MXPA99004918A - Circuit switch combined of loss of burial and failure by a - Google Patents

Abstract

A combined circuit breaker for ground loss and arc failure includes a differential current transformer, an active electrical line and a neutral electrical line that pass through the current transformer and form its primary, an asymmetrical current transformer that has a core through which the active and common electrical cables pass, and a secondary coiled in the core to produce a signal more strongly sensitive to the current in the common line than in the active line. An arc fault detector is connected to the secondary of the second transformer, while a ground fault detector is connected to the secondary of the first transformer, and the outputs of the earth leakage and arc fault detectors are connected to a circuit breaker arranged to disconnect the circuit when a ground fault or an earth fault is detected

Description

CIRCUIT SWITCH COMBINED FROM LOSS TO EARTH AND FAULT BY ARC Field of the invention This invention relates in general to devices for protecting electrical circuits in the event of faults, and more particularly to a device for protecting a circuit against earth losses and arc faults. Earth leakage circuit breakers have been used quite widely and perform the very useful function of disconnecting a source of electrical current from a load when an earth leakage is detected. Among the most common types of ground losses detected by known ground loss circuit breakers are those produced when a person accidentally contacts an active electrical wire and ground. In the absence of a ground fault circuit interrupter, currents could flow in life-threatening quantities through the person's body. Virtually all earth leakage circuit breakers use a differential current transformer to detect ground losses. The current transformer is arranged with the active and neutral cables of an electrical circuit passing through the center of a transformer, preferably a toroidal transformer, in which a secondary winding has been symmetrically formed. In the normal operation of an electrical circuit, the currents flowing through the active electrical conductor and the neutral electrical conductor are equal and opposite, and the differential transformer produces no output signal. When a loss to ground occurs, the currents are no longer the same, and the differential transformer produces a signal that can be detected by suitable signal conditioning circuitry to activate a relay or contactor or the like to interrupt the electrical circuit. Earth leakage circuit breakers must also deal with a fundamentally different type of fault that occurs when the neutral conductor is connected to the ground conductor, in the downstream circuit powered by the ground fault circuit interrupter where the Neutral and ground connections are intentionally connected, but in the wrong way, or by accidental short circuits formed for example when a strand of a braided electrical conductor accidentally bypasses the neutral and ground connections. If a "neutral to ground" fault of the type just described occurs in a circuit in which the earth and neutral lines are connected, for example, and a person inadvertently makes contact with an active power cord while also being connected to the fault From neutral to ground, the return current is divided between the neutral electrical conductor and the ground conductor. Of the two, the neutral conductor passes through the differential transformer, and only a fraction of the ground loss current is available to be detected. The neutral electrical conductor may be a wire of greater caliber than the ground conductor, and will not include resistive connections such as in duct grounding, and therefore the largest portion of the ground leakage current flows frequently in the neutral thread. In a circuit where an earth leakage current of 6 milliamperes flows through a person, for example, it can cause three quarters of the current to flow through the neutral wire where it is considered a load current and is not detected, and only a quarter flows through the ground conductor. Therefore, a much larger leakage current must flow to the ground before the fault is detected, all to the detriment of the person through which the leakage current flows to ground. The aforementioned problem has been commonly faced with the provision of a second transformer, sometimes referred to as a grounded neutral transformer. The second transformer is arranged with the active and neutral lines that extend through the transformer core forming a first winding, and forming another wound winding in the toroidal core the second. However, instead of detecting the differential current through the second winding wound on the toroidal core, an oscillator is connected thereto, the second winding of the neutral-to-ground transformer forming part of the resonant circuit of the oscillator. In the absence of a neutral ground connection, there is insufficient feedback on the oscillator to initiate and maintain the oscillation. However, when a neutral-to-ground fault occurs, a closed coupling loop is formed between the differential and neutral transformers, a feedback path is created and oscillation is initiated. The oscillation induces in the neutral wire a current that is detected in the same way as a loss to ground by the primary differential transformer. Earth leakage circuit breakers of the type just described detect both conventional ground losses and ground losses in the presence of intentional or accidental neutral ground faults. It is desirable to provide circuitry for detecting arc faults as well as ground faults. Arc faults are typically undetectable by the differential transformer or neutral transformer grounded from a ground fault circuit interrupter, because the waveforms produced by an arc fault appear in both the active and neutral lines. One approach to detecting faults from neutral to ground is to provide a transformer, through which only the neutral line of the electrical circuit passes. If a neutral grounded transformer is provided through which only the neutral line passes, it will not be able to detect ground-neutral conditions that arise when an electrical circuit is accidentally connected to the loss-to-ground circuit interrupter with the wires active and neutral inverted. Therefore, a neutral grounded transformer is preferably arranged by passing both the active and neutral lines through the neutral transformer and forming its two secondary ones. An arc fault can not easily be detected in such a transformer, and a third transformer is usually provided whose primary is only one of the active and neutral cables for detecting arc faults. The need for three transformers, a primary differential transformer to detect ground losses, a grounded neutral transformer, and an arc fault detection transformer, creates a particular problem. Often there simply is not enough space to include the three transformers and their associated circuitry in a package that fits in the space provided for a double plug, for example. A combined circuit breaker for arc failure and ground loss is required. The amount of circuitry required to detect ground and arc faults, and open an electrical circuit in response to it, makes it difficult to physically pack all the necessary components in a double socket, for example. Since a differential earth leakage transformer must be as symmetrical as possible to reduce the common mode response, signals indicating arc faults can not be detected from the secondary winding of the differential transformer. Applicants have discovered, however, that it is possible to detect signals representing arc faults with a carefully designed asymmetric transformer that is also suitable for use as a grounded neutral transformer, as described above. Although theoretically perfect transformers of the toroidal type with active and neutral cables passing through to form the primary, are not sensitive to arc faults to produce a useful signal, applicants have discovered that an asymmetric transformer can be constructed that produces Useful signs indicative of arc faults, and at the same time it can be used to couple a detection signal from the neutral conditions to ground to the neutral conductor of an electric circuit. An object of this invention is to provide a combined circuit breaker for ground loss and arc failure in a compact package including a first differential transformer for producing signals indicative of a ground loss, and a second asymmetric transformer having a winding arranged in it to produce both signals indicative of an arc fault and to couple a signal from an oscillator to the neutral wire that passes through the transformer to produce a fault condition in the event of a ground-fault. Another object is to provide a second winding for injecting an arc test signal to test the function AFCI_ - Expressed succinctly, and according to a presently preferred embodiment of the invention, a combined circuit breaker for earth loss and arc failure includes a transformer of differential current, an active electrical line and a neutral electrical line that pass through the current transformer and form its primary, an asymmetric toroidal transformer that has a core through which the active and neutral electric cables pass, and a coiled secondary in the core to produce a signal more strongly sensitive to current in one of the line conductors than in the other. An arc fault detector is connected to the secondary of the second transformer, while an earth leakage detector is connected to the secondary of the first transformer, and the outputs of the earth leakage and arc fault detectors are connected to an earth fault switch. circuit arranged to disconnect the circuit when a ground fault or arc fault is detected.

