US3155790A

US3155790A - Reversing relays having increased cut-off power

Info

Publication number: US3155790A
Application number: US137106A
Authority: US
Inventors: Pierre L A Lemonnier
Original assignee: M T I LE MATERIEL TECH IND
Current assignee: M T I LE MATERIEL TECH IND; M T I LE MATERIEL TECHNIQUE INDUSTRIEL
Priority date: 1960-09-09
Filing date: 1961-09-11
Publication date: 1964-11-03
Anticipated expiration: 1981-11-03
Also published as: DE1258957B; FR78975E; CH372754A; FR1274543A; LU40566A1; GB997014A

Description

3,155,790 REvERsING RELAYS HAVING INCREASED CUT-OFF POWER Filed sept. 11, 1961 Nov. 3, 1964 P. A. LEMONNIER 3 SheetshSheet 1 INVENTOR P/ERRELALEMONN/E? ATTORN Nov. 3, 1964 P. 1 A. LEMoNNn-:R 3,155,790

REvERsING RELAYS HAVING INCREASED cuT-oFF POWER Filed sept. 11, 1961 s sheets-sheet 2 INV ENT OR P/ERRE LA. LEMONN/ER ATTORNEY Nov. 3, 1964 P. l.. A. LEMONNIER 3,155,790

REvERsING RELAYS HAVING INCREASED CUT-oFF POWER 3 Sheets-Sheet 5 Filed Sept. l1. 1961 P/ERRE LA. maw/m BY 4m f ATTORNEY @0 23a sa United States Patent O 3,155,796 REVIERSHNG RELAYS HAVNG ECREASED @UT-@FF PWER Pierre L. A. lLeinonnier, Paris, France, assigner to Milli. Le Materiel Technique industriei, Faris, France, a conn pany of France Filed Sept. iii, 196i, Ser. No. i37,i6 @latins priorit application France, Sept. 9, 119ml, $38,256; dan. i961, 345,595 Ciaims. (Ci. Zitti- 87) The present invention relates to improvements in reversing relays having increased cut-off power.

It is known that, in order to avoid using a flexible connection for joining a terminal to a moving part of a reversing relay, such a relay can be made by providing the moving part with two contact bridges, which are arranged opposite to one another and separated -by a spring, the bridges moving between four fixed contacts with which they work alternately two by two. The two iixed contacts on one side of the bridge forming the current feed, are interconnected, and the two iixed contacts on the other side are the output lterminals for the current, and are connected to two distinct circuits.

Known makes of reversing relays of this kind have the disadvantage that they have a weak cutting oit power.

In fact, when the pair of contact bridges passes from one position to the other, the two fixed contacts forming the current feed being at the same potential, and the two contacts of the bridges which work with these fixed contacts being interconnected, it is possible for arc to strike between the two contacts white being brought together before an arc which has formed between the two contacts being separated is quenched.

So, in spite of appearances, the current cut-offs made by the contact bridges are not doubie, but simple-a fact which for the same displacement of the moving part much reduces the cutting oit power.

The present invention deals with a reversing relay which in comparison with known relays is capable of considerably increased cutting ofi1 power.

in a reversing relay where the moving part carries at least one pair of contacts bridges supported and separated by a spring, the bridges moving between four fixed contacts arranged in a rectangle with which they work alternately two by two, two of the said xed contacts situated on one side being interconnected and the other two serving two distinct circuits, the two bridges of the said pair are carried on an insulating support, and the spring which separates them is insulated electrically from one at least of these bridges.

Thus the two bridges are insulated from one another while the reversing relay passes from one position to the other, and the cut-oit is in fact double for each contact bridge.

In one advantageous method of carrying outthe invention the two contact bridges are in the shape of a T and are hinged on the insulating support by their vertical parallel arms, while the contacts carried by the cross-arm work into fixed Contact blades, parallel to one another and perpendicular to these cross-arms.

Further, the spacing of the blades is at least equal to the gap distance (i.e. the displacement of the moving contacts carried by a bridge with regard to the lixed contacts with which they work), but less than twice this gap distance.

So the two parallel adjacent arcs, which appear between fixed contacts and moving contacts while being separated, mutually repel one another, thus Iblowing one another out.

So that this result is obtained with certainty, whatever current strengths are used, the contacts carried by the bridges are swelled out, the one towards the other, so

3,155,790 Patented Nov. 3, 1964 that the two arcs start between the iixed and moving contacts as near to one another as possible, but nevertheless at such a distance that they do not tend to be united to a single one.

