EP0190304A1

EP0190304A1 - Hybrid power switch

Info

Publication number: EP0190304A1
Application number: EP19850904123
Authority: EP
Inventors: Allen David Muirhead
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-08-20
Filing date: 1985-08-20
Publication date: 1986-08-13
Also published as: AU4727885A; WO1986001334A1; GB8421070D0

Abstract

Commutateur hybride de puissance pour courant alternatif utilisé avec une alimentation de fréquence prédéterminée, comportant des contacts mécaniques de commutation (16, 18) capables de supporter la puissance nominale de charge et de fonctionner entre une position d'ouverture et une position de fermeture, ainsi qu'un commutateur électronique (20) caractérisé par un flux de courant unidirectionnel normalement ouvert et connecté électriquement en parallèle aux contacts mécaniques de commutation (16, 18). Un dispositif de commande électrique (32) du commutateur hybride de puissancepermet de fournir un signal de mise sous tension et un signal de mise hors tension pour déclencher le mouvement des contacts (16, 18) entre les positions d'ouverture et de fermeture. L'actionnement du dispositif de commande électrique (32) rend le commutateur électronique (20) conducteur pendant la durée de déplacement des contacts (16, 18) à travers une zone où il peut se produire un amorçage d'arc. Si la durée de commutation des contacts mécaniques (16, 18) reste inférieure à la durée de conduction dudit commutateur électronique (20) devenu conducteur pendant une demi-période de tension de secteur, le mouvement des contacts (16, 18) et la fermeture du commutateur électronique (20) se déclenchent simultanément. En revanche, si la durée de commutation excède une demi-période, la synchronisation relative entre le déclenchement de la fermeture des commutateurs électronique et mécanique est réglée de sorte que les contacts (16, 18) se déplacent à travers la zone d'amorçage d'arc lorsque le commutateur électronique (20) est conducteur.Hybrid power switch for alternating current used with a power supply of predetermined frequency, comprising mechanical switching contacts (16, 18) capable of supporting the rated load power and operating between an open position and a closed position, as well an electronic switch (20) characterized by a normally open unidirectional current flow and electrically connected in parallel to the mechanical switching contacts (16, 18). An electrical control device (32) of the hybrid power switch provides a power on signal and a power off signal to initiate movement of the contacts (16, 18) between the open and closed positions. Actuation of the electrical control device (32) makes the electronic switch (20) conductive for the duration of movement of the contacts (16, 18) through an area where arcing can occur. If the duration of switching of the mechanical contacts (16, 18) remains less than the duration of conduction of said electronic switch (20) which has become conductive during a half-period of mains voltage, the movement of the contacts (16, 18) and the closing of the electronic switch (20) trip simultaneously. On the other hand, if the switching time exceeds half a period, the relative synchronization between the triggering of the closing of the electronic and mechanical switches is adjusted so that the contacts (16, 18) move through the priming zone d arc when the electronic switch (20) is conductive.

Description

"Hybrid Power Switch"

The present invention relates to improvements in a hybrid electrical power switching device.

The switching of high current electrical loads has conventionally been performed by mechanical contacts moving between open and closed positions. However, as the contacts of the switch approach one another there is a risk that arcing will occur which, over time will erode the contacts.

It is known to use solid state electronic switches, but these tend to generate excessive heat leading to additional expense and bulk. It is therefore preferred to utilise mechanical contacts due to their simplicity and low heat generation.

Attempts have been made to utilise an electronic switch and mechanical contacts connected in parallel (hereinafter referred to as a hybrid switch) which can lead to a number of advantages over the use of either type of device in isolation. By closing (i.e. render conductive) the normally open electronic switch during the period that the mechanical contacts are undergoing a switching operation, the current is diverted from the mechanical contacts. The *ON' state voltage drop (saturation voltage) of the electronic switch is of the order of 1 to 2 volts and this appears across the mechanical contacts during the switching operation irrespective of the main circuit voltage which is present across the contacts when in the fully open condition and with the electronic switch open. This saturation voltage is too low to sustain any significant arcing between the mechanical contacts irrespective of the magnitude of the electric current being switched. The contacts therefore effectively perform arc-free switching.

