US3588605A - Alternating current switching apparatus with improved electrical contact protection and alternating current load circuits embodying same - Google Patents

Abstract

A SWITCHING APPARATUS FOR AN ALTERNATING LOAD CIRCUIT IS PROVIDED WITH A BIDIRECTIONAL OR BILATERAL SEMICONDUCTOR TRIODE DEVICE OF THE THYRISTOR TYPE CONNECTED FOR CONDUCTION IN SHUNT WITH THE CONTACTS OF AN ELECTROMECHANICAL SWITCH FOR ARC SUPPRESSION, AND HAS A GATE CIRCUIT COMPRISED IN WHOLE OR IN PART BY REACTIVE PASSIVE ELEMENTS WHICH EXHIBIT IMPEDANCE CHARACTERISTICS TO RENDER THE SOLID STATE TRIODE DEVICE CONDUCTIVE IN RESPONSE TO ARCING CONDITIONS AT THE SWITCH CONTACTS BUT WHICH PRECLUDE SUCH A RESPONSE DUE TO CIRCUIT TRANSIENTS. THE SOLID STATE TRIODE DEVICE IS ALSO SUBSTANTIALLY INSTANTANEOUSLY RESPONSIVE TO RELATIVELY LARGE DV/DT CHARACTERISTICS OR EXTREME EXCESSES (EXCEEDING THE BREAKOVER VOLTAGE OF SOLID STATE DEVICE) OF ARCING VOLTAGE APPEARING ACROSS THE SWITCH CONTACTS.

Description

United States Patent [72] Inventor Charles F. Casson Winslow, Ind. [21] Appl. No. 766,504 [22] Filed Oct. 10, 1968 [45] Patented June 28, 1971 [73] Assignee AMF Incorporated [541 ALTERNATING CURRENT SWITCHING APPARATUS WITH IMPROVED ELECTRICAL CONTACT PROTECTION AND ALTERNATING CURRENT LOAD CIRCUITS EMBODYING SAME 8 Claims, 5 Drawing Figs.

[52] U.S.C1 317/11, 307/305. 317/33 [51] Int. Cl 1102b 7/22 [50] FieldotSearch 317/11,33, (lnquired); 307/252 (Inquired, 305 (A), 305

[56] References Cited UNITED STATES PATENTS 3,389,301 6/1968 Siwko 317/11 P IC' Primary Examiner.lames D. Trammell AttorneysGeorge W. Price and Charles J. Worth ABSTRACT: A switching apparatus for an alternating load circuit is provided with a bidirectional or bilateral semiconductor triode device of the thyristor type connected for conduction in shunt with the contacts of an electromechanical switch for are suppression, and has a gate circuit comprised in whole or in part by reactive passive elements which exhibit impedance characteristics to render the solid state triode device conductive in response to arcing conditions at the switch contacts but which preclude such a response due to circuit transients. The solid state triode device is also substantially instantaneously responsive to relatively large dv/dt characteristics or extreme excesses (exceeding the breakover voltage of solid state device) of arcing voltage appearing across the switch contacts.

LOAD LC ALTERNATING CURRENTSWITCI-IING APPARATUS WITH-I IMPROVED ELECTRICAL CONTACT PROTECTION AND ALTERNATING CURRENT LOAD CIRCUITS EMBODYING SAME This invention relates to electrical switching means for selectively coupling a load to an alternating current source, and more particularly to such switching means including arcsuppression means effecting improved contact life and current ratings for such switching means and load circuits embodying same.

Mechanical switches are presently made which are capable of conducting relatively high continuous currents, but they are severely limited in their ability to survive the efiects of breaking such currents. This limitation is particularly severe at voltages above the characteristic arc voltage of any specific contact material used for contacts. To increase contact life under such adverse conditions has usually involved increasing the mass of the contacts to absorb the are energy and making the contacts of a precious metal.

It is well ,known that when contacts separate during intended normal switching, harmful arcing usually occurs therebetween. Also, after contacts first engage during switch closing, they may repeatedly bounce (commonly called chatter) which is attended by arcing until final engagement is attained. Such arcing accelerates contact consumption by pitting and burning which contributes to relatively short contact life and reduces the reliability of the overall circuit.