BRIEF DESCRIPTION OF THE DRAWINGS The new aspects of the invention are set forth in detail in the appended claims. The invention itself along with other objects and their advantages can be easily understood by reference to the following detailed description of a presently preferred embodiment of the invention, taken in conjunction with the accompanying drawing, in which: Figure 1 is a schematic diagram of a combined switch for ground loss and arc failure according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to Figure 1, there is shown a schematic diagram of a ground fault circuit interrupter and a combined arc fault interrupter. An active electric line 12 and a neutral electric line 14 are connected between a primary current source and a load (not shown). A normally open electrical relay 20 or contactor is connected in circuit relationship between the power supply and the load to interrupt the active and neutral lines in case of failure. A first differential current transformer 22, preferably formed in a toroidal core 24 of permeable material, is disposed with the active electrical conductors 12 and neutral 14 extending through to form a differential primary. A symmetrical secondary winding 25 is preferably formed in the toroidal core 24 to produce an electrical signal in its secondary corresponding to the differential current between the active and neutral lines. Preferably, the core and the winding are symmetrically constructed to maximize common mode rejection, that is, to produce a signal output as close to zero as possible when the active and neutral lines carry identical but opposite currents. A ground loss circuit detector 38 is connected to the secondary winding 25 of the differential current transformer 22. The ground loss detecting circuit can be of any of the assemblies known to those skilled in the art, and preferably includes an oscillator that produces a signal in an output. A second asymmetric toroidal transformer 26 is also arranged with active lines 12 and neutral 14 extending through a toroidal core to form a differential primary. An asymmetric winding 30, such as a winding formed on a portion of the circumference of the core only, can be used to provide at its output a differential signal having low common mode rejection, that is, a signal that is more sensitive to the current flowing in one of the conductors that in the other. Alternatively, the toroidal core can be made asymmetric, for example, by nicking a notch or otherwise removing a portion of the core in one or more discrete positions, to reduce the common mode rejection of the transformer. A capacitor 34 is connected to one end of the secondary winding 30 to form a resonant circuit. The other end of the capacitor is connected to an output of the oscillator 36 in the ground loss detector 38 to provide an oscillating signal to the secondary of the transformer. The ground loss detector 38 produces an output signal at an output 39 when there is a neutral ground condition or a conventional ground loss, or both. With the exception of the use of an asymmetric transformer, the circuitry for detecting a neutral condition to ground is known to those skilled in the art. However, its use in connection with an asymmetric transformer is considered new. The secondary winding 30 of the transformer 26 is also connected to an arc fault circuit detector 40. The detector .40 is sensitive to the signals produced in the transformer 26 in response to arcing, which have a distinctive signature or configuration. which can be discriminated by the detector 40. Detectors of this type are known. Since the transformer 26 is asymmetric, arc faults that do not produce a differential current are detected. The leakage detector output to ground / neutral to ground 38 and the output of the arc fault detector 40 are connected in parallel by means of bias diodes 42, 44 to the input 46 of a switch 48, such as an SCR. The SCR has its anode 50 connected to the base 52 of a switching transistor 54 which is connected to the coil 56 of the relay or contactor 20. When the SCR is triggered by a ground fault or an arc fault, the connection to the The base of the relay switching transistor 54 is bypassed, disconnecting the transistor 54 and the conduction relay 20, disconnecting the load. The activation of the reset switch 111 resets the device. The combined earth leakage and arc fault circuit interrupter of this invention includes circuitry to periodically check