Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of ground fault indication
Reexamination Certificate
2001-04-04
2002-10-15
Le, N. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of ground fault indication
C508S133000, C508S424000, C361S042000, C361S096000
Reexamination Certificate
active
06466029
ABSTRACT:
FIELD OF INVENTION
The present invention relates generally to AC power line test equipment and, more particularly, to a power line testing apparatus capable of transmitting a tracer signal easily distinguishable from environmental noise.
BACKGROUND OF THE INVENTION
When electrical power line outlets (receptacles) are installed in residential, commercial or industrial environments, it is extremely important that they are properly wired. If such outlets are incorrectly wired, they may be absolutely useless and/or can cause significant damage to equipment and property or, even worse, they can result in electrical shock, thereby possibly causing serious harm to, or death of, individuals. A further extremely hazardous situation occurs, even if the wiring scenario is correct, but where the integrity of the connected earth ground conductor is compromised. A proper ground connection is meant to shunt leakage current, which can appear at conductive enclosures of appliances due to defects or poor insulation, to earth ground. The integrity of earth ground conductors can be compromised by an increase in electrical impedance due to corrosion of conductors and/or conduits (when used as earth ground) or by degradation of conductor-to-conductor connections in commonly used wire nuts or screw terminals, which may loosen over time. If sufficient impedance is present in a ground system, the ground conductor is no longer a real earth ground.
Ground fault interrupting devices are known in the art. Those devices are usually equipped with an internal test function which works quite reliably, yet they cannot test the integrity of fixed or temporary branch circuit extensions. Therefore, numerous external testing devices have become available which are to be connected to such extensions and to perform the required test from there. Almost all known devices in this art generate a leakage current flow of a certain amount from the hot to the ground conductor, but they ignore the time that elapses until the device under test reacts. This leads in many cases to the false assumption that the device under test works properly and reliably while, in fact, in many cases it does not since the device indeed reacts, but exceeds the permissible period of time for reaction.
Fairly new in the art are so-called arc-fault interrupting devices. These devices are meant to interrupt an electrical branch circuit if serial or parallel arcing occurs along or between hot and neutral conductors in the power line system. Like ground fault circuit interrupting devices, they usually have an internal test function. In practice, there is a significant problem with these test functions because the activation of such a test is limited to either giving an incorporated microcontroller a command to activate the associated trip mechanism, or “inject” an electronic signal to the system in order to simulate arcing. As a result of the first such “test”, it only gets proven that the microcontroller is “alive” and that the mechanical parts of the apparatus work properly. A second type of “test” is apparently more enhanced than the first. However, in both of these tests, there is no reliable confirmation whatever that the devices will work properly and reliably under real arcing conditions.
When work needs to be performed on an electrical branch circuit, it is first necessary to unenergize that circuit or interrupt the current flow to the circuit. This is commonly achieved by opening or unscrewing the circuit-interrupting device in the distribution panel that is associated with that particular branch circuit. In many cases, it is not known which circuit-interrupting device, out of a plurality of such devices (as commonly occurs on a panel of circuit breakers), is actually the one in question. Absolute methods of determining the correct device are often not only inconvenient and time consuming but, in many circumstances, not even feasible. A “classic” method used to find the associated device needed is to sequentially unscrew fuses or to open circuit breakers in a distribution panel until the one in question has been found. Subsequently, all outlets in the branch circuit under test need to be checked for an unenergized condition. In certain environments, such as hospitals or manufacturing plants, this or similar methods are totally impractical. In other environments (e.g. where computers without backup power are used), such methods can be, at the very least, disturbing and/or annoying. As alternatives to these methods a variety of electronic devices have been developed that accurately determine which is the particular circuit-interrupting device in question. By examining those devices, it becomes apparent that they all have significant drawbacks.
The invention disclosed in Virgilio U.S. Pat. No. 5,625,285 describes a device for monitoring the present wiring scenario and acceptable grounding properties on a standard 3-Wire 120 volt AC electrical outlet. The circuit for this device is reproduced as FIG.
2
. The described device can detect and indicate the following wiring possibilities:
1. Correct wiring
2. Defective ground
3. Open neutral
4. Hot and neutral reversed
5. Hot on neutral with open neutral
One of the most hazardous situations, hot and ground reversed, cannot be detected and indicated. Further, it sometimes occurs that a second hot wire is mistakenly connected to a receptacle. In such a case, the voltage across the two hot conductors is twice that of the nominal voltage. If someone intends to perform work on such a miswired circuit, he or she faces an undesirable and potentially dangerous situation, since the circuit is still energized even if one associated circuit-interrupting device has been deactivated.
In order for the Virgilio structure to analyze the integrity of the earth ground conductor, a high current pulse of short duration is drawn over the power line system. A pickup coil senses the strength of the thereby generated magnetic field. The induced voltage in the pickup coil is then used to trigger a semiconductor device, which then activates a visible indicator. This circuit requires exact calibration during the manufacturing process. If parasitic impedance is present in the ground conductor, the magnetic field loses strength, therefore the induced voltage in the pickup coil is no longer sufficient to trigger the semiconductor control device. In practice, line resistance and capacitance are subject to continuous changes due to frequent on and off switching of heavy electrical loads. This has major impact on the proper performance of that device. These line and ground impedances impact the reliability of Virgilio's ground integrity test.
Another structure, disclosed in Robitaille U.S. Pat. No. 4,929,887, describes an electrical outlet monitor that recognizes the fact that more than one hot conductor can be, possibly, connected to an outlet. However, the unit does not indicate in detail what the present wiring scenario is, but only that it is incorrect. The circuit for this unit is shown in FIG.
1
.
No patented prior art is known to the applicant which covers GFCI testing devices that include the measurement and consideration of elapsed time as part of testing criteria. However, there is a device available, designed and manufactured by a German company named BEHA GmbH and distributed in North America by Greenlee which does take elapsed time into consideration.
Another structure, disclosed in Spencer et al. U.S. Pat. No. 5,875,087, describes an enhanced digital circuit breaker that includes an AFCI and an associated AFCI test function. As it is apparent from the block diagram of this breaker (FIG.
14
), activation of the incorporated momentary push button
20
does not in any way introduce an actual arcing condition in the power line system under test, but rather, simply instructs the microcontroller to mechanically trip the device through D
2
and thyristor
22
. In effect, this is not at all a “test” of the reliability of the device to respond to a genuinely dangerous condition; it is simply a demonstration of the “des
Stroth John E.
Stroth Peter
Wottrich Joachim
Contact Technology Systems, Inc.
Hamdan Wasseem H.
Le N.
Perkins Jefferson
Rudnick Piper
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