Arc fault detector with protection against nuisance trips and circuit breaker incorporating same

An arc fault detector of the type which generates a trip signal when a time attenuated accumulation of pulses produced each time an arc is struck reaches a selected value, includes a pulse conditioner which discriminates against false trips induced by phenomena such as tungsten bulb burn out and turn on of a cold tungsten bulb controlled by a dimmer. The pulse conditioner includes a zener diode which limits the amplitude of the pulses applied to the integrator, and stretches all of the pulses to reduce the variation in pulse duration. The latter can be accomplished by a peak detector which stretches pulses which reach the zener limit to a duration of at least about a half cycle.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to detection and interruption of currents and 
circuits experiencing arc faults. More particularly, it relates to an arc 
fault detector and a circuit breaker incorporating such an arc fault 
detector which minimize the effects of other phenomena such as the burnout 
of tungsten bulbs, which can falsely provide an indication of an arc 
fault. 
2. Background Information 
Arc faults can occur in electrical systems for instance between adjacent 
bared conductors, between exposed ends of broken conductors, at a faulty 
connection, and in other situations where conducting elements are in close 
proximity. Arc faults in ac systems can be intermittent as the magnetic 
repulsion forces generated by the arc current force the conductors apart 
to extinguish the arc. Mechanical forces then bring the conductors 
together again so that another arc is struck. 
Arc faults typically have high resistance so that the arc current is below 
the instantaneous or magnetic trip thresholds of conventional circuit 
breakers. Also, the intermittent nature of an arc fault can create an 
average 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. It is not practical to simply lower 
the pickup currents on conventional circuit breakers as there are many 
typical loads which draw similar currents, and would therefore, cause 
nuisance trips. 
Much attention has been directed toward trying to distinguish arc currents 
from other intermittent currents. It has been recognized that arc currents 
generate a step increase in current when the arc is struck. However, many 
typical loads generate a similar step increase, such as for instance when 
the device is turned on. In many instances, the step increases generated 
by these loads are singular events while an arc fault generates a series 
of step increases. The arc fault detector described in U.S. Pat. No. 
5,224,006 counts the step increases in current and generates a trip signal 
if a selected number of step increases occur within a given interval. 
However, there are loads, such as a solid state dimmer switch with the 
firing angle phased back substantially, which also generate repetitive 
step increases in current. This problem is addressed by the arc fault 
detector in U.S. Pat. No. 5,691,869 in which the arc current is passed 
through a bandwidth limited filter which generates pulses having an 
amplitude proportional to the step increases. An arc indication is 
generated when a time attenuated accumulation of these pulses reaches a 
predetermined value. Thus, a few very large magnitude step increases 
within a period of time, or a larger number of more modest step increases 
within a similar time period, generate a trip signal. The trip level can 
be set so that the cyclic pulses generated by a dimmer do not generate the 
time attenuated accumulation which reaches the trip level. 
There is at least one arc condition which can occur in a protected circuit 
to which it is desired that the arc fault circuit not respond. This is an 
arc created by the burnout of a tungsten filament such as in a light bulb. 
When the filament burns through, a small gap is created between the burned 
out ends of the filament. An arc is struck across this gap and can quickly 
envelop the entire filament so that it extends between the two conductors 
thereby drawing a very large arc current. In order to terminate this arc, 
tungsten bulbs are provided with a small fuse in the base. Even so, 
burnout of the filament and blowing of the fuse results typically in a 
pair of current pulses of opposite polarity. This pair of pulses can be of 
sufficient magnitude that the threshold value of the time attenuated 
accumulation of pulses in the circuit breaker described in U.S. Pat. No. 
5,691,869 is exceeded and the circuit breaker is tripped. This is 
considered a nuisance trip as the fuse has interrupted the arc. 
