US3754202A

US3754202A - Grounding plug

Info

Publication number: US3754202A
Application number: US00185123A
Authority: US
Inventors: R Nelson
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-09-30
Filing date: 1971-09-30
Publication date: 1973-08-21
Anticipated expiration: 1990-08-21

Abstract

An electrical connector plug for providing connection to two or more sides of a circuit with conventional prong means and having a grounding prong which can be inserted into a grounding socket in a receptacle. The grounding prong is provided with a protruding sensing finger that is in an inactive position when the receptacle has a grounding socket, but if there is no grounding socket present, then the protruding sensing finger strikes a solid surface of the receptacle and moves backward to trigger a latch means that unlatches the grounding prong and allows it to be pushed back into the body of the plug, allowing the circuit contact prongs to enter the receptacle''s live sockets to complete the electrical circuit. In order to prevent accidental triggering of this sensing finger by finger pressure on the end of the sensing finger or by force exerted on the axially-shiftable portion of the grounding prong, which might overcome the safety feature of the grounding prong, by finger pressure on the end of the grounding prong, the mechanism may be safetied by an arrangement for the latch means, such that pressure applied to the protruding prong by a yieldable means such as a finger, or other yieldable portion of the body, will push the entire grounding prong back toward the plug but will prevent the protruding sensor finger from triggering the lever, therefore preventing the retraction of the grounding prong into the plug. As an additional feature, the grounding prong can be designed so that it will never be completely retracted into the plug and a grounding plate is provided for attachment to a receptacle so that when the grounding prong is used in conjunction with a receptacle not having a matching grounding socket, the retracted grounding prong will still contact the grounding plate and provide for the safe grounding of the circuit.

Description

United States Patent [191 Nelson [451 Aug. 21, 1973 1 1 GROUNDING PLUG Richard B. Nelson, 3160 Hollycrest Dr., Hollywood, Calif. 90028 [22] Filed: Sept. 30, 1971 [21] Appl. No.: 185,123

[76] Inventor:

[52] 11.8. C1. 339/14 P, 339/31 R [51] Int. Cl H0lr 3/06 [58] Field of Search 339/14 R, 14 P, 14 RP,

[56] References Cited UNITED STATES PATENTS 3,308,415 3/1967 Cramer et a1. 339/14 R 2,984,808 5/1961 Bender 339/14 RP 2,299,390 1/1967 Eckelkamp..... 339/12 3,025,486 3/1962 Falconer 339/31 R 3,440,591 4/1969 Whalen 339/14 R Primary Examiner-Marvin A. Champion Assistant ExaminerRobert A. l-laffer [57] ABSTRACT cle has a grounding socket, but if there is no grounding socket present, then the protruding sensing finger strikes a solid surface of the receptacle and moves backward to trigger a latch means that unlatches the grounding prong and allows it to be pushed back into the body of the plug, allowing the circuit contact prongs to enter the receptacles live sockets to complete the electrical circuit. In order to prevent accidental triggering of this sensing finger by finger pressure on the end of the sensing finger or by force exerted on the axially-shiftable portion of the grounding prong, which might overcome the safety feature of the grounding prong, by finger pressure on the end of the grounding prong, the mechanism may be safetied by an arrangement for the latch means, such that pressure applied to the protruding prong by a yieldable means such as a tinger, or other yieldable portion of the body, will push the entire grounding prong back toward the plug but will-prevent the protruding sensor finger from triggering the lever, therefore preventing the retraction of the grounding prong into the plug. As an additional feature, the grounding prong can be designed so that it will never be completely retracted into the plug and a grounding plate is provided for attachment to a receptacle so that when the grounding prong is used in conjunction with a receptacle not having a matching grounding socket, the retracted grounding prong will still contact the grounding plate and provide for the safe grounding of the circuit.

28 Claims, 19 Drawing Figures 34 H 4 55 5 Z8 39 I/ 4Vll\\\\\ m [24 ,1,

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PAIENIEDmcznm 3754;202- sum 2 or 2 i INVENTOR. RICHARD 5. NELSON v b.6-2a- ATTOR/VA'YS.

