Patent Application
20090027146
1. Field of the Invention
This invention relates generally to electrical switching apparatus and, more particularly, to circuit interrupters, such as, for example, aircraft or aerospace circuit breakers. The invention also relates to methods of interrupting overcurrents of a power circuit.
2. Background Information
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
It is known to provide a cantilevered ambient compensation bimetal operatively associated with the bimetal. The bimetal, when heated, moves an insulative shuttle, which pulls on the ambient compensation bimetal that, in turn, is attached to a trip latch member. An increase or decrease in ambient temperature conditions causes the free end of the bimetal and the free end of the ambient compensation bimetal to move in the same direction and, thereby, maintain the appropriate gap between the two bimetal free ends, in order to eliminate the effects of changes in ambient temperature. Under overcurrent conditions, the bimetal and insulative shuttle pull on the ambient bimetal, which moves the trip latch member to trip open the operating mechanism.
Subminiature circuit breakers are used, for example, in aircraft or aerospace electrical systems where they not only provide overcurrent protection but also serve as switches for turning equipment on and off. Such circuit breakers must be small to accommodate the high-density layout of circuit breaker panels, which make circuit breakers for numerous circuits accessible to a user. Aircraft electrical systems, for example, usually consist of hundreds of circuit breakers, each of which is used for a circuit protection function as well as a circuit disconnection function through a push-pull handle.
Typically, subminiature circuit breakers have provided protection against persistent overcurrents implemented by a latch triggered by the bimetal responsive to I2R heating resulting from the overcurrent. There is a growing interest in providing additional protection, and most importantly arc fault protection.
During sporadic arc fault conditions, the overload capability of the circuit breaker will not function since the root-mean-squared (RMS) value of the fault current is too small to actuate the automatic trip circuit. The addition of electronic arc fault sensing to a circuit breaker can add one of the elements required for sputtering arc fault protection—ideally, the output of an electronic arc fault sensing circuit directly trips and, thus, opens the circuit breaker. See, for example, U.S. Pat. Nos. 6,710,688; 6,542,056; 6,522,509; 6,522,228; 5,691,869; and 5,224,006.
U.S. Pat. Nos. 6,864,765, 6,813,131, 6,710,688, 6,650,515, and 6,542,056 disclose a circuit breaker including three different trip modes, all of which employ a trip latch to actuate an operating mechanism and trip open separable contacts. The three trip modes include: (1) overcurrent conditions (thermal trip) detected by a bimetal, which actuates a trip latch through a shuttle and an ambient compensation bimetal; (2) arc fault (and/or ground fault) conditions detected by electronic circuits, which energize a trip motor to actuate the trip latch; and (3) relatively high current conditions (instantaneous trip) also attract the trip latch.
U.S. Pat. No. 7,170,376 discloses a miniature coil assembly including a coil controlled by an arc fault detection circuit and a plunger. An elongated ambient temperature compensating bimetal is interlocked to an ambient temperature slide, whereby lateral movement of such slide is controlled, in part, by the ambient temperature compensating bimetal. The plunger is coupled to the ambient temperature slide, in order to effect an arc fault trip function therewith.
If a circuit breaker operating mechanism does not open the separable contacts relatively quickly, then the internal components of the circuit breaker may be damaged. For example, it is known that separable contacts can weld closed if an overcurrent or fault condition persists for too long a time. Furthermore, an excessive trip time can produce carbon when the separable contacts break the power circuit. This carbon may cause dielectric breakdown after the fault and allow a current carrying path when the circuit breaker is intended to be open. Also, installed circuit breakers may become corroded, stuck or otherwise damaged. This can cause major changes in the ability of the circuit breaker to protect the corresponding power circuit against thermal overloads.
A known circuit breaker includes a fusible link to prevent the fusing of the separable contacts and, thus, the inability to break the power circuit. The fusible link opens if the separable contacts weld or if a dielectric breakdown occurs.
There is room for improvement in electrical switching apparatus such as circuit interrupters.
There is also room for improvement in methods of interrupting overcurrents of a power circuit.
These needs and others are met by embodiments of the invention, which employ a thermal overload mechanism to actuate an operating mechanism latch responsive to a thermal fault caused by current flowing through separable contacts. An electromagnetic device cooperates with the thermal overload mechanism to actuate the latch responsive to the electromagnetic device being energized. A processor repetitively determines a value of the current flowing through the separable contacts, determines if the value exceeds a predetermined value for a number of occurrences, and responsively energizes the electromagnetic device. This actuates the latch contemporaneous with actuation of the latch by the thermal overload mechanism, in order to decrease the time to trip open the separable contacts.
