Heat sensing electrical receptacle

Abstract

An electrical receptacle has an outlet and senses the operating temperature and automatically turns off when the temperature rises above a predetermined threshold. The receptacle has a reset button that must be manually operated to enable operation of the outlet to resume.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a receptacle having at least one electrical outlet, and more particularly, is directed to an electrical outlet that senses the ambient temperature, the receptacle temperature and the temperature of a prong of an electrical plug inserted into the outlet, and that automatically shuts off when any of these temperatures is too hot, and has a reset button for resuming operation.

Many fires are believed to be caused by overloaded electrical outlets, that is, outlets operated with more power transfer than the outlet was designed for. Fires are sometimes caused by a loose connection, a glowing connection and/or a high resistance path. A glowing connection occurs when copper oxide is formed between a copper wire and a steel screw in a small air gap creating carbon which glows.

The condition of too much power usage is always accompanied by increased temperature in at least one of the ambient temperature, the receptacle temperature and the temperature of a prong of an electrical plug inserted into the receptacle, collectively referred to herein as “operating temperature”. To avoid fires, it is desirable for the outlet to sense when the operating temperature is too hot, and to cease operation.

Bimetallic switches are electromechanical thermal sensors. The bimetallic or bi-metal portion consists of two different metals bonded together such as brass and Invar. The metals expand at different rates as they warm, causing the element to twist or curve. The changing geometry is used to make or break an electrical contact. Once temperature has returned to normal levels, they revert back to their original geometry.

For a bi-metal comprising brass and invar, the bending occurs at a metal temperature of about 200° F.; the actual temperature threshold is determined by the design of the bimetal and its materials. The metal can be heated by a loose connection or by ambient air temperature. Typical plastic household wiring insulation and outlet housing melts at a temperature of about 300° F. but operation above 200° F. is not recommended due to its high probability of material distortion.

U.S. Pat. No. 6,166,618 (Robertson) discloses an outlet having a bimetallic dome that interrupts electrical contact when the temperature rises above a predetermined threshold. FIGS. 9 and 10 of the Robertson patent are reproduced as FIGS. 9 and 10 hereof. FIG. 9 shows electrical contacts 76c, 66c. At the bottom of FIG. 9, bimetallic dome 106 is shown in its reset (conducting) state. As the temperature rises above the operating threshold of bimetallic dome 106, it flips from a convex to a concave form. At the top of FIG. 9, there is a bimetallic dome in its tripped (non-conducting state), wherein the section of electrical contact 76c is electrically disconnected from contact member 66c. When the bimetallic dome changes shape, it pushes dielectric rod 110 outwards through hole 108, as shown in FIG. 10. Dielectric rod 110 can be manually depressed to reset the bimetallic dome.

The Robertson configuration has several drawbacks. First, a bimetallic dome is associated with each of the outlets in a duplex receptacle, increasing the cost of the receptacle. Second, the dielectric rod is positioned such that the faceplate of the receptacle must be removed to access the dielectric rod, which is inconvenient. Also, the location of the dielectric rod makes it impossible to quickly see that it has tripped. Third, as the bimetallic dome cools below its operating threshold, it can reset itself back to its original configuration. This automatic resetting can be dangerous to a person working around the outlet; in particular, a worker can be electrocuted by the sudden resumption of current. Fourth, although one outlet of a duplex outlet may be tripped, the other outlet will continue functioning, implying to a casual observer that the first outlet is dead rather than tripped, which could result in worker electrocution.

The Robertson patent also discloses another embodiment, shown in FIG. 11, having dish-shaped bimetallic portions 80 that resets on their own as the operating temperature cools. A reset button is absent.

Thus, there is a need for an outlet that is sensitive to heat and avoids undesirable operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a three-dimensional view of the electrical receptacle package;

FIGS. 2 and 3 are three-dimensional views of the underside of the electrical receptacle package shown in FIG. 1 showing different types of wire connections;

FIGS. 4A-4G are three-dimensional views of subassemblies of the electrical receptacle package shown in FIG. 1;

FIG. 5A is a top-down view of the electrical receptacle with its outer package removed;

FIG. 5B is a depth view across reference line AA in FIG. 5A;

FIG. 6A is a three-dimensional view of the thermal interrupt in its reset state;

FIG. 6B is a three-dimensional view of the thermal interrupt in its tripped state.