Heretofore, attempts to reduce harmful contact arcing have involved mechanical means for reducing the amount of contact bounce such as biasing springs, cam systems, or the like, which increases switch size and costs, and introduce reliability problems into the overall switch circuitry. In recent years, various solid state devices such as transistors, controlled rectifiers, AC switches and the like, have been used in an attempt to provide more durable switches and to increase circuit reliability. While some solid state switch devices are capable of switching currents on the order of or more times greater than they can continuously conduct and can accomplish such switching without the arcing associated with mechanical switches, such devices have certain limitations which make them less than completely satisfactory for switching and conducting of high currents. Some of these limitations are relatively low continuous current conducting capability, susceptibility to damage by sustained current and voltage transients, and the need for complex associated circuitry such as biasing networks, triggering networks, and the like.

Accordingly, an object of this invention is to provide an electrical switching apparatus having improved switch contact protection, which obviates problems heretofore associated with attempts to increase switch contact life.

Another object is to provide a switching apparatus of the type described wherein substantially all arcing resulting from contact disengagement is eliminated, thereby. increasing contact life.

Another object of this invention is to provide a new and novel electrical switching means and load circuits for same for increasing switch contact life and current ratings of any given electromechanical switch incorporated therein, which is especially suitable for switching alternating current loads, and new and novel alternating current load circuits embodying such switching means.

Another object of the invention is to provide a new and novel means for sensing the presence of an arc to be suppressed and effecting suppression of said arc in a switch apparatus of the single-pole, single-throw type or the like.

Another object is to provide a switching apparatus of the type described which is of rugged and uncomplicated construction, is relatively economical to manufacture, and provides increased reliability of the overall switch circuitry.

A further object is to devise such a switching apparatus employing a solid state device and in which no external triggering circuit is needed.

Another object is to provide a switching unit of the type described for connecting an alternating current power source to a load, wherein the magnitude of currents switched is greater than that possible for contacts alone, and wherein the magnitude of currents continuously conducted is greater than that possible for solid state switch devices alone. A related object is to provide such a switching unit which uses a minimum mass of contact material.

Still another object of this invention is to provide new and novel switching apparatus including both solid state and electromechanical switching devices and new and novel passive gate circuitry for said solid state switching devices and new and novel alternating current load circuits embodying such apparatus.

Yet another object of this invention is to provide new and novel switching apparatus including both solid state and electromechanical switching devices and new and novel passive gate circuitry for said solid state switching devices and new and novel alternating current load circuits embodying such apparatus; and wherein said gate circuitry includes at least one reactive, passive, impedance element.

The foregoing and other objects and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawings wherein several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustration purposes only and are not to be construed as defining the limits of the invention.

FIG. I is a schematic diagram of one embodiment of the switching apparatus of the invention and an alternating current load circuit embodying same;

FIG. 2 is a schematic diagram of a second embodiment of the switching apparatus of the invention and an alternating current load circuit embodying same,

FIG. 3 is a schematic diagram of a modification of the embodiment of FIG. 2;

FIG. 4 is a schematic diagram of another modification of the embodiment of FIG. 2; and

FIG. 5 is a schematic diagram of a third embodiment of the switching apparatus of the invention and an alternating current load circuit embodying same, said switching apparatus including a gated monostable solid state device and a singlepole, double-throw switch in selective cooperation with the controlled current path and gate circuit of said solid device. BasicaLly, this invention contemplates the protection of electrical switch contacts in simple make/break switch configurations such as single-pole, single-throw types, by the use of monostable, quiescently nonconductive, bilateral semiconductor devices of triode configuration and having associated passive impedance gate circuitry, said semiconductor device or devices and said gate circuitry thereof being connected such that the said bilateral semiconductor device is rendered conductive substantially instantaneously upon the occurrence of an arcing condition at the switch. The bilateral semiconductor device is connected in shunt with the switch terminals to suppress or quench arcing when rendered conductive. Such arcing occurs during periods of contact bounce or break in the switch. Therefore, when the contacts disengage during normal intended disengagement or to contact bounce after engagement, the bilateral semiconductor device is triggered substantially instantaneously to the conductive state by the voltage conditions at the contacts. The current in the circuit following the disengagement of the contacts, is conducted through the triode connected bilateral semiconductor device (which operates as a bidirectional controlled rectifier) until the contacts reengage or the normal cycle of current being conducted drops below the holding current required to maintain the bilateral semiconductor device in its conducting state. With the electrical switching means or apparatus of this invention, continuous conduction is provided by the contacts, and conduction during switching disengagement and bounce is provided by the semiconductor device. This permits a combination of high-current capability, for both switching an continuous conducting, heretofore not available.

the switch S and one side of the bilateral semiconcuctor triode D, at a common terminal 14, and the second power lead P2 is connected to one side of a load L. The other side of the load L is connected through a third lead P3 to the other side of the bilateral semiconductor triode D and the other side of the switch S, the latter connection comprising a second common circuit terminal 16 on the third lead P3.