Tungsten filament bulbs can also generate false trips when used with a 
dimmer. As mentioned, a dimmer which is phased back can generate 
repetitive step increases in current on each half cycle. As also 
discussed, the circuit breaker can be set so that the threshold of the 
time attenuated accumulation of pulses generated by the dimmer do not 
reach the trip level with normal loads. However, when a tungsten filament 
lamp is first turned on, the cold filament has a very low resistance and 
can draw up to fifteen times normal current. This can result in a nuisance 
trip when a tungsten lamp controlled by a dimmer switch is first turned 
on. 
Commonly owned U.S. patent application Ser. No. 08/939,976 filed on Sep. 
29, 1997, addresses the problems created by a tungsten filament bulb by 
disclosing an arc fault detector in which a zener diode places amplitude 
limits on pulses having an amplitude proportional to the magnitude of step 
increases in current in the protected circuit so that the time attenuated 
accumulation of these pulses does not reach the trip level based upon a 
few very large pulses which can be generated by burnout of a tungsten lamp 
or turn on of a tungsten lamp controlled by a dimmer switch. In 
particular, the zener diode clips the pulses generated in response to the 
step increases in current in the protected circuit before the time 
attenuated accumulation of the pulses. As true arc faults will continue to 
strike at a random rate, and therefore raise the time attenuated 
accumulation of pulses to the trip threshold, false trips due to burnout 
of a tungsten lamp or turn on of a cold tungsten bulb are avoided. While 
the arc detector described in the cited patent application reduces false 
trips due to tungsten lamp burnout and turn on of dimmer controlled 
tungsten bulbs, there is room for improvement. 
There is a need for an arc fault detector and circuit breaker incorporating 
such a detector which can respond faster to true arc faults, yet not 
falsely trip on other phenomena such as burnout of a tungsten bulb or turn 
on of a tungsten bulb controlled by a dimmer. 
There is a need for achieving such a result with a simple, reliable and low 
cost detector. 
In particular there is a need for such apparatus which can respond sooner 
to step increases in current in a protected circuit, whether small or 
large and yet discriminate against current discontinuities caused by 
tungsten bulbs. 
SUMMARY OF THE INVENTION 
These needs and others are satisfied by the arc fault detector and circuit 
breakers incorporating such arc fault detectors which are derived from 
recognition that pulses generated by tungsten bulbs are normally not only 
large in amplitude, but are also longer in duration than the pulses 
generated by true arc faults. Thus, in an arc fault detector in which a 
pulse generator generates a pulse signal containing pulses related in 
amplitude to step increases in current each time an arc is struck and 
containing pulses related in amplitude to other current pulses such as can 
be generated by a tungsten bulb, and means generating a trip signal as a 
function of a time attenuated accumulation of the pulses, pulse 
conditioning means is provided which not only limits the amplitude of the 
pulses generated by the pulse generator but also stretches the pulses 
generated by the pulse generating means to reduce variations in duration 
of the pulses that are applied to the means generating the trip signal. By 
reducing the variations in pulse duration, the disproportionate effect of 
the pulses generated by a tungsten bulb over pulses generated by a true 
arc fault are reduced. 
The trip means which generates a time attenuated accumulation of the 
conditioned pulses only responds to conditioned pulses having an amplitude 
above a predetermined threshold amplitude. The pulse conditioning means 
limits the amplitude of the conditioned pulses to a selected amplitude 
which is above the threshold amplitude. Preferably, the peak detector has 
a time constant which is selected to provide a predetermined pulse 
duration for a conditioned pulse to decay in amplitude from the selected 
amplitude to the predetermined threshold amplitude. Preferably, this 
predetermined pulse duration is about one half cycle of the ac system. 
In a preferred form, the peak detector comprises a peak detector capacitor 
to which pulses from the pulse generating means are applied through a 
first resistor and a second resistor shunting the capacitor. In addition, 
a diode between the pulse generating means and the peak detector capacitor 
prevents discharge of the capacitor through the pulse generating means. 
Also preferably, the means limiting the conditioned pulses to the selected 
amplitude is a zener diode connected in parallel with the peak detector 
capacitor between the first resistor and the peak detector capacitor. 