In accordance with one aspect of the invention, an electrical switching apparatus comprises: a housing; separable contacts; an operating mechanism comprising a latch, the operating mechanism being structured to open the separable contacts responsive to actuation of the latch; and a trip mechanism cooperating with the latch of the operating mechanism to trip open the separable contacts, the trip mechanism comprising: a thermal overload mechanism structured to actuate the latch responsive to a thermal fault caused by current flowing through the separable contacts, an electromagnetic device cooperating with the thermal overload mechanism to actuate the latch responsive to the electromagnetic device being energized, and a processor structured to repetitively determine a value of the current flowing through the separable contacts, to determine if the value exceeds a predetermined value for a number of occurrences, and to responsively energize the electromagnetic device.
The electrical switching apparatus may have a rated current, and the predetermined value may be about twelve times the rated current.
As another aspect of the invention, a circuit interrupter comprises: a housing; separable contacts; an operating mechanism comprising a latch, the operating mechanism being structured to open the separable contacts responsive to actuation of the latch; and a trip mechanism cooperating with the latch of the operating mechanism to trip open the separable contacts, the trip mechanism comprising: a thermal overload mechanism structured to actuate the latch responsive to a thermal fault caused by current flowing through the separable contacts, an electromagnetic device cooperating with the thermal overload mechanism to actuate the latch responsive to the electromagnetic device being energized, and a processor structured to repetitively determine a value of the current flowing through the separable contacts, to determine if the value exceeds a predetermined value for a number of occurrences, and to responsively energize the electromagnetic device, in order to actuate the latch contemporaneous with actuation of the latch by the thermal overload mechanism, in order to decrease the time to trip open the separable contacts.
The processor may be further structured to periodically measure the voltage and to determine the peak value of the current flowing through the separable contacts.
As another aspect of the invention, a method of interrupting current flowing through a power circuit comprises: sensing the current flowing through the power circuit; repetitively determining a value of the current flowing through the power circuit; determining if the value exceeds a predetermined value for a number of occurrences and responsively energizing an electromagnetic device; actuating a latch responsive to the electromagnetic device being energized; contemporaneously actuating the latch responsive to a thermal fault operatively associated with the current flowing through the power circuit; and opening separable contacts responsive to the latch being actuated.
The method may employ as the predetermined value a first predetermined value; add a second predetermined value to an accumulator responsive to the value exceeding the first predetermined value; and energize the electromagnetic device when the accumulator exceeds a third predetermined value.
The method may periodically subtract a fourth predetermined value from the accumulator.
The method may, after a first predetermined time, add the second predetermined value to the accumulator when the value exceeds the first predetermined value; and after a second predetermined time, subtract the fourth predetermined value from the accumulator.
The method may subtract the fourth predetermined value from the accumulator regardless whether the value exceeds the first predetermined value.
A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:
As employed herein, the term “number” shall mean one or an integer greater than one (i.e., a plurality).
As employed herein, the statement that two or more parts are “connected” or “coupled” together shall mean that the parts are joined together either directly or joined through one or more intermediate parts. Further, as employed herein, the statement that two or more parts are “attached” shall mean that the parts are joined together directly.
As employed herein, the term “thermal fault” shall mean a thermal overload current condition or other overcurrent condition.
The invention is described in association with an aircraft or aerospace arc fault circuit breaker, although the invention is applicable to a wide range of electrical switching apparatus, such as, for example, circuit interrupters adapted to detect a wide range of faults, such as, for example, arc faults and/or ground faults in power circuits.
Referring to
The circuit breaker 1 is also provided with an arc fault detector (AFD) 27. The AFD 27 senses the current in the electric power system 11 by monitoring the voltage across the bimetal 23 through the lead 31 with respect to a local ground reference 47. This voltage represents the current flowing through the separable contacts 17. If the AFD 27 detects an arc fault in the electric power system 11, then a trip signal 35 is generated, which turns on a switch such as the silicon controlled rectifier (SCR) 37 to energize a trip coil 39. When energized, the trip coil 39 actuates the operating mechanism 19 to open the separable contacts 17. A resistor 41 in series with the trip coil 39 limits the coil current and a capacitor 43 protects the gate of the SCR 37 from voltage spikes and false tripping due to noise. Alternatively, the resistor 41 need not be employed.
The AFD 27 cooperates with the operating mechanism 19 to trip open the separable contacts 17 in response to an arc fault condition. The AFD 27 includes an active rectifier and gain stage 45, which rectifies and suitably amplifies the voltage across the bimetal 23 through the lead 31 and the local ground reference 47. The active rectifier and gain stage 45 outputs a rectified signal 49 on output 51 representative of the current in the bimetal 23. The rectified signal 49 is input by a peak detector circuit 53 and a microcontroller (μC) 55.
