This invention relates to an heating assembly comprising an electrical connector and an electrical lead to electrical plugs which make an electrical connection between an electrical heater and an electrical power outlet.
It is often necessary to make an electrical connection to an electrical lead, e.g. to splice two leads together or to attach an electrical plug to a lead in order to apply power. It is also known to insert a light, or other signal, into an electrical circuit so that it is easy to determine whether current is flowing through a heater or other operational electrical device which forms part of the circuit. The light can, for example, be placed in a plug through which the device is connected to a wall outlet in a building or to another power source. It is also known to insert a fuse, circuit breaker, ground fault circuit interrupter (GFCI), or other circuit protection device into an electrical circuit so that if the current exceeds a safe level or otherwise behaves in an abnormal fashion, the protection device is converted from its normal state into an abnormal state in which it disconnects the circuit from the power supply or otherwise prevents maintenance of the abnormal current. The protection device can, for example, be placed in a plug through which a heater or other operational device is connected to a wall outlet in a building or to another power source. When an electrical circuit contains both a signal light and a protection device, absence of the signal light may signify failure of the operational device itself, or failure of the electrical connections to the operational device, or conversion of the protective device into its abnormal state, e.g. failure ("blowing") of the fuse.
Self-regulating strip heaters which comprise elongate electrodes connected by a conductive polymer resistive element are well-known. In operation, the thermal output of these strip heaters varies in response to changes in the thermal environment and thus serves to limit the maximum temperature which the heater achieves. Such heaters are often used to provide freeze protection of pipes, such as domestic or commercial water lines, or to maintain a constant temperature for pipes in process industries. A number of measures have been proposed for assisting the safe operation of such heaters. Reference may be made, for example, to U.S. Patent No. 4,436,986 (Carlson) and U.S. Patent No. 4,822,983 (Bremner et al).
DE-A-320-3651 discloses a two part connector. Conductors are positioned in bores in the first part. Cut-clamps are positioned in the second part. When the first and second parts are placed together, the cut-clamps cut through the insulation of the conductors and make electrical contact. The connector is described as being useful for cables such as flat cables which have multiple conductors (see Figure 1).
US-A-4,684,195 discloses an electrical connector in which a solderless connection can be made to a wire by inserting the wire into a wire-receiving tunnel and then making connection via U-shaped contact member. The connector is particularly useful for making in-line splices.
US-A-3,184,569 discloses an electrical plug which provides overload protection by means of two fuses. In order to tell whether the overload protection is active, the plug includes a signal light which goes out when the fuse blows as a result of an overcurrent condition. The light is installed within a transparent plastic portion which is provided with an opaque hinged cover for use when it is desired that the light not be visible. This plug is specifically designed to be connected to another plug.
I have now realised that when a conductive polymer strip heater is connected to a power supply through an electrical plug which is inserted into a convential wall outlet and which contains a fuse or other protective device, it is very useful to include, as a part of the plug, a signal device, e.g. a light-emitting diode, which is connected between the two legs of the heater circuit so that it will provide a signal (e.g. will be lit up) only when the power is on and the protective device is in its normal state.
In accordance with the present invention, there is provided a heating assembly which comprises :
The heating assembly of the invention is in the form of an electrical plug which is used to connect an elongate electrical heater to an electrical power outlet. The power outlet may be a wall outlet either inside or outside of a building, or it may be part of a power supply or other suitable source of power. The outlet may be two-hole for connection to a phase and a neutral wire, or three-hole for connection to a phase, a neutral, and a ground (earth) wire. The electrical lead may be any electrical component with a first conductor and a second conductor, e.g. an electrical power cord. The plug is particularly useful in making an electrical connection to a strip heater, i.e. an elongate heating element which comprises at least two electrodes. For a connection made with this type of heater, the first conductor is one electrode of the heater and the second conductor is the other electrode of the heater. The strip heater may comprise a conductive polymer, i.e. a composition in which a particulate conductive filler is dispersed or otherwise distributed in a polymeric component. Particularly preferred are self-regulating conductive polymer heaters, i.e. those which exhibit PTC (positive temperature coefficient) behavior. The electrodes of a self-regulating heater are generally elongate metal wires or braid which are parallel and spaced apart. They are attached to or embedded in a resistive element which comprises the conductive polymer and is often in the form of a continuous strip. In order to provide environmental protection and electrical insulation, it is common for the resistive element and the electrodes to be covered by a dielectric layer, e.g. a polymeric jacket. A metallic grounding braid is often present over the dielectric layer in order to provide physical reinforcement and a means of electrically grounding the strip heater.
