Electricity: electrical systems and devices – Safety and protection of systems and devices – Ground fault protection
Reexamination Certificate
2001-07-19
2003-06-10
Riley, Shawn (Department: 2838)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Ground fault protection
C361S078000
Reexamination Certificate
active
06577478
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a power distribution system, and more particularly, to an overload circuit interrupter capable of detecting overload and interrupting the overload circuit interrupter, and to a circuit breaker with the same.
BACKGROUND OF THE INVENTION
Low voltage networks, typically 600 volts and below, are used to distribute electric power in a specified area, such as part of a city or an industrial or commercial area. Often, the cables in such networks are located underground. Typically, the network is designed to feed at more than one point, and therefore, has multiple sources. Occasionally, the cables have failed, due to various causes such as thermal degradation, age, moisture or rodent damage.
The networks are protected by circuit breakers. In order to isolate the faulty cable and therefore minimize disruption of the networks, cable limiters are provided at the ends of the cables. Cable limiters are fuse-like devices, which assure safe reactions to high voltage and low impedance faults, such as are created by phase-to-phase faults.
Wiring (miniature) circuit interrupters and current leakage circuit interrupter are commonly used devices for protecting people and property from fire and dangerous electrical faults. A wiring circuit interrupter is used to protect a power line. First, when excessive current passing through circuit breaker is converted to heat in the use of an electrical device, the circuit interrupter is tripped by a bending of bimetal in it. Second, when an electrical tool or other metallic object on the load shorts the power line, high current is passed through instantaneously. Therefore, bimetal in the circuit breaker is heated by high current, so the electrical device is interrupted by operation of an inner magnet of the circuit breaker.
It is known in the art that a current leakage circuit interrupter has the ability to detect current leakage, which may be present on the power line, and trip the circuit interrupter, so that the circuit interrupter prevents people from receiving an electric shock from the leakage current.
In America, ground fault circuit interrupters (GFCI) are required to be used, which contact people's hands directly, in the wiring (miniature) circuit interrupter. The GFCI, which is able to detect leakage current with high sensitivity, belongs to the category of current leakage circuit interrupters. Thus GFCI must be installed in kitchens, bathrooms, parking places and basements, which easily may become damp and wet.
In spite of the wiring circuit interrupter and current leakage circuit interrupter, large numbers of fires occur all over the world every year. This is due to the fact that often an arcing type fault to ground occurs rather than a phase-to-phase fault. Arcing faults typically create current with low root mean square (RMS) value, which is below the thermal threshold for such circuit breakers. Even so, such arcs can cause damage or start a fire if they occur near combustible material.
Arcs are potentially dangerous due to the high temperatures. An arc, however, will only trip a GFCI when it produces sufficient current leakage to ground. In addition, an arc will trip a circuit breaker only if the current, flowing through the arc, exceeds the trip parameters of the thermal/magnetic mechanism of the breaker. Therefore, an additional type of protective device is needed to detect and interrupt arcs that do not fit these criteria. An arc detector whose output is used to trigger a circuit interrupting mechanism is referred to as an arc fault circuit interrupter (AFCI).
The U.S. Consumer Product Safety Commission (CPSC) estimated that 40% of fires in 1997 were caused by arc faults. Also, the National Electric Code (NEC) includes a regulation requiring installation of the AFCI in residential buildings from January 2002.
The causes of arc faults are numerous. For example, they include aged or worn insulation and wiring, mechanical and electrical stress caused by overuse, over currents or lightning strikes, loose connections, and excessive mechanical damage to insulation and wires.
Three types of arcing may occur in residential or commercial buildings: serial arcing, parallel arcing and ground arcing.
Serial (or contact) arcing occurs between two contacts in series with a load. The conductors comprising the cable are separated and surrounded by an insulator. A portion of the conductor is broken, creating a series gap in the conductor. Under certain conditions, arcing will occur across this gap, producing a large amount of localized heat. The heat produced by the arcing might be sufficient to break down and carbonize insulation close to the arcing. If the arc is allowed to continue, enough heat will be generated to start a fire. Under these conditions, current flowing through the arc is controlled by load.
Parallel (line) arcing is the second. Cable comprises electrical conductors covered by outer insulation and separated by inner insulation. Deterioration or damage to the inner insulation at any point may cause parallel fault arcing to occur between the two conductors. The inner insulation could have been carbonized by an earlier lightning strike to the wiring system, or it could have been cut by mechanical action such as a metal chair leg cutting into an extension cord.
Ground arcing occurring between a conductor and ground is the third. If the outer insulation used for protecting conductors is damaged, the conductor contacting ground due to a damaged portion will result in ground arcing.
Current flowing through the arcing may be changed by impedance because parallel arcing and ground arcing occur parallel to the load. Long time deterioration causes cable carbonization and damage to coating. The cable is further deteriorated by Joule heat, which is induced by arcing current. The arcing generates joule heat according to the ** in a relation of J (Joule heat)=I
2
(arcing current)×t(Time).
One major problem associated with any type of arc detection is false tripping. False tripping occurs when an arc detector produces a warning output, or disconnects a section of wiring from the voltage source, when a dangerous arcing condition does not actually exist. This problem is caused by the fact that an arc signal (arcing current and arcing voltage) is not generated in the form of a correct sine wave, and has various types of waveform. Specifically, the arc signal is similar to the driving pulse, which is created in appliances, such as an electric fan and dryer, which have an electric motor inside.
Also, if you use an electrical device, at the beginning of a cycle, a high pulse similar to the arc signal is generated, but after some time passes, the output signal has a normal amplitude. Therefore, it is difficult to detect arcing because the arc signal is similar to driving pulse at the beginning of a cycle.
The arc fault detector (AFD) disclosed in U.S. Pat. No. 5,805,397 uses a method of detecting arcing by multiple channel sensing. The prior patent discloses the method of detecting arcing in several bandwidths, and the AFD trips the circuit in a condition of arcing generation in all of the bandwidths.
A schematic diagram in block form of the prior art is shown in FIG.
1
. The electrical system
100
protected by the circuit breaker
103
includes a line conductor
105
and a neutral conductor
107
connected to provide power to a load
109
. The circuit breaker
103
includes separable contacts
111
that can be tripped open by a spring operated by trip mechanism
101
. The trip mechanism
101
may be actuated by a conventional thermal-magnetic overcurrent device
116
. This thermal-magnetic overcurrent device
116
includes a bimetal connected in series with the line conductor
105
. Persistent overcurrents heat up the bimetal causing it to bend and release a latch
113
, which actuates the trip mechanism
101
. Short circuit currents through the bimetal
115
magnetically attract an armature
114
, which alternatively releases the latch
113
to actuate the trip mechanism
101
.
A sch
Ban Gi-Jong
Kim Cheon-Youn
Kim Dong-Sub
Kim Jeong-Wan
Human El-Tech, Inc.
Rabin & Berdo P.C.
Riley Shawn
LandOfFree
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