the earth leakage and arc fault detectors, the switching circuits, and the relay automatically, without the need for operator intervention. An automatic test timer 80 periodically produces pulses at the ground loss output 82 and an arc fault output 84. Preferably the pulses are staggered, so that the ground loss circuit is verified first, then the circuit is checked of failure by arc, and the cycle is repeated continuously. The timer outputs are connected to a ground loss test circuit 86 and an arc fault test circuit 88, respectively. The ground loss test circuit 86 produces a high level signal 92 which is connected to an input of a gate NI 73, and produces an energizing signal on a second output 90, which activates the fault relay 96, to simulate a ground loss current, between the active electrical conductor 12 and the neutral conductor 14 on opposite sides of the transformer 22. Preferably, the resistor 98 generates a current of approximately 10 milliamperes, and therefore can be a resistance of approximately 15. k-ohms The second output 84 of the timer 80 is connected to an input 100 of the arc test simulator 88. The arc test simulator 88 produces a high logic level at its first output 102 which is connected to the other input of the NI gate 73 , and a simulated arc signal in its second outputs 104, 105 which are connected to a winding 106 in the asymmetric transformer 26 which is coupled to the detection winding 30. The arc test simulator 88 will generate a signal that simulates the shape of the wave produced by a real arc to verify the functioning of the arc detector. The bypass fault current flowing through the resistor 98, simulating a loss to ground, causes a differential current to pass through the transformer 24, activating the block gfi 38. The block gfi 38 produces at 39 a signal of output that activates the gate 46 of the SCR 48. The conduction of the SCR 48 lowers the junction 107 and resets the self-check timer 80. A monostable timer 72 briefly maintains the active low state of the signal 92, which appears in the output 108 of the gate NI 73, when the self-reset timer 80 resets the verification block gfi 86. The output of the timer 72 is connected to an input of a NO-Y gate 62. The activation of the SCR 48 eliminates the base activation of the transistor 54, which serves to disconnect the relay current from the relay coil 56, and to decrease to zero the voltage across the zener diode 109 and the transistor 112. The input of the inverter 60 is protected with ntra overvoltage by the zener diode 121 and the resistance 122. When the voltage at 58 falls to zero, the inverter 60 activates the other input of the NO-Y gate 62. The NO-Y gate 62, keeping high, both inputs, produces a low trigger signal 123, and activates a monostable timer 110 which reconnects the base activation to the transistor 54, re-energizing the relay coil 56, before the relay contacts 20 can be opened. At the same time, the timer 110 activates the transistor 66, by bypassing the SCR 48, and deactivating the condition SCR. The diode 120 ensures that all current of the SCR is bypassed through the transistor 66. The time delay of the timer 110 is set to be greater than the combined time delay of the relay 96 plus any delay of the block. gfi 38 to remove the door drive from the SCR 48. The above allows the circuitry to be checked without actually disconnecting the load. Similarly, when the arc fault test 88 is activated, the afi block 40 produces the same momentary disconnection sequence of the relay coil 56. The transistor 112, the resistors 117 and 118, the capacitor 119, and the timer 114 form a fail-safe circuit. The timer 114 is a type that resets its time base to each shot without causing the output to be low. Each time the junction 107 is made low by the self-verification sequence, the input of the timer activator 116 is made low by the action of charging the capacitor 119. This keeps the timer constantly on, and the output of the timer high, maintaining the transistor 112 in conduction by the resistor 117. The resistor 118 serves to keep the input of the activator 116 high, and inactive, if the capacitor 119 is not periodically charged. The periodic charging must occur in a time less than the time constant of the timer 114, or the output of the timer 115 is low, and the transistor 112 is disabled. In this manner, the transistor 112 will stop driving, disengaging the relay coil 56, if the test sequence is not functional.

