US3600530A

US3600530A - Ignition distributor having a radio frequency interference shield

Info

Publication number: US3600530A
Application number: US858729A
Authority: US
Inventors: Charles L Dusenberry; Donald G Guethersloh
Original assignee: General Motors Corp
Current assignee: Motors Liquidation Co
Priority date: 1969-09-17
Filing date: 1969-09-17
Publication date: 1971-08-17
Anticipated expiration: 1988-08-17

Abstract

A radio frequency interference shield of an ignition distributor of the type having a fixed base and an adjustable breaker plate which is shifted by a vacuum unit relative to the base. The distributor is of the type wherein the centrifugal advance mechanism is disposed above the breaker can and located within a rotor assembly for the distributor. A radio frequency interference shield is provided which comprises a generally cupshaped metallic housing that is attached to the breaker plate of the distributor. The cup-shaped housing is formed of two complementary interfitting metallic parts which are attached to the breaker plate and which have matching semicircular openings that together form a circular opening disposed about a rotatable part of the distributor that drives the rotor. The two housing parts are secured to the breaker plate by screws and with the breaker plate form a closed housing except for the opening that receives a rotatable part of the distributor and an opening for adjusting the breaker contact set of the distributor. The compartment defined by the breaker plate and the two housing shield parts enclose the distributor breaker contacts, a capacitor, and the capacitor lead wire connecting the capacitor to one of the breaker contacts.

Description

United States Patent [72] Inventors Charles L. Dus enberry v Chesterfield; Donald G. Guethersloh, Anderson, both of,

lnd. [21] Appl. No. 858,729 [22] Filed Sept. 17, 1969 [45] Patented Aug. 17, 1971 [73]- Assignee General Motors Corporation Detroit, Mich.

[54] lGNlTION DISTRIBUTOR HAVING A RADIO FREQUENCY INTERFERENCE SHIELD 3 Claims, 5 Drawing Figs. p

[52] US. Cl 200/19, 123/148 P, 200/27, 200/168 6, 310/70 [51] Int. Cl HO-1h 19/00, H02h 9/02 [501 Field otSearch 310/70, 153; 123/1481; 200/l932, 168

[56] References Cited UNITED STATES PATENTS 2,291,652 8/1942 Rose 123/148 X 2,768,227 10/1956 Ruckelshaus.. 123/148 X 3,025,362 3/1962 Thomas 200/27 X 3,342,955 9/1967 200/19 Smith et al Primary Examiner-Milton O. Hirshfield AssistantExaminer-Mark O. Budd Attorneys-E. W. Christen, C. R. Meland and Richard G.

Stahr ABSTRACT: A radio frequency interference shield of an ignition distributor of the type having a fixed base and an adjustable breaker plate which is shifted by a vacuum unit relative to the base. The distributor is of the type wherein the centrifugal advance mechanism is disposed above the breaker can and located within a rotor assembly for the distributor. A radio frequency interference shield is provided which comprises a generally cup-shaped metallic housing that is attached to the breaker plate of the distributor. The cup-shaped housing is formed of two complementary interfitting metallic parts which are attached to the breaker plate and which have matching semicircular openings that together form a circular opening disposed about a rotatable part of the distributor that drives the rotor. The two housing parts are secured to the breaker plate by screws and with the breaker plate form a closed housing except for the opening that receives a rotatable part of the distributor and an opening for adjusting the breaker contact set of the distributor. The compartment defined by the breaker plate and the two housing shield parts enclose the distributor breaker contacts, a capacitor, and the capacitor lead wire connecting the capacitor to one of the breaker contacts.

40 zsm Q 4 58 2 1, 254 2 5 152- 56A 'i 1 r 50 i v IGNITION DISTRIBUTOR HAVING A'RADIO FREQUENCY INTERFERENCE SHIELD This invention relates to ignition distributors for internal combustion engines and more particularlyto an ignition distributor that has a shield for preventing signals developed by the ignition apparatus from interfering with proper operation of communications equipment on a motor vehicle such as a radio.