The active rectifier and gain stage 45 and the peak detector circuit 53 form a first circuit 57 adapted to determine a peak amplitude 59 of a rectified alternating current pulse based upon the current flowing in the electric power system 11. The peak amplitude 59 is stored by the peak detector circuit 53.
The μC 55 includes an analog-to-digital converter (ADC) 61, a microprocessor (μP) 63 and a comparator 65. The μP 63 includes one or more arc fault algorithms 67 and a trip routine 100 (
The μP 63 includes an output 71 adapted to reset the peak detector circuit 59. The second circuit 69 also includes the comparator 65 to determine a change of state (or a negative (i.e., negative-going) zero crossing) of the alternating current pulse of the current flowing in the electric power system 11 based upon the rectified signal 49 transitioning from above or below (or from above to below) a suitable reference 73 (e.g., a suitable positive value of slightly greater than zero). Responsive to this negative zero crossing, as determined by the comparator 65, the μP 63 causes the ADC 61 to convert the peak amplitude 59 to a corresponding digital value.
The example arc fault detection method employed by the AFD 27 is “event-driven” in that it is inactive (e.g., dormant) until a current pulse occurs as detected by the comparator 65. When such a current pulse occurs, the algorithm(s) 67 record the peak amplitude 59 of the current pulse as determined by the peak detector circuit 53 and the ADC 61, along with the time since the last current pulse occurred as measured by a timer (not shown) associated with the μP 63. The arc fault detection method then uses the algorithm(s) 67 to process the current amplitude and time information to determine whether a hazardous arc fault condition exists. Although an example AFD method and circuit are shown, the invention is applicable to a wide range of AFD methods and circuits. See, for example, U.S. Pat. Nos. 6,710,688; 6,542,056; 6,522,509; 6,522,228; 5,691,869; and 5,224,006.
A digital output 79 of μP 63 of μC 55 includes the trip signal 35. An analog input 81 of μC 55 receives the peak amplitude 59 for the ADC 61. Hence, the μP 63 measures the voltage of the bimetal 23, determines the value of the current flowing through the separable contacts 17, and generates the trip signal 35.
As will be discussed, below, in connection with
As will be discussed, below, in connection with
Referring to
The manual operator 120 is preferably provided with a trip indicator 122. The manual operator 120 and trip indicator 122 are capable of sliding axial movement with respect to the ferrule 118. The manual operator 120 is provided with a central portion 124 having a central slot 126 extending approximately half the length thereof.
A clevis or thermal latch element 136 is provided with a latch surface 138 and a depending portion 140. The clevis 136 is pivotally supported by a pin 142, which is movable relative to the manual operator 120 in a slot (not shown). The end portions of the pin 142 are retained within grooves (not shown) in the central housing 112, which grooves guide axial movement thereof.
The mechanical latch elements 146 (only one latch element 146 is shown in
The mechanical latch elements 146 have camming apertures 151 (only one aperture 151 is shown) therein defining camming surfaces 152 (only one camming surface 152 is shown) which are disposed at an acute angle with respect to the axis of reciprocation of the manual operator 120 thereby to effect manual opening of the circuit breaker 1. Two lower camming surfaces 154 (only one camming surface 154 is shown) are disposed at substantially a right angle with respect to the axis of reciprocation of the manual operator 120 to provide positive locking of the circuit breaker 1. The central portion 124 carries a camming pin 156 which extends across the slot 126 therein and through the camming apertures 151 of the mechanical latch elements 146, in order to be in operative engagement therewith.
A spring 162 is provided to resiliently bias the manual operator 120, clevis 136 and latch elements 146 upwardly with respect to the ferrule 118.
A movable contact carrier or plunger 164 of a contact plunger assembly 165 has a central opening 166 therein for acceptance of the clevis 136. The contact carrier 164 carries a contact bridge 168 (shown in
The contact carrier 164 has a laterally extending slot 178 therein for the acceptance of a thermal or overload slide 180 and the ambient temperature slide 182. The overload slide 180 is movable internally of the contact carrier 164 under the influence of the elongated current responsive bimetal 184, which is retained within the housing 112 by end supports 185 at each end thereof.
A clevis guide assembly (e.g., made of ceramic) 186 couples the overload slide 180 to and insulates it from the bimetal 184. The overload slide 180 is provided with a slot 188, which accepts and closely cooperates with the clevis 136 to effect actuation of the latch 20 and release of the clevis 136 in response to lateral movement (e.g., right with respect to
The ambient temperature slide 182 underlies the overload slide 180 and is movable internally of the contact carrier 164 under the influence of the elongated ambient temperature compensating bimetal 190, which is part of an ambient compensator assembly 192 including an adjustable screw guide 193, a calibrate screw 194 and a compensator spring 195.