The periodic activation of the gfi test, and then the arc test, is set at a rate so that if there is no indication of failure at junction 107, the periodic trigger pulse frequency that appears in the timer trigger 116 falls to the middle, which is too slow to keep the timer 114 activated; this causes the fail-safe transistor 112 to open, de-energizing the relay 20 and opening the load. During the occurrence of a ground fault or actual arc fault, junction 107 will be low when the SCR 50 is made conductive by the gfi block 38 or the arc detector block 40. This keeps the self-test timer 80 in the state of deactivate and deactivate the autoreposition function. The transistor 54 now stops driving followed shortly by the opening of the fail-safe transistor 112. One of said transistors opens the relay 20. The device is in this state until a reset occurs at 111. It is to be understood that many of the timing and logic blocks and some or all of the test blocks and detectors could be replaced by a microprocessor. Although the invention has been described in connection with its presently preferred embodiment, those skilled in the art will recognize that modifications and changes can be made therein without departing from the true spirit and scope of the invention, which is therefore intended to be defined only by the appended claims.

Claims (27)

1. A circuit protector that includes: a differential current transformer to produce a signal in response to a ground loss in the circuit; an asymmetric transformer to produce a signal in response to an arc fault in the circuit; an earth leakage detector connected to the differential current transformer; and an arc fault detector connected to the asymmetric transformer.