The switch S includes first and second contacts 18 and 20, respectively, the first contact 18 being connected with the first common circuit terminal 14 and the second contact 20 being connected with the second common circuit terminal I6.

The bilateral thyristor triode D is a gate and/or breakdown actuated semiconductor which has first and second power electrodes 22 and 24 connected, respectively, at the first and second common circuit terminals 14 and 16, and a gate electrode 26, the latter being connected through the capacitance C to the second power electrode 24 (and second common terminal 16).

As embodied in the circuit shown, the bilateral semiconductor triode D provides a shunt path 14-22-24-16-13, which includes the power conducting path of the triode, around the switch S when the triode is rendered conductive, for a purpose to be hereinafter more fully described.

The bilateral semiconductor triode D is a solid state switching device which will conduct current through its power electrodes 22 and 24 in either direction and can be rendered conductive (turned ON) by a small gate signal applied to the gate electrode 26 or by the voltage across the said power electrodes 22-24 exceeding the rated breakover voltage in either direction of conduction. Once rendered conductive, the bilateral semiconductor triode D will remain in the conductive state until the current flow through the power electrodes 22-24 thereof drops below a minimum holding value. In AC application, such a cutoff condition is effected on every halfcycle when the current reverses or if a shunt is placed across the power electrodes 22-24.

Although triggering of such devices as the bilateral semiconductor diode D can be effected by the application, across its power electrodes 22-24, of a voltage having a magnitude greater than its rated breakover voltage, which can be considered as a relatively static effect, triggering may actually be effected by other phenomena, either alone or in combination. An additional triggering effect can be provided when a voltage having a high time rate of change, i.e., high dv/dt characteristic or a steep wavefront, is applied across the power electrodes 22-24.

In the present application, in the embodiment of FIG. 1 and in the other embodiments to be hereinafter described, the response, for practical purposes, of the bilateral semiconductor triode D and its equivalent in the other embodiments, will be to either agate signal applied to the gate electrode 26 or a steep wavefront voltage applied across the power electrodes 22-24. The gate electrode 26 of the bilateral semiconductor triode D does not effect any cutoff function and is only utilized as a means for effecting the conductive state in the said triode at a desired point in time.

The time required to gate or actuate the bilateral thyristor triode D to the ON state is relatively short, i.e., on the order of a few microseconds. Such actuation can be accomplished by an applied voltage of either positive or negative polarity at the gate electrode 26 or across the power electrodes 22-24 as described above.

For the purpose of protecting mechanical switch contacts, the bilateral thyristor triode D acts with such a high relative rate of speed as compared to the contacts 18 and 20 of the switch S as to be substantially instantaneously responsive devices for all practical purposes.

The mechanical switch S, bilateral thyristor triode D, capacitance C (or the other additional gate impedances of the following embodiments) can be included in an integral structure or modular unit switching means 10 with only two external connections, namely, the common terminals 14 and 16. Then, by merely placing the common terminals 14 and 16 in series with an AC source 12 and a load L, the switching means 10 is fully operable to control the energization of the load L in a self-protected, simple switching configuration.

Referring now to FIG. 2, wherein like elements to the embodiment of FIG. I bear like legends with the suffix A, a switching means 10A and the load circuit in which it is embodied are shown as being identical to the embodiment of FIG. 1, with the exception that a second capacitance C2 is connected from the gate electrode 26A of the bilateral thyristor triode DA to the first power electrode 22A thereof.

Referring next to FIG. 3, like elements to the embodiments of FIGS. l and 2 bear like legends with the suffix B. The switching means 108 and the load circuit in which it is embodied are shown as being identical to the embodiment of FIG. 2, with the exception that a resistance R is connected from the gate electrode 268 of the bilateral thyristor triode DB to the first power electrode 248 thereof, instead of the capacitance CA2, and the position of the first and second common circuit terminals 143 and 16B and the power electrodes 22B and 248 have been reversed in the circuit. In FIG. 4, like elements to the embodiments of FIGS. 1, 2 and 3 bear like legends with the suffix C. The switching means 10C and the load circuit in which it is embodied are shown as being identical to the embodiment of FIG. 3, with the exception that an inductive impedance Lll is connected from the gate electrode 26C to the first power electrode 22C instead of the resistance R.