1 GROUNDING PLUG BACKGROUND OF THE INVENTION This invention relates to an electrical plug for attachment of a source of electrical power through an electrical cord to an electrical circuit or machine, with means provided for a separate grounding contact.

Household electrical appliances, portable machinery, and other devices utilizing electrical circuitry, which are connected to a power source through electrical cords having a plug installed on the end with which to engage an electrical receptacle have conventionally had two or more conductors in the electrical cord connected to prongs in a plug, which engage contacts in sockets in the electrical receptacle. In order to reduce the hazards of electrical shock to the user of the appliance, a separate grounding wire has been used in the electrical cord, the grounding wire being attached to the frame of the appliance and to a grounding prong on the electrical plug, which will be engaged with a grounding socket in the electrical receptacle. In some other varieties of plugs, the grounding wire has protruded from one side of the plug, and was adapted to be attached to a grounded screw on the receptacle after the plug had been inserted into the normal power sockets in the electrical receptacle. However, attaching the ground wire after plugging the plug into the receptacle is dangerous if a short circuit exists which would energize the frame of the appliance, and it was often inconvenient to attach the wire before the plug was inserted into the receptacle.

Further, an appliance equipped with a grounding plug having a protruding grounding prong could not be engaged into a receptacle that did not have provision for receiving the grounding prong into a grounding opening for completing the grounding function. Therefore, adaptors have to be provided and kept on hand in such cases. These adaptors are sometimes supplied as three pronged grounding plugs, but often they become separated and lost causing the user to accomplish the insertion of the plugs by breaking off the grounding prongs of the connector, or by other hazardous means.

Prior art such as US. Pat. No. 3,171,113 dated Feb. 23, 1965 for An Electric Plug with Ground Indicating Light" provides for manual retraction of a grounding prong for use in a socket not adapted for grounding. However, such a device would leave the grounding prong retracted until such time as it is manually released. Thus, the user could use the plug in a three prong grounded socket with the grounding prong held in a retracted position, thus defeating the purposes of an automatic safety grounding device.

Similar disadvantages may be seen in the Three Way Electric Plug, patented Mar. 13, 1962 under U.S. Pat. No. 3,025,486 which achieves the manual retraction of a grounding prong by means of a push button which retracts and locks the grounding pin in a retracted position and thus can defeat the safety purpose of the grounding prong.

U.S. Pat. No. 2,323,736 issued on July 6, 1943 for an Attachment Plug," has similar propensities for overcoming the safety features of the grounding device as do US. Pat. No. 3,308,415 issued on Mar. 7, 1967 for a Self-Adjusting Three Pronged Electric Plug Adapted for Either Two or Three Pronged Electric Sockets," and US. Pat. No. 2,922,134 for an Electrical Connector Plug issued on Jan. 19, 1960.

In contrast to the above reference background patents, an object of this invention is to provide a grounding electrical attachment plug which can be used with socket receptacles having two or more receiving apertures, but providing for automatic retraction of the grounding prong when a corresponding receptacle aperture is not present.

A further object of the invention is to provide for spring biasing of the grounding prong to hold the prong in an extended position unless it is forcibly retracted and held in a retracted position by the mechanical urging of a contact with a receptacle surface. A further object of the invention is to provide for a mechanical latch means which will hold an extended grounding prong in position in a manner such that pressure of insertion of the grounding prong into the receptacle will not cause it to retract. A further object of the invention is to provide a sensing finger protruding from the grounding prong to release the latch means if no matching aperture exists in the electrical receptacle.

A further object of the invention is to provide a safety device such that pressure applied to the sensing finger of the grounding prong by resilient means such as a human tissue, will simply move the grounding prong backwards together with the sensing finger and prevent release of the grounding prong for telescoping motion into the plug body.

Still another object of the invention is to provide an electric grounding means for a receptacle that is not provided with a grounding aperture. Yet another object is to provide for automatic contact of the grounding prong with an electrical grounding means provided externally on an electrical receptacle.

All of these objects and advantages of the present invention are designed to provide a grounding plug which cannot have its safety features easily overcome by the user and which will provide automatic grounding of plugs in either grounded receptacles having grounding prong receiving means, or in grounded receptacles to which external grounding means can be added for automatic contact with partially retracted grounding prongs. In all cases the grounding prong retraction is independent of the user of the grounding plug and is accomplished automatically when the prongs of the connector plug are inserted into an electrical receptacle.

