Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2002-08-22
2003-12-23
Tong, Nina (Department: 2858)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S636130, C340S638000, C340S652000, C340S653000, C361S042000, C324S522000, C324S536000
Reexamination Certificate
active
06667691
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to means for providing safety and reliability to electric circuits. More particularly, the invention relates to a method and apparatus for providing an early detection and alert of an arcing fault in electrical systems that are used in residences, commercial areas, industries, automotive vehicles, etc.
BACKGROUND OF THE INVENTION
Automatic and continuous protection of electric supply systems is accomplished today, by: a) fuses and circuit breakers that protect against dangerous over-currents by terminating the current flow in the event of a short-circuit or an overload; and b) ground fault interrupters which terminate the current flow whenever there is a dangerous leakage of current from the “line” Chase) conductor to the “ground” conductor.
Another kind of electrical fault is the arcing fault which appears when two conductors in the circuit become separated by a small air gap that allows current to pass through it. In most instances of arcing fault, the current flows intermittently, as if there is a switch that is turned alternately and irregularly “on” and “off”.
Arcing fault poses the following threats and problems:
1) A fire hazard resulting from excessive heat that is liberated unexpectedly at an insufficiently protected arcing zone.
2) An arcing fault which develops into a major fault, damaging the electric appliance or machine in which it occurs and leading to a final short circuit and current termination.
3) An arcing fault which leads to bursts of over currents and to irregular current supply, which accelerates the wear of the electric appliances involved.
A series arcing fault is not protected by the above-mentioned protective means, since the current is limited by the load resistance, and is consequently below the threshold value predefined as dangerous. A parallel arcing fault is also not completely protected, since as long as the average current is below the predefined threshold value, it continues to constitute a fire hazard. Also, there is the potential for a short-circuit build-up, followed by current termination.
The need for an early warning device against the build-up of arcing conditions is clear, as such a device may be helpful in preventing fires, uncontrolled current termination, and damage to electrical appliances and machines.
In the last two decades, many devices have been introduced for the detection of arcing fault.
Most of them use the fact that discontinuities in the conduction parameters of an electric circuit lead to fast transients, and sense the resulting fast current oscillations. These high-frequency oscillations are then analyzed, by utilizing various features that differentiate between an arcing fault event and an event caused by a valid use of a load of the electric network. To facilitate the analysis, the transients are commonly converted to digital data or to other simple signals. All these suggestions differ principally, by the method of discrimination between the various events; their capabilities and efficiencies are derived accordingly.
For example, U.S. Pat. No. 4,466,071 (Russell et al.), issued in August 1984, is directed to a method and system for detecting high impedance arcing faults. The detection “is realized by monitoring the high frequency components of the alternating current and evaluating the high frequency components of each cycle”. The occurrence of a significant increase in the magnitude of the high frequency components signifies either an arcing fault or a normal switching operation. The duration of time over which this increase exists, discriminates between the various possibilities and can determine the presence of an arcing fault.
The method of U.S. Pat. No. 4,466,071 poses three particular drawbacks.
1. The switching of a dimmer might be erroneously identified by this system as an arcing fault event, as such can generate a high level of high frequency current components for a sufficient time.
2. If the minimal period for determining an arcing fault is too short, for example, less than 0.5 second, then a long switching event like pressing or releasing the trigger of an electric drill might be identified as a fault; and
3. If, on the other hand, the minimal period that defines an arcing fault is too long, for example, more than 0.5 second, then short signals which characterize the early stages of the evolution towards a fault might be ignored. Thus, the opportunity for providing an early warning would be lost.
U.S. Pat. No. 5,682,101 (Brooks et al.) issued in October 1997, discloses a detector that monitors the rate-of-change of the current in the line and produces a signal which is proportional to it. The detector produces a pulse whenever the rate-of-change signal exceeds a certain threshold. Each high-frequency disturbance in the current produces steep oscillations which are filtered into a selected high-frequency band, and are transformed by the detector to pulses. The pulses are “counted” by charging a capacitor by an amount that is proportional to the number of the pulses. Upon exceeding a selected charge level, the detector signifies an arcing fault. The time constant for the discharge of the capacitor, in the example given therein, is 33 msec.
About the discrimination between various similar phenomena, Brooks states as follows: “The pattern in the rate-of-change signal produced by the sensor
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indicates whether the condition of the circuit is a normal load, a normal switching event . . . or an arcing fault event”. But though this detector covers a wide range of events, still, a long switching event might load the capacitor beyond the selected charge level, and cause a false alarm.
Other drawbacks of the Brooks et al. system are: 1) the use of a blocking filter for each load in order to diminish spurious noise, which also diminishes the protected portion of the circuit; and 2) the discharge of the capacitor deletes valuable information, about suspicious events, that might enable an early detection of evolving problems.
U.S. Pat. No. 5,726,577 (Engel), issued in March 1998, discloses a detector for series arcs in AC circuits. This detector generates signals representing the second derivative of the current, which signals contain pulses in response to discontinuities in the current. The continuity properties of arcing current are very regular at currents too low to blow the arc apart; indeed, “the current is more continuous, except for discontinuities at current zero crossings”. This leads to a unique pattern of pulses, wherein a pair of pulses of opposite polarity appears, each half a cycle. On the other hand, the signal from a dimmer has a triple pulse of alternating polarities which also appears once per each half a cycle. This difference is the basis for the discrimination between pulses due to arcing and other pulses due to normal loads. However, some restrictions and drawbacks must be mentioned: This method is appropriate only for series arcing with quite low levels of currents which enable a steady and persistent arcing. But it is less appropriate for casual arcing or intermittent arcing, which have irregular behavior, with the consequence that the pair pattern is not repeated. Also, the method of U.S. Pat. No. 5,726,577 is targeted for just one dimmer and for detecting faults only in AC circuits.
U.S. Pat. No. 5,818,237 (Zuercher et al.), issued in October 1998, discloses a detector for arcing faults. It tracks the envelope of the current signal, and differentiates it. Thus a di/dt signal is generated that contains pulses in response to step increases in the current. The detector analyzes the pulses and rejects those that appear at a rate equal to, or higher than, the frequency of the power supply. In this way, the detector succeeds in eliminating pulses produced by loads such as dimmers; while the other pulses are counted, and after a predetermined number is attained within a predefined time interval, the detector actuates the current breaker. This detector cannot discriminate between signals of a true arcing fault and those of a normal load, s
Frommer William S.
Frommer & Lawrence & Haug LLP
Tong Nina
LandOfFree
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