The present invention will now be described in greater detail by way of example with reference to the accompanying drawings, wherein:-

FIG. 1 is a diagrammatic representation of a current 'cut-off device using a bridge member shaped like a T.

FlG. la is an end-view of such a device.

FIG. 2 shows a plan View of a practical design for a reversing relay,

FIGS. 3 and 4 are sections along III- HI and IV-V of FIG. 2.

FIG. 5 is an axial section taken along line V-V of FiG. 6 of an electromagnetic relay controlling a four pole double throw reversing switch.

FIG. 6 is an end-View of the base of the relay shown in FlG. 5.

FiG. 7 is a plan View of the relay shown in FIG. 5.

FIG. 8 is a perspective view of the relay position indicator of the relay shown in FIG. 5.

Referring iirst to FIGS. l and la, the method of breaking the current circuit will -be described using a bridge- Contact, shaped like a T, carrying unsyinmetrical contacts.

The xed contacts are two xed parallel conducting strips l and 2 situated in the same plane, the upper sides of which are silvered. These strips 1 and 2 can be bridged by a cross-strip 3, whose semi-spherical contacts 4- and 5 touch strips i and 2 respectively in the closed position. The strip 3 carries a lateral extension 3a which allows the strip 3 to be hinged about the axis X-X on the armature of the relay, the strip 3 moving in the sense of the double arrow F. A spring 6 provides the contact pressure necessary to maintain the current circuit in the closed position.

At the moment of breaking the circuit, first a decompression of the spring 5 occurs with rotation of the

contacts

4 and 5 in contact with the strips 1 and 2, then a separation of the contacts and the strips.

During this opening movement arcs strike on the one hand between contact 4 and strip 1 and on the other hand between contact 5 and strip 2, so that as long as these arcs are not quenched the current follows the path shown by the chain line a, b, c, d, e, f, of which parts b, c and d, e represent the arcs.

As the two arcs are close to one another, each is subjected to the magnetic field set up by the other, and the resulting forces are directed in opposite directions F1 and F2.

The forces F1 and F2 are proportional to the square of the strength of the current, and are inversely proportional to the distance separating the two arcs.

In order to ensure that the arcs are effectively blown out, it is desirable to make the distance between the

contacts

4 and 5 as small as possible, and this involves bringing the strips i and 2 closer together. The distance k between these strips cannot become less than the opening distance lz of the contacts, so that no arc can strike between these strips in the air ionised by the cut-off arcs.

Since the magnetic forces F1 and F2 are proportional to the square of the current strength, very high strengt arcs b, c and d, e cause the arcs to be quickly blown out in opposite directions, so that the space between the strips 1 and 2 is de-ionised very quickly.

When a low current is used, the arcs can be blown out satisfactory by reducing the distance of these arcs as far as possible by using contacts which are shifted towards one another so as to engage with the edges in respect of the fixed strips 1 and 2. As is shown in FIG. 1a, the

contacts

4 and 5 of the bridge member 3 are so shaped that they take the shape of two drops of liquid whose most swollen or bulged out parts face one another.

The

tips

4a and 5a of these contacts thus meet the fixed contacts 1 and 2 at their inner edges when the switch is closed. Therefore, at the moment of opening the device at which the arcs strike, the distance K in 1a is only a little greater than the distance k between the edges of the members 1 and 2. Thus the mutually repelling force of the arcs, which is inversely proportional to the distance, is increased, and even arcs of weak strength are shifted in opposite directions, a fact which prevents localisation of the arcs point of attack and the rapid wear of the contacts.

In the case of strong arcs the contacts behave like a horn gap, that is to say that by their shape (see the dotted lines) they cause the elongation of the arc which results in its cooling and final extinction. The arcs are always quenched without any risk of re-striking, if the distance k between the edges of the two strips 1 and 2 is at least equal to the distance lz of the contact opening and less than twice this distance. Furthermore, as the strips 1 and 2 have in practice a width corresponding to the diameter of the

contacts

4 and 5, good results are obtained by spacing the strips 1 and 2 at an interval k which is nearly equal to their own width; this holds as long as the spacing at the end of the contact-throw is then about equal to the diameter of the

contacts

4 and 5.

