Electricity: electrical systems and devices – Safety and protection of systems and devices – Ground fault protection
Reexamination Certificate
1999-05-27
2001-07-17
Leja, Ronald W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Ground fault protection
C324S424000
Reexamination Certificate
active
06262871
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an electronic test circuit that attaches to a ground fault and/or arc fault interrupt device and automatically checks for the proper functioning of the ground fault and/or arc fault interrupt device, without the need for user attention, and disables the ground fault or arc fault interrupt device in a safe condition in case of a malfunction of the device.
2. Background of the Invention
A common source of electrical injuries in the home occurs when people place radios or similar electrical devices that are operated using household AC electrical current near a swimming pool or bathtub while swimming or bathing. If a radio is knocked into water, it can create undesirable electrical leakage current through the water to ground creating what is known as a ground fault. A ground fault can also occur when a person touches an electrically hot conductor while standing on or touching a grounded conductive surface. When sufficient electrical current passes through a person, electrical burns or electrocution may result. Many electrical appliances such as heaters, hair dryers, electric razors and pumps are used near water and can present this type of hazard. Outdoor appliances such as power drills and electrically powered lawn care equipment are often subject to cut or frayed power cords that can present an electrical leakage hazard. Even a relatively low level of electrical current leakage can be dangerous to a human. Underwriters Laboratories, in their 943 standard for ground fault interrupt devices, requires that listed devices must open in response to any leakage current exceeding six milliamperes.
A ground fault is not the only class of potentially dangerous abnormal operating conditions. Another type of undesirable operating condition occurs when an electrical spark jumps between two conductors or from one conductor to ground. This spark represents an electrical discharge through the air and is objectionable because heat is produced as a byproduct of this unintentional “arcing” path. Such arcing faults are a leading cause of electrical fires. Arcing faults can occur in the same places that ground faults can occur—in fact, a ground fault would be called an arcing fault if it resulted in an electrical discharge, or spark, across an air gap. As such, a device that protects against ground faults can also prevent many classes of arcing faults.
In the United States, protection circuits known as ground fault current interrupters or GFCI's, are presently required by code for the bathrooms of most new homes and commercial buildings, as well as for garage and outdoor outlets in residential applications. In Europe, a similar device called a residual current detector, or RCD, is used for detecting and interrupting dangerous electrical leakage to ground. The theory of operation behind commercial GFCI and RCDs is the same. The major difference between the two devices is that in the U.S., most GFCIs are required to trip when leakage currents exceed 6 milliamperes while European RCDs are generally set for a trip level of 30 milliamperes.
In commercial GFCI circuits, the current carrying conductors that connect the AC source to the load will pass through a current transformer, thereby acting as primary windings for that transformer. The transformer has a secondary winding with many turns that goes to an amplifier. In a two wire system, when no electrical leakage path to ground is present, all of the electrical current that goes out one wire returns in the other wire. Accordingly, the two currents, forward and reverse, balance out one another in terms of the magnetic flux that is generated in the current transformer and so no signal is generated in the transformer secondary. On the other hand, if there is leakage to ground at the load or from the conductors connecting the source to the load, then there will be an imbalance in the currents. In other words, more electrical current goes out one wire than returns in the other, the difference being the component of current that takes another path (the ground fault current). This results in a net magnetic flux in the transformer and this will serve to generate an induced voltage in the secondary of the transformer. That secondary voltage is amplified and filtered and used to trip a relay or circuit breaker, thus removing power from the load and removing power from the leakage path. Most GFCI circuits include a reset actuator that allows the circuit breaker to be reset. Most GFCI circuits include a test button that implements an artificial leakage path around the current transformer, allowing a user to manually test the GFCI circuit, by pressing the test button and confirming operation if the internal breaker appears to open. This test does not actually confirm that an internal breaker has opened. Usually, a reset button that is mechanically connected to the relay contacts pops out and the user must assume that this means that the circuit breaker opened and the GFCI is functional.
All of the above comments also apply to GFCIs (or RCDs) that are used for multiphase systems having more than two current carrying wires. The only difference is that the additional current carrying wires are also passed through the current transformer so that in the absence of an electrical leakage path, the currents going through the transformer sum to zero.
In many electrical systems, one of the current carrying conductors will be grounded at some point in the electrical system. This grounded conductor is known as the “neutral” conductor. GFCI's will often have a circuit to detect electrical leakages from this conductor to ground by means of a second current transformer. Typically, neutral to ground faults are detected by injecting a signal onto the neutral conductor which produces an oscillation if feedback is provided through the loop completed by the neutral to ground fault.
In the United States, the National Electric Code has mandated that Arc Fault Current Interrupters, or AFCI's, be installed in certain new construction starting in the year 2002. The requirements of AFCI circuits are under development under the Underwriters Laboratories standard 1699. That standard will require that AFCI's be provided with a test circuit that simulates the arc detection circuitry output to exercise the remaining portions of the device. Since in most implementations, the AFCI will be combined with a GFCI and share much of the detection and interruption electronics, the use of a single test button for manual testing of both ground fault detection and arc fault detection will probably be common.
Ground fault current interrupts that use a differential transformer to detect the current imbalance that is indicative of a fault condition have been in use since the 1960's. U.S. Pat. No. 3,683,302 (Butler et al.) discloses a sensor for a ground fault interrupter that is operative to detect current imbalances by means of a differential transformer. U.S. Pat. No. 3,736,468 (Reeves et al.) discloses a GFCI that uses a differential sense transformer, the secondary of which is amplified to trip a circuit breaker. Other designs that combine a differential sense coil and amplifier combination to trip a circuit breaker upon fault detection include U.S. Pat. Nos. 3,852,642 (Engel et al.), 3,859,567 (Allard), 3,936,699 (Adams), 4,216,515 (Van Zeeland), 4,216,516 (Howell), 4,255,773 (Jabbal) and 4,353,103 (Whitlow).
GFCI's (or equivalently, RCD's) are required by code in many settings. In the United States, GFCI protection is required for the bathroom, garage and outdoor outlets on all new construction. The lifetime of an electrical outlet may be measured in decades but there is no assurance that a GFCI will continue to function properly over that time interval. GFCI outlets that are installed outdoors, or portable GFCI's used with construction equipment, may become encrusted with dirt or corrosion. Electronic components and insulation will age with time and this may cause a degradation in perf
Hirsh Stanley S.
Nemir David C.
Rubio Edward
Leja Ronald W.
Myers Jeffrey D.
X-L Synergy, LLC
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