2. The circuit protector of claim 1 including an active electrical conductor and a neutral electrical conductor, each conductor forming a primary winding of each of the differential current transformer and the asymmetric transformer.

The circuit protector of claim 2, wherein the asymmetric current transformer produces an output signal that is more sensitive to the signals in a selected conductor of the active and neutral conductor than to the signals in the other conductor.

4. The circuit protector of claim 2, wherein the differential transformer includes a toroidal core.

5. The circuit protector of claim 2, wherein the asymmetric transformer includes a core selected from the group consisting of toroidal cores, section cores and square cores.

The circuit protector of claim 5, wherein the asymmetric transformer includes a first winding coupled to an oscillator to produce a ground loss that indicates current in the differential transformer when the secondary neutral conductor is connected to a ground conductor .

The circuit protector of claim 6, wherein the first winding of the asymmetric transformer, which is used to generate in the differential transformer a current indicating loss to ground, is also used as an output winding for the signal of failure detection by arc.

The circuit protector of claim 1, wherein the differential current transformer is characterized by a greater common mode rejection characteristic than the asymmetric transformer.

9. The circuit protector of claim 1, wherein the asymmetric transformer includes a core having circumferentially non-uniform magnetic characteristics.

10. The circuit protector of claim 1, wherein the asymmetric transformer includes a core in which a groove is formed.

11. The circuit protector of claim 1, including a circuit breaker.

12. The circuit protector of claim 11, wherein the circuit breaker includes a relay.

The circuit protector of claim 12, including a switching device connected to the relay for selectively energizing the relay.

14. The circuit protector of claim 1 including an arc fault simulator coupled to the asymmetric transformer to simulate an arc fault to verify the operation of the circuit protector.

15. The circuit protector of claim 1, including a ground loss simulator coupled to the active and neutral lines on opposite sides of the differential current transformer to simulate a ground loss to verify the operation of the circuit protector.

16. The circuit protector of claim 15, including a ground loss simulator coupled to the active and neutral lines to simulate a loss to ground to verify the operation of the circuit protector.

17. The circuit protector of claim 14, including a timer for periodically energizing the arc fault simulator.

18. The circuit protector of claim 15, including a timer for periodically energizing the ground loss simulator.

19. The circuit protector of claim 15, including a timer for periodically energizing the ground loss simulator and the arc fault simulator.

20. The circuit protector of claim 1, in which the asymmetric transformer includes a non-symmetrical winding.

21. The circuit protector of claim 12, including a ground loss simulator coupled to the active and neutral lines on opposite sides of the differential current transformer to simulate a ground loss to verify the operation of the circuit protector.

22. The circuit protector of claim 12, including an arc fault simulator coupled to the asymmetric transformer to simulate an arc fault to verify the operation of the circuit protector.

23. The circuit protector of claim 21, wherein the relay coil is de-energized momentarily and then reenergized, before the relay contacts can be opened, to test the proper operation of the electronic interruption circuits.

24. The circuit protector of claim 22, wherein the relay coil is de-energized momentarily and then reenergized, before the relay contacts can be opened, to test the proper operation of the electronic interruption circuits.

25. A circuit protector that includes a relay that has a coil, and contacts arranged to interrupt power to a load when a fault is detected; and a test circuit to simulate a fault and momentarily de-energize the coil to test the protector and then re-energize the coil before the contacts open.

26. The circuit protector of claim 14 including a repositionable timer that is repositioned each time the arc detector activates the circuitry's excitation circuitry, and if it is not reset, causes the circuit breaker to interrupt the power to the load.

27. The circuit protector of claim 15 including a repositionable timer that is repositioned each time that the earth leakage detector activates the circuitry's excitation circuitry, and that if it is not reset, causes the circuit breaker interrupt the power to the load.