A grounding connection for a wiring device includes a mounting strap having a screw-receiving aperture by which the strap is secured to an outlet box and a single spring wire extending across the aperture and adapted to exert pressure on the shank of a screw inserted between an edge of the aperture and the wire. The ends of the wire are restrained from movement in a direction opposing the force applied to the wire by the screw shank, but at least one end of the wire is capable of lateral movement with respect to the screw so that bowing of the wire is possible.
What is claimed is
1. A grounding connection for use with a wiring device which is adapted to be mounted in an outlet box, comprising, in combination, a mounting strap secured to the wiring device and including a screw receiving aperture, at least one screw having a shank which can be loosely received within said aperture for securing said mounting strap to said outlet box, a spring wire extending across said aperture and adapted to exert pressure on the threaded portion of the shank when it is inserted between an edge of said aperture and said wire, said mounting strap including openings for receiving the ends of said wire, said openings restraining the wire ends from movement in the direction of the force applied to the wire by said shank, at least one of said openings permitting its associated wire end to move in a direction generally transverse to said first direction.
2. A grounding connection according to claim 1, wherein the portion of the mounting strap forming said edge is recessed so that it lies substantially in the same plane as said wire.
3. A grounding connection according to claim 2, wherein said openings comprise a first opening approximately the same size as the diameter of the wire and an elongated slot in an edge of the strap.
4. A grounding connection according to claim 1, wherein said screw includes at least one enlarged lobe having a radial dimension greater than the radial dimension of the remaining portion of the screw.
5. A grounding connection according to claim 4, wherein said screw is coated with a soft metallic material.
This invention relates to grounding connections for use with wiring devices. More specifically, the present invention relates to a wiring device, such as a wall receptacle, having an improved ground connection between the mounting strap of the device and a standard outlet box.
It is common practice to provide for the grounding of electrical appliances so that in the event of a circuit malfunction (for example), the exposed metallic portions of the appliance cannot reach a dangerous voltage. Consequently, many (if not most) electrical receptacles include a grounding contact through which the exposed metallic portions of the appliance can be connected to the system ground. Generally, this system ground is the potential of the standard outlet box in which the receptacle or other wiring device is mounted. Since the grounding contact is usually electrically connected to the mounting strap, such receptacles include some means for electrically connecting the mounting strap to the outlet box. The present invention provides an advantageous way of making this connection.
A device related to this invention is illustrated in Muska U.S. Pat. No. 3,432,793. The Muska patent discloses various types of metallic clips capable of exerting a yieldable pressure on the shank of the screw used to fasten the wiring device to the outlet box. The Muska device is relatively simple and inexpensive, and provides the desired low resistance connection between the mounting strap and outlet box.
The present invention is an improvement over the devices illustrated and described in the Muska patent in that it is of simpler construction (and thus easier to manufacture) and capable of providing a more secure contact with the screw shank. Hence, a superior connection can be made with this invention. Moreover, because of its simplicity, it is easier to assemble than the devices illustrated in the Muska patent.
The invention is described in detail below with reference to the annexed drawings wherein:
FIG. 1 is a front elevation view, partially broken away, showing a typical wiring device incorporating the invention;
FIG. 2 is a slightly enlarged side sectional view along the line 2--2 of FIG. 1;
FIG. 3 is a rear sectional view along the line 3--3 of FIG. 2;
FIG. 4 is a substantially enlarged side view of a fastening screw according to the invention; and
FIG. 5 is a bottom view of the screw.
The invention is illustrated and described for use with a common duplex wall receptacle; however, the invention is not limited to any particular type of wiring device and may be used with any wiring device (such as a switch or other type of receptacle) where it is necessary or desirable to electrically connect a mounting strap or the like to an outlet box.
In FIG. 1 a standard outlet box 10 is shown. Typically, outlet box 10 will be electrically connected to the ground of the electrical system in which it is connected by means which are not illustrated. The wiring device comprises a duplex receptacle 12 which is mounted within the outlet box 10 by means of a mounting strap 14. Mounting strap 14 is secured both electrically and mechanically to the outlet box 10 by means of a pair of screws 16 (only one of which is illustrated) passing through screw-receiving apertures 18 in strap 14 and then into threaded engagement with a suitably tapped bore 19 in the outlet box.