The ignition system for the internal combustion engine of a I motor vehicle is known to be a source of electrical radiation which may interfere with the satisfactory operation of communications equipment such as radios.located on'a motor vehicle. The amount of interference to some extent depends upon the distance between the antenna of-the radio receiver and the source of radio interference. The problem may become particularly troublesome where the radio antenna is embedded in the windshield and where the distributor for the internal combustion engine islocated at the rear of the engine and therefore closely adjacent the windshield mounted antenna. A proposed solution to this problem which has beensuggested by those skilled in the art is to provide a grounded metallic shield which is disposed between the interfering signal source and the antenna of the radio. The shield has taken various configurations but has followed the central idea of disposing a grounded metallic shield about the entire distributor cap in order to reduce the radiation of signals generated by the ignition apparatus. Typical examples of radiofrequency' inter ference shields which enclose the entire distributor cap are disclosed in the U.S. Pats. to Arthur No., 2,347,019 and to I'Iallett No. 2,465,447. 1

Where a radio frequency interference shield is provided that entirely encloses thedistributor cap, such as those shown in the above-mentioned patents, the shield must beprovided with a. plurality of openings to accommodate the spark plug In contrast to known radio frequency interference shieldsa which entirely enclose the distributor cap'and which are attached to the base of the distributor the present invention contemplates providing a shield whichiis located closely adjacent the source of interfering signals and which is located within thedistributor cap. In carrying this object forward an ignition distributor which has a rotatable breaker plate that supports the breaker contacts'and a capacitor is fitted with a metallic shield which is attached to the breaker plate. The shield and breaker plate define a compartment of relatively small volume that encloses the breakercontacts, the capacitor, and the capacitor lead wire of the ignition distributor.

It accordingly is one of the objects of this invention to provide a shield for preventing the radiation of signals developed by ignition apparatus from interfering with radio receiving equipment wherein the shield is located within the distributor cap of an ignition distributor and attached to a rotatable breaker plate of the distributor and further wherein the breaker plate and shield form a housing which contains the breaker contacts, the capacitor and the capacitor lead wire of a distributor.

It will be appreciated that by using the shield of this inven-- tion the shield does not-need to be provided with a plurality of openings accommodating the sparkplug wires and the shield is located closely adjacent to the apparatus generating the'interfering signals with a result that improved radio frequency suppression is provided.

Another object of this invention is to provide a radio frequency interference shield for an ignition distributor wherein acup-shaped housingcomprised, of two interfitting metallic parts is secured to the breaker plate of the distributor to provide an enclosed compartment that contains the breaker points, the capacitor and the capacitor lead of .the distributor. By providing a two part shield it is possible to remove the shield to service the distributor, for example, to replace the breaker contact set without the; necessity of removing the distributor shaft from the distributorbase.

A further object of this invention is to provide a radio frequency interference shield foran ignition distributor which is comprised of two complementary interfitting metallic shield parts which are secured to the breaker plate of an ignition distributor by fastener means such asscrews and wherein portions of said shield parts engage the. outer peripheral edge of the breaker plate to prevent rotation of the shield parts during operation of the distributor.

Still another object of this invention is to provide a radio vfrequency interference shield for an ignition distributor wherein the shield comprises a pair of metallic parts which are fixed to the breaker plate of a distributor which have complementary engaging surfaces and wherein the shield and breaker plate form an enclosed compartment containing the distributor breaker contacts and capacitor and further wherein an opening is provided in the shield for the reception of a tool that engages means for adjusting the breaker contact set.

IN THE DRAWINGS FIG. 1 is a sectional view of an ignition distributor that is fitted with the radio frequency interference shield of this invention.

FIG. 2 is a sectional view taken along line 2-2 of FIG. 1 with the distributor cap removed.

FIG. 3'is a plan view of two complementary metallic shields that form the radio frequency interference shield of this invention;

FIG. 4 is a sectional view taken along line 4-4 of FIG. 3.

FIG. 5 is a sectional view taken along line 5-5 of FIG. 3.

Refe'rringnow to the drawings and more particularly to FIG. 1, the referencenumeral 10 generally designates a base of an ignition distributor. This base 10 is formed of metal material and has an annular elongated section 12.and radially extending support section designated by reference numeral 14. The

annular section 12 extends axially and contains a shaft 16 which'is driven by the cam shaft of an internal combustion engine. The base portion 12 has an elongated portion not illustrated which fits within the internal combustion engine and which alsosupports an internal lower bearing not illustrated for rotatably supporting the lower end of shaft 16. The lower end of the shaft is connected to a gear (not shown) for driving the shaft.