The notable advantages of this hybrid switch ^•arrangement are the extension of the electrical life of the mechanical contacts to equal their mechanical life, in terms of number of switching operation at full load, and the absence of heat generation in the electronic switch which would otherwise occur if it were to be used to carry full load current continuously. Present methods of achieving hybrid switching for alternating current generally employ a bi-directional electronic switch such as a TRIAC which is closed for several cycles of the power line voltage waveform whilst the mechanical contacts are switching. The TRIAC exhibits a poor commutating dv/dt performance and because of this can fail to commutate if a low power factor inductive load is being switched.

Unidirectional electronic switching devices exhibit more desirable switching characteristics, but cannot handle alternating voltages and thus are effectively open only during alternative half cycles. Thus, if the mechanical contacts pass through the portion of their travel in which arcing may occur whilst the electronic switch is effectively open, arcing will still occur.

It is an object of this invention to produce a hybrid switch that obviates or mitigates the above disadvantages.

The invention uses a single uni-directional device such as a silicon controlled rectifier (SCR) connected in parallel - 3 - with mechanical contacts whose switching period is less than one half cycle of the power line voltage waveform. The SCR (since it is a uni-directional device) has a much improved performance when switching inductive loads and is generally cheaper than a TRIAC with the same current carrying capability. If the SCR is fired at or near the zero crossing point of the voltage waveform and in particular at the same polarity transition (e.g. line positive to negative, depending on the orientation of the SCR) and is allowed to commutate at the next zero crossing transition, it will only "see" uni-directional current flow. Provided the mechanical contacts can pass through the arcing zone (including bouncing) within this period the hybrid switching function as hereinbefore described is successfully achieved. A hybrid a.c. power switch for use with a power line of predetermined frequency according to this invention comprises a hybrid a.c. power switch for controlling an a.c. power supply comprising a mechanical switch having contacts moveable through an arcing zone from a first position in which said mechanical switch is in a closed state to a second position in which said mechanical switch is in an open state, mechanical switch operating means operable upon the mechanical switch to change the state thereof, an electronic switch in parallel with said mechanical switch and having an open state and a closed state, electronic switch operating means to change the state of said electronic switch, initiating means operable to initiate a change of state of said mechanical switch, control means operable between said initiating means and said operating means to control the operation of said operating means by said initiating means, sensing means responsive to a predetermined stat of said ^"power supply ^• and operable upon said control means upon 5 detection of said predetermined state to enable said operating means and said control means enabling said operating means to ensure passage of said contacts through said arcing zone with said electronic switch in a closed state.

Embodiments of the invention will now be described with 1Q reference to the accompanying drawings in which:

Figure 1 is a schematic illustration of a hybrid switch, connected to a load.

Figure 2 is an illustration of the voltage waveform of the power line which is to be connected to a load by a power 15 switch.

Figure 3 is a representation of a zero-crossing detector output which is derived from the voltage of the power line. Each corner of the square waveform represents a zero-crossing of the power line voltage. 20. Figure 4 is a schematic illustration of the output of the power control switch, there being a low or logic zero output when the switch is OFF and a high or logic 1 output when the switch is ON;

Figure 5 is an illustration graphically of the 25 condution periods of power through the uni-directional switch in a switch ON and a switch OFF operation. The lower level indicating no power condution and the upper level indicating full conduction.

Figure 6 is an illustration of the conduction of power through the relay that controls the operation of the mechanical contacts, the lower level indicating no conduction and the upper level indicating condution to operate the relay to move the contacts to the closed position.

Figure 7 is an indication of the time sequence- of opening and closing of the mechanical switching contacts, the lower level indicating the open position and the upper level indicating the closed position.

Figure 8 is an illustration of the times during which power is supplied to the load, the lower level indicating no power and the upper level indicating substantially full power.

