GB2170654A - Switching arrangement - Google Patents

Abstract

An electrical switching arrangement comprises a solid state relay 4, an electro-mechanical relay 6 having contacts 5 to complete a by-pass across the solid state relay, and actuating means 14 and 16 such as a switch which has two sets of contacts to switch on the solid state relay and after a short dwell the electro-magnetic relay and to switch off the electro-magnetic relay and then after a short dwell the solid state relay. In the illustrated embodiment, the electromagnetic relay is bi-stable so on a first actuation of the means the electro-magnetic relay enters a stable conducting state whilst the solid state relay drops out on the cessation of the actuation. On a subsequent actuation the solid state relay is again rendered conductive before the electromagnetic relay is reset and again on cessation of the actuation drops out. The electro- magnetic relay is caused to alter state by a capacitor 17 which when the relay is not conducting is discharged through a relay contact and which when the relay is conducting is charged up through a resistor 18. <IMAGE>

Description

SPECIFICATION Switching arrangement FIELD OF THE INVENTION The present invention concerns switching arrangements especially for use in energy conserving systems wherein loads are periodically switched off and users have to reconnect the loads if wanted.

BACKGROUND OF THE INVENTION it has long been known to have switches incorporating mechanical delays for illuminating passages in premises having multiple tenancies. These switches were bulky and expensive and gave only short, usually too short, periods of illumination. With the ever-increasing cost of electricity, it has become desirable to consider similar arrangements in buildings especially large office blocks. Mainly this has involved the use of electro-mechanical relays to switch the individual loads but such relays suffer from arc erosion and need frequent maintenance or replacement. The present inventors have proposed the replacement of the electro-mechanical relay by a solid state relay but it is expensive to use a solid state relay capable of coping with current overloads.

SUMMARY OF THE INVENTION The present invention provides a switching arrangement comprising a solid state relay, an electro-mechanical relay having contacts to complete a by-pass across the solid state relay, and actuating means arranged to switch on the solid state relay and after a short dwell the electro-mechanical relay contacts and to switch off the electro-mechanical relay contacts and then after a short dwell the solid state relay so that during each change of state of the electro-mechanical relay contacts the solid state relay is conducting and thus preventing arcing across the electro-mechanical relay contacts.

The solid state relay need only be conducting when the contacts are changing state.

Thus this relay has a highly intermittent duty cycle and is unlikely to be damaged by the thermal heating due to current overloads and can be small. Equally the electro-mechanical relay does not have to take any current during a change of state when the contact impedance is high and this reduces contact heating as well as avoiding arc erosion. This means that smallish contacts can be used. Thus the cost of using two relays is counter-balanced by being able to use small relays.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a circuit diagram of a switching arrangement.

BEST MODE OF PERFORMING THE INVENTION One embodiment of the invention will now be described, by way of example, with reference to the drawing.

The circuit arrangement illustrated has connection points, one 1 for connection to the neutral of a mains supply, a second 2 for connection to the line of the mains supply, and a third 3 for connection to a "line" side of a load (not shown). Connected across the second and third in parallel are a solid state relay 4 such as a triac and contacts 5 of a relay 6, together with an arc-snubbing resistor-capacitor branch 7.The relay 6 is an electro-mechanical relay with an operating coil 8 with a series resistor 9 to limit the current to below the trip-in level across a smoothed-halfwave rectifier consisting of a diode 10, a re- sistor 11 and a capacitor 12 connected between the first and second points A first set of contacts 14 of a switch, which has two sets of contacts, the first set of which closes before and opens after the other set, such as a "Licon" switch (Licon is a trademark) is connected between the point 2 and, through a protective resistor 15 to a triggering eiectrode of the solid state relay 4.The second set of contacts 16 of the switch has two poles and in the unswitched state a capacitor 17 is connected between the second point 2 and a tapping on a resistor chain 18 between the first point 1 through the diode 10 and the second point; the tapping is also connected to the second point through contacts of the relay when the relay is is the off state. This capacitor 17 is connected in parailel with the resistor 9 when the second set of contacts is operated.

Also connected between the first connection point and the second point is a series branch of a rectifier 19 and a high-value resistor 20 and between the first and third points a capacitor 21.

In operation, initially the various contacts are as shown and a direct current flows through the coil 6 but this current is limited by the resistors 9 and 11 to a value below the trip-in level but above the trip-out level. No voltage exists across the capacitor 1 7. To switch a load such as a fluorescent lamp at a work station, the "Licon" switch is actuated manually as by a pull-cord bringing contacts 14 together firing the solid state relay and shortly thereafter bringing contacts 16 together thus shunting the resistor 9 with the capacitor 17 and tripping in the relay. The "Licon" switch contacts then return to their initial position. In this condition the capacitor 17 is charged up through resistor 18 and on a subsequent actuation of the "Licon" switch, this capacitor applies a trip-out pulse to the relay coil.Periodically all the loads are to be disconnected except for essential supplies. In an office, work station lights are often left on all day even when no one is present and they should be off but other lights are needed for safety in passageways. Switching off inessential lights is achieved by having a separate mains circuit for the essential lights and one or more for the inessential loads and then periodically interrupting briefly the inessential mains circuit or circuits say at lunch time or going-home time or more frequently at say hourly intervals. When the mains circuit is interrupted, the relay trips out; although there should be no arcing since there is no supply, any arcing is suppressed by the snubber branch 7. The arrangement is thus restored to the initial condition and the loads can be restored by operating the "icon" switch again. With some loads and supplies, there may be transients and these are suppressed by the rectifier 19, resistor 20 and capacitor 21. Thus if the load goes into oscillation on failure of the mains supply and the oscillating voltage is rectified by rectifier 10 to hold the electro-magnetic relay energised, the polarity of the oscillating voltage tending to hold that relay energised is shunted by the rectifier 19 and resistor 20.

When the contacts 5 open, point 3 is not permitted to float but is tied to point 1 by the capacitor 21 and to point 2 by the snubbing branch 7; any transients are thus shunted and dissipated without any possibility of the transients triggering the solid state relay.

Claims (4)

1. A switching arrangement comprising a solid state relay, an electro-magnetic relay having contacts to complete a by-pass across the solid state relay, and actuating means arranged to switch on the solid state relay and after a short dwell the electro-mechanical relay contacts and to switch off the electro-me chanical relay contacts and after a short dwell the solid state relay so that during each change of state of the electro-mechanical relay contacts the solid state relay is conducting and thus preventing arcing across the electromechanical realy contacts.

2. A switching arrangement according to claim 1 wherein the actuating means comprises two manually operable sets of contacts ganged together so first one set is rendered conductive and then the other so providing the short dwell, the first set of contacts switching the solid state relay on and the second set providing a triggering pulse to the electro-magnetic relay's operating coil which coil is in a bistable circuit so its contacts remain conductive even when on cessation of the actuation the solid state relay ceases to be conductive until on a second actuation of the actuating means the solid state relay is again conductive and the coil receives a further triggering pulse whereafter it remains non-conductive until a third pulse is received.

3. A switching arrangement according to claim 2 wherein the second set of contacts causes a capacitor to supply the triggering pulse, the capacitor when the relays are nonconducting being shunted by contacts of the electro-magnetic relay so the next pulse is a zero pulse but when the last mentioned contacts are opened being charged up so the next pulse is a definite pulse.

4. A switching arrangement substantially as herein described with reference to the accompanying drawing.