However, so that the blowing out effect stays effective, the spacing of the strips 1 and 2 must not be more than twice their own width, that is to say that on the crossstrip 3 the spacing of the

contacts

4 and 5 is understood to be between one and two times the diameter of these contacts.

FIGS. 2, 3 and 4 show the application of the invention to a reversing relay.

Referring to these figures,

strips

8 and 9 are connected to the same pole P of a supply and serve as the currentfeed, whilst strips 10 and 11 are the outputs to two distinct circuits. As shown in FlG. 4, the strips S and 9 are preferably connected together by means of' a rivet 13 inside the xed insulating support 12 which is integral with the relay frame, so that the part 8a of the strip is the only current-feed for the two strips S and 9.

The reversing current breaker consists of the two crossstrips 14 and 15, which by moving alternately according to the direction of the arrow F (FIG. 3) are adapted to either

bridge strips

8 and 10 or strips 9 and 11. Nevertheless, if the cross-strips 14 and 15 are electrically interconnected, as is usually the case in known constructions, the following disadvantage results:

When the set of two strips 14 and 15, starting for example, from the top position where the circuit 8, 1t) is closed, move towards the lower position so as to open the

circuit

8, 10 and close the circuit 9, 11 they cause two arcs to appear, one between the contacts 8 and 14a, and the other between contacts 1 and 14h.

However, if these two strips 14 and 15 are interconnected, then since strip 9 is at the same potential as strip 8, and contact 15a is at the same potential as contact 14a, before the arc between contact 14a and strip 8 is extinguished, a new arc strikes between contact 15 and strip 9, so that the breaking of the circuit 8, l@ only occurs between contact 14b and strip 10, on a single arc of much greater power than the two initial arcs. The cut-off power of the device is therefore very much reduced.

To remedy this, the insulated movable support 17 associated with the reversing relay and carrying strips 14 and 15 has a cavity 17a. The lateral extensions 14C and 15C of the two strips 14 and 15 penetrate into the cavity 17a. These extensions carry on. their opposing faces insulating discs 1S which serve to support the ends of a compression spring 19. Thus, the strips 14 and 15 although elastically connected to one another,

il are electrically insulated from one another, so that when one functions at the cut-off, the other cannot restore the current between one of the strips t5 or 9 and the strips 11 or 10 which are respectively diagonally opposite to the former strips.

With this arrangement the cut-off is effected on two arcs which are mutually blown out, and not on a single arc in which the energy of the two initial arcs is accumulated. Each strip 14 and 15 rests at its back edge 19 on the bottom of the cavity 17a and carries at its end remote from the contacts two flanges Ztl, which are inserted in holes 21 in the insulating support 1'7.

As a result, when the two fixed strips, such as 8 and 1t) or 9 and 11, are not strictly in the same plane parallel to the upper and lower faces of the cavity 17a, each of the strips 14 or 15 can tilt slightly, so as to be placed against the pair of corresponding fixed strips, only one of its corners actually resting on the bottom of the cavity 17a. The length of the extensions 14C and 15e being known and also the position of the spring 19 in relation to these extensions, the straight lines D, D (F16. 2) joining the flanges 2G to the axis of the spring 19, pass between the contacts of the strips 14 and 15, so that if these strips are tilted transversely the pressure of the spring 19 continues to be distributed between the two contacts of each of the strips.

FlGS. 5, 6 and 7 illustrate an embodiment of an electromagnetic relay having a plurality of reversing contacts.

Referring to these figures the base 12 of this relay is approximately square, and has at each corner (FlG. 6) a set of

strips

8, 9, 1t? and 11. The insulating support 17 is shaped like a double T, and each of its four ends have a cavity 17a to receive the extensions 14e and 15o of two T strips 14 and 15.

The support 17 is fixed by means of a screw 22 to a flange 23a of the moving armature 23 of the relay. The armature 23 consists of a sheet of metal bent at right angles to form two parts, a cranked part 23b of which is joined to the relay housing 25 by one end of a thin steel strip 24. The strip 24 acts as a hinge being fixed to both the yoke 25 and the part 23h of the armature 23 by means of

plates

26 and 27 riveted thereto.

The magnet yoke 25 is U-shaped, and carries the magnet core 23 in its centre lixed by a screw 29. The core 2S carries the exciting coil 30.