BRIEF DESCRIPTION OF THE DRAWING 
A full understanding of the invention can be gained from the following 
description of the preferred embodiment when read in conjunction with the 
accompanying drawing which is a schematic diagram of an arc fault circuit 
breaker incorporating the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The FIGURE illustrates an arc fault circuit breaker 1 in accordance with 
the invention providing protection for an electrical circuit 3 which 
includes a line conductor 5 and a neutral conductor 7. The circuit breaker 
1 provides overcurrent and short circuit protection, arc fault protection 
and ground fault protection for the electrical system 3. Overcurrent and 
short circuit protection are provided by the conventional thermal-magnetic 
trip mechanism (not shown) which includes a bimetal 11. As is well known, 
the bimetal responds to persistent overcurrent conditions to actuate a 
spring-powered operating mechanism 9 to open a set of separable contacts 
13 connected in series with the line conductor 5 to interrupt current flow 
in the electrical system. 
Ground fault protection is provided by a ground fault circuit 15. Such 
circuits are well known in the art. The ground fault circuit 15 shown is 
the well known dormant oscillator type which utilizes two sensing coils 17 
and 19 which sense current in the line and neutral conductors 5 and 7. In 
response to a line to ground or a neutral to ground fault, the ground 
fault circuit 15 generates a ground fault trip signal which turns on the 
silicon controlled rectifier (SCR) 21. This energizes a trip solenoid 23 
connected by the SCR 21 between the neutral conductor 7 and circuit 
breaker common which is referenced to the line conductor through the lead 
24. A resistor 25 limits the current through the coil 23 and a capacitor 
27 protects the gate of the SCR 21 from being falsely tripped on noise. 
Energization of the shunt trip coil 23 actuates the spring-powered 
operating mechanism 9 to open the separable contacts 13. Current is drawn 
through the coil 23 to provide power to the ground fault circuit 15 
through the lead 26. However, this current is insufficient to actuate to 
the trip coil. 
Arc fault protection is provided by an arc fault circuit 29. This arc fault 
circuit 29 utilizes a pair of leads 31 connected across the bimetal 11 to 
sense current in the protected electrical system 3. As taught by U.S. Pat. 
No. 5,519,561, as the resistance of the bimetal 11 is known, the voltage 
drop across this bimetal provides a measure of the current flowing in the 
line conductor 5. The arc fault circuit 29 also includes a pulse generator 
33, a circuit 35 which provides a time attenuated accumulation of the 
pulses generated by the pulse generator 33, and an output circuit 37 which 
provides a trip signal which through the lead 39 turns on the SCR at 21 to 
open the separable contacts 13 in the same manner as the ground fault 
circuit 15. 
The pulse generator 33 includes a high pass filter 41 formed by the series 
connected capacitor 43 and resistor 45, followed by a low pass filter 47 
formed by the parallel connected capacitor 49 and resistor 51. The high 
pass filter 41 and low pass filter 47 have a band pass in a range which 
generates pulses in response to the step increases in current caused by 
striking of an arc and yet is below any carrier frequencies that may be on 
the power line. In the exemplary circuit, this pass band is in a range of 
about 290 to 1,540 Hz for the 3 db points and could go up to 10 KHz or 
more. 
An operational amplifier (op amp) 53 provides gain for the pulses. A 
capacitor 54 reduces high frequency noise in the pulses. The op amp 53 is 
biased at its non-inverting input by a 13 vdc supply voltage. A resistor 
55 and capacitor 57 delay application of the bias to prevent false trip 
signals during power up. The positive and negative pulses generated by the 
band pass filter ride on the plus 13 vdc volt bias applied to the op amp 
53. This bias is removed by the ac coupling capacitor 59 which along with 
the resistor 61 forms another high pass filter stage. The bi-polar pulse 
signal resulting is rectified by a rectifier circuit 63 which includes 
another op amp 65. Positive pulses are applied to the non-inverting input 
of the op amp 65 through the diode 67 while negative pulses are applied to 
the inverting input through the diode 69. The output of the op amp 65 is a 
pulse signal having pulses of a single polarity. 