The ambient temperature compensating bimetal 190 is interlocked to the ambient temperature slide 182, whereby lateral movement of such slide 182 is controlled, in part, by such bimetal 190. The ambient temperature slide 182 is provided with a slot 196, which, when the circuit breaker 1 is in the contacts closed position, as shown, accepts the hooked end depending portion 140 of the clevis 136. In the contacts closed position, the latch surface 138 of the clevis 136 engages the upper surface of the ambient temperature slide 182 adjacent the periphery of the slot 196 with a pressure determined by the upward resilient bias provided by spring 174.
As an important aspect of the invention, the clevis 136 is released responsive to the overload slide 180, and the ambient temperature slide 182 is structured to contemporaneously release the clevis 136 responsive to the plunger 102 when the trip coil 39 is energized by the μP output 79 (
If the tests fail at either 204 or 208, then a predetermined value (K4) (e.g., without limitation, one) is subtracted from the accumulator. Since the routine 100 runs periodically, this periodically subtracts the predetermined value (K4) from the accumulator. After 212, the interrupt service routine returns, at 214, to a background routine (not shown) of the μP 63. Alternatively, if the test fails at 204, then step 212 may be skipped and the interrupt service routine returns, at 214.
The circuit breaker 1 senses the load current through the bimetal 23, which is series with the line conductor 13 and, thus, the load conductor 14. When the μP 63 determines that the sensed current exceeds about twelve times (12×) rated current for a suitable number of occurrences, it outputs the trip signal 35 to the trip coil 39, which causes the separable contacts 17 to open. Hence, the routine 100 permits the μP 63 to sense a rapid current spike through the voltage across the bimetal 23 and actuate the trip coil 39 in response thereto.
For example, the μC 55 (e.g., without limitation, a Peripheral Interrupt Controller (PIC) 16F684 Microcontroller marketed by Microchip Technology Inc. of Chandler, Ariz.) samples the peak current from the bimetal 23 about every 1.25 mS (e.g., without limitation, synchronized with every zero crossing (positive or negative) of the 120 VAC line cycle at 400 Hz). If the sampled peak current is greater than twelve times the circuit breaker rating, then the μP 63 fills an accumulator (bucket). For example, the trip threshold of the accumulator is set to be, for example, greater than 20 units. The μP 63 adds five units for every half-cycle (e.g., every 1.25 mS) that the sampled peak current is greater than twelve times the circuit breaker rating. Also, every cycle (e.g., 2.5 mS), the μP 63 subtracts one unit. Thus, after five example half-cycles (e.g., 6.25 mS), the μP 63 has subtracted two units (since only 2.5 full cycles have elapsed) and has added 25 units (five units per half-cycle times five half-cycles) for a net increase of 23 units (=25 units−2 units), which exceeds the trip threshold.
This is a redundant mechanism to open the separable contacts 17 and typically provides relatively quicker trip times in order to prevent internal component damage. Also, if the separable contacts 17 are bound together or if the operating mechanism 19 is hung up on burrs or foreign debris, then the miniature coil assembly 98 will “hammer” the contacts 17 open with the solenoid force.
Any number of known or suitable arc fault trip algorithms may be employed by the μP 63 in combination with the example trip routine 100 (
In the case of overcurrents at the maximum potential short circuit current at rated voltage, the μP 63 rapidly opens the operating mechanism 19 by pulling (e.g., without limitation, left with respect to
The disclosed circuit breaker 1 provides a fail-safe and redundant mechanism to initiate a trip and interrupt current flow. If the bimetal 23 (thermal overload mechanism 184,190) or operating mechanism 19 become damaged and unable (e.g., without limitation, the mechanical trip mechanism may hang up on burrs and/or foreign debris) to thermally trip the operating mechanism 19, then the fail-safe/redundant mechanism reliably initiates the trip. This provides additional safety without the additional cost of a fusible link. This protects the bimetal 23 of the circuit breaker 1 by ensuring a rapid, repeatable trip time. This mitigates damage to the circuit breaker 1, aircraft wiring and surrounding equipment.
Although separable contacts 17,170,172 are disclosed, suitable solid state separable contacts may be employed. For example, the disclosed circuit breaker 1 includes a suitable circuit interrupter mechanism, such as the separable contacts 17 that are opened and closed by the operating mechanism 19, although the invention is applicable to a wide range of circuit interruption mechanisms (e.g., without limitation, solid state or FET switches; contactor contacts) and/or solid state based control/protection devices (e.g., without limitation, drives; soft-starters).
While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.