FIG. 7 is a three-dimensional view of the bi-metal portion of the thermal interrupt mechanism;

FIG. 8 is a three-dimensional view of the contact portion of the thermal interrupt mechanism;

FIGS. 9 and 10 are views of a prior art electrical safety receptacle; and

FIG. 11 is a view of another prior art electrical safety receptacle.

DETAILED DESCRIPTION

An electrical receptacle has an outlet and senses the operating temperature and automatically turns off when the temperature rises above a predetermined threshold. The receptacle has a reset button that must be manually operated to enable operation of the outlet to resume.

FIG. 1 is a three-dimensional view of the electrical receptacle package of the present invention. The receptacle has a top outlet and a bottom outlet. Each outlet is adapted to receive a 3-prong plug comprising a neutral (N) terminal, a load (L) terminal and a ground terminal for receiving the blades of an electrical plug, or a 2-prong plug comprising a neutral terminal and a load terminal. Specifically, the top outlet has neutral slot 16A, live slot 17A and ground slot 18A, while the bottom outlet has neutral slot 16B, live slot 17B and ground slot 18B. Screw 7 indicates the line terminal, screw 9G indicates the feed terminal, a screw (not shown) on the occluded side of the receptacle indicates the neutral terminal, and mounting tabs 31A, 31B are provided. Typically, electrical receptacles are connected in parallel via the household wiring. Generally, the line terminal serially couples to a thermal interrupt which serially couples to the feed (load) terminal.

Reset button 10 is located between the top outlet and the bottom outlet. During normal operation, the top of reset button 10 is approximately flush with the receptacle packaging. A thermal interrupt, discussed below, is located between the line terminal of the receptacle and the live terminals of the outlets. The thermal interrupt functions to interrupt the contact between the household wiring and the portion of the receptacle in contact with the blades of the electrical plug inserted into the top outlet or the bottom outlet. The thermal interrupt also prevents power from reaching any downstream outlets connected via the household wiring; downstream outlets are assumed to be on the feed (load) side. When the thermal interrupt triggers, reset button 10 pops outward. Manually depressing reset button 10, when the temperature is sufficiently cool, returns the reset button to its flush configuration and the thermal interrupt to its reset condition.

The receptacle package shown in FIG. 1 is approximately the same size as a standard two-outlet electrical receptacle having dimensions 2.64×1.33 inches, with a depth of 1.1 inches.

FIGS. 2 and 3 are three-dimensional views of the underside of the electrical receptacle package shown in FIG. 1 showing different types of wire connections. Generally, wires can be coupled to receptacles via the side-wire method, in which wire is wrapped under a screwhead, the back-wire method, in which wire is inserted from behind through a hole or slot and clamped under a clamping plate as the screw is tightened, or the push-wire method, in which a wire is simply pushed into a terminal and clamped by a spring-loaded brass member inside the terminal. The push-wire method causes many loose connections, and is not favored for this reason. FIG. 2 shows a back-wire configuration with holes 41A-47A; FIG. 3 shows a back-wire configuration with slots 41B-47B.

FIGS. 4A-4G are three-dimensional views of subassemblies of the electrical receptacle package shown in FIG. 1.

FIG. 4A shows cover 15 having neutral slots 16A, 16B, live slots 17A, 17B, ground slots 18A, 18B, and hole 14 for reset button 10.

FIG. 4B shows reset button 10 having contact arm 12 and extension 13. Reset button 10 has an internal spring located at its base, best seen in FIGS. 6A and 6B.

FIG. 4C shows the bimetallic line terminal subassembly. As shown in FIG. 7, bimetallic dish 2 has base 4 and silver contact 3 fastened to its top, such as by spot welding. Practically, the silver contact is usually a silver coating on a nickel backing. Instead of silver, any other conductive substance may be used, such as gold. Base 4 is fastened to line terminal 6 such as by spot welding. Returning to FIG. 4C, screw 7 passes through line terminal 6 and is threaded into clamping plate 8.