Referring now to FIG. 5, wherein like elements to the embodiments of FIGS. 1 to 4 bear like legends with the suffix D, a switching means 10D and the load circuit in which it is embodied are shown as being generally similar to all of the previous embodiments. In the embodiment of FIG. 5, however, the gate electrode 26D is connected to the second power electrode 24D of the bilateral thyristor DD through a RC impedance comprising a capacitance C3 and resistance R2 is series. Further, the switch SD is provided with a back-contact 28, cooperation with the moving contact 18D thereof. This back-contact 28 is connected through a shunt lead PS directly to the gate electrode 26D. Also the positions of the first and second power electrodes 22D and 24D and the first and second common terminals 14D and 16D are shown in the reversed configuration already described for FIGS. 3 and 4.

The purpose and function of all of the various interconnections and of the passive impedance elements associated with the gate electrodes 26, 26A, 26, 26C and 26D of all of the foregoing embodiments will now be described with reference to the operation of the several embodiments of the invention.

OPERATION Referring to the embodiment of FIG. 1, inoperation of switch means 10, assuming that the movable or operable contact 18 is engaged with the fixed contact 20 of the switch S and the load L is energized by the AC source 12 through the path Pl-l4-8-20-16-P3, the bilateral thyristor triode D is shunted through the path 14-18-20-16 and is in a nonconductive, quiescent (OFF") state.

Now, if the switch contacts 18 and 20 of the switch S are caused to break and an arc is formed therebetween, the high transient voltage of the arc, having high dv/dt characteristics, is applied across the power electrodes 22-24 of the bilateral thyristor triode D as well as across the impedance path 22-26-C24. The capacitance C is chosen with a value that is large enough to permit the high frequency arcing currents to pass therethrough, either into or out of the gate electrode 26,

depending upon the polarity of the applied waveform when the break occurs between the contacts 18 and 20 of the switch S. On the other hand, the capacitance C is small enough so as to preclude flow of gating current into or out of the gate electrode 26 in an amount sufiicient to render the bilateral thyristor triode conductive when the switch S is in the break condition and no arcing voltage is present, i.e., under normal circuit voltage and circuit transient conditions.

Thus, the bilateral thyristor triode D will respond substantially instantaneously to either or both the arcing voltage across its power electrodes or gate current through the capacitance C to effect a shunt path around the switch S for the remainder of the present half-cycle of the applied alternating current waveform and substantially instantaneously suppresses or extinguish es the arcing condition at the contacts 18 and 20 of the switchS. As the half-cycle of applied current waveform terminates (approaches zero crossover), the current through the power electrodes 22 and 24 of the bilateral thyristor triode D drops below the required minimum holding value and the triode D is rendered nonconductive (returns to the OFF state) dee'nergizing the load L.

Because of the substantially instantaneous provision of a shunt path to suppress or quench arcing at the contacts 18 and 20 of the switch S, similar arc suppression and quenching are effected during making or bounce conditions of the switch contacts 18 and 20. Thus, increased contact life, increased reliability and higher current ratings are effected in the switching means than would be possible for the switch S alone.

Since a half-cycle of a 60-cycle. AC wave consumes approximately 8.3 milliseconds, the bilateral thyristor triode D may be rendered conductive for a time period in the range of something greater than 0 seconds to 8.3 milliseconds, depending upon the point in the current wave at which the bilateral thyristor triode Dis rendered conductive. The relative time period of particular concern, when the contacts of the switch S are to be brought into engagement from the open position. is the bounce time which is measured from the moments such contacts commence to reopen after initial engagement until all contact bounce or chatter has ceased, the bounce time sometimes consuming several milliseconds.

The are voltage appearing across the contacts of the switch S has a greater magnitude and/or higher dv/dt characteristic than does the AC wave from the source 12, resulting in the initiation of the arc suppression function in the switching means 10 as previously described herein. Thus, in the case of contact making or contact bounce, the bilateral thyristor triode D is turned off by the shunting action of the switch S through the shunt path 14-1'8-20-16 when the contacts 18 and 20 achieve a fully closed, stable relationship.

The foregoing description of operation is generally applicable to all of the embodiments of the present invention and the operating and response characteristics of the bilateral semiconductor (thyristor) triodes D, DA, DB, DC and DD are identical. The differences in operation of the following embodiments reside in the passive gate impedance networks and/or the switching mode involving the gate electrode 26D in FIG. 5.

