CIRCUIT BREAKER WITH IMPROVED ARC INTERRUPTION FUNCTION

Abstract: A fault current interrupter is provided by the parallel combination of a polymer current limiter and a voltage dependent resistor connected across a pair of separable contacts to permit the interruption of current without the occurrence of arcing between the contacts when the contacts first become separated. The polymer current limiter is selected to have a relatively low resistance at quiescent operating currents and a substantially higher resistance at short circuit overcurrents. This allows the current to transfer away from the contacts through the polymer current limiter until the voltage across the voltage dependent resistor causes the voltage dependent resistor to become conductive and thereby transfer the current away from the polymer current limiter.

Description: CIRCUIT BREAKER WITH IMPROVED ARC INTERRUPTION FUNCTION BACKGROUND OF THE INVENTION U.S. Patent Application Serial No. 06/610,947 filed May 16, 1984 entitled "Solid State Current Limiting Circuit Interrupter" in the name of E. K. Howell discloses the use of semiconductor elements in combination with circuit interrupting contacts to allow the contacts to separate without the occurrence of an arc between the contacts. In the Howell application, which is incorporated herein for purposes of reference, a transistor element is employed in combination with a voltage dependent resistor to transfer the current away from the separating contacts to the transistor and thence from the transistor to the voltage dependent resistor. Some means is required for switching the transistor between conductive and non-conductive states in order for the transistor to be conductive when the contacts are first opened and for the transistor to become non-conductive shortly after contact separation. The Howell application advantageously employs a saturable core current transformer for switching the power transistor on and off within predetermined time intervals. It has since been determined that the same function which the transistor performs can be accomplished by means of a resistor fabricated from a positive temperature coefficient material (PTC) having a relatively low resistance value at low temperatures and a substantially higher resistance at a predetermined higher temperature. U.S. Patent Nos. 4,329,726 and 4,413,301 to L.M. Middleman et al. disclose PTC materials operational in the range of 5 to 100 amperes which are employed in series with separable contacts in order to provide circuit protection by the increased series resistance within the circuit when the PTC material carries current higher than a predetermined value. The use of a material having a negative temperature coefficient within circuit interrupting devices is disclosed within U.S. Patent No. 4,019,097 entitled "Circuit Breaker with Solid State Passive Overcurrent Sensing Device." This patent teaches the use of a material such as vanadium dioxide or lanthanum cobalt oxide in series with a flux transfer trip mechanism. The thermal response properties of the aforementioned materials are used to sense the presence of an overcurrent condition and to allow the current through a trip mechanism to increase to an operational value. All the aforementioned patents are incorporated herein for purposes of reference. The materials described within the patents to Middleman et al. are incapable of carrying sufficient current to provide overcurrent protection in a circuit such as protected by a molded case circuit breaker. U.S. Patent No. 4,583,146 entitled "Fault Current Interrupter" describes a parallel combination of a PTC resistor and a voltage dependent resistor connected across a pair of separable contacts to effectuate fault current interruption without damaging or destroying the PTC resistor. U.S. Patent No. 5,614,881 entitled "Current Limiting Device" describes a polymeric current limiting element that is not dependent on a positive temperature coefficient of resistance (PTCR) characteristic, and is herein incorporated by reference. U.S. Patent Application Serial No. 08/797,151 filed 10 February 1997 entitled "Current Suppressing Circuit Breaker Unit for Inductive Motor Protection" describes the aforementioned polymeric current limiting element employed in conjunction with a molded case circuit breaker to rapidly suppress and extinguish high short circuit currents. U.S. Patent Application Serial No. 08/797,152 filed 10 February 1997 entitled "Circuit Breaker Current Limiting Arc Runner" describes the aforementioned polymeric current limiting element employed in a circuit breaker arc runner to rapidly suppress high short circuit currents without heating up during quiescent operating conditions. U.S. Patent Application Serial No. 08/932,486 filed 18 September 1997 entitled "Current Limiting Circuit Breaker with Current Commutation" describes the aforementioned polymeric current limiting element electrically connected in parallel with one of two pairs of separable contacts of a rotary breaker for rapid commutation of the short circuit current into the current limiter. The purpose of the instant invention is to provide a fault current interrupter employing polymer current limiters, not dependent on PTCR characteristics, within circuits capable of interrupting current within residential and industrial power buses without damaging or destroying the polymer current limiter in the process. SUMMARY OF THE INVENTION Fault current interruption circuits capable of repeatedly interrupting fault currents within certain molded case circuit breaker ratings are made possible by the arrangement of a polymer current limiter (PCL) and a voltage dependent resistor (VDR) in parallel with a pair of mechanically switched contacts. Upon separation of the contacts, the current first transfers through the PCL having an initially low resistance. The passage of current through the PCL causes the resistance of the PCL to increase by several orders of magnitude in a manner described in the aforementioned U.S. Patent No. 5,614,881 . The voltage across the PCL and the VDR, in parallel, rapidly increases to the clamping voltage of the VDR, turning on the VDR and transferring the current thereto. Since the voltage across the VDR is substantially higher than supply voltage, the current then rapidly drops to a low value, allowing a pair of auxiliary contacts to complete the interruption process. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a circuit diagram of a circuit interruption arrangement according to the invention; Figure 2 is a circuit diagram of a further embodiment of the interruption arrangement depicted in Figure 1 . DESCRIPTION OF THE PREFERRED EMBODIMENT Although the use of a PCL as a series or parallel element to a pair of separable contacts in circuit interruption devices is known, the use of such a material as a parallel circuit element for transferring current away from separating contacts to a voltage dependent resistor for eliminating arc occurrence between the contacts has not heretofore been disclosed. While various materials may be used in PCL devices , each providing unique characteristics, nickel-filled epoxy systems have been shown to be suitable for high short circuit current interruption. Other material systems for PCL devices are described in the aforementioned U.S. Patent No. 5,614,881 . One such fault current interrupter using a PCL is shown in FIG. 1 . The fault current interrupter 10 is connected across a main contact assembly 15 consisting of fixed contacts 1 1 , 12 and bridging contacts 13, 14 which are separated upon overload current through a power bus 16. The current through the power bus is sensed by means of a current transformer arranged with its primary winding comprising the power bus and with its secondary winding connected with an operating mechanism to rapidly open the contact assembly 15 when the current reaches a predetermined value. The use of one such current transformer and operating mechanism within a protected circuit is described, for example, in U.S. Patent No. 4,115,829 to E. K. Howell and U.S. Patent No. 4,001 ,742 to C. L. Jencks et al. and reference should be made to these patents for a detailed description. The fault current interrupter 10 provides a function similar to the solid state current limiting circuit interrupter within the aforementioned E. K. Howell application wherein "arcless interruption" occurs between separable contacts by transferring the current away from the contacts via a solid state switch. An auxiliary contact assembly 17 having a fixed contact 19 and a movable contact 18 can also be employed in combination with the fault current interrupter 10 if so desired, The power bus 16 is connected to a power source by means of line terminal 20 and to an operating load by means of load terminal 21. A PCL 22, which operates in the manner described in aforementioned U.S. Patent No. 5,614,881 , is connected in parallel with the separable contact assembly 15 and with a voltage dependent resistor, hereafter VDR, such as a metal oxide varistor 23, hereafter MOV, by means of lines 24 and 25. In operating the fault current interrupter 10, upon separation of the contact assembly 15, the current immediately transfers through the PCL 22 having a low initial resistance. The current passes through the PCL causing its resistance to rapidly increase in a manner described in the aforementioned U.S. Patent No. 5,614,881 such that the voltage across the parallel combination of the PCL 22 and the MOV 23 correspondingly increases to the clamping voltage of the MOV causing the current to immediately transfer through the MOV. The voltage, now being substantially higher than the supply voltage, rapidly causes the current through the MOV to drop to a very low value. The MOV can have the composition described within U.S. Patent No. 4,374,049 in the names of J. Ellis et al. whereby the clamping voltage can be adjusted by varying the composition of the MOV materials as well as the process of fabrication. The electrode interface of the PCL 22 as shown in Fig. 1 (and Figure 2) reference should be made to aforementioned U.S. Patent No. 5,614,881 for a detailed description of how the PCL operates, is heated by l 2 Rt adiabatic joule heating, where R is the resistance of the PCL electrode interface. When current first transfers to the PCL, R is low, hence the power loss is low and the interface temperature rises slowly. As temperature rises, R increases resulting in higher power loss and faster heating. However, because the power is a function of the square of the current, the heating rate is quite sensitive to current magnitude. The fault current interrupter 10 shown in FIG. 2 is similar to that within FIG. 1 wherein the fault current interrupter is connected across a contact assembly 15 within a power bus 16. The PCL 22 is connected in parallel within the contact assembly and with the MOV 23 by means of lines 24, 25. The PCL 22 has a thin MOV layer 27 fused to one end which exhibits a very low clamping voltage in the order of approximately 5 volts. When the current transfers from the contact assembly 15 to the PCL 22, the heating power within the MOV layer 27 is generated by the product of the voltage across the MOV layer 27 