Hybrid switching device employing liquid metal contact

A hybrid electrical current switching device comprises a triggerable solid-state current switch connected in parallel with a mechanical switch in which the current flow depends on the relative positioning of a liquid metal conducting medium. The solid-state switching device operates as a crowbar switch to mitigate effects of arcing.

BACKGROUND OF THE INVENTION 
The present invention relates to hybrid switching devices employing 
solid-state crowbar protection circuitry and, more particularly, to such 
hybrid electrical switches in which a mechanical current switching element 
employs one or more liquid metal contacts. 
Electromechanical switches employed in the interruption and initiation of 
electrical current flow paths generally have certain problems associated 
with them. When such current-carrying switches are removed from their 
closed to their open positions, inherent inductive effects amost always 
result in some arcing between the electrical contacts. This arcing can 
erode and degrade the contacts so that, eventually, the electromechanical 
switch no longer operates in an acceptable fashion. The principle function 
of these separate electromechanical contacts is to withstand circuit 
voltages when the contacts are separated and to assure low contactor 
impedance when they are closed. The contact resistance of the separable 
contacts is a critical variable applicable to the closed position. 
Stability of this contact resistance is often affected by chemical 
reaction between the contacts and the ambient atmosphere, espcially during 
arcing. Furthermore, for composite contact materials, segregation of 
materials near the contact surface with repeated melting and 
solidification caused by arcing also adversely affects the contact 
resistance. Moreover, the electromechanical device must be designed so 
that significant mechanical pressure is applied to the contacts in the 
closed position to provide the low contact resistance desired. For proper 
operation, the mechanism must maintain enough contact force to provide 
microdeformations in the contacts to increase the effective area of the 
contact. In general, the contact resistance is proportional to the 
resistivity of the material employed, directly proportional to the square 
root of the hardness of the material and inversely proportional to the 
square root of the contact pressure. Accordingly, it is seen that contact 
material properties such as hardness and resistivity play a significant 
role in determining contact resistance in electromechanical switches. 
Furthermore, it is also highly desirable that such materials exhibit low 
oxidation rates since switches are often exposed to atmospheric 
conditions. If oxidation of the contact material does occur to any 
significant degree, it is furthermore necessary that any of the oxidation 
products not significantly interfere with the functioning of the contacts. 
These criteria have generally limited the viable contact materials to such 
substances as silver and gold, alloys thereof, and/or materials employing 
significant amounts of these expensive materials. 
In order to mitigate arcing effects in particular, many switching devices 
have been proposed in which electromechanical contacts are connected in 
parallel with a solid-state switching device, such as a thyristor or 
silicon-controlled rectifier. The solid-state switching element is used to 
divert or "crowbar" the current away from the contacts either just before 
or just after contact separation, thereby reducing arcing and easing the 
requirements of the electromechanical device and its contact materials. 
Crowbar circuits are described by Horowitz and Hill in their text "The Art 
of Electronics", Cambridge University Press, 1980, on pages 176-177 
thereof. The solid-state switching element is also used to initiate 
current flow through the switching device just prior to closing the solid 
contacts, thereby lowering the device voltage and reducing the probability 
of arcing between the contacts. However, studies conducted by others have 
indicated that even with the use of additional crowbar circuitry, solid 
contacts for hybrid circuit switching devices are still likely to involve 
silver-base materials, at least as an overlay material on another base 
material such as copper. This conclusion is based primarly on the ability 
of silver to provide and maintain low contact resistance due to its low 
electrical resistivity, medium to low hardness, and its relative inertness 
to surface-contaminating chemical reactions. 
A large number of individuals skilled in the art of designing electrical 
circuit switching devices have proposed the use of solid-state circuitry 
in parallel with electro-mechanical contacts to reduce arcing on these 
contacts when opening or closing under load or fault conditions. The 
following list of patents all appear to disclose such crowbar circuitry in 
conjunction with electromechanical switching devices. However, all of the 
patents listed below further appear to involve the use of solid metal 
contacts employing an essentially standard design. This list includes the 
following patents: U.S. Pat. No. 2,058,808, issued Nov. 1, 1960 to W. 