The circuit 35 generates a time attenuated accumulation of the pulses in 
the pulse signal generated by the pulse generator 33. The pulses are 
accumulated on a capacitor 71 connected to the 26 vdc supply. A bleed 
resistor 73 connected across the capacitor provides the time attenuation. 
The pulses are applied to the capacitor 71 through a differential 
amplifier formed by the pair of transistors 75a and 75b. When no pulses 
are generated, both electrodes of the capacitor 71 are at 26 volts. The 
pulses from the pulse generator 33 provide base drive current for the 
transistor 75a. A voltage divider formed by the resistor 77 and 79 
connected at their midpoint to the base of the transistor 75b set the 
minimum amplitude for the pulses to turn on the transistor 75a. In the 
absence of pulses, the transistor 75b is on which holds the transistor 75a 
off due to the voltage developed across the resistor 80. When the 
amplitude of a pulse exceeds the threshold, the transistor 75a is turned 
on (which turns the transistor 75b off). This threshold is selected so 
that pulses which could be generated by some normal loads, such as for 
instance a dimmer switch operating at normal loads, are not accumulated. 
The amplitude of the pulses is set by the gain of the op amp 65 which in 
turn is determined by the ratio of the feed back resistor 81 and input 
resistor 83. The amplitude and duration of each pulse determine the amount 
of charge which is applied to the capacitor 71. The successive pulses are 
accumulated through the summation of the charge they add to the capacitor 
71. The resistor 73 continuously bleeds the charge on the capacitor 71 
with a time constant determined by the values of the capacitor 71 and 
resistor 73 to time attenuate the accumulation of the pulses. It can be 
appreciated that the magnitude and time interval between pulses determines 
the instantaneous voltage that appears across the capacitor 71. 
The output circuit 37 monitors the voltage across the capacitor 71 
representing the time attenuated accumulation of the pulses in the pulse 
signal generated by the pulse generator. Each pulse lowers the voltage on 
the capacitor which is applied to the base of a transistor 85 in the 
output circuit. A voltage is applied to the emitter of the transistor 85 
by the 13 vdc supply through a resistor 87 and diode 89. With no pulses 
being generated, the voltage on the base of the transistor 85 is 26 volts. 
Without the diode 89, the 13 volt reverse bias would destroy the base to 
emitter junction of the transistor 85. The diode 89 withstands this 
voltage. When the voltage at the lower end of the capacitor 71, and 
therefore on the base of the transistor 85, falls below the 13 volts minus 
the forward drop across the diode 89, the transistor 85 is turned on. 
Feedback provided through the lead 91 and the resistors 93 and 95 holds 
the transistor 85 on by providing a continuous output of the op amp 65 
which holds the transistor 75a on. Turn on of the transistor 85 provides 
base drive current for the transistor 97 which draws current limited by 
the resistor 99 to generate an arc fault trip signal which turns on the 
SCR 21 and trips the separable contacts 13 open. 
The larger the pulses in the pulse signal generated by the pulse generator 
33 the harder the transistor 75a is turned on, and hence, the faster 
charge is accumulated on the capacitor 71. As mentioned, burnout of a 
tungsten bulb 101 energized by the electrical system 3 protected by the 
circuit breaker 1 can generate, typically, two large amplitude pulses in 
consecutive half cycles which can by themselves accumulate sufficient 
charge on the capacitor 71 to reduce the voltage on the base of the 
transistor 85 to the threshold voltage which generates the trip signal. As 
also mentioned, turn on of a cold tungsten bulb controlled by a dimmer can 
also produce large initial pulses which can accumulate sufficient charge 
on the capacitor 71 to generate a trip signal. 
