases. To open the main switch a further ripple signal is sent, the heater 15 is energized as before, and, after an interval of, say 30 seconds, opens the main switch. The ripple signal is stopped simultaneously, i.e. before the auxiliary relay has operated. In a modification, Fig. 2, the auxiliary relay 13 is also of the two-position stable type having two heaters 18, 22 and the moving contact 16 of the main switch 16, 17 also determines which of the auxiliary heater coils is energized by an incoming signal. As shown, heater 14 is short-circuited and the closure of switch 10 energizes auxiliary winding 18 which after a time of, say 30 seconds, changes over contact 19 whereupon heater 14 energizes and, after say, a further 30 seconds, closes the main switch to energize the load. A subsequent signal will energize coil 22 which after a delay of, say 30 seconds, moves the contact 19 back to the position shown to remove the short circuit from heater coil 15 which will open the main switch if the signal persists for say a further 30 seconds. If the signal persists only long enough to operate the auxiliary relay a third signal of 30 seconds duration may be sent subsequently to open the main switch. Specifications 574,372, [Group XXXVII], and 648,789 also are referred to.