The upper end of the shaft 16 is rotatably journaled in a bearing, designated by reference numeral 18 which is supported by the base 10. Disposed within radially spaced chambers or pockets located about the bearing 18 is a lubricating compound generally designated by reference numeral 20. The bearing l8has a-portion 18A which projects above the section l4'of the base of the distributor and which forms a journal for the breaker plate of the distributor in a manner to be more fully described hereinafter.

The portion I4 of the base of the distributor supports a distributor cap which isv generally designated by reference numeral 22. The distributor cap 22 is conventional and is formed of a molded insulating material 24 which carries a plurality of circumferentially spaced conductive inserts 26 which respectively receive the terminals of conductors connected with the spark plugs of an internal combustion epgine. The distributor cap 22 also includes the conventional center terminal 28 which is connected with the secondary of an ignition coil. The distributor cap 22 is attached to the portion 14 of the base of the distributor by a pair of spring biased clamp arms 30, one of which is illustrated in FIG. 1.

The center electrode 28 an the circumferentially spaced inserts 26" are periodically connected by a conductor means 32. The conductor means 32 is supported by a rotor 34 which is formed of insulating material and which contains a centrifugal advance mechanism generally by reference numeral 36. The centrifugal advance mechanism includes a weight plate 38 connected to'an annular part 40 which in turn is connected a breaker cam 42 which operates the breaker contacts of the distributor. The centrifugal advance mechanism 36 further includes flyweights 44 which are engaged by a cam plate not i1- lustrated that is rigidly connected to the top end of the shaft 16. The shaft 16 passes through the cam 42 and the annular part 40 and is connected with the cam plate in a manner illustrated'in the l-Iartzell et al., U.S. Pat. No. 2,872,537. As explained in that patent movement of the flyweights 4 adjusts the cam 42 relative to the shaft 16 to shift the cam as a function of engine'speed. The rotor part 34 which is formed of insulating material is fixed to the weights plate 38 by a pair of screws, one of which is illustrated in FIG. 1 and designated by reference numeral 46.

The ignition distributor of this invention includes a conventional breaker plate generally designated by reference numeral 50. A cylindrical bearing member 52 is secured to the breaker plate 50 and fits around the'top end 18A of the bearing 18. The bearing 52 is secured to an inner wall of a central opening formed in the breaker plate 50. The breaker plate 50 can' rotate about the top end 18A of the bearing 18 since the top end 18A of the bearing forms a journal for rotatable movement of the breaker plate 50.

The breaker plate 50 is rotatably adjusted by a conventional vacuumunit generally designated by reference number 54 and containing a diaphragm which is not illustrated but which is connected to the rod 56 shown in FIG. 1. The rod 56 has an upstanding portion 56A which fits within an opening formed in the breaker plate 50. The tubular connector 58 .of the vacuum unit is connected with the intake manifold of an internalcombustion engine as is well known to those skilled in the art', and the breaker plate 50 is rotated relative to the base 12 of the distributor during operation of the internal combustion engine.

The vacuum unit 54 has a support section 60 which is secured to the base section 14 of the distributor by

screws

62 and 64. The screw 64 also secures a terminal clip 66 to the vacuum unit which is connected with a conductor 68. The opposite end of the conductor 68 is welded or otherwise secured to the underside of breaker plate 50 and the screw 64 therefore connects the breaker plate 50 to the metallic portion 60 of the vacuum unit and therefore electrically connects the breaker plate 50 to ground. This is because the base 12 of the distributor contacts the block of the internal combustion engine and therefore is at electrical ground in a motor vehicle electrical system.

The breaker plate 50 supports a conventional breaker contact set which is generally designated by reference numeral 70. The breaker contact set 70 comprises a support plate 72 which is fixed to the breaker plate 70 by means of screws 74 and 76 which pass through openings formed in the support plate 72 and which are threaded into the breaker plate 50. The support plate 72 carries post 78 which pivotally supports a breaker arm designated by reference numeral 80. The breaker arm 80 is electrically insulated from the post by insulation 81. The breaker arm 80 carries a rubbing block 82 which is engaged by the cam 42 during rotation of the shaft 16 and carries the movable contact 84 of the breaker contact set. The fixed contact 86 of the breaker contact set is supported by a support 88 having an upstanding flange'90 which supports the fixed contact 86. Support 88 has an opening disposed about the post 78 and with this arrangement the support 88 can be adjusted relative to the fixed support 72 by pivotal movement around post 78. This adjustment is accomplished by an adjusting screw generally designated by reference numeral 91 having a head 92 formed with a hexagonal recess for receiving an adjusting tooland having a threaded end 94 fitted within a part 96 which is fixed to another upstanding flange 98 of the support 88. Access to the adjusting screw 91. can be provided by providing an opening (not illustrated) formed in the side of the distributor cap which is opened orclosed by a slidable door as disclosedin the Pat. I-Iartzell et al., U.S. Pat No. 2,872,537.Thecontact set that has been described may be of the type shown in Norris et al., U.S. Pat. 'No. 2,965,726 granted on Dec. 20, 1960. d