Figure 9 is a schematic illustration of a hybrid switch with an alternative relay operating means and uni-directional switch operating means to the one illustrated in Figure 1;

Figure 10 is an illustration of the voltage of the power line waveform for the arrangement of Figure 9;

Figure 11 is an illustration of the ON/OFF operation of the power control means for the embodiment of Figure 9;

Figure 12 is a representation of the output of the zero-crossing detector as supplied to the relay control means and electronics switch control means for the embodiment illustrated in Figure 9; Figure 13 is an illustration of the logic state of the

D input of the flip flop 64 of Figure 9;

Figure 14 is an illustration of the logic state of the Q output of flip flop 64 and D input of flip flop 66 of Figure 9;

Figure 15 is an illustration of the logic state of the Q output of flip flop 66 of Figure 9;

Figure 16 is an illustration of the conduction through the. electronic switch 72 of Figure 9;

Figure 17 is an illustration of the signal of the drive of the transistor 84 of relay 82 of Figure 9; the upper line indicating conduction through the transistor and relay and the lower line indicating when conduction is not taking place through the relay; and

Figure 18 is an illustration of the closed and open conditions of the mechanical contacts of the hybrid switch of Figure 9.

Figure 19 is a schematic diagram of a further embodiment of a hybrid switch.

Referring to the drawings. Figure 1 is a schematic illustration of an a.c. power switch connected to a a.c. power source 10 to control the application of power from the source to a load 12 by the manipulation of an ON-OFF initiating switch 14. Under conditions of steady load operation, power flows from the line 10 to the load 12 through the closed mechanical contacts 16 and 18 but on start-up and on shut-down, power flows substantially through a uni-directional electronic switching member 20 as the contacts 16 and 18 are making and breaking respectively. It will be noted that the uni-directional device 20 is connected in parallel with the mechanical switching contacts 16 and 18. The device 20 is preferably a silicon controlled rectifier (SCR) that has a gating contact 22 which permits the rectifier to close and conduct when gating voltage is applied and the voltage bias is in the appropriate direction. Continued application of the voltage bias in the same direction maintains the SCR conducting even if the gating voltage is removed. Upon application of a reverse voltage across the SCR, the SCR opens and prevents further conduction. Other suitably rated uni-directional devices such as transistors could be used in place of the device 20.

The resistor 24 and capacitor 26 are components of a snubber network to limit the applied dv/dt and prevent ungated turn-on.

The contact 18 is mounted on an armature and moves between open and closed positions with respect to the contact 16 under the influence of a relay indicated by the numeral 26. When relay 26 is activated the armature moves to the closed position to close the contacts as illustrated in solid lines. When the relay is de-activated the armature moves to the dotted line position illustrated in the drawings.

Numeral 27 refers to a voltage source for operating the relay 26 and the exclusive-or gate generally indicated by numeral 28 of the control circuit for the rectifier 20.

Control means are provided between the initiating switch 14 and electronic switch 20 for applying a voltage to the control gate 22 of the silicon control rectifier 20 only during the first negative going half cycle of the power supply after the control switch 14 has been closed or opened. In this way, the electronic switch can conduct electricity to the load only during this half cycle.

In the embodiment of Figure 1, the armature that carries the mechanical contact 18 into and out of electrical connection with the contract 16 has a period of operation which is no longer than the period of conduction of rectifier 20 during the one half cycle of the voltage of the power supply so that as the connectors 16 and 18 are opening and closing, there is an alternative path for power through the silicon controlled rectifier.

The provision of this alternative path for power from the voltage source to the load while the contacts are closing and opening effectively eliminates a voltage drop across the contacts during the closing and opening operations and thereby eliminates arcing in the closing and opening operations.

The operation of the relay 26 and of the silicon controlled rectifier 20 to achieve this purpose is illustrated by the graphs of Figures 4 to 10 inclusive assuming the voltage waveform of the power line 10 is of the form shown in Figure 2. A zero-crossing detector 30 operates as a sensing device to detect a predetermined condition (i.e. zero crossing) of the power supply and has an electrical output similar in form to Figure 3 wherein each high corner of the square waveform corresponds to a zero-crossing point of the voltage of the applied power waveform. The zero-crossing detector output voltage is in anti-phase with the voltage of the applied power waveform. A similar arrangement using an in phase zero crossing detector could be used in which case the conducting direction of the SCR 20 would be reversed.

The output of the zero-crossing detector 30 is applied to the clock input 35 of the D-type flip flop 32 which controls operation of the relay 26 and gating contact 22. Flip Flop 32 is designed to transfer the logic from its D input 34 to its Q output 36 on the occurrence of the first positive going pulse produced by the output of the crossing detector following a change of logic at the D input 34. Thus, if the logic 1 appears at the D input 34, the logic 1 will be transferred to the O output 36 upon the occurrence of the first positive going signal from the crossing detector 30 following the application of the logic 1 to the input 34. As seen in Figure 4, closing of the switch 14 to initiate closing of the contact 18 changes the D input 34 of the flip flop 32 from a no voltage, logic zero, to an applied voltage, logic one.