Miller--"Electrical Arc Suppressor"; U.S. Pat. No. 3,237,030, issued Feb. 
22, 1966 to R. J. Coburn--"Radio Noise-Free Switch"; U.S. Pat. No. 
3,321,668, issued May 23, 1967 to E. S. Baker--"Current Control 
Apparatus"; U.S. Pat. No. 3,330,992, issued July 11, 1967 to A. R. 
Perrins--"Electric Switch"; U.S. Pat. No. 3,339,110, issued Aug. 29, 1967 
to J. P. Jones--"Relay Circuits"; U.S. Pat. No. 3,389,301, issued June 18, 
1968 to E. I. Siwko--"Arc Suppressing Circuit"; U.S. Pat. No. 3,395,316, 
issued June 30, 1968 to P. A. Denes et al.--"Electrical Switch With 
Contact Protector"; U.S. Pat. No. 3,402,302, issed Sept. 17, 1968 to E. J. 
Coburn--"Radio Noise-Free Switch"; U.S. Pat. No. 3,466,503, issued Sept. 
9, 1969 to L. F. Goldberg--"Assisted Arc AC Circuit Interruption"; U.S. 
Pat. No. 3,474,293, issued Oct. 21, 1969 to E. I. Siwko et al.--"Arc 
Suppressing Circuits"; U.S. Pat. No. 3,504,233, issued Mar. 31, 1970 to E. 
L. Hurtle--"Electric Circuit Interrupting Device With Solid State Shorting 
Means"; U.S. Pat. No. 3,539,775, issued Nov. 10, 1970 to C. F. 
Casson--"Double-Make Contact Switching Apparatus With Improved AC Arc 
Suppression Means"; U.S. Pat. No. 3,555,353, issued Jan. 12, 1971 to C. F. 
Casson--"Means Effecting Relay Contact Arc Suppression in Relay Controlled 
Alternating Load Circuits"; U.S. Pat. No. 3,558,910, issued Jan. 26, 1971 
to R. G. Dale et al.--"Relay Circuits Employing a Triac to Prevent 
Arcing"; U.S. Pat. No. 3,588,605, issued June 28, 1971 to C. F. 
Casson--"Alternating Current Switching Apparatus With Improved Electrical 
Contact Protection"; U.S. Pat. No. 3,614,464, issued Oct. 19, 1971 to W. 
V. Chumakov--"Arcless Tap- or Source-Switching Apparatus Using 
Series-Connected Semiconductors"; --U.S. Pat. No. 3,633,069, issued Jan. 
4, 1972 to G. Bernard--"Alternating Current Circuit-Interrupting System 
Comprising a Rectifier Shunting Path"; --U.S. Pat. No. 3,639,808, issed 
Feb. 1, 1972 to G. R. Ritzow, "Relay Contact Protecting Circuit"; U.S. 
Pat. No. 3,783,305, issued Jan 1, 1974 to P. Lefferts--"Arc Elimination 
Circuit"; U.S. Pat. No. 3,982,137, issued Sept. 21, 1976 to J. K. 
Penrod--"Arc Suppressor Circuit"; U.S. Pat. No. 4,025,820, issued May 24, 
1977 to J. K. Penrod--"Contactor Device Including Arc Suppression Means"; 
U.S. Pat. No. 4,074,333, issued Feb. 14, 1978 to K. Kurakami et al.--"AC 
Relay System"; U.S. Pat. No. 4,152,634, issued May 1, 1979 to J. K. 
Penrod--"Power Contactor and Control Circuit"; U.S. Pat. No. 4,068,273, 
issued Jan. 10, 1978 to A. Metzler--"Hybrid Power Switch". 