Referring first to FIG. 2, the capacitance C2 is in parallel with the impedance that 26A-26A between the first power electrode 22A and the gate electrode 26A of the bilateral thyristor triode DA. In this configuration, the arc voltage caused by break or bounce of the contacts 18 and 20 is divided across the capacitances C1 and C2. By selection of the proper relative values of capacitance for CA and C2, the capacitance of CA can be large enough to effect gating of the bilateral thyristor triode DA when the switch S is opened under arcing conditions, but small enough so that it will not effect gating thereof on succeeding half-cycles with S1 already in the open condition. Further, the capacitance of C2 will be large enough to allow the bilateral thyristor triode DA to shunt off under conditions, for example, when the load LA is inductive and to provide transient protection for the bilateral thyristor triode DA to preclude spurious triggering thereof.

Referring next to FIG. 3, the requirements for the value of the capacitance CD are those previously described for the embodiments of FIGS. 1 and 2. As for the resistance R, the resistance value is made large enough to effect firing or triggering of the bilateral thyristor triode DB during arcing conditions at the contacts 188 and 20B of the switch SB. The resistance value of the resistance R is further made small enough such that the bilateral thyristor triode DB will not respond to normal circuit voltages and transients, thereby precluding spurious firing or triggering when the switch SB is in the break (open) condition.

Referring now to FIG. 4, the requirements for the value of the capacitance CC are those previously described for the embodiments of FIGS. 1, 2 and 3. As for the inductance L1, the value of inductance is selected such that, in conjunction with the capacitance CC the bilateral thyristor triode DC will be triggered in response to arcing conditions at the contacts 18C and 20C of the switch SC. Thisinductance value, however, is also within such a range as to preclude spurious triggering of the bilateral thyristor triode DC due to normal circuit voltages and transients when the switch SC is in the break (open) condition.

In the embodiments of FIGS. 2, 3 and 4 to use with 60-cycle power, the respective triodes DA, DB and DC each may be a General Electric TRIAC SC46B bilateral thyristor triode or the equivalent thereof, and the relative for each of the capacitance CA, CB and CC may be 0.10 microfarads. The relative values of capacitance C2 of FIG. 2, resistance R of FIG. 3 and inductance L1 of FIG. 4 may be 1.0 microfarads, 0.5 ohms and I0 millihenrys, respectively.

Referring next to FIG. 5, the operation is substantially identical to that of FIG. 1 during make and bounce conditions at the contacts 18D and 20D of the switch SD. In this embodiment, the resistance-capacitance gate impedance R2-C3 acts in similar manner to the capacitance C of FIG. 1. However, the resistance R2 precludes high-current surges from reaching the gate 26D.

In effecting a break condition of the switch contacts I8D-20D, the movable contact 18D effects the break, when operated, substantially instantaneously triggering the bilateral thyristor triode DD, as previously described with respect to the operation of FIG. I, and then proceeds to engage the back-contact 28 thereby shunting the gate terminal 26D to the first power electrode 22D through the shunt PS by closing the path 26DPS-28-18D14D-22D. This precludes spurious signals from triggering the bilateral thyristor triode DD by precluding any potential difference between the gate terminal 26D and the first power electrode 22D when the switch SD is in the open (back-contact closed) condition with contacts 18D and 28 made.

In the embodiment of FIG. 5 representative values for the circuit parameters of the switching means 10D may be as follows:

Capacitance C3-200 volt, 0.0l microfaradj Resistance R2-l watt, ohm; and

Bilateral Thyristor Triode DD-General Electric TRIAC type SC46B of equivalent.

Thus, in all of the foregoing embodiments, it can be readily seen that the bilateral thyristor triode D, DA, DB, DC or DD actually carries the load current during bounce or break conditions at the mechanical switch S, SA, SB, SC or SD, respectively, until the end of the present half-cycle of the applied alternating current waveform, at which time the said bilateral thyristor triode is rendered nonconductive and breaks the load circuit.

The simplicity and self-contained nature of the switching means 10, 10A, 10B, 10C and 10D is readily apparent in that no external triggering or gating circuitry is needed to effect the highly beneficial arc suppression functions. In all of the foregoing embodiments the modular structure of the switching means 10, 10A, 10B, 10C and 10D is also readily apparent in that a pair of common terminals, external to the module, is all that is necessary to incorporate the switching means in an alternating current load circuit. These common terminal pairs are 14-16, l4A-16A, 1413-168, 14C-16C and l4D-16D in the embodiments of FIGS. 1, 2, 3, 4 and 5, respectively.