The iniating signal from the switch 14 is transferred by an opto isolator 38 to the D input 34. A d.c. control voltage of about 15 volts controlled by rectifier 50 is applied across the terminals 40 and 42 with the switch 14 closed to energise a light emitting diode 44. A photo transistor 46 responds to the emission of the LED 44 to provide a high signal at input 34. Numeral 48 is a current limiting resistor. The control voltage across the contacts 40 and 42 can conveniently be between three to thirty volts and can be derived from the power line.

Upon a positive going voltage from the output of the zero-crossing detector 30 appearing at the clock input of the flip flop 32, logic 1 on its D input terminal 34 is transferred to its Q output terminal 36. The appearance of a logic 1 at the Q output terminal 36 of the flip flop 32 causes two reactions.

It starts and maintains the flow of current through the relay 26 by applying a gating voltage to the gate 55 of transistor 52. Thus, it puts relay 26 into operation and maintains it in operation as long as the logic 1 appears at the Q output terminal 36 of the flip flop. When relay 26 is in -operation, the mechanical contacts 16 and 18 are drawn towards each other to the closed solid line position.

The appearance of a logic 1 at the Q output terminal 36 of the flip flop 32 also causes a difference of voltage to appear across the terminals 51 and 53 of the exclusive-or gate 56 for a period of time determined by the time constant of resistor 58 and capacitor 60. This time constant is made less than the period of one half cycle of the power line waveform so that the output of gate 56 is a pulse of duration determined by the time constant.

While the difference in voltage appears across the terminals 51 and 53, base current is provided for the transistor 62 which conducts and provides current to the gate 22 of SCR 20 to close the SCR and allow it to conduct. Once current is flowing, the SCR is held closed by the current until a voltage reversal occurs or the current drops below the holding current. Thus during the first positive half cycle of the power supply, the SCR 20 is enabled to conduct current across the contacts of the relay. Once, however, capacitor 60 becomes fully charged and has changed the output at terminal 53 from logic zero to logic 1, the difference in voltage across the inputs 51 and 53 of the exclusive-or gate 56 disappears and the output reverts to zero. The gating current to transistor 62 and to SCR 20 is removed so that SCR 20 will open if the current through it falls below its minimum holding current or the voltage applied across it reverses.

By the time a voltage reversal occurs, i.e. a maximum of one half cycle, however, the contacts 16 and 18 have fully closed due to relay action without arcing and are themselves then able to carry full load current. The period of closure of the contacts is less than one half cycle of the power supply so that closure takes place during the time that power is supplied to the load 12 through the silicon control SCR 20.

A consideration of Figure 5 and 10 show the operation of the electronic SCR 20 and the mechanical switching contact 16 and 18. As seen in Figure 5, the electronic switch 20 becomes operative during the first positive going output of the zero-crossing detector and stays operative until the end of the first half cycle. An examination of Figure 8 shows that power to the load is continuous from the appearance of the logic 1 signal at the Q output of the output terminal 36 of the flip flop 32, being first through the SCR 20 and subsequently through the mechanical contacts 16 and 18. When the switch 14 is opened the logic 1 reverts to logic 0 at the D input 34 of the flip flop 32. Upon the next positive going voltage signal from the zero-crossing detector

30, this zero logic is transferred to the Q output terminal 36 of the flip flop. Transistor 52 loses its gating current and switches off, the relay opens and the contacts 16 and 18 begin to open. Simultaneously and due to the change from logic 1 to logic 0 at the Q output of flip flop 32, a voltage differential is developed across the input terminals 51 and 53 of the exclusive-or gate 56 to provide a gate drive once again to transistor 62 to the gate of silicon controlled rectifier 20 for the period of the time constant of the resistor 58 and capacitor 60. The SCR 20 is closed by the output of transistor 62 to conduct and will turn OFF once the load current falls below its minimum holding current or a voltage reversal occurs. In this time the relay contacts 16 and 18 have opened without arcing, the current during the opening period having been carried by the rectifier 20. This is illustrated in Figures 5 to 10 graphically. As illustrated in Figure 5 the gating pulse to the rectifier is less than a half cycle as controlled by the time constant of the resistor 58 and condensor 60.