The prior art bimetallic device is dome-shaped, to ensure that when the device trips, the dielectric rod is pushed from a recessed position to a projecting position; that is, the height of the dome should be relatively large as the height difference has a mechanical purpose. In contrast, the present invention can use a shallow dish-shaped bimetallic device as the reset button is moved by its internal spring, not by the flexing of the bimetallic device.

FIG. 4D shows neutral terminal subassembly 19 having a left triple wipe basket with prongs 19A, 19B, 19C and a right triple wipe basket with prongs 19D, 19E, 19F. The triple wipe baskets are configured for a 15 amp, 120 volt plug, but in other embodiments also accommodate a 15 amp, 240 volt plug; a 20 amp, 120 volt plug; or a 20 amp, 240 volt plug.

In the United States, a 240 volt plug has two hot legs each having 120 volts. In Europe, a 240 volt plug has one neutral leg and one hot leg having 240 volts. Accordingly, for a United States 240 volt plug, a single bimetal thermal interrupt must be configured to open the contacts corresponding to both of the hot legs, or a bimetal thermal interrupt must be associated with each of the hot legs.

FIG. 4E shows plastic base 20 having reset button compartment 21 and ground terminal holes 22A, 22B. Neutral terminal subassembly 19 fits into the left side of plastic base 20, while feed terminal subassembly 9 fits into the right side of plastic base 20.

FIG. 4F shows feed terminal subassembly 9 having a left triple wipe basket with prongs 9A, 9B, 9C and a right triple wipe basket with prongs 9D, 1E, 9F. Feed terminal subassembly 9 also has screw 9G inserted therein. As shown in FIG. 8, silver contact 5 is spot welded on feed terminal subassembly 9.

FIG. 4G shows grounding strap subassembly 30 having mounting tabs 31A, 31B and ground prongs 32A, 32B. After the screws in mounting tabs 31A, 31B are tightened, grounding wire 33 serves to electrically connect grounding strap subassembly 30 to a metal box (not shown) placed in the wall.

The neutral, live and ground blades of a three-prong plug are inserted through slots 16A, 17A, 18A of FIG. 4A. The neutral blade then rests in right triple wipe basket having prongs 19D, 19E, 19F of FIG. 4D, while the live blade then rests in right triple wipe basket having prongs 9D, 9E, 9F of FIG. 4F. The ground blade passes through ground terminal hole 22A of base 20 of FIG. 4E and thence to ground prongs 32A of ground strap 30 of FIG. 4G.

FIG. 5A is a top-down view of the electrical receptacle with its outer package removed. At the left, neutral subassembly 19 includes screws 19G, 19K, clamping plates 19H, 19N, and holes for neutral wire 191, 19J, 19L, 19M. A similar configuration exists at the right for feed subassembly 9. Part of feed subassembly 9 is occluded by the bimetal subassembly and reset button, which are better viewed in FIG. 5B.

FIG. 5B is a depth view across reference line AA in FIG. 5A. Reset button 10 has spring 11 at its interior base, extension 13 and contact arm 12. Bimetallic dish 2 has base 4 and silver contact 3 fastened to its top. Base 4 is fastened to line terminal 6. Silver contact 5 opposes silver contact 3; silver contact 5 is welded to feed terminal 9. Screw 7 is inserted through a hole in line terminal 6 and fastened with clamping plate 8.

FIG. 6A is a three-dimensional view of the thermal interrupt in its reset state, that is, its normally closed state. Arm 12 of reset button 10 is seen to be below contacts 3 and 5 that are in contact with each other. At a normal operating temperature, bimetallic dish 2 bends inwards, resisting the tendency of arm 12 to move upwards. As the ambient temperature increases, bimetallic dish 2 bends so as to move contact 3 away from contact 5, allowing spring 11 of reset button 10 to push upwards between silver contacts 3, 5, thus interrupting power flow.