However, it is apparent that hybrid switching devices must, of necessity, 
incur an added cost associated with the circuitry for performing the 
crowbar function. Accordingly, for greater cost competitiveness with 
conventional electromechanical switches, it is highly desirable that the 
cost of the solid-state crowbar circuitry be compensated by changes in the 
design of the electromechanical portion of the hybrid switching device. In 
particular, two of the large cost elements associated with most low 
voltage (less than 1,500 volts) switches are the contact material and the 
driving mechanism. The contact material is expensive because it preferably 
employs a noble metal such as silver or gold. The drive mechanism also 
tends to be expensive in that it requires mechanical devices for holding 
the switch contacts in a forcibly closed position with sufficient pressure 
to cause microdeformations and yet, in the next instant of time, to 
quickly separate the contacts. 
In sum, it is seen that electromechanical switching devices generally 
require the use of relatively expensive contact material. Furthermore, it 
is seen that even in situations employing crowbar circuitry to mitigate 
arcing effects, expensive contact material is also generally required. 
Furthermore, it is seen that the cost of the crowbar circuitry has in the 
past added significantly to the cost of hybrid switching circuit devices 
without concomitant savings associated with the electromechanical portion 
of the switch. It is further seen that while liquid metal contact 
switching devices have been employed in the past, they have generally not 
been employed in circuits in which high arcing currents are a 
consideration. This is generally the result of vapor pressure problems 
associated with liquid metal contact devices. In liquid metal switches 
which are opened to the atmosphere, arcing can rapidly contribute to 
vaporization of the liquid metal. In the case that the liquid metal is 
mercury, it is generally appreciated that the escape of mercury vapor to 
the surrounding atmospheric environment would generally be detrimental. In 
the situation in which the liquid metal is contained within a sealed 
environment, normal arcing in the switch results in the build up of 
significant vapor pressures from the volatilized liquid metal. Containment 
of these high vapor pressures is a significant design challenge, solved 
only at added cost to the device. Accordingly, for these reasons it is 
seen, particularly from the list of patents cited above, that the use of 
liquid metals in switches carrying high levels of arcing current has not 
been employed. 
SUMMARY OF THE INVENTION 
In accordance with a preferred embodiment of the present invention, a 
hybrid electrical current switching device comprises a triggerable 
solid-state current switching device connected in parallel with a 
mechanical current switching device in which current flow depends on the 
relative positioning of a liquid metal conducting medium. In a preferred 
embodiment of the present invention, the liquid metal conducting medium is 
disposed within a sealed housing. The present invention also preferably 
includes triggering means which operate to trigger the solid-state device 
into a low resistance state at approximately the same time that the 
mechanical switch is moved to an open position. Furthermore, the 
triggering means also preferably operates to switch the solid-state device 
into a low resistance state immediately prior to closing of the mechanical 
switch. 
In the hybrid switch described above, there is no means required for 
holding the mechanical switch contacts together under pressure. 
Furthermore, there is no need to provide expensive contacts. Additionally, 
the presence of the solid-state current diverting circuitry, that is the 
crowbar circuitry, acts to mitigate the effects of arcing and the 
concomitant problem of pressure buildup within a sealed mechanical switch 
housing. Furthermore, several different forms of liquid metal switching 
elements may be employed, depending upon the load current and speed 
requirements of the switch.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates a preferred embodiment of the present invention 
employing mechanical current switching device 10, 20, 30 or 40 in which 
current flow depends on the relative positioning of a liquid metal 
conducting medium. The switch device may comprise any of the embodiments 
10, 20, 30 or 40 illustrated in FIGS. 2, 3, 4 and 5 and which are more 
particularly described below. Switching device 10, 20, 30 or 40 is 
electrically connected in a parallel configuration with a triggerable 
solid-state current switching device 60, such as back-to-back thyristors 
or silicon-controlled rectifiers. It is this solid-state device which 
provides crowbar protection in the present invention. The trigger 
electrode for the solid-state switch 60 is driven by triggering means 50. 