The above and other features of this invention will be fully understood from the following detailed description and the accompanying drawings, in which;

FIG. 1 is an axial view in compound cross-section of the presently preferred embodiment according to the invention;

FIG. 2 is a right hand end view of FIG. 1;

FIG. 3 is a cross-section taken on line 3-3 of FIG.

FIG. 4 is a cross-section taken on line 4-4 of FIG.

FIG. 5 is a fragmentary cross-section of the embodiment of FIG. 1 with parts shown in displaced position;

FIG. 6 is a fragmentary cross-section of another embodiment of the invention;

FIG. 7 is a cross-sectional view taken on line 7-7 of FIG. 6;

FIG. 8 is a side view of the embodiment of FIG. 1 shown in partial cross-section, and in assembly with an electrical receptacle;

FIG. 9 is a view taken on line 9-9 of FIG. 8.

FIG. 10 is a section taken at line 10-10 of FIG. 9;

FIG. 11 is a bottom view of a part of the assembly of FIG. 9;

FIGS. 12 and 13 are plan views of alternate constructions for a part of the assembly in FIG. 9',

FIG. 14 is an end view of another connector useful with the invention;

FIG. 15 is a side view of another receptacle useful with the invention;

FIG. 16 is a side view of a connector complementary to the device of FIG. 15;

FIG. 17 is an end view taken at line 17-17 of FIG. 15; and

FIGS. 18 and 19 are side and top views, respectively, of an alternate embodiment for the device of FIG. 15.

The preferred embodiment of this invention is shown as connector plug 14. This plug includes a body 15, made of insulative material, with an entry port 16, which is adapted to receive an electrical cord 17. The electrical cord 17 contains three conductors 18, 19 and 20 in the form of insulated wire. The body 15 has two electrically conductive prong members 21 and 22 embedded within the body 15 and extending therefrom. The body 15 is an elongated structure having a longitudinal axis 23 and the prong members 21 and 22 are held parallel to each other and to axis 23. Electrical conductors 18 and 19 are secured to the prong members 21 and 22 for electrical contact and transmission of electrical current flow. Conductor 20 is attached to v a grounding prong 24a.

Grounding prong member 24a extends parallel to axis 23 of the plug 14, and includes an axially shiftable conductive element 24 and an outer tube 28. Outer tube 28 may be conveniently molded or otherwise fixed in position when the body 15 is initially manufactured. It is often convenient to make a body such as body 15 in a plastic molding process, in which case outer tube 28 can be molded in position by preplacement within a die cavity. Intermediate tube 29 is inserted into the outer tube 28. The outer tube 28 is then swaged to fold over the protruding end 30 of outer tube 28, and the swedged end prevents the intermediate tube 29 from being withdrawn from the assembly. Outer tube 28 has one end 31 flattened and modified for attachment of the conductor 20. The combination of the outer tube 28, intermediate tube 29, and axially shiftable conductive element 24 form a grounding prong member. The combination of the outer tube 28 and the intermediate tube 29 is sometimes referred to as an outer tubuler member which is fixed to the body. Intermediate tube 29 is provided with a rectangular slot 32 piercing one wall from the exterior surface to an axial passage 33. A cross pin 34 (FIG. 4) is installed transverse to the axis of the outer tube 28 and the intermediate tube 29 to hold them in a relative fixed position so that the intermediate tube 29 which is already restrained from being withdrawn from the outer tube 28 by the swedged end 30 will be prevented from being pushed further into the body 15. A compression spring 35 is contained within the passage 33 of intermediate tube 29 and is reacted against the transverse pin 34. The axially shiftable conductive element 24 is slidably inserted into the bore 33 of the intermediate tube 29 that is, it is shiftable in, and embraced by, the wall of passage 33.