The term "PTC behavior" is used in this specification to denote a composition or an electrical device which has an R₁₄ value of at least 2.5 or an R₁₀₀ value of at least 10, and preferably both, and particularly one which has an R₃₀ value of at least 6, where R₁₄ is the ratio of the resistivities at the end and the beginning of a 14°C range, R₁₀₀ is the ratio of the resistivities at the end and the beginning of a 100°C range, and R₃₀ is the ratio of the resistivities at the end and the beginning of a 30°C range. Self-regulating conductive polymer heaters which exhibit PTC behavior, and appropriate conductive polymer compositions, are disclosed in U.S. Patents Nos. 3,858,144 (Bedard et al), 3,861,029 9 (Smith-Johannsen et al), 4,188,276 (Lyons et al), 4,388,607 (Toy et al), and 4,426,339 (Kamath et al).
The invention is illustrated by the drawings. Figure 1 shows a schematic circuit diagram for an electrical connector in the form of a plug which is designed for insertion into an electrical power outlet. A first contact member 2 comprises a first prong 4 which can be inserted into one socket of the outlet and a first fuse-contact section 6. A second contact member 8 comprises a second fuse-contact section 10 and a first conductor-contact section 12 to which a first conductor of an electrical lead can be physically and electrically connected. A third contact member 14 comprises a second conductor-contact section 16 to which the second conductor of an electrical lead can be physically and electrically connected and a second prong 18 which can be inserted into the other socket of the outlet. A signal member 20 is electrically connected between the second and third contact members. In this embodiment, the signal member 20 is electrically in series with a first resistor 22 and a second resistor 24. When the first and second prongs 4,18 are inserted into the power outlet, the signal member 20 provides a signal if a fuse 26 connects the first and second fuse-contact sections 6,10. No signal is provided if there is no electrical connection between the first and second fuse-contact sections 6,10. The signal member 20 may be a light, e.g. a light emitting diode (LED) which provides a visual signal, a bell or other apparatus which provides an audio signal, or an electrical switch which can generate an alarm signal. Other types of signal members may be appropriate in different circumstances.
The first and second fuse-contact sections 6,10 are positioned to receive a fuse 26. The selection of a specific fuse is dependent on the normal operating conditions and the anticipated fault conditions. Since the connection is made to a strip heater a very fast acting fuse, i.e. a fuse which has little, if any intentional delay in the overload region and which "trips" (opens) very rapidly when the current in the circuit comprising the fuse exceeds the rated value of the fuse. Appropriate fuses are very fast-acting ceramic ferrule fuses with a current rating of 10 amperes and a voltage rating of 125/250 volts. Such fuses are available, for example, from the Bussman Division of Cooper Industries under the name Buss GBB™-10. While it is possible to use one of the measures disclosed in U.S. Patent No. 4,822,983 (Bremner et al) in conjunction with this invention, I have found that, when a fuse as just described is employed, excellent results can be obtained without using a sensor conductor as disclosed in that patent.