A non-magnetic piece 31 (eg. brass) in the general shape of a cross (see FIG. 8) is positioned against the side of the yoke 25 facing the base. The upper arm 31a of this piece has an aperture 32 through which the flange 23a passes. The arm 31a also has a flange 33 which projects into the aperture 32. The flange 33 acts as a stop for the armature in the opening direction.

The lower arm 31b of the cross 31 is bent at right angles, the end of this right angle, having a circular boss stamped on it, serves as the support for one end of a com pression spring 35. The spring 35 acts as the return spring for the armature 23. The other end of the spring 35 presses against the upper end of the spring casing 36, which forms part of the double T support 17. The upper end of the spring is centered by the nut of the screw 22.

For fixing the magnet yoke 25 to the base 12, the latter has two bosses 33 which tit into lateral recesses in the double T support 17. Bolts 39, pass through both the support 17 and the cross 31 and are screwed into two threaded bores on the side of the yoke 25 facing the base 12. Thus the two bolts 39 secure both the yoke 25 and the cross 31 to the base 12.

In addition to the

stips

8a, 10 and 11, the outer face of the base 12 carries two strips 41D and 41, whose positions 0n the outer face correspond to the two upper strips 9 on the inside face (see FIG. 5). The strips ttl and 41, which are bent in opposite directions in the material forming the base 12 serve as terminals for energising the coil 31D. The inner parts 40a and 41a of these strips are in fact extended by means of iiexible conductors (not shown), which after passing through the grooves 44 formed in the bosses 35, are wound round the side of the yoke 25 facing the base and terminate at the coil.

The moving contacts carried by strips 14 and 15 are at the greatest possible distance from the hinge 24. For the same angular swing of the armature the throw of these contacts is then a maximum, and the speed of movement of these contacts is then also as great as possible; which together assist the power of the cut-off. Further, the stress on the return spring 35 is almost equal and opposite to the stress on the four springs 19 which exert the contact pressure of the moving contacts on the fixed strips. Likewise, the stress on the return spring occurs on the same side in relation to the hinge 24 as the attractive force of the magnet in its energised condition, and the forces acting on the hinge are then much reduced, and consequently the strip 24 allows the moving armature 23 to swing without being fatigued or deformed. Under these conditions it is possible to use a very thin and very flexible piece of steel for this hinge.

The relay which has just been described is enclosed in a parallel piped cover Sil nesting in a peripheral groove l at the edge of the base.

So that it is possible to ascertain whether or not the relay is energised, the relay is provided with an indicating device which is visible through a window Sila in the side of the cover 5@ opposite the base l2. This indicating device consists of a U shaped metal band 52 provided with slots at its two ends which lit over the rectangular flanges at the ends of the horizontal arms 31C and Sid of the cross 31. The base portion of the U shaped tlange has a threaded bore 53 for iixing screws to hold the cover 5d. Each side of the band 52 carries two

tongues

54 and 55. The tongues 54 each have a hole which supports the pivots 56a of a U shaped frame S6. The frame 56 is made from a single piece of metal and has on its side opposite the base, a shutter Seb. The two ends of the U shaped frame 56 carry two catches 56C which face one another. The catches 56C are inserted in the grooves 57 formed laterally at the ends of the upper arm of the support 17 (FIG. 6). p

When the relay is energised, the support 17 causes the frame 55 to tilt about its pivots 56a, so that the shutter 56h becomes visible in the window Stia. The tongues 55 impart rigidity to the corresponding part of the cover Sti, so that deformation of the latter does not prevent the frame 56 from tilting. ln order to avoid the risk of short circuiting between the strips situated on both sides of the vertical plane of symmetry of the relay (line V-V in FG. 6) when the arcs are blown out in the direction of this piane of symmetry, an insulating plane partition 59 (FIG. 7) can be provided on the side of the base l2 facing the relay in the plane of symmetry.

It goes without saying that many modifications and changes can be made to the present invention by those skilled in the art without departing from the true spirit and scope of the present invention, as defined in the appended claims.