Axially shiftable conductive element 24, which is of hollow tubular construction, has a transverse bellcrank lever pin 36 (FIGS. 1 and 3) mounted to it on a diameter transverse to the longitudinal axis of the grounding prong 24a. Pin 36 extends at either or both of its ends into one or both of slots 36a and 36b in intermediate tube 29. This holds the axially shiftable conductive element 24 against rotation and against expulsion from the body. Swivally mounted to the pin 36 is a bell-crank lever 37 which has two protruding

lever arms

38 and 39, which are displaced at right angles to each other. The lever 37 and its arm 39 are sometimes called latch means." Slot 32 in the intermediate tube 29 is axially aligned with slot 40 of axially shiftable conductive element 24. Bias spring 35 presses against lever arm 39 (sometimes called an upstanding" lever arm) to bias it toward its extended position sometimes called an upstanding position as illustrated in FIG. 1) wherein it stands in

slots

32 and 40 and exerts a latching action yet to be described. A cap plug 42 (sometimes called cap plug means) is installed in the clyindrical bore 43 of the axially shiftable conductive element 24. Capplug 42 has a longitudinal bore 44 which slidably mounts a sensing pin 45. Cap plug 42 is constructed of electrically conductive material. Sensing pin 45 protrudes beyond the face 46 of cap plug 42 but is prevented from withdrawal from cap plug 42 by a formed protuberance 47 intermediate on the shank of sensing finger 45 (sometimes called sensing finger means"). Sensing finger 45 is restrained at its internal end by lever arm 39 of a bell-crank lever 37.

For convenience of construction, and of making electrical connections, the body 15 of the electrical plug may be made in two

sections

50 and 51 and these sections may conventionally be held together with screws such as 53, or any other convenient clamping method. The cap plug 42 should make a tight fit in the grounding prong 24 to prevent accidental expulsion.

In use in a conventional grounding receptacle where there would be socket apertures to receive the electrical contact prongs 21 and 22, and the grounding prong 24a, the plug 14 would act as a conventional three conductor grounding plug. However if there is no provision for the grounding prong member 240 in the receptacle then the end of sensing finger 45 would be pushed back to the solid face of the receptacle as is shown in FIG. 5. Pushing back sensing finger 45 removes the lever arm 39 of the bell-crank lever 37 from the slot 32 of intermediate tubular member 29, thus permitting axially shiftable conductive element 24 to bepushed backwards against compression spring 35 into the bore 33 of the intermediate tubular member 29. It will be observed that bias spring 35 yieldingly biases element 24 toward the protruding position of FIG. 1, and that it also yieldingly rotatably biases bell-crank lever 37 to ward its latched (upstanding) position shown in FIG. 1. However, if an individual tried to defeat the protection of the grounding prong 24 by applying finger pressure (the finger being yieldable tissue) to the sensing finger 45, the entire element 24 with the sensing pin 45 would travel backwards for the length of slot 32 in intermediate tubular member 29 without causing sufficient retraction of the sensing pin to turn lever 37 sufficiently to remove it from slot 32 before the finger also contacts the end of element 24. Then element 24 moves back along with the sensing finger. As soon as lever arm 39 contacted the end of slot 32, it would be impossible for the sensing finger 45 to release the lever arm 39 from engagement with slot 32 and therefore the axially shiftable conductive element 24 would remain locked in relative position to the intermediate tubular member 29 and could not be retracted into the bore 33. This prevents manual override of the safety system. This same result would occur were retraction attempted by pushing on element 24 without first retracting the sensing finger. This prevents retraction by frictional engagement with the wall of the receptacle as the element 24 is pressed into it. When the plug 14 is withdrawn from the receptacle a retracted element 24 will automatically spring forward to protrude from the body and be locked in its grounding position by bell-crank lever 37.

As an ancillary improvement, a butterfly shaped grounding plate 55 may be attached by means of the center plate mounting screw 56 which is conventionally used in wiring receptacle boxes, and the grounding plate 55 is made of the length that will contact the faces of receptacle sockets 57 and 58 (FIG. 13). Since a receptacle box 59 is normally a grounded portion of the circuit, the ground will be transmitted through the screw 56 and through the conductive member 55. Therefore, when the plug 14 is inserted into the receptacle 57 the axially shiftable conductive element 24 is pressed into the bore 33 against the compressive spring 35 bias, which in turn presses the element 24 into contact with the grounding plate member 55, therefore providing an automatic ground. There can be no defeat for this system because the element 24 is spring loaded to protrude for whatever distance is allowed by the receptacle.