Triggering means 50 preferably operates in the following manner. When the 
mechanical liquid metal switch is at rest in either the closed and 
conducting or open and nonconducting condition, the semiconductor device 
is in a high impedance (ungated) state. After mechanical switch 10, 20, 
30, 40 has been opened for a relatively long period of time, a gating 
signal is sent to solid-state switch 60 upon initiation of mechanical 
closing of the liquid switch. Furthermore, this gating signal is 
maintained for a period of time sufficient to ensure good electrical 
continuity through the liquid metal within the switch housing. Lastly, 
when mechanical switch 10, 20, 30, 40 is being changed from a closed to an 
open state, a gating signal is sent to solid-state switch 60 at or about 
the instant of arc development so as to switch the semiconductor device 
into a low impedance state. In accordance with one embodiment of the 
present invention, a gating signal may be sent to semiconductor device 60 
upon the development of a voltage of greater than about 5 volts across the 
liquid metal switch. This is the approximate voltage developed during the 
early stages of arcing. In accordance with another embodiment of the 
present invention, initiation of a gating signal to trigger a low 
impedance condition may be made to depend upon a joint condition, such as 
signals indicating that the liquid metal switch is in the open condition 
and there is a detectable current flowing through the liquid metal switch. 
Still another method would initiate the gating signal after a fixed and 
predetermined delay from the signal to activate the mechanical switch 10, 
20, 30, 40. The selection of the most appropriate logic for initiation of 
the gating signal depends, in part, on the specific electromechanical 
characteristics of switch 10, 20, 30, 40. Furthermore, the embodiment 
illustrated in FIG. 1 illustrates the use of a bipolar semiconductor 
device 60. However, monopolar solid-state switches may be employed. 
Triggering means 50 are conventionally employed in the electrical 
interruption arts, particularly in those situations employing crowbar-type 
protection circuitry. Such triggering means can be found in several of the 
circuits described in the patents listed above. 
An important aspect of the present invention is the construction of a 
hybrid switching device in which a triggerable solid-state switch is 
disposed in parallel across an electromechanical switch in which the 
electrical connection is made through a liquid metal conductor, such as 
mercury. While mercury is the preferable liquid metal employed in the 
present invention, other related materials such as alkali metals having 
similar properties may be substituted for mercury. In such liquid metal 
switches, electrical connection is made through a liquid metal conductor 
in one of several methods. For example, in FIG. 2, connection is made by 
inserting a solid contact into a liquid pool. In FIG. 3, contact is made 
by flowing liquid metal up to a solid metal contact. In FIG. 4, circuit 
formation is accomplished by completing electrical continuity through an 
aperture connecting two liquid metal pools. In FIG. 5, circuit formation 
is accomplished by rotating the housing to lower a terminal contact into a 
liquid metal pool. 
In FIG. 2, liquid metal switch 10 comprises conductive housing 14 forming 
one of the two switch contacts. Liquid metal 12 is disposed within housing 
14 and contact is made by slidably moving a second solid metal electrode 
contact 16 through insulating plug 18. FIG. 2A shows switch 10 in a closed 
position and FIG. 2B illustrates the same switch in an open position. The 
motion of the solid contact may be affected either through a sliding 
motion or through a bellows action. The action of switch 10 is rapid and 
under direct control. Furthermore, housing 14 is preferably sealed by plug 
18 to prevent the escape of volatized liquid metal vapor to the 
atmosphere, particularly under conditions of relatively high internal 
pressures. 
A second form of liquid metal switch is illustrated in FIGS. 3A and 3B. 