What l claim is:

l. A reversing electromagnetic relay comprising a magnet core; a yoke upon which said core and winding are fixedly mounted; a winding for energizing said core; an armature pivotally connected to said yoke and magnetically associated with said core for magnetically induced pivotal movement in one direction relatively to said core when the latter is energized; means for pivotally moving the armature in an opposite direction relatively to the core when the latter is not energized, a rst insulating support member secured to a portion of said armature remote from its pivotal connection to said yoke, said support member having a recess opening in a direction facing away from said connection; a pair of opposed, spaced, T-shaped, substantially flat, conducting contact bridges, each end of opposite faces of the cross-bars of the two bridges carrying a substantially rounded contact and the leg of each bridge extending into said recess and being pivotally connected therein for pivotal move ment of said bridges about the ends of the legs thereof, the plane of each bridge being substantially parallel to the axis of said pivotal connection; insulated spring means yieldably holding said bridges apart; a second insulating support member which is stationary and spaced from said first support member; a rst pair of interconnected, fixed, strip contacts insulatedly secured to said second support member for alternate engagement with the two rounded contacts at corresponding ends of the two bridges and a second pair of xed, strip contacts insulated, one from the other, and secured to said second support member for alternate engagement with the other two rounded contacts of said bridges.

2. A reversing relay according to claim l, wherein the pair of rounded contacts are drop-shaped, the bulges of which face towards one another.

3. A reversing relay according to claim l, wherein said fixed strip contacts are perpendicular to the cross-bars of said T-bridges and wherein the spacing of the facing edges of the strip contacts cooperating with both rounded contacts of a bridge is somewhat greater than the distance of one rounded contact to the corresponding xed strip contact in open condition of said contacts.

4. A reversing relay according to claim l, wherein the distance of the facing edges of the rounded contacts carried by a T-bridge is smaller than twice the diameter of said substantially rounded con-tacts measured in the direction of the cross-bar of said bridge.

5. A reversing electromagnet relay comprising a yoke having a plurality of parallel legs; a winding carried by one of said legs tor energizing said yoke; a thin flexible blade secured to an external leg of said yoke along a first edge of said blade; an armature magnetically associated with said yoke, one end of said armature being secured to a second edge of said blade parallel -to said first edge; a flat lirst insulating support member secured to the other end of said armature perpendicularly thereto, said support member being provided with substantially rectangular recesses inthe front face thereof remote from said blade; similar pairs of opposed, spaced, T-shaped, substantially flat, conducting contact bridges, each end of the opposite faces of the cross-bars of each T-bridge carrying a substantially rounded contact and the legs of said "-bridges of each pair being in pivotal engagement within a different one of said recesses with the dat faces of said bridges facing walls of said recess which are parallel to said armature, said legs being arranged in said recess for pivotal movement about their ends; an insulated coil spring between both bridges of each pair for yieldably keeping said bridges apart, said springs of all the pairs being similarly arranged in a plane parallel to said front face; a second insulating support member which is stationary and spaced from said first support member; rst pairs of interconnected fixed strip contacts insulatedly secured to said second support member for alternate cooperation with the two rounded contacts carried by the corresponding ends of each pair of bridges; second pairs of insulated fixed strip contacts insulatedly secured to said second support member for alternate cooperation with the two other rounded contacts of said pair of bridges; and spring means located in said plane and acting on said i'irst support member for urging the latter and said armature in an opposite direction to the direction of magnetic attraction of said yoke.

6. A reversing relay according to claim 5, wherein each lateral edge of the leg of the T-bridge ends in an outwardly bent tongue and wherein the walls of said recesses are provided with holes for accommodation of said tongues, whereby each T-bridge is adapted for pivotal movement about its end and in the plane of the longitudinal axis of its leg.

7. A reversing relay according to claim 5, further comprising a non-magnetic ilat member externally secured to another external leg of the yoke, said ilat member having an aperture through which said armature passes and a bent flange on 'the side opposite to said aperture, and wherein said latter spring means is a coil spring, One end of which acts yon said first support member and the other on said bent iange, whereby the edge of the aperture remote from said bent iiange acts as a stop for said armature.

8. A reversing relay according to claim 5, wherein said flat rst support member comprises a lateral recess on each side thereof and the relay further comprising a flat U-shaped member substantially surrounding said yoke and having facing ends engaging within said recesses, a transverse pivotal connection with said yoke and a bent Si flange in the transverse portion of said U, whereby said U-member and bent iiange occupy two angularly spaced positions according to the energized and nie-energized conditions of the relay and a cover surrounding said relay and having a Window registering with one position of said nent iiange.