The support 88 has another upstanding flange 100 which has an opening that receives a plastic fastener designated by reference numeral 102. The fastener 102 has a cylindrical axially extending portion 104 which receives terminals 106 and 108 connected respectively with wire conductors 110 and 112. The terminals 106 and 108 have open-ended slots which are disposed about the cylindrical 'portion 104 and the terminals are held in engagement with each other by a flat spring 114 which also serves to bias the breaker arm 80 toward the cam 42. The conductor 112 passes through a grommet located within an opening in the base of the distributor and is connected with the primary of an ignition coil as is well know to those skilled in theart. The conductor. 110 is connected with a capacitor 122, one side of which is grounded to the breaker plate 50 by a clamp 124. The means for connecting the terminal clips 106 and 108 may be similar to that disclosed in the Julian U.S. Pat. No. 3,162,732 granted on Dec. 22, 1964.

The ignition apparatus that as thus far been described in conventional and the radio frequency interference shield for preventing the radiation of signals which would otherwise interfere with proper operation of a radio receiver on a motor vehicle will now be described. The radio interference shield as previously pointed out is disposed within the distributor cap 24 as opposed to prior art arrangements where the interference shield would be disposed entirely about the distributor cap 24 and secured to the base 14. r

The radio interference shield of this invention comprises a pair of metallic parts and 132 which are preferably formed of a tin-plated steel approximately .023 inches thick and which when secured to the breaker plate 50 form a generally cup-shaped housing having an-opening 134 located closely adjacent to part 40 of the weight plate assembly of the distributor. The

parts

130 and 132 are shown in detail in FIG. 3 and in the sectional views of FIGS. 4 and 5. The shield part 130 has the top wall 130A connected with an axially extending wall 1308. The wall 130B is indented over an area designated by reference numeral 130C and the part 130 has a flange 130D which forms a support flange for the shield part 130. The flange 130D is formed with an opening 13013 which receives a screw 134 that is threaded into the breaker plate 50 and which serves to secure the metal shield 130 to the breaker plate 50. The wall 1308 of the shield 130 is struck-in at 130F and 130G to provide ledges which engage the top surface of the breaker plate 50 when the shield 130 is attached to the breaker plate 50. The lower edge l30I-I of the part 130 engages the outer peripheral edge 50A of the breaker plate 50 when the shield 130 is assembled to the breaker plate. This prevents the shield 130 from rotating relative to the breaker plate 50 when it is attached to the breaker plate. The flange 130D has no projecting edge to engage the edge of the breaker plate and is secured directly to the top edge of the breaker plate 50 when the part 130 is secured to the breaker plate. The shield 130 has edges or borders 130] which fit within complementary borders formed on the part 132 when the part 132 is secured to the breaker plate.The shield part 130 has an opening 130K illustrated in FIG. 4 which is aligned with the head 92 of the adjusting screw 90 to permit adjustment of the breaker contact set when the shield is in place on the breaker plate.

' Referring now more particularly to FIGS. 3 and 5 the shield partl32 is illustrated which is complementary to the part 130 and which interfits with the part 130 when the part 132 is secured to the breaker plate 50. The part 132 has a top wall 132A formed with edges or borders 1328 which extend or are bent to be slightly above the top wall 132A as is illustrated in FIG. 5. When the

parts

130 and 132 are assembled to the breaker plate 50 the edges 132B overlap the edges 1301 of the shield 130 to provide an enclosure for the breaker contact set and the capacitor. Moreover, the top of the edges 130] engage the lower surfaces of the edges 1328 of the shield part 132. It is seen that the shield part 132 has an axially extending wall 132C which merges into a flange section 132D.The section 132D engages the top surface of the breaker plate 50.and has an opening 132E which receives a screw 136 for securing the shield 132 to the breaker plate 50. The screw 136 is threaded into an opening formed in the breaker plate 50. The lower edge 132F of the shield 132 engages the outer peripheral edge 50A of the breaker plate 50 over the periphery of the shield 132 except for the section 132D which lies flat against the top surface of the breaker plate 50.