The outlet box 10 is customarily mounted within the wall of a house or other building and covered by means of a cover plate 20. Each of the two receptacles comprising the duplex receptacle 12 includes two sockets 22 for making the usual electrical connection and a grounding socket 24 which, as mentioned briefly above, permits the exposed portions of the appliance (for example) to be grounded to the outlet box 10 when a plug (not shown) is inserted into the receptacle.
In accordance with the invention, a single spring wire 26 extends across the screw-receiving aperture 18 behind the strap 14. One end 26a of wire 26 is anchored within a small aperture 28 within mounting strap 14. The other end 26b of the wire 26 is free to slide in an elongated slot 30 extending inwardly from an edge of mounting strap 14. The wire end 26a within opening 28 may be bent back over itself and the end 26b may be bent at right angles to the wire to prevent accidental dislodging.
The edge of the screw-receiving aperture 18 facing wire 26 is shown at 32 in FIGS. 2 and 3. It is formed on a recessed portion 33 which is formed within mounting strap 14 so that it extends into approximately the same plane as wire 26. Screw 16 is held within aperture 18 between edge 32 and the central portion of wire 26. Because portion 32 is recessed, the tendency of the screw to deviate from the illustrated horizontal position (FIG. 2) is minimized.
A problem can arise in making a good electrical connection between screw 16 and outlet box 10 if the screw is undersized and/or the bore 19 is oversized. This problem is particularly aggravated where the installer has made an oversized hole in the wall to receive outlet box 10, and the mounting strap 14 is "floating" within the hole. According to a further feature of the invention, this problem is minimized by the use of a screw having (for example) two enlarged lobes 38a and 38b (FIGS. 4 and 5) which create an interference fit with possible oversized bores 19. In the preferred embodiment, these lobes are provided by squeezing the screw shank in a suitable press so that the radii of opposite lobes 38a and 38b are enlarged slightly (e.g., about 10 percent). One or more of these lobes may be shaped to increase its radial dimension(s). It has been observed that a screw formed as described provides advantages compared to a simple oversized screw in that it is capable of providing an interference fit after it has been used a number of times. As much of the shank may be shaped as required.
The aperture 28 and slot 30 in which the wire 26 is mounted restrain the ends of the wire from movement in the direction of the force applied to the wire by screw 16. However, although the wire end 26a is anchored within aperture 28, the other end 26b is free to move laterally (i.e., generally transverse to the direction of such force) within the elongated slot 30. Consequently, insertion of screw 16 between wire 26 and edge 32 causes the wire to bow or flex, applying a force which biases the screw against the edge 32. As a result, a secure electrical connection is made between the mounting strap 14 and screw 16 (and, thus, outlet box 10).
Wire 26 may be made of steel music wire with tin or other suitable plating. A typical diameter would be 0.030 inch. It must be stiff enough to provide a good electric connection to the screw 16, yet not so stiff as to prevent manipulation of the screw when seeking the threaded mounting hole within the outlet box into which the screw is to be inserted. The spring wire 26 can also serve the function of retaining the screw within the aperture 18 so that the usual cardboard washers used for this purpose will not be necessary. As mentioned in the Muska patent, this eliminates a possibility of error since such washers are sometimes inadvertently left in the screw-receiving aperture during installation.
To further improve the electrical connection between the screw and threaded bore 19, the screw may be coated with a soft metallic material such as solder. This coating will fill the threads of the screw to an extent so that when the screw is inserted into the tapped bore 19 within the outlet box, the soft plating will "smear," providing intimate contact between the screw and bore. The plating may also help to improve the connection between the shank of the screw and spring wire 26.
Obviously, different wire sizes and materials can, and will, be used to accomplish the desired result. Other wire configurations may also be used. For example, it is possible for both ends of the wire to be retained within elongated edge slots such as 30. Conversely, both wire ends may be retained within apertures so long as the wire can be flexed by the screw. It is also possible for the wire to be mounted so that it bows toward the screw (i.e., spring pressure is applied by decreasing the curve of the wire).