It is to be understood that the magnitude and frequency of the power source are not to be construed as limiting the invention, since switching devices of the types described are presently commercially available that will operate effectively with power sources of other magnitudes and over a wide range of frequencies.

Although several embodiments of the invention have been illustrated and described in detail, it is to be expressly understood that the invention is not limited thereto. Various changes may be made in the design and arrangement of the parts without departing from the spirit and scope of the invention as the same will now be understood by those skilled in the art.

I claim:

1. Means substantially instantaneously quenching an arcing condition across the contacts of electric switch means having alternating current directed therethrough, comprising:

normally nonconductive bidirectional semiconductor means having first and second power terminals adapted to be connected in shunt with such a switch means and a gate terminal adapted to receive a gating signal, said bidirectional semiconductor means being rendered substantially instantaneously conductive between said power terminals in response to a gating signal and an arcing condition across said contacts;

' circuit means connected with said gate terminal and adapted to receive the electrical energy of an arcing condition across the said contacts of a said switch means; and

said circuit means including impedance means comprising capacitance means connected in circuit from said gate terminal to one of said power terminals and inductance means connected in circuit from said gate terminal to the other of said power terminals of said bidirectional semiconductor means.

, 2. The invention defined in claim 1, wherein said bidirectional semiconductor means comprises a bilateral gated thyristor.

3. Means substantially instantaneously quenching an arcing condition across the contacts of electric switch means having alternating current directed therethrough, comprising:

normally nonconductive bidirectional semiconductor means having first and second power terminals adapted to be connected in shunt with such a switch means and a gate terminal adapted to receive a gating signal, said bidirectional semiconductor means being rendered substantially instantaneously conductive between said power terminals in response to a gating signal and an arcing condition across said contacts; and

circuit means connected with said gate terminal and adapted to receive the electrical energy of an arcing condition across the said contacts of a said switch means;

said circuit means including impedance means comprising resistance and capacitance means connected in series from said gate terminal to one of said power terminals of said bidirectional semiconductor means; and

said circuit means providing a shunt path from said gate terminal to the other of said power terminals of said bidirectional semiconductor means adapted to be selectively effected by the said switch means when the latter is in an open condition.

4. The invention defined in claim 3, wherein said bidirectional semiconductor means comprises a bilateral gated thyristor.

5. An alternating current load circuit for respectively connecting a load to a alternating current source comprising:

first and second input terminals adapted to be connected with an alternating current source; switch means having a first and second contact means, one

of said contact means being directly connected to one of said input terminals;

bidirectional semiconductor means having first and second power terminals and a gate terminal one of said power terminals being directly connected with said one of said input terminals;

load means connnected from said other of said input terminals to said other of said power terminals and said other of said contact means;

impedance means connected from said gate terminal to at least one of said power terminals and to the side of said load means;

said switch means effecting selective energization of said load means from a source of alternating current when same is applied to said input terminals;

.said bidirectional semiconductor means being substantially instantaneously responsive to the occurrence of an arcing condition at said contact means of said switch means to shunt said switch means through said power terminals and extinguished such an arcing condition; and

said impedance means comprising capacitance means connected in circuit from said gate terminal to one of said power tenninals and inductance means connected in circuit from said gate terminal to the other of said power terminals of said bidirectional semiconductor means.

6. The invention defined in claim 5, wherein said bidirectional semiconductor means comprises a bilateral gated thyristor.

7. An alternating current load circuit for selectively connecting a load to an alternating current source comprising:

first and second input terminals adapted to be connected with an alternating current source;

switch means having a first and second contact means, one

of said contact means being directly connected to one of said input terminals; bidirectional semiconductor mean having first and second power terminals and a gate terminal one of said power terminals being directly connected with said one of said input terminals;

load means connected from said other of said input terminals to said other of said power terminals and said other of said contact means; and

impedance means connected from said gate terminal to at least one of said power terminals and to one side of said load means;

said switch means effecting selective energization of said load means from a source of alternating current when same is applied to said input terminals;

said bidirectional semiconductor means being substantially instantaneously responsive to the occurrence of an arcing condition at said contact means of said switch means to shunt said switch means through said power terminals and extinguish such an arcing condition;

said impedance means comprising resistance and capacitance means connected in series from said gate terminal to one of said power terminals of said bidirectional semiconductor means; and

said circuit means providing a shunt path from said gate terminal to the other of said power terminals of said bidirectional semiconductor means adapted to be selectively effected by the said switch means when the latter is in an open condition.

8. The invention defined in claim 7, wherein said bidirectional semiconductor means comprises a bilateral gated thyristor.

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