In order to discriminate against tungsten bulb burn out, since such a 
condition is addressed by a fuse 103 in the bulb 101, and to eliminate 
false arc indications due to turn on of a cold tungsten bulb controlled by 
a dimmer, a pulse conditioner 105 is provided which modifies the pulses 
output by the full wave rectifier 63. This pulse conditioner 105 
conditions the pulses in two respects. First, it limits the amplitude of 
the pulses and second, it stretches the pulses to reduce variations in 
pulse duration. As the pulses generated by the two tungsten bulb phenomena 
tend to be greater in amplitude and wider than the pulses generated by 
true arcs, this conditioning of the pulses reduces the contribution to 
charge accumulation on the capacitor 71 produced by the tungsten bulb 
effects. As tungsten bulb burn out typically only produces two pulses and 
cold turn on of the tungsten bulb only produces a couple of large pulses 
before the filament warms up, while true arcs continue to randomly 
generate pulses, the parameters can be set to ignore tungsten bulb 
phenomena and still respond quickly to arc faults. 
Limiting of pulse amplitude is provide by a zener diode 107. The breaker 
over voltage of the zener diode 107 is selected to be above the threshold 
voltage required to turn on the transistor 75a but less than the amplitude 
that is typically generated by tungsten bulb burn out or cold turn on. 
Stretching of the pulses is provided by pulse stretching circuit 109. The 
pulse stretching circuit 109 includes a capacitor 111 connected between 
the output of the pulse generator 33 and ground. The pulses from the pulse 
generator are applied to the capacitor 111 through a first resistor 113. A 
second resistor 115 connected in shunt bleeds charge from the capacitor 
111. A diode 117 prevents discharge of the capacitor 111 back through the 
rectifier 63 of the pulse generator. This pulse stretching circuit 109 
forms a peak detector having a time constant which is determined by the 
values of the capacitor 111 and the resistor 115. This time constant is 
selected such that a pulse from the pulse generator 35 having an amplitude 
equal to the selected limiting voltage set by the zener diode 107 will 
decay to the threshold voltage set by the resistors 77 and 79 in about one 
half cycle, i.e., about 8.3 milliseconds in a 60 cycle system. In the 
exemplary embodiment of the invention, the time constant was selected to 
be about 23 milliseconds. This time constant, combined with the clipping 
provided by the zener diode 107 and the threshold set by the resistors 77 
and 79 stretches a 70 amp peak arc current to about a full half cycle of 
integration conduction on the capacitor 71. Tungsten bulb burn out 
produces a pair of pulses one half cycle apart which are much larger than 
the limit set by the zener diode, e.g., about twice as large. These pulses 
also have a certain time that they remain above the clipping voltage so 
that the capacitor 111 remains charged and only begins to discharge when 
the pulse voltage falls below the clipping voltage set by the zener diode 
107. Therefore, a pulse generated by tungsten burn out will tend to last 
longer than a half cycle by the time that it remains above the clipping 
voltage. With the two pulses generated by tungsten burn out occurring one 
half cycle apart, the second pulse will occur before the first pulse has 
decayed to the threshold voltage, and hence the integration capacitor 71 
will be continuously charged until the second pulse terminates. The total 
integration time for these pulses will be two half cycles plus the 
additional time that the second pulse remains above the clipping voltage. 
On the other hand, the clipping voltages selected while the two pulses 
produced by tungsten bulb burn out will generate more integrating current 
for the capacitor 71 than two pulses generated by arc faults, the trip 
circuit can be set to trip on a third arc fault pulse which has an 
amplitude of at least about the clipping voltage. Pulses generated by the 
step changes in current produced by smaller arc faults which exceed the 
threshold voltage but not the clipping voltage are also stretched, but not 
to a full half cycle. However, since the pulses generated by a tungsten 
bulb are not stretched proportionately as much as pulses generated by arc 
faults, the arc fault detector of the invention can be made to trip sooner 
on arc faults without generating false trips in response to tungsten bulb 
burn out and cold turn on when dimmer controlled. 
While specific embodiments of the invention have been described in detail, 
it will be appreciated by those skilled in the art that various 
modifications and alternatives to those details could be developed in 
light of the overall teachings of the disclosure. Accordingly, the 
particular arrangements disclosed are meant to be illustrative only and 
not limiting as to the scope of invention which is to be given the full 
breadth of the claims appended and any and all equivalents thereof.