This invention relates generally to devices for protecting electrical circuits in the event of faults, and more particularly to a device for protecting a circuit against ground faults and arc faults.
Ground fault circuit interrupters have become quite widely used and they provide the very useful function of disconnecting an electrical power source from a load when a ground fault is detected. Among the more common types of ground faults sensed by known ground fault circuit interrupters are those caused when a person accidentally makes contact with a hot electrical lead and ground. In the absence of a ground fault circuit interrupter, life threatening amounts of currents could flow through the body of the person.
Virtually all ground fault circuit interrupters use a differential current transformer for sensing ground faults. The current transformer is arranged with the hot and neutral leads of an electrical circuit passing through the center of a transformer, preferably a toroidal transformer with a symmetrically wound secondary formed thereon. In normal operation of an electrical circuit, the currents flowing through the hot electrical conductor and neutral electrical conductor are equal and opposite, and no output signal is produced by the differential transformer. When a ground fault occurs, the currents are no longer the same, and the differential transformer produces a signal that can be sensed by appropriate signal conditioning circuitry to activate a relay or contactor or the like to interrupt the electrical circuit.
Ground fault circuit interrupters 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 but wrongly connected together, or by accidental short circuits formed for example when a strand of a stranded electrical conductor accidentally bridges the neutral and ground connections.
If a xe2x80x9cgrounded neutralxe2x80x9d fault of the type just discussed occurs in a circuit in which the ground and neutral lines are connected together, for example, and a person inadvertently makes contact with a hot electrical lead while also connected to the grounded neutral fault, the return current is divided between the neutral electrical conductor and ground conductor. Of the two, the neutral conductor passes through the differential transformer, and only a fraction of the ground fault current is available to be sensed. The neutral electrical conductor can be a larger gauge wire than the ground conductor, and will not include resistive connections such as in conduit grounds, and therefore the larger portion of the ground fault current often flows in the neutral wire. In a circuit where a 6 milli-amp ground fault current flows through a person, for example, it may result in three-quarters of the current flowing through the neutral wire where it is seen as a load current and not detected and only one quarter flowing through the ground conductor. Therefore, a much larger ground fault current must flow before the fault will be detected, all to the detriment of the person through whom the ground fault current flows.
The problem just mentioned has been commonly addressed by providing a second transformer, sometimes referred to as a grounded neutral transformer. The second transformer is arranged with the hot and neutral lines extending through the core of the transformer, forming a first winding, and another winding wound on the toroidal core forming the second. Rather than sensing differential current through the second winding wound on the toroidal core, however, an oscillator is connected thereto with the second winding of the grounded neutral transformer forming a part of the resonant circuit of the oscillator. In the absence of a ground neutral connection, there is insufficient feedback in the oscillator to initiate and sustain oscillation. However, when a grounded neutral fault occurs, it forms a closed coupling loop between differential and neutral transformers, a feedback path is created and oscillation is initiated. The oscillation induces a current in the neutral lead that is detected in the same manner as a ground fault by the primary differential transformer.
Ground fault circuit interrupters of the type just described detect both conventional ground faults, and ground faults in the presence of intentional or accidental grounded-neutral faults.
It is desirable to provide a circuitry for detecting arc faults as well as ground faults. Arc faults are typically undetectable by the differential transformer or the grounded neutral transformer of a ground fault circuit interrupter, because the wave forms produced by an arc fault appear on both the hot and neutral lines.
One approach to sensing grounded neutral faults is to provide a transformer, through which only the neutral line of the electrical circuit passes.
If a grounded neutral transformer is provided that has only the neutral line passing therethrough, it will not be able to sense grounded neutral conditions that arise when an electrical circuit is inadvertently connected to the ground fault circuit interrupter with the hot and neutral cable wires reversed. Therefore, a grounded neutral transformer is preferably arranged with both the hot and neutral lines passing through the neutral transformer and forming two secondaries thereof.
An arc fault cannot be readily sensed at such a transformer, and a third transformer is ordinarily provided whose primary is only one of the hot and neutral leads for detecting arc faults.
The need for three transformers, a primary differential transformer for sensing ground faults, a grounded neutral transformer, and an arc fault sensing transformer, creates a particular problem. Often there is simply not enough room for all three transformers and their associated circuitry to be included in a package that will fit in the space provided for a duplex receptacle, for example.
There is a need for a combined arc fault and ground fault circuit interrupter. The amount of circuitry required for sensing ground faults and arc faults, and opening an electrical circuit in response thereto, makes it difficult to physically package all of the necessary components in a duplex receptacle, for example.
Because a differential ground fault sensing transformer must be as symmetrical as possible to reduce common mode response, signals indicating arc faults cannot be sensed from the secondary winding of the differential transformer. Applicants have discovered, however, that it is possible to sense signals representing arc faults with a carefully designed asymmetrical transformer that is also suitable for use as a grounded neutral transformer as described above.
Although theoretically perfect transformers of the toroidal type having hot and neutral leads passing therethrough to form the primary, are not responsive to arc faults for producing a usable signal, applicants have discovered that an asymmetrical transformer can be built that produces usable signals indicative of arc faults, and can at the same time be used for coupling a signal for sensing grounded neutral conditions to the neutral conductor of an electrical circuit.
It is an object of this invention to provide a combined ground fault and arc fault circuit interrupter in a compact package that includes a first differential transformer for producing signals indicative of a ground fault and a second asymmetrical transformer having a winding disposed thereon for both producing signals indicative of an arc fault and for coupling a signal from an oscillator to the neutral wire passing through the transformer for causing a fault condition in the event of a grounded neutral fault.
It is another object to provide a second winding for injecting an arc test signal for testing the AFCI function.
Briefly stated, and in accordance with a presently preferred embodiment of the invention, a combined ground fault and arc fault circuit interrupter includes a differential current transformer, a hot electrical line and a neutral electrical line passing through the current transformer and forming the primary thereof, an asymmetrical toroidal transformer having a core through which the hot and neutral electrical leads pass, and a secondary wound on the core for producing a signal more strongly responsive to the current in one of the line conductors than the other. 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 ground fault and arc fault detectors are both connected to a circuit interrupter arranged for disconnecting the circuit when either a ground fault or an arc fault is detected.