The device includes means for generating from the power supply, the voltage 27 for operating the relay 26, the d.c. control voltage across 40 and 42 for operating the flip flop and for the zero-crossing device. These voltages can be derived by any one of several available well-known means not referred to in detail. In the embodiment, the period of operation of the mechanical contacts 16 and 18 of the relay is less than one half cycle of the power supply and during the half cycle in which the ^"•• relay opens provision is made for conducting the supply of power to the load around the contacts and through a uni-directional electronics device.

Other embodiments will be apparent to those skilled in the art and one further embodiment is illus trated in Figure 9 of the drawings. This embodiment has two flip flops 64 and 66, an exclusive-or gate 68, a transistor 70 and a silicon controlled rectifier 72 which is in parallel to the mechanical switch contacts 74 and 76. One of the flip flops 64 acts to control the operation of the SCR 20 and relay 82 and the other 66 acts to generate a pulse output for the SCR gate. The load 78 is -- similarly supplied from an a. c. supply 80 and a relay 82 connects through a transistor 84 with the Q output of flip flop 64 and D2 input 88 of flip flop 66. The output of the zero-crossing detector 90 connects to the clock input CK of each of the flip flops 64 and 66. The d.c. control voltage is similarly applied as in Figure 1 from terminal similarly numbered 40 and 42.

The operation of this embodiment is illustrated in the waveforms of Figures 10 to 18. When the control voltage is applied (Figure 11) , a logic 1 immediately appears on the Dl : input of the flip flop 64. (Figure 13) . This input is transf erred to the Q output of flip flop 64 and D input of flip flop 66 upon the occurrence of the f irst positive going signal from the voltage crossing device to the clock input CK of flip flop 64 as illustrated in Figure 14.

Trie occurrence of the logic 1 at the output of flip flop 64 and input of flip flop 66 causes operation of the relay 82 through the gating of transistor 84 as indicated in Figure 17 and the consequent commencement of closure of the mechanical switch contacts 74 and 76.

At the same time, it has caused a difference in voltage across the inputs of the exclusive-or gate 68 which as will be noted is connected across the input and the output of flip flop 66. This difference in output causes a voltage to occur at the output of the exclusive-or gate 68 and transistor 70 conducts gate current into the uni-directional SCR 72 to conduct current to the load while the contacts 74 and 76 are closing. On the occurrence of the next following positive going signal from the zero-crossing detector, the logic signal on the D2 input terminal of flip flop 66 is transferred to the output Q2 terminal so that a logic 1 appears at the input and output terminals of flip flop 66. There is then no difference across the terminals of exclusive-or gate 68 and the gating current of transistor 70 terminates so that current flow through the SCR 72 is terminated upon a voltage reversal.

A similar sequence of events occurs as the d.c. control voltage is removed. A logic zero is applied to the input of flip flop 64, transferred to the output to commence flow through the relay 82 and render the silicon controlled rectifier 72 conductive by reason of the difference in output and input on the flip flop 66. With the occurrence of the next following positive going signal, the input and output of flip flop 66 is made to have the same zero logic and gating current to the SCR 72 is terminated. By this time, the relay has opened without arcing.

Hybrid switching devices are commonly used for loads of about 40 amperes but can be used for loads of any reasonable magnitude.

The embodiments of Figures 1 through 18 illustrate a hybrid swith in which the mechanical contact has a switching period less than a half cycle of the supply. This enables the firing of the SCR and the relay to be accomplished simultaneously. However, for switches in which the switching period is greater than one half cycle a modification of the embodiments illustrated above may be used to ensure that arcing of the contacts does not occur. In this arrangement the gating voltage to the silicon controlled rectifier is supplied such that the SCR will be closed during passage of the contacts through an arcing zone, which is a zone in which there is the potential for arcing to occur.. Thus, movement of the contacts can be initiated prior to closing of the SCR as the switch moves from an open to a closed position.

An embodiment of a hybrid switch for use with a mechanical switch having a closing period greater than one half cycle is shown in Figure 19. In this embodiment similar components to those shown in Figures 1 and 9 will be identified with like reference numerals with a suffix "a" added for clarity of description.

