1. Field of the Invention
The present invention relates generally to the field of electrical circuit interrupting devices adapted to complete and interrupt electrical current carrying paths between a source of electrical power and a load. More particularly, the invention relates to a novel technique for rapidly interrupting an electrical circuit and for dissipating energy in a circuit interrupter upon interruption of a current carrying path.
2. Description of the Related Art
A great number of applications exist for circuit interrupting devices which selectively complete and interrupt current carrying paths between a source of electrical power and a load. In most conventional devices of this type, such as circuit breakers, a movable member carries a contact and is biased into a normal operating position against a stationary member which carries a similar contact. A current carrying path is thereby defined between the movable and stationary members. Such devices may be configured as single-phase structures, or may include several parallel mechanisms, such as for use in three-phase circuits.
Actuating assemblies in circuit interrupters have been developed to provide for extremely rapid circuit interruption in response to overload conditions, over current conditions, heating, and other interrupt-triggering events. A variety of such triggering mechanisms are known. For example, in conventional circuit breakers, bi-metallic structures may be employed in conjunction with toggling mechanisms to rapidly displace the movable contacts from the stationary contacts upon sufficient differential heating between the bi-metallic members. Electromechanical operator structures are also known which may initiate displacement of a movable contact member upon the application of sufficient current to the operator. These may also be used in conjunction with rapid-response mechanical structures such as toggle mechanisms, to increase the rapidity of the interrupter response.
In such circuit interrupters, a general goal is to interrupt at current close to zero as rapidly as possible. Certain conventional structures have made use of natural zero crossings in the input power source to effectively interrupt the current through the interrupter device. However, the total let-through energy in such devices may be entirely unacceptable in many applications and can lead to excessive heating or failure of the device or damage to devices coupled downstream from the interrupter in a power distribution circuit. Other techniques have been devised which force the current through the interrupter to a zero level more rapidly. In one known device, for example, a light-weight conductive spanner is displaced extremely rapidly under the influence of an electromagnetic field generated by a core and winding arrangement. The rapid displacement of the spanner causes significant investment in the expanding arcs and effectively extinguishes the arcs through the intermediary of a stack of conductive splitter plates. A device of this type is described in U.S. Pat. No. 5,587,861, issued on Dec. 24, 1996 to Wieloch et al.
While currently known devices are generally successful at interrupting current upon demand, further improvement is still needed. For example, in devices that do not depend upon a natural zero crossing in the incoming power, back-EMF is generally relied upon to extinguish the arcs generated upon opening, which, themselves, define a transient current carrying path. The provision of spaced-apart splitter plates establishes a portion of this transient current carrying path and represents resistance to flow of the transient current, producing needed back-EMF. However, depending upon the level of power applied to the device, such sources of back-EMF may be insufficient to provide sufficient resistance to current flow to limit the let-through energy to desired levels. In particular, splitter plates, as one of the sources of back-EMF, may fail at higher voltage levels (current tending to shunt around the plates, for example), imposing a limitation to the back-EMF achievable by conventional structures. As a result, depending upon the nature of the event triggering the circuit interruption, the excessive let through energy can degrade or even render inoperative the interrupter device.
There is a need, therefore, for an improved circuit interrupting technique which can provide efficient current carrying capabilities during normal operation, and which can rapidly interrupt current carrying paths, while limiting let through energy to reduced levels by virtue of rapid arc extinction. There is a particular need for a method that can be employed economically in a variety of interrupter structures while providing improved circuit interruption characteristics over a range of voltage and current ratings.
The invention provides a novel technique for interrupting an electrical current carrying path and for dissipating energy in a circuit interrupter designed to respond to these needs. The technique may be employed in a wide variety of circuit interrupting devices, such as circuit breakers, motor controllers, switch gear, and so forth. While the method is particularly well suited to very fast-acting devices, such as devices employing light-weight spanners or movable contacts structures, it may be used to improve circuit interruption of other interrupter types, including devices having various triggering mechanisms to initiate circuit interruption.
In accordance with the technique, a normal or first current carrying path is defined in an interrupter, along with a transient or alternative current carrying path. The transient current carrying path includes circuit components which establish a preferred current path during an initial phase of circuit interruption, and which change a conductive state to enhance the energy-dissipating capabilities of the transient circuit thereafter. In a preferred configuration, variable resistive structures are positioned adjacent to incoming and outgoing conductors, and are in a relatively conductive state during the initial phase of circuit interruption. Prior to interruption, the transient current carrying path may be an essentially open circuit, passing substantially no current, with all current being directed through the normal current carrying path. In the initial phase of interruption, arcs are created in parallel with the variable resistance elements, and the relatively lower resistance of the elements causes current to flow preferentially through the transient current carrying path. A rapid change in the resistive state of the elements then ensues, such as due to heating by the transient current. Thereafter, the elements contribute to the rapid interruption of the transient currents by contributing to the back EMF through the device. The elements which establish the preferred current carrying path, and which then change their resistive state, may be static components, such as a polymer in which a dispersion of conductive material is doped.