Grounding plate 55 is preferably made so it can be snapped into place. Accordingly, its central region 70 has a hole 71 therethrough to pass the grounding screw, and also four fingers 72 bent away from the central region to clamp onto the edges which bound

apertures

73, 74 in a cover plate. Contact surfaces 75, 76 project to the area where they will be contacted by the element 24. The surfaces are aligned by the contact made between the cover plate and the fingers. Plate 55 can thereby be snapped into place before being held by the screw.

Depending on the orientation of the apertures in the receptacle which are exposed through

apertures

73 and 74, it is possible that there may be a situation where a contact surface may not be contacted by the element 24. Accordingly, an extension 80 (FIG. 12) may be provided at one side of the central region, or two

extensions

81, 82, one at each side (FIG. 13). The arrangements of FIGS. 12 and 13 can assure that there is no receptacle arrangement wherein a grounding action does not occur.

FIG. 14 shows a plug which is coded so there will be no possible reversal of polarity, and grounding would always be assured. In this construction, a plug 85 includes a grounding prong member 86 according to this invention, plus a larger and a smaller

conductive prong

87, 88, respectively whose sizes are coded to the hole size in the receptacle so prongs 87 and 88 cannot be reversed. i

FIGS. l5, l6 and 17 show a reversal of parts such as might occur where a cord set is used with an appliance. In such an arrangement, an appliance 90 such as a coffee pot, is provided with two circuit prongs 91, 92 and a retractable grounding prong member of the type shown in FIG. 1. There are mounted to the appliance rather than to a cord-carried plug.

A cord-carried receptacle 94 is attached to a threeconductor cord, and includes

apertures

95, 96 for prongs 91 and 92, and an aperture 97 for member 93. When aperture 97 is present, the prong enters it. If it is absent, prong 93 retracts.

Incidentally, an extra ground lead 100 is shown attached to the ground lead. It has a spade attachment 101 to be attached to a grounded screw if desired. If this feature is not used, lead 100 is merely cut off.

FIGS. 18 and 19 illustrate another possible usage of the device of FIG. 16. It may be that, rather than insert the ground prong into a grounded aperture, it would be preferred for a springy abutment contact to be made. Accordingly, a grounded strap 102 may be bent over the end of receptacle 94, and when it is attached to the appliance, prongs 91 and 92 enter the respective apertures, but member 93 will be triggered, and will be pressed inwardly against its bias spring. This will constitute a reliable ground for the appliance.

An alternate construction to accommodate the sensing finger 45 is shown in FIG. 6 where the sensing finger 45 lies adjacent to the element 24 in a channel formed in the exterior of the element 24, rather than totally within the element. What does exist in common is that the end of the sensing finger is spaced axially farther from the face of the plug than is the end of element 24.

As an additional feature, cap plug 42 may be made of a magnetized material and the conductor 55 may be made of a magnetically permeable material so that the element 24 will be magnetically held in firm, mutual contact with the conductor member 55.

It is to be understood that the grounding conductor 20 would be grounded to the frame of an appliance or other electrical device, and that the ground receptacle or contact within the receptacle to receive the plug 14 would be connected to electrical ground.

In all embodiments, where the element 24 and the intermediate tube 29 or their equivalent are defined as tubular, the term tubular is not intended to be limited to a structure having concentric walls, or even a cylindrical wall. It is intended to define elements of an assembly which will telescope, and which will receive such internal elements as are required.

The shape of the outside wall of the element 24 can be selected to fit apertures of any shape, all of which fall within the scope of this invention. 1

Therefore, this electrically grounded plug provided the convenience of a multiple pronged plug system having an automatically retractable grounding prong member that is actuated only when a receptacle is not provided for such a member, but yet the prong can be safe from manual tampering by the user since it cannot be actuated except by the surface of the receptacle, unless willfully done with additional tooling. In addition the grounding prong member is spring-biased so that it may engage grounding means placed on a receptacle for grounding safety when a receptacle does not have provision for receiving a grounding prong within an aperture.