Referring therefore to Figure 19 the operation of the circuit is substantially as described above with respect to Figure 1 in that an output from the zero crossing detector 30A 5 clocks a signal from the D inpu t 34A of flip flop 32A to operate relay 26A and initiate movement of the mechanical contacts 18A .

The output of zero crossing detector 30A is also passed through a pulse generator in the form of an exclusive OR-GATE 100 having one terminal 102 connected directly to the output of

10 the zero crossing detector 30A and the other input 104 connected through an RC c ircuit 10 6 , 108 to provide a momentary diff erence in the inputs of the term inals 102, 104.- The output of the exclusive OR-GATE 100 is applied to the clock input 108 of a D type flip flop 110. The input to the D terminal of the flip

15 flop 110 is the outpu t 36A of the Q terminal of the flip flop 32A. Thus, upon a clock ing pulse appearing at input 108 the Q output o f flip flop 35A is transf erred to the Q output of the flip flop 110 . The Q output of flip flop 110 is connected to one inpu t of an AND gate 112 , the o ther input of which i s

20. derived from the Q output of flip flop 35A. The output of the AND-gate 112 is applied to the puls e generator in the form of an exclusive OR-GATE 56A and the RC c ircuit 58A , 60a to trigger the SCR 20A in the manner described above.

The purpose of the fl ip flop 110 is to provide a delay

25. between the control signal to activate the relay 26A and the contro l s ignal to activate the s ilicon contro l rectif ier 20A. The delay provided by the D type flip flop can be increased by utilising a number of flip flops in s eries wi th each one being clocked by the output of the exclusive OR-GATE 100 and the receiving as its D input the Q output from the preceding flip flop. The clock input 35A from the zero crossing detector provides a clocking pulse , i . e . a positive going edge every cycle. However, the output from the zero crossing detector provides a change of state every half cycle and as a result the inputs 102, 104 to exclusive OR-GATE 100 d iffer each half cycle . Thus , the output from exclusive OR-GATE is a positive going pulse each half cycle so that each of the D type flip flops 110 employed will provide a delay of one half cycle.

In operation there fore the closing of switch 14 will produce a high signal at the D input 34A of flip flop 32A. Upon the next positive going edge from the zero crossing detector the high D inpu t 34A i s clo cked to the Q output 36A and initiates movement of the mechanical contacts 18 from an open position towa rd the closed position . At the same time a high input appears on the D input to flip flop 110 and a lso on one input to the AND-gate 112. On the initia l clock pulse from the zero crossing detector , the Q output of flip flop 110 remains low as the D inpu t at tha t time is also low. However , after one half cycle the state of the zero cross ing detector 30A changes producing a clocking pulse from the output of the exclus ive OR-GATE 100. The input to the D type flip flop 110 is now high and this is clocked through to th e Q outpu t to appea r a t the second term inal of the AND-ga te 112. The output of AND-gate 112 thus goes from low to high to initiate switching of the silicon control rectifier 20A. Thus, the open state of the SCR 20A has been maintained for a further one half cycle relative to the initiation of the movement to the contacts of the switch 18 and the silicon controlled rectifier 20A is closed permitting conduction of the current during the latter part of the movement of the contacts of switch 18 so that as they pass through the arcing zone the current is carried by the. silicon controlled rectifier 20A. Upon opening of the switch 14A the D input to flip flop 34A goes low and upon the next clocking pulse from the zero crossing detector 30A the Q output of flip flop 32A also goes low. This terminates the control signal to relay 26A and so initiates movement of the contacts of switch 18 from a closed to an open position. At the same time, however, one of the inputs to AND-gate 112 goes low, so changing the output of AND-gate 112 from high to low. There then appears a difference on the inputs 51A, 53A of gate 56A which produces a pulse output to initiate closing of the silicon controlled rectifier 20A. Thus, as the contacts commence their travel from the closed towards the opened position, the silicon controlled rectifier is closed to handle the current. The SCR will remain closed for the remainder of the half cycle by which time the contacts will have moved through the arcing zone although they may not at that time have completed their travel. It will be seen therefore that by introducing the flip flop 110 into the circuit a delay can be built into the control function to allow switching periods greater than one half cycle. In order to match the period in which the contacts pass through the arcing zone with the period of conduction of the SCR, the delay may be adjusted by employing an appropriate number of flip flops 110. Also, the output of the zero crossing detector may be inverted or reversed as appropriate to shift the initiation of the relay by one half cycle.