It can be seen in FIG. 3 that the

complementary shield parts

130 and 132 have semicircular openings 1301. and 1326 which together form the circular opening 134 illustrated in FIG. l-which receives the part 40 of the weight plate assembly of the distributor. This opening 134 is sized such that it has a slightly larger diameter than the annular part 40 when the

shield parts

130 and 132 are secured to the breaker plate 50 with the edges 132B overlying and engaging the edges 130.]. i

As previously pointed out the

shield parts

130 and 132 are secured to the breaker plate 50 by the screws or fasteners 134 and 136 and certain portions of the lower edge of the shield parts engage the outer peripheral edge 50A of the breaker plate to prevent'rotation of these parts. it will beappreciated that the axially extending walls of the shield parts follow the outer contour of the outer periphery 50A of the breaker plate 50 and that the shield parts and the breaker plate form a substantially closed compartment which is designated by reference numeral 140. It will further be appreciated that the walls defining the compartment 140 enclose the breaker cont act set including the breaker contacts 84 and 86,. the capacitor 122, and the capacitor lead 110 connecting the capacitor to the moveable breaker contact 84. This provides an efficient shield for preventing the radiation of signals which might otherwise interfere with proper reception by a radio receiver on a motor vehicle. These signals have a frequency of approximately 1,000 kilocycles and are at times radiated by the conductor 110 which operates as a small transmitting antenna. This conductor 110 is completely enclosed by the breaker plate 50 and the cup-shaped housing provided by the

shield parts

130 and 132 and these parts are located closely adjacent to conductor 110, the capacitor 122 and the breaker contacts as opposed to prior art arrangements where the shield is located about the distributor cap and therefore at a substantial distance from the source of signals that interfere -with radio reception.

It has been found that by providing a shield that is located within the distributor cap an improved radio interference suppression is provided as compared to prior art arrangements where the shield is located on the exterior of the distributor cap 24.

The two part or two piece shield is provided in order that the shield may be removed without removing the shaft 16 from the base when it is desired to service the distributor, as for example, when it is desired to replace the contact set 72. it is pointed out that the shield part 130 forms an axial support for the shield part 132 when the parts are assembled to the breaker plate 50 since the edges 132B rest on the top of edges 130.1. The

shield parts

130 and 132 are at electrical ground since they contact metallic breaker plate 50 which is grounded by wire 68.

The capacitor lead 110 passes over the top of breaker arm spring 114 and is positioned closely adjacent the top wall 132A of shield 132.

As previously pointed out the shield and breaker plate form an enclosure of relatively small volume so that the plate and shield parts are located closely adjacent the source of interference. In this regard, and in a preferred embodiment of this and the inner surface of shield top walls 130A and 1332A is approximately .95 inches. The diameter of the shield parts across the overlapping edges 132B and 130.] is approximately 3.3 inches. The relative dimensions of the other parts of the distributor will be evident from an inspection of the drawings since the parts have the same full scale.

What we claim is:

1. A shielded ignition distributor for an internal combustion engine comprising, a' base, said base having a laterally extending support section, a distributor cap comprised of insulation material secured to said support section of said base extending axially therefrom-said distributor cap having a center electrode anda plurality of circumferentially spaced electrodes which are adapted to be connected with wires leading to spark plugs of an internal combustion engine, shaft means rotatably supported by said base extending into said cap, a metallic breaker plate supported by said base and adjustable relative thereto, a breaker cam driven by said shaft, rotor means driven by said shaft including conductive means operative to connect the center electrode and said circumferentially spaced electrodes of said cap, breaker contacts supported from one wall of said breaker plate operated by said breaker cam, a capacitor supported from said one wall of said breaker plate including a capacitor lead wire connected to one of said breaker contacts, and a generally cup-shaped metallic radio interference shield supported by said breaker plate and defining with said breaker plate a substantially closed metallic housing containing said breaker contacts, said capacitor, said capacitor lead wire and said breaker cam, said metallic housing comprising first and second complementary metallic shield parts removably secured to said breaker plate by removable fastener means, said shield parts having overlapping edges which engage each other and having semicircular openings defining a central opening that receives said shaft means for driving said rotor of said distributor, said shield located within said distributor cap and having an outer wall spaced from the inner wall of said cap.