Figure 2 shows an exploded view of one specific embodiment of a connector in the form of a plug which is designed to connect the conductors of a first electrical lead 28 to an electrical power outlet. The electrical lead 28 is a strip heater which comprises a resistive element 30, and embedded therein, a first conductor 32 and a second conductor 34. The heater is surrounded by a metallic grounding braid 36. Both the first conductor 32 and the second conductor 34 have been stripped of the conductive polymer which comprises the resistive element 30 to allow easy insertion into the conductor-receiving member 38. The conductor-receiving member 38 comprises a first channel 40, a second channel 42, and a third channel 44, designed for insertion of the first conductor 32, the second conductor 34, and the grounding lead 36, respectively. Each channel is a tunnel which has a frusto-conical opening, both features sized so that the conductors inserted in them can be held by frictional forces. The walls of the channels serve to prevent contact between the conductors. A radial opening 46,48,50 is cut through each tunnel to allow electrical connection of the inserted conductor to the conductor-contact sections. Once the conductors are inserted into the conductor-receiving member 38, it is mated to the conductor-connecting member 52. As a result of the design of the conductor-receiving member, which may include positioning pins or other design elements which are not shown, there is a unique mated configuration. When the two pieces are mated, the first conductor 32 is in physical and electrical contact with the first conductor-contact section 12, the second conductor 34 is in physical and electrical contact with the second conductor-contact section 16, and the grounding lead 36 is in physical and electrical contact with the grounding contact section 54. The first and second conductor-contact sections 12, 16 and the grounding contact section 54 may comprise insulation-piercing means for use when the conductors are insulated with a polymeric jacket. The conductor-connecting member further comprises first and second fuse-contact sections 6,10 designed to receive a fuse. The first connection-making section 4 corresponds to the first prong of Figure 1 and the second connection-making section 18 corresponds to the second prong. A ground-connection-making section 56 is also present to connect to the ground in the outlet. The embodiment shown also comprises a base 58 for the conductor-connecting member 52 which can be welded, glued, or snapped into place. A signal port 60 is positioned so that the signal member 20 will be visible. Figure 2B shows the bottom of the conductor-connecting member 52 with the base 58 removed. The signal member 20, an LED, and first and second resistors 22,24 are visible. It is apparent that the first fuse-contact section 6 and the first connection-making section 4 are made from a single piece of metal, e.g. brass. In this embodiment, the second conductor-contact section 16 and the second connection-making section 18 are also made from a single piece of metal, as are the second fuse-contact section 10 and the fuse conductor-contact section 12.
The conductor-receiving member 38 is preferably made from an insulating material, e.g. a polymer. It is particularly preferred that it be made from a transparent polymer, e.g. polycarbonate, so that there is a visual indication that the conductors are properly positioned. The conductor-receiving member 38 and the conductor-connecting member 52 are maintained in position by a closure means 62, e.g. a screw or a clamp. When mated properly, the plug provides adequate strain relief to the electrical lead. A minimum pull force of 20 pounds on the electrical lead is achieved. (The "pull force" or "pullout force" measures the amount of force required to pull the electrical lead 0.125 inch (0.318 cm) out of the plug. This procedure is described in U.S. Patent No. 4,883,945 (Bautista)). Proper mating of the conductor-receiving member 38 and the conductor-connecting member 52 cannot be achieved if either the first conductor 32 or the second conductor 34 has been directly connected to the respective conductor-contact section 12,16.
When the fuse 26 is in place but no conductors are inserted into the conductor-receiving member, the plug may be used to determine whether wall outlets are properly wired and/or live. If the fuse is electrically intact and there is no signal when the plug is placed into the outlet, that is an indication that the outlet is defective. Additional signal members can be incorporated into the plug to signal whether the phase and neutral wires are wired correctly. When there is a ground connection to the plug, additional signal members will indicate whether there is actually a ground.
Figure 3 illustrates a connector 64 which is designed to connect a first electrical lead 28 of a heater comprising first and second conductors 32,34 and a second electrical lead 66 comprising third and fourth conductors 68,70. In this connector 64, there are two conductor-receiving members 38,72. The second conductor-receiving member 72 comprises two channels 74,76 for insertion of the third and fourth conductors 66,68. A second closure means 78 maintains the second conductor-receiving member 72 in the mated configuration. In the mated configuration, the third conductor 68 makes physical and electrical contact with the first connection-making section 4 and the fourth conductor 70 makes physical and electrical contact with the second connection-making section 18.