References Cited in the le of this patent UNITED STATES PATENTS 1,141,119 {aisling lune 1, 1915 2,585,824 Noyes Feb, 12, 1952 2,825,013 Krenke Feb. 25, 1958 2,956,140 Walter Oct. 11, 1960 2,995,638 Souehet Aug. 8, 1961 3,014,103 Moran et al Dec. 19, 1961 3,021,519 McLaughlin Feb. 13, 1962 UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No 3, 155y790 November` 3V 1964 Piense L, A., Lemonnier 1t s hereby certified that error appears in the above numbered paten't, requiring correction and that the said Letters Patent should read as eorec'ted below Column 3Y line 57, for "contacts 1 and 1410" read contacts IOV and 14h column 5S1 lines 62 and 63V strike ou'rJ "net core; a yoke upon which said core and winding are fixedly mounted; a winding for energizing said Core; and insert instead net, Core; a winding for energizing said core; a yoke upon which said Core and winding are fixedly mounted y Signed and sealed this 30th day of March 1965.,

(SEAL) Attest:

ERNEST lW. SWIDER' EDWARD J. BRENNER Attesting fficer n Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OE CORRECTION Patent No., 3, 155Y79O November 3 1964 Pierre L, A. Lemonnier It is hereby certified that error appears in the above numbered patent requiring correction and that the Said Letters Patent should read as corrected below.

Column 3 line 57, for "contacts l and 14h" read contacts IO and 14h column 5xl lines 62 and 63v strike out "net core; a yoke upon which said core and winding are fixedly mounted; a winding for energizing said corefg" and insert instead net core; a winding for energizing said core; a yoke upon which said Core and winding are fixedly A' Signed and sealed this 30th day of March 1965..

(SEAL) Attest:

EDWARD J. BRENNER Attesting Officer

Claims

1. A REVERSING ELECTROMAGNETIC RELAY COMPRISING A MAGNET CORE; A YOKE UPON WHICH SAID CORE AND WINDING ARE FIXEDLY MOUNTED; A WINDING FOR ENERGIZING SAID CORE; AN ARMATURE PIVOTALLY CONNECTED TO SAID YOKE AND MAGNETICALLY ASSOCIATED WITH SAID CORE FOR MAGNETICALLY INDUCED PIVOTAL MOVEMENT IN ONE DIRECTION RELATIVELY TO SAID CORE WHEN THE LATTER IS ENERGIZED; MEANS FOR PIVOTALLY MOVING THE ARMATURE IN AN OPPOSITE DIRECTION RELATIVELY TO THE CORE WHEN THE LATTER IS NOT ENERGIZED, A FIRST INSULATING SUPPORT MEMBER SECURED TO A PORTION OF SAID ARMATURE REMOTE FROM ITS PIVOTAL CONNECTION TO SAID YOKE, SAID SUPPORT MEMBER HAVING A RECESS OPENING IN A DIRECTION FACING AWAY FROM SAID CONNECTION; A PAIR OF OPPOSED, SPACED, T-SHAPED, SUBSTANTIALLY FLAT, CONDUCTING CONTACT BRIDGES, EACH END OF OPPOSITE FACES OF THE CROSS-BARS OF THE TWO BRIDGES CARRYING A SUBSTANTIALLY ROUNDED CONTACT AND THE LEG OF EACH BRIDGE EXTENDING INTO SAID RECESS AND BEING PIVOTALLY CONNECTED THEREIN FOR PIVOTAL MOVEMENT OF SAID BRIDGES ABOUT THE ENDS OF THE LEGS THEREOF, THE PLANE OF EACH BRIDGE BEING SUBSTANTIALLY PARALLEL TO THE AXIS OF SAID PIVOTAL CONNECTION; INSULATED SPRING MEANS YIELDABLY HOLDING SAID BRIDGES APART; A SECOND INSULATING SUPPORT MEMBER WHICH IS STATIONARY AND SPACED FROM SAID FIRST SUPPORT MEMBER; A FIRST PAIR OF INTERCONNECTED, FIXED, STRIP CONTACTS INSULATEDLY SECURED TO SAID SECOND SUPPORT MEMBER FOR ALTERNATE ENGAGEMENT WITH THE TWO ROUNDED CONTACTS AT CORRESPONDING ENDS OF THE TWO BRIDGES AND A SECOND PAIR OF FIXED, STRIP CONTACTS INSULATED, ONE FROM THE OTHER, AND SECURED TO SAID SECOND SUPPORT MEMBER FOR ALTERNATE ENGAGEMENT WITH THE OTHER TWO ROUNDED CONTACTS OF SAID BRIDGES.