2. The ignition distributor according to claim 1 where said first and second metallic shield parts each have lower edges engaging the peripheral edge of said breaker plate, and wherein said first shield part has inwardly struck portions engaging the top surface of said breaker plate and wherein an edge of the second shield part overlaps a complementary edge "of the first shield part and further wherein said shield parts are secured to said breaker plate by screws threaded into said breaker plate.

3. A shielded breaker plate and breaker contact assembly for an ignition distributor comprising, a laterally extending metallic breaker plate having an opening for receiving a distributor shaft, breaker contact means supported from one wall of said breaker plate, a capacitor supported from said one wall of said breaker plate, a capacitor lead wire connected to one breaker contact of said breaker contact means, and a generally cup-shaped metallic shield secured to said breaker plate having a central opening aligned with theopening in said breaker plate, said metallic shield comprised of first and second shield parts having overlapping engaging edges and having semicircular complementary openings defining said central opening in said shield, said first and second shield parts each having lower edges engaging the peripheral edge of said breaker plate and at least one of said shield parts having an inwardly struck portion engaging said one wall of said breaker plate, and removable fastener means for respectively removably securing said shield parts to said breaker plate, said shield and breaker plate defining a metallic enclosure containing saidbreaker contact means, said capacitor and said capacitor lead wire, said enclosure serving to reduce radiation of radio interference signals developed within said enclosure during use of said assembly.

Claims

1. A shielded ignition distributor for an internal combustion engine comprising, a base, said base having a laterally extending support section, a distributor cap comprised of insulation material secured to said support section of said base extending axially therefrom, said distributor cap having a center electrode and a plurality of circumferentially spaced electrodes which are adapted to be connected with wires leading to spark plugs of an internal combustion engine, shaft means rotatably supported by said base extending into said cap, a metallic breaker plate supported by said base and adjustable relative thereto, a breaker cam driven by said shaft, rotor means driven by said shaft including conductive means operative to connect the center electrode and saId circumferentially spaced electrodes of said cap, breaker contacts supported from one wall of said breaker plate operated by said breaker cam, a capacitor supported from said one wall of said breaker plate including a capacitor lead wire connected to one of said breaker contacts, and a generally cup-shaped metallic radio interference shield supported by said breaker plate and defining with said breaker plate a substantially closed metallic housing containing said breaker contacts, said capacitor, said capacitor lead wire and said breaker cam, said metallic housing comprising first and second complementary metallic shield parts removably secured to said breaker plate by removable fastener means, said shield parts having overlapping edges which engage each other and having semicircular openings defining a central opening that receives said shaft means for driving said rotor of said distributor, said shield located within said distributor cap and having an outer wall spaced from the inner wall of said cap.

2. The ignition distributor according to claim 1 where said first and second metallic shield parts each have lower edges engaging the peripheral edge of said breaker plate, and wherein said first shield part has inwardly struck portions engaging the top surface of said breaker plate and wherein an edge of the second shield part overlaps a complementary edge of the first shield part and further wherein said shield parts are secured to said breaker plate by screws threaded into said breaker plate.

3. A shielded breaker plate and breaker contact assembly for an ignition distributor comprising, a laterally extending metallic breaker plate having an opening for receiving a distributor shaft, breaker contact means supported from one wall of said breaker plate, a capacitor supported from said one wall of said breaker plate, a capacitor lead wire connected to one breaker contact of said breaker contact means, and a generally cup-shaped metallic shield secured to said breaker plate having a central opening aligned with the opening in said breaker plate, said metallic shield comprised of first and second shield parts having overlapping engaging edges and having semicircular complementary openings defining said central opening in said shield, said first and second shield parts each having lower edges engaging the peripheral edge of said breaker plate and at least one of said shield parts having an inwardly struck portion engaging said one wall of said breaker plate, and removable fastener means for respectively removably securing said shield parts to said breaker plate, said shield and breaker plate defining a metallic enclosure containing said breaker contact means, said capacitor and said capacitor lead wire, said enclosure serving to reduce radiation of radio interference signals developed within said enclosure during use of said assembly.