Electricity: electrical systems and devices – Safety and protection of systems and devices – Ground fault protection
Reexamination Certificate
2001-05-10
2003-03-11
Huynh, Kim (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Ground fault protection
C361S093100, C361S078000
Reexamination Certificate
active
06532139
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a power distribution system, and more particularly, to a circuit breaker for triggering operation of the power distribution system when at least one of an arc fault, a ground fault or an overload is detected in the system.
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, an industrial or a 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 fail due to various causes such as thermal degradation, age, moisture or rodent damage. The networks are protected by circuit breakers. However, in order to isolate the faulty cable and therefore to minimize disruption of the networks, cable limiters are provided at the ends of the cables. Cable limiters are fuse-like devices that only react safely to high voltage and low impedance faults, such as those created by phase-to-phase faults.
Wiring (miniature) circuit interrupters and current leakage circuit interrupters are commonly used devices for protecting people and property from fire and dangerous electrical faults. Wiring circuit interrupters are used to protect power lines. First, when excessive current passing through a circuit breaker is converted to heat, the circuit interrupter is tripped by the bending of an internal bimetal. Second, when an electric tool or other metallic object on the load shorts the power line, high current is passed through instantaneously, causing the bimetal to heat up and bend. This causes the electric device to be interrupted by the inner magnet of the circuit interrupter.
It is known in this field that the current leakage circuit interrupter has the ability to detect current leakage that may be present in the power line. It trips the circuit interrupter and so protects people from electric shock resulting from current leakage.
In America, according to current regulations, a ground fault circuit interrupter (GFCI) is presently used in applications where direct human contact is possible. The GFCI, which is able to detect current leakage with high sensitivity, is used in current leakage circuit interrupters. Thus, a GFCI must be installed in all kitchens, bathrooms, parking places, basements or other damp places.
In spite of the wiring circuit interrupter and current leakage circuit interrupter, many electrical fires occur all over the world every year. These are often occurred by an arcing type fault to ground occurs rather than a phase-to-phase fault. Arcing faults typically create root mean square (RMS) current value, which is below the thermal threshold for such circuit breakers. Even so, the arcs can cause damage or start a fire if they occur near combustible material.
Arcs are potentially dangerous due to their high temperatures. An arc, however, will only trip a GFCI if 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 circuit breaker. Therefore, an additional type of protection 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).
According to the Consumer Product Safety Commission (CPSC), it was estimated that 40% of the fires in 1997 were due to arc faults. The National Electric Code (NEC) requires AFCI installation in all the residential buildings beginning in January 2002. The causes of arcing are numerous. For example, it may be caused by: aged or worn insulation and wiring; mechanical and electrical stress caused by overuse, excessive currents or lightning strikes; loose connection; or excessive mechanical damage to insulation and wires.
Three types of arcing may occur in residential or commercial buildings: series arcing, parallel arcing and ground arcing.
Series (or contact) arcing occurs between two contacts in series with a load. An example of series arcing is illustrated in FIG.
1
. The conductors
14
,
16
comprising the cable
10
, are separated and surrounded by an insulator
12
. A portion of the conductor
14
is broken, creating a series gap
18
in the conductor
14
. 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 the insulation
19
close to the point of 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.
A schematic diagram illustrating an example of parallel (line) arcing is shown in FIG.
2
. The cable
20
comprises electrical conductors
24
,
26
covered by outer insulation
22
and separated by inner insulation
28
. Deterioration or damage to the inner insulation
28
at
21
may cause parallel fault arcing
23
to occur between the two conductors
24
,
26
. The inner insulation could have been carbonized by an earlier lightning strike to the wiring system, or it could have been cut by some mechanical action such as a metal chair leg cutting into an extension cord.
A schematic diagram illustrating an example of ground arcing occurring between a conductor and the ground is shown in FIG.
3
. If the outer insulation
38
for protecting conductors
34
,
36
is damaged, the conductor
36
contacting ground at the damaged portion
39
produces ground arcing.
The arcing current may be changed by impedance because parallel arcing and ground arcing occur parallel to the load. The long-term deterioration causes cable carbonization and damage to the coating. The cable is further deteriorated by Joule heat, which is induced by arcing current. The arcing is generated in the following manner: J (Joule heat)=I
2
(arcing current)×t (Time).
An example of static current and arcing current in the resistor load are illustrated in FIG.
4
. The arcing current
42
is not normal sine wave but is distorted at the phase changing point. According to the distortion of arcing current, arcing voltage is also distorted.
FIG. 5
shows the relation between arcing current and arcing voltage.
An example of distorted AC line voltage caused by arcing current is illustrated in FIG.
6
. The Joule heat is increased against the decrease of RMS AC line voltage value
61
caused by irregular arcing current
62
. An arc is superposed on the AC line voltage. The frequency of harmonic or overtone is extended to GHz, and it can be seen by spectrum analysis of the frequency of arcing current.
The 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 arcing current and arcing voltage are not generated in the form of correct sine wave, and have various types of waveforms. Specifically, arcing current and arcing voltage are similar to the driving pulse created in appliances, such as fans and dryers that have electric motors inside.
FIG. 7
illustrates the signals related to output voltage in the resistor load, and
FIG. 8
illustrates the output voltage with arcing. And,
FIG. 9
illustrates output voltage waveform in a driving electric device.
The signals in
FIG. 7
show that under a normal load, the output voltage is generated to pulse every {fraction (1/60)} sec. The signals in
FIG. 8
show that under arcing conditions, arcing voltage with high amplitude is detected every {fraction (1/60)} sec. Also, if you use an electric device, you can see that at the beginning of a cycle, high pulse similar to the arcing voltage is generated, and after a period of time, outp
Kim Cheon-Youn
Kim Jeong-Wan
Human El-Tech, Inc.
Huynh Kim
Rabin & Berdo P.C.
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