This invention is not to be limited by the embodiment shown in the drawings and described in the description, which are given by way of example and not of limitation, but only in accordance with the scope of the appended claims.

I claim:

1. An electrical connector plug which comprises:

a. a body member, said body member having a longitudinal axis;

b. a plurality of electrically conductive contact prong members, said electrical contact prong members being mounted to said body member in position to protrude from said body member parallel to said longitudinal axis, said electrical contact prong members being adapted to be attached to electrical conductors for transmission of electrical current;

c. at least one grounding prong member, said grounding prong member comprising an axially shiftable conductive element being carried by the body member, and shiftable within the body member, said grounding prong member being adapted to protrude axially from and to be retracted into said body member, said grounding prong member being adapted to be attached to a grounding conductor;

d. latch means movably mounted to the plug selectively to permit or to limit axial movement of the axially shiftable conductive element;

e. bias spring means, said bias spring means biasing the axially shiftable conductive element toward its protruding position;

f. sensing finger means movably mounted to the plug and so disposed and arranged relative to the latch means as to move the latch means to permit axial movement of the axially shiftable conductive element into the body member when said sensing finger undergoes an axial force, said sensing finger means projecting beyond the end of said axially shiftable conductive element unless it is retracted by said axial force.

2. A device according to claim 1 in which said sensing finger means is disposed adjacent to said grounding prong member.

3. A device according to claim 1 in which said grounding prong member includes a conductive outer tubular member fixed to said body member, said axially shiftable conductive element being slidably mounted within said outer tubular member, said axially shiftable conductive element being adapted to protrude from said body member parallel to said longitudinal axis, the outer tubular member being adapted to be attached to a grounding conductor.

4. A device according to claim 3 in which the axially shiftable tubular element includes a bore through which the sensing finger means passes, and in which it is axially shiftable, the biasing on the sensing finger means biasing one of its ends to protrude beyond the respective end of the axially shiftable conductive element.

5. A device according to claim 4 in which the latch means is pivotally mounted to the axially shiftable conductive element, in which the outer tubular member includes an axially extending slot, and in which the latch means includes an upstanding arm adapted to enter said last-named slot and engage said wall of said axial passage to prevent retraction of the axially shiftable conductive element except when the sensing finger means is retracted into the axially shiftable conductive element.

6. A device according to claim 5 in which the bias spring means also biases the latch means toward its inter-engaged position.

7. A device according to claim 5 in which the said last-named slot is of such length as to permit a limited, but not total, amount of retraction of the axially shiftable conductive element even when the sensing finger means is not retracted. Y

8. A device according to claim 3 in which said latch means is formed as a bell-crank lever means adapted to lock said axially shiftable conductive element in a protruding axial position with respect to said body memher.

9. A device according to claim 3 in which cap plug means of electrically conductive material is fixedly inserted in said axially shiftable conductive element, said cap plug means being adapted to enter an electrical receptacle as a leading part of said grounding prong member.

10. A device according to claim 9 in which said cap plug means is constructed of a magnetically permeable material having electrically conductive properties.

11. A device according to claim 9 in which said cap plug means is constructed of a magnetized material having eletrically conductive properties.

12. A device according to claim 9 in which said cap plug means has an axial bore therethrough, said bore being adapted to slidably mount said sensing finger means, said sensing finger means being adapted to protrude from said cap plug means.

13. A device according to claim 8 in which said outer tubular member and said axially shiftable conductive element have co-registering axially extending slots, and the bell-crank lever means includes an upstanding arm, said slot in said outer tubular member being aligned with said slot in said axially shiftable conductive element, said slots being adapted to engage the upstanding lever arm of said bell-crank lever means for locking said axially shiftable conductive element in a protruding position with respect to said body member, said spring bias means being adapted to prevent said sensing finger means from releasing said bell-crank lever means when said sensing finger means is contacted by a fleshlike resilient material whereby said fleshlike resilient material will yield until contact is made with said grounding prong to push back said axially shiftable conductive element against said spring bias means thereby to lock said upstanding arm of said bell-crank lever means in said slot of'said outer tubular member to prevent sliding of said axially shiftable conductive element into said body member.