FIG. 6B is a three-dimensional view of the thermal interrupt in its tripped state. Reset button 10 is elevated such that arm 12 abuts against contact 5. The body of arm 12 prevents contact 3 from touching contact 5 even if bimetallic dish 2 tries to change shape on its own, such as after the temperature cools. Reset button 10 must be manually depressed to return the electrical receptacle to its reset state. If the temperature is too high, bimetallic dish 2 will still bend away from contact 5 so that power conduction will not occur. When the temperature has cooled sufficiently, then resetting can occur.

The present invention has various advantages. There is only one bimetallic device per duplex receptacle, reducing the cost of thermal overload protection. The reset button is readily accessible, making it easy to see when the device has tripped and convenient to reset the device. The device cannot automatically reset under any circumstances, that is, under all circumstances, manual action must occur to reset the device.

The present invention has been described with respect to a duplex receptacle. In another embodiment, the present invention is applied in a wall adapter outlet. Specifically, a portable unit having duplex outlets with thermal interrupt protection is plugged into a wall receptacle having duplex outlets lacking thermal interrupt protection.

In yet another embodiment, the present invention is applied in a power strip comprising a plurality of receptacles, the power strip being plugged into a standard outlet. The power strip has one bimetallic subassembly for all of its receptacles. If the power strip is long, a sensor and relay are provided so that the bimetallic subassembly can react to operating temperatures throughout the power strip.

Most households include ground fault interrupt (GFI) electrical receptacles in areas that are moist, such as bathrooms. A ground fault is an unintended leakage of current to ground, possibly through a person. The regular grounding system protects the equipment that is attached (or plugged in) to the circuit against a ground fault in it. GFI devices are designed to protect people, not equipment.

A GFI receptacle shuts down the protected electric circuit—opens it—when it senses an unexpected loss of power, to ground. GFI protection devices constantly monitor and compare the amount of power flowing from the panel on the hot or phase wire and the amount returning on the neutral wire. Any time thel current on the hot leg and the neutral leg are unequal, the protection device will trip and open the circuit. GFI devices work by passing both the hot wire and the neutral wire through a sensor such as a differential transformer and connecting the sensor to a solenoid or relay that opens switch contacts built into the power conductors inside the device—in front of the transformer. When it is working properly, a GFI device will open its protected circuit when the difference between the current coming in and the current going out reaches 0.005 ampere.

A GFI receptacle typically has a reset button. Due to its elaborate circuitry, a GFI receptacle is substantially more expensive than a regular receptacle.

The present temperature sensing features could be added to a GFI receptacle.

Although an illustrative embodiment of the present invention, and various modifications thereof, have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to this precise embodiment and the described modifications, and that various changes and further modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.

Claims

1. An electrical receptacle, comprising: a live terminal having a first contact, a power interruption device with a bimetallic portion having a second contact, and a resettable arm for preventing the first contact from touching the second contact when the power interruption device is in a tripped state.

2. The electrical receptacle of claim 1, wherein the resettable arm is attached to a reset button that must be manually operated to switch the power interruption device from the tripped state to a reset state.

3. The electrical receptacle of claim 2, wherein the reset button is visible when the electrical receptacle is in its normal operating configuration.

4. The electrical receptacle of claim 2, wherein the reset button is accessible when the electrical receptacle is in its normal operational package.

5. The electrical receptacle of claim 1, wherein the bimetallic portion is dish-shaped.

6. A plurality of electrical receptacles, comprising: a live terminal having a first contact, a power interruption device with a bimetallic portion having a second contact, and a resettable arm for preventing the first contact from touching the second contact when the power interruption device is in a tripped state.

7. The electrical receptacles of claim 6, wherein the resettable arm is attached to a reset button that must be manually operated to switch the power interruption device from the tripped state to a reset state.

8. The electrical receptacles of claim 7, wherein the reset button is visible when the electrical receptacles are in their normal operating configuration.

9. The electrical receptacles of claim 7, wherein the reset button is accessible when the electrical receptacles are in their normal operational package.

10. The electrical receptacle of claim 6, wherein the bimetallic portion is dish-shaped.