Here, switch 20 comprises a housing having electrically conductive portion 
24 and electrically insulating portion 28 electrically separating 
conductive end plate 26 from conductive housing portion 24. Liquid metal 
12 is disposed within the housing. End plate 26 and housing portion 24 
form two stationary solid electrodes which, in the closed position, are 
bridged by a liquid metal column in one position. However, reorientation 
of the container to a position such as that shown in FIG. 3B causes liquid 
metal 12 to flow away from end plate 26 and thus breaks the electrical 
circuit. Because there are no mechanical feedthroughs in this embodiment, 
such a container is easily sealed, thus restricting the environment to 
which the contact surfaces are exposed and preventing the release of 
internally generated, vaporous material. Normal arcing occurring in such a 
switch can result in significant pressure buildup under high arc current 
conditions. However, crowbar protection circuitry diverts the arcing 
current rapidly enough so that pressure containment is not a significant 
problem. Since switch 20 is opened and closed by a simple rotation of the 
housing, mechanism requirements are minimal. However, since the liquid 
flow contact separation is effected by gravity, it is limited in speed and 
may have greater than usual shot-to-shot timing variations. 
FIG. 4 illustrates the construction and operation of liquid metal switch 30 
comprising solid end electrodes 34 and 36 which, together with annular 
insulating portion 31, define a housing holding liquid metal 12 as a 
conducting contact medium. Within the thus-defined housing, there is 
disposed rotatable insulating disk 32 having an eccentrically-positioned 
flow channel 33 defined therein. In the closed position illustrated in 
FIGS. 4A and 4B, disk 32 is positioned with channel 33 in a lower position 
so that channel 33 completes an electrical circuit between two liquid 
metal pools which are in contact with end electrodes 34 and 36, thus 
completing the circuit. As disk 32 is rotated so that channel 33 is above 
the level of at least one of the liquid metal pools, electrical contact is 
broken. This latter, switch-open situation is illustrated in FIG. 4C. 
Accordingly, switch 30 exhibits many of the features exhibited by switch 
20. However, because of the presence of disk 32 which operates as an 
additional insulating barrier between the liquid metal pools, switch 30 
exhibits greater hold-off voltages than switch 20, under similar 
dimensional constraints. 
FIG. 5 illustrates the construction of liquid metal switch 40 including an 
insulating housing comprising portions 42 and 48. Housing portions 42 and 
48 are rotatable together in a manner similar to switch 20 in FIG. 4A. The 
housing contains a liquid metal pool 12 such as mercury, together with an 
atmosphere 43, preferably comprising air, argon, or mixtures thereof. Such 
an atmosphere may be employed not only in the switch shown in FIG. 5, but 
also in the other liquid metal switches shown in FIGS. 2-3. Terminal 
contact 45 is disposed through housing portion 48. A second terminal 
contact 46 is disposed through housing portion 42, in the manner shown. 
When switch 40 is in the closed position, as is illustrated in FIG. 5, 
current flows through conductive contacts 46, liquid metal pool 12 and 
terminal contact 45. Additionally, in the embodiment shown, contact 45 is 
electrically connected to terminal plate 44 by any conventional means, 
such as by the nut and washer illustrated. Thus, the mechanical stress of 
rapid rotation of the housing is born by metallic members 44 and 46. In a 
fashion similar to the switch shown in FIG. 4, terminal contact 45 is 
mounted eccentrically with respect to the center of rotation of the 
housing. Accordingly, as the housing is rotated, contact 45 is removed 
from liquid metal pool 12, thus breaking the electrical connection. 
From the above it may be appreciated that the present invention provides a 
heretofore unemployed form of hybrid switch. In particular, it is seen 
that the hybrid switch of the present invention is economical in that 
conventionally required electromechanical switching mechanisms are not 
required and because the present invention does not require the use of 
expensive electrode contact materials employing precious metals. It is 
further seen that the hybrid switch of the present invention significantly 
mitigates the effect of arcing and its concomitant effects upon pressure 
buildup in liquid metal switch housings. 
While the invention has been described in detail herein in accord with 
certain preferred embodiments thereof, many changes and modifications 
therein may be effected by those skilled in the art. Accordingly, it is 
intended by the appended claims to cover all such modifications and 
changes as fall within the true spirit and scope of the invention.