and the current through the MOV layer 27. Alternatively, the fixed voltage drop provided by the MOV layer 27 can be distributed in grain boundaries within the material comprising the PCL 22, or in combination with the MOV layer 27 if more rapid electrode interface heating is desired. Since the initial heating power within the MOV layer 27 is a linear function of current the initial rate of temperature rise in this embodiment is greater and is less sensitive to current magnitude than in the embodiment of FIG. 1. Thereby effecting a greater rate of resistance rise in the PCL, and a greater rate of current transfer into the MOV 23. When high current composite metal insulator PCLs are arranged such that the conductive metal is encapsulated within a matrix of MOV material to form a PCL-MOV resistor, the separate MOV 23 is no longer required. The metal would provide initial low temperature and low resistance conductive properties to the PCL-MOV resistor to rapidly transfer the current initially away from the contact assembly 15. As the current through the PCL-MOV resistor increases and the electrode-interface temperature increases, the PCL would operate as described in aforementioned U.S. Patent No. 5,614,881 causing the current upon transfer through the PCL-MOV material to rapidly decrease. Although the fault current interrupter of the instant invention is described for purposes of protecting equipment and wiring within a power bus, this is by way of example only. The fault current interrupter can be used in any situation where "arcless" switching is required such as explosive atmosphere in mines for example, and when "noise-free" switching is required such as with sensitive electronic components within computers.

Claims: WHAT IS CLAIMED IS: 1 . A fault current interrupter comprising: a pair of separable electric contacts arranged for interrupting current flow through an electric circuit; a polymer current limiter electrically connected in parallel across said electric contacts for transferring said current through said polymer current limiter when said electric contacts first become separated, wherein said polymer current limiter comprises a first and second electrode arranged on opposite sides of a polymeric conductor, a polymeric binder with a vaporization temperature at which significant gas evolution occurs below 800°C and an electrically conductive filler, and an interface element in series with said polymeric conductor, said interface element having a higher resistivity than said polymeric conductor; and a resistor electrically connected in parallel across said electrical contacts and said polymer current limiter. 2. The fault current interrupter of claim 1 wherein said resistor comprises a material of ohmic resistance characteristics. 3. The fault current interrupter of claim 1 wherein said resistor comprises a material of voltage dependent characteristics. 4. A fault current interrupter comprising: a pair of separable electric contacts arranged for interrupting current flow through an electric circuit; a polymer current limiter electrically connected in parallel across said electric contacts for transferring said current through said polymer current limiter when said electric contacts first become separated, wherein said polymer current limiter comprises a first and second electrode arranged on opposite sides of a polymeric conductor, a polymeric binder with a vaporization temperature at which significant gas evolution occurs below 800°C and an electrically conductive filler, and an interface element in series with said polymeric conductor, said interface element having a higher resistivity than said polymeric conductor, and a layer of material having grain boundaries and wherein said grain boundaries include a material having voltage dependent properties to increase the rate at which said layer of polymeric conducting material reaches a predetermined temperature. 5. A fault current interrupter comprising: pair of contacts and a resistor connected across said contacts; said resistor comprising a polymer current limiter having a first and second electrode arranged on opposite sides of a polymeric conductor, a polymeric binder with a vaporization temperature at which significant gas evolution occurs below 800°C and an electrically conductive filler, and an interface element in series with said polymeric conductor, said interface element having a higher resistivity than said polymeric conductor, whereby said resistor exhibits a first resistance at a first quiescent current and a second higher resistance at a second higher short circuit overcurrent, said resistor also having voltage dependent properties whereby said resistor exhibits a third resistance at a first voltage drop across said resistor and a fourth lower resistance at a second higher voltage drop across said resistor. 6. The fault current interrupter of claim 5 wherein said polymer current limiter and said voltage dependent resistor comprises a composite material whereby said current transfers through one component of said composite material at a first quiescent current where said electric contacts first becomes separated and then transfers through another component of said composite material at a second short circuit overcurrent higher than said first quiescent current. 7. The fault current interrupter of claim 5 wherein said resistor comprises a first material of said polymer current limiter and a second material having said voltage dependent properties.

Inventors: ARNOLD DAVID, ARNOLD, DAVID
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1999
Main classification: H01H9
IPC codes: H01H9