14. In combination: an electrical receptacle having sockets for receiving electrical conductor prongs, an electrical plug, and interconnecting grounding means, said combination comprising:

grounding plate means mounted to the electrical receptacle and having a conductive contact surface facing away from the sockets; V

a plug body member, said plug body member having a longitudinal axis;

a plurality of electrically conductive contact prong members, said electrical contact prong members being mounted to said body member and protruding from said body member parallel to said longitudinal axis, said electrical contact prong members being adapted to be attached to electrical conductors for transmission of electrical current;

at least one grounding prong member, said grounding prong member comprising an axially shiftable conductive element being carried by the body member, and shiftable in and embraced by the wall of an axial passage in the body member, said grounding prong member being adapted to protrude axially from and to be retracted into said body member, said grounding prong member being adapted to be attached to a grounding conductor;

latch means movably mounted to the plug selectively to permit or to limit axial movement of the axially shiftable conductive element;

bias spring means, said bias spring means biasing the axially shiftable conductive element toward its protruding position;

sensing finger means movably mounted to the plug and so disposed and arranged relative to the latch means as to move the latch means to permit axial movement of the axially shiftable conductive element into the body member when said sensing finger undergoes an axial force, said sensing finger means projecting beyond the end of said axially shiftable conductive element unless it is retracted by said axial force.

15. A combination according to claim 14 in which said sensing finger means is disposed adjacent to said grounding prong member.

16. A combination according to claim 14 in which said grounding prong member includes a conductive outertubular member fixed to said body member, said axially shiftable conductive element being slidably mounted within said outer tubular member, said axially shiftable conductive element being adapted to protrude from said body member parallel to said longitudinal axis, said grounding prong member being adapted to be attached to a grounding conductor.

17. A combination according to claim 16 in which the axially shiftable tubular element includes a bore through which the sensing finger means passes, and in which it is axially shiftable, the biasing on the sensing finger means biasing one of its ends to protrude beyond the respective end of the axially shiftable conductive element.

18. A combination according to claim 17 in which the latch means is pivotally mounted to the axially shiftable conductive element, in which the outer tubular member includes an axially extending slot, and in which the latch means includes an upstanding arm adapted to enter said last-named slot and engage said wall of said axial passage to prevent retraction of the axially shiftable conductive element except when the sensing finger means is retracted into the axially shiftable conductive element.

19. A combination according to claim 18 in which the bias spring means also biases the latch means toward its inter-engaged position.

20. A combination according to claim 18 in which the said last-named slot is of such length as to permit a limited, but not total, amount of retraction of the axially shiftable conductive element even when the sensing finger means is not retracted.

21. A combination according to claim 16 in which said latch means is formed as a bell-crank lever means adapted to lock said axially shiftable conductive element in a protruding axial position with respect to said body member.

22. A combination according to claim 16 in which cap plug means of electrically conductive material is fixedly inserted in said axially shiftable conductive element, said cap plug means being adapted to enter an electrical receptacle as a leading part of said grounding prong member.

23. A combination according to claim 22 in which said cap plug means is constructed of a magnetically permeable material having electrically conductive properties.

24. A combination according to claim 22 in which said cap plug means is constructed of a magnetized material having electrically conductive properties.

25. A combination according to claim 22 in which cap plug means has an axial bore therethrough, said bore being adapted to slidably mount said sensing finger means, said sensing finger means being adapted to protrude from said cap plug means.

26. A combination according to claim 21 in which said outer tubular member and said axially shiftable conductive element have co-registering axially extending slots, and the bell-crank lever means includes an upstanding area, said slot in said outer tubular member being aligned with said slot in said axially shiftable conductive element, said slots being adapted to engage the upstanding lever arm of said bell-crank lever means for locking said axially shiftable conductive element in a protruding position with respect to said body member, said spring bias means being adapted to prevent said sensing finger means from releasing said bell-crank lever means when said sensing finger means is contacted by a fleshlike resilient material whereby said fleshlike resilient material will yield until contact is made with said grounding prong to push back said axially shiftable conductive element against said spring bias means thereby to lock said upstanding arm of said bell-crank lever means in said slot of said outer tubular member to prevent sliding of said axially shiftable conductive element into said body member.