The circuit of Figure 19 has also been modified to ensure that upon failure of the SCR 20A the switch 18 cannot be closed. This is accomplished using a reset signal derived from the open pole indicated at 114 of the mechanical switch 18. The open pole 114 is connected through a signal line 116 to the reset terminals of each of the D type flip flops 32A and 110. The open pole 114 is also connected through a high value resistor 120 to the load 12A and through capacitor 122 to the line 10A. As live line 10A serves as a zero volt reference line for the d.c. supply under normal conditions the relative voltage in the signal line 116 is low. This allows the D type flip flops to operate in the normal manner. However, if the switch contacts move from the open position toward the closed position without the silicon control rectifier 20A closing, the voltage at the pole 114 is pulled towards neutral which is high with respect to the zero volt reference line and thus produces after a period determined by CR network 120, 122 a high reset signal for each of the D type flip flops 110. This effectively impresses a low signal on the Q outputs of each of the flip flops and thus will terminate the control signal to the relay 26A. The relay is de-energised and the switch reverts to its open position. The switch will then cycle from the open position toward the closed position, but in each case will not be able to complete the movement due to the removal of the control signal to the relay. In this way the mechanical switch will not be subject to arcing in the event of failure of the SCR 20.

This signal line may also be utilised in the embodiments of Figures 1 and 9 by employing a normally closed contact of the relay as a reset line.

It is believed that it will be apparent from the above embodiments that a simple hybrid switch has been disclosed that takes advantage of a uni-directional switching characteristics of the silicon controlled rectifier and ensures that the contacts of the mechanical switch are not subjected to arcing. Moreover, the hybrid switch can be used with switching periods greater than one half cycle if necessary and also may incorporate a safety feature to inhibit operation of the switch in the event that the SCR fails as shown in the embodiment of Figure 19.

Claims

CLAIMS :

1. A hybrid a.c. power switch for controlling an a.c. power supply comprising a mechanical switch having contacts moveable through an arcing zone from a first position in which said mechanical switch is in a closed state to a second position in which said mechanical switch is in an open state, mechanical switch operating means operable upon the mechanical switch to change the state thereof, an electronic switch in parallel with said mechanical switch and having an open state and a closed state, electronic switch operating means to change the state of said electronic switch, initiating means operable to initiate a change of state of said mechanical switch, control means operable between said initiating means and said operating means to control the operation of said operating means by said initiating means, sensing means responsive to a predetermined stat of said power supply . and operable upon said control means upon detection of said predetermined state to enable said operating means and ensure passage of said contacts through said arcing zone with said electronic switch in a closed state.

2. A hybrid a.c. power switch according to claim 1 wherein said control means includes delay means to delay enablement of said electronic switch operating means relative to said mechanical switch operating means upon said mechanical switch changing from an open state to a closed state.

3. A hybrid a.c. power switch according to claim 2 wherein said delay means are inoperable upon said mechanical switch changing from a closed to an open state.

4. A hybrid a.c. power switch according to claim 1, 2 or 3 wherein said electronic switch has a unidirectional current flow characteristic.

5. A hybrid a.c. power switch according to any one of claims 1 to 4 wherein said electronic switch operating means includes a pulse generator to provide a limited duration pulse to said electronic switch to change said switch from an open to a closed state.

6. A hybrid a.c. power switch according to any one of claims 1 to 5 wherein said sensing means includes a zero crossing detector to provide a signal to said control means upon the voltage of said power supply crossing zero.

7. A hybrid a.c. power switch according to claim 6 wherein said control means includes a flip flop and the signal of said zero crossing detector provides a clocking signal to said flip flop.

8. A hybrid a.c. power switch according to any one of claims 1 to 7 including inhibiting means to inhibit operation of said operating means upon failure of said electronic switch.

9. . A hybrid a.c. power switch according to claim 8 wherein said inhibiting means is responsive to a change of voltage across said mechanical switch upon change of said switch from said open state.

10. A hybrid a.c. power switch according to claim 9 wherein said control means includes a flip flop and said inhibiting means resets flip flop to inhibit said operating means.