27. A combination according to claim 14 in which said grounding plate means is constructed of magnetically permeable electrically conductive material.

28. A combination according to claim 14 in which said grounding plate means is constructed of magnetized electrically conductive material.

Claims

1. An electrical connector plug which comprises: a. a body member, said body member having a longitudinal axis; b. a plurality of electrically conductive contact prong members, said electrical contact prong members being mounted to said body member in position to protrude from said body member parallel to said longitudinal axis, said electrical contact prong members being adapted to be attached to electrical conductors for transmission of electrical current; c. at least one grounding prong member, said grounding prong member comprising an axially shiftable conductive element being carried by the body member, and shiftable within the body member, said grounding prong member being adapted to protrude axially from and to be retracted into said body member, said grounding prong member being adapted to be attached to a grounding conductor; d. latch means movably mounted to the plug selectively to permit or to limit axial movement of the axially shiftable conductive element; e. bias spring means, said bias spring means biasing the axially shiftable conductive element toWard its protruding position; f. sensing finger means movably mounted to the plug and so disposed and arranged relative to the latch means as to move the latch means to permit axial movement of the axially shiftable conductive element into the body member when said sensing finger undergoes an axial force, said sensing finger means projecting beyond the end of said axially shiftable conductive element unless it is retracted by said axial force.

7. A device according to claim 5 in which the said last-named slot is of such length as to permit a limited, but not total, amount of retraction of the axially shiftable conductive element even when the sensing finger means is not retracted.

8. A device according to claim 3 in which said latch means is formed as a bell-crank lever means adapted to lock said axially shiftable conductive element in a protruding axial position with respect to said body member.

13. A device according to claim 8 in which said outer tubular member and said axially shiftable conductive element have co-registering axially extending slots, and the bell-crank lever means includes an upstanding arm, said slot in said outer tubular member being aligned with said slot in said axially shiftable conductive element, said slots being adapted to engage the upstanding lever arm of said bell-crank lever means for locking said axially shiftable conductive element in a protruding position with respect to said body member, said spring bias means being adapted to prevent said sensing finger means from releasing said bell-crank lever means when said sensing finger means is contacted by a fleshlike resilient material whereby said fleshlike resilient material will yield until contact is made with said grounding prong to push back said axially shiftable conductive element against said spring bias means thereby to lock said upstanding arm of said bell-crank lever means in said slot of said outer tubular member to prevent sliding of said axially shiftable conductive element into said body member.

14. In combination: an electrical receptacle having sockets for receiving electrical conductor prongs, an electrical plug, and interconnecting grounding means, said combination comprising: grounding plate means mounted to the electrical receptacle and having a conductive contact surface facing away from the sockets; a plug body member, said plug body member having a longitudinal axis; a plurality of electrically conductive contact prong members, said electrical contact prong members being mounted to said body member and protruding from said body member parallel to said longitudinal axis, said electrical contact prong members being adapted to be attached to electrical conductors for transmission of electrical current; at least one grounding prong member, said grounding prong member comprising an axially shiftable conductive element being carried by the body member, and shiftable in and embraced by the wall of an axial passage in the body member, said grounding prong member being adapted to protrude axially from and to be retracted into said body member, said grounding prong member being adapted to be attached to a grounding conductor; latch means movably mounted to the plug selectively to permit or to limit axial movement of the axially shiftable conductive element; bias spring means, said bias spring means biasing the axially shiftable conductive element toward its protruding position; sensing finger means movably mounted to the plug and so disposed and arranged relative to the latch means as to move the latch means to permit axial movement of the axially shiftable conductive element into the body member when said sensing finger undergoes an axial force, said sensing finger means projecting beyond the end of said axially shiftable conductive element unless it is retracted by said axial force.

16. A combination according to claim 14 in which said grounding prong member includes a conductive outer tubular member fixed to said body member, said axially shiftable conductive element being slidably mounted within said outer tubular member, said axially shiftable conductive element being adapted to protrude from said body member parallel to said longitudinal axis, said grounding prong member being adapted to be attached to a grounding conductor.

24. A combination according to claim 22 in which said cap plug means is constructed of a magnetized material having electrically conductive properties. said