Method and apparatus for detecting arcing in alternating-current power systems by monitoring high-frequency noise

An arc detector for detecting potentially hazardous arcing in electrical connections comprises detection and signal processing circuitry for monitoring high-frequency noise characteristic of arcing on the power line and distinguishable from other sources of high-frequency noise.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an inexpensive detector of electrical arcs on 
power lines, for providing advance warning of potentially dangerous 
conditions. 
2. Discussion of the Prior Art 
Electrical arcs can develop temperatures well above the ignition level of 
most common flammable materials, and therefore pose a significant fire 
hazard. For example, worn power cords in the home may arc sufficiently to 
start a fire. Fortunately, arcing is an inherently unstable phenomena and 
does not usually persist long enough to start a fire. Under certain 
conditions, reflected in particular characteristics of the electrical 
disturbance produced, the likelihood of the arc persisting and starting a 
fire is much higher. It is one object of the present invention to provide 
a method and apparatus to detect such potentially dangerous arcs by 
monitoring voltage disturbances on the power lines. 
Two basic types of arcing are likely to occur in the home: line fault 
arcing and contact arcing. An arc due to a line fault results from either 
a line-to-line short or a line-to-ground short. When a fault of this type 
occurs several things are likely to happen: 1) the fault will draw current 
up to or beyond the capacity of the circuit; 2) lights will dim indicating 
an excessive load is being drawn; and 3) if the circuit is properly 
protected by a circuit breaker, the breaker will trip, interrupting supply 
of power to the arc. All of this will likely occur and be completed in 
less than a second. The resulting arcing will be explosive but 
short-lived, reducing the total heat to be dissipated by surrounding 
materials and thus reducing the likelihood of a fire. 
Contact arcing results from a high resistance connection in series with a 
load. This may occur due to loose connections, oxidized contacts, or 
foreign non-conducting material interfering with the conduction path. 
One example of a condition that may cause contact arcing is a well-used 
wall outlet wherein the spring pressure provided by the contacts has been 
reduced through age and use, so that insufficient pressure is applied to 
the inserted plug contacts to ensure low-resistance connection. 
Contact arcing is also commonly caused by use of extension cords of 
insufficient current-carrying capacity. For example, the plug may be 
heated by resistance heating, gradually decomposing elastomeric insulating 
material around the contacts until the material partially flows into the 
contact area, preventing proper contact from being made. This process may 
become regenerative as the initial arcing produces more heat, carbonizing 
the insulation, producing a hard insulative layer on the contact surface. 
A third cause of contact arcing often observed in aluminum wiring involves 
the oxidation of contacts. In this case a chemical 6 process, principally 
oxidation, builds up a semi-conductive or non-conductive layer on the 
surface of the contacts. Preferably, when the material of the conductors 
is susceptible to oxidation, the contacts are gas-tight, preventing oxygen 
from entering and promoting oxidation. However, if the connections become 
loose over time, oxidation begins and arcing can result. 
Many instances of contact arcing result from the gradual degeneration of 
the current-carrying contacts. Dangerous arcs may begin as small 
occasional arcing, gradually building up over time until the arcing become 
persistent enough to start a fire. For this reason, it would be highly 
advantageous if contact arcing conditions could be detected early, and a 
warning provided before the fault reaches a dangerous level. 
It will thus be appreciated that there are fundamental differences between 
line fault arcing and contact arcing. The former will generally involve 
high currents (&gt;20 A), be somewhat explosive and either burn itself out or 
trip a circuit breaker. Conventional circuit protection devices are 
normally adequate to guard against line fault arcing. By comparison, the 
average current drawn in contact arcing is no more than the current drawn 
by the load itself. Nevertheless, even low-current contact arcing, for 
example, a 60 watt light bulb on the end of a faulty extension cord, or a 
set of Christmas tree lights with faulty contacts, may release sufficient 
heat to cause a fire. Accordingly, conventional circuit breakers are 
inadequate to prevent dangerous conditions due to contact arcing. 
A need therefore exists for an inexpensive plug-in monitoring device 
capable of detecting arcing that may result in a fire. The most convenient 
device would be a small `night-light` style plug-in module capable of 
monitoring an entire house and providing a warning if potentially 
dangerous arcing occurs. The detection, in this case, must be made by 
monitoring voltage alone; to monitor current would require a current 
sensor to be placed around a conductor, thereby requiring the device to be 
wired in place. Ideally, such an arc detector would be the electrical arc 
equivalent to the smoke detectors widely in use today, with the further 
advantage of warning of a potential fire days, weeks or even months in 
advance of its occurrence. 
When the arc detector as above senses that potentially dangerous arcing is 
present, the homeowner should be alerted to have an electrician check the 
wiring to determine the cause of the arcing. A hand-held, battery-powered 
diagnostic device for use by an electrician in locating the origin of the 
arcing and correcting the fault would be highly useful. Such a device 
might sense electrical arcing by monitoring radio-frequency emissions from 
the power line due to arcing and as such would require no direct 
connection. If the source of the arcing was not immediately apparent, the 
electrician could place this device near suspect areas and leave it for a 
period of time. If arcing occurred again, the diagnostic device would 
record parameters of the arcing such as the number of occurrences, the 
time of occurrence, the direction of maximum intensity, the duration, 
etc., to assist in locating the source of the arcing. 
A need also exists for a circuit breaker that in addition to detecting 
arcing that may result in a fire removes power from the load in the event 
that it detects sufficient arcing to present a hazard. Such a device could 
be conveniently packaged in much the same style as a conventional circuit 
breaker, or could be installed in an outlet similar to the currently 
available ground fault interrupters. Because the load current flows 
through the circuit breaker, it is convenient in this application to 
monitor load current. 
The arc detector in each embodiment must be immune to noise commonly 
present on household power lines, e.g., due to lamp dimmers, motors, 
carrier-current communications systems, switching transients, and the 
like. 
While there have been a number of devices proposed to detect arcing, most 
address arcs caused by line faults. U.S. Pat. No. 5,121,282 issued to 
White, for example, describes a system that monitors both line voltage and 
current for characteristics particular to arcing and trips a circuit 
breaker if enough of these characteristics are present. The White device 
assumes, however, that the arc is the result of a line fault. One 
characteristic of a line fault is that the fault current will lag the 
voltage by 70.degree.-90.degree.. This is because under line fault 
conditions, the current flow will depend almost entirely on the power 
distribution wiring which is generally highly inductive. A plug feeding a 
heater that is arcing in the socket--that is, exhibiting a contact 
fault--will not exhibit this characteristic and thus the fault will not be 
detected by the White device. 
U.S. Pat. No. 4,639,817 to Cooper et al shows an arc detector for "grid" or 
"spot" type power networks as used in large commercial or industrial 
installations. The Cooper circuit interrupts the power if high-frequency 
(10 KHz-100 KHz) noise of more than a threshold amplitude is detected for 
more than 0.7 seconds. If adapted to home use, this detector would be 
tripped by continuous high-frequency noise, such as from electric drills 
and the like. 
U.S. Pat. No. 4,858,054 to Franklin recognizes that arc short circuits 
differ from dead short circuits, as described above, and indicates that 
different detection techniques should be employed. However, Franklin's 
device still monitors the current and trips only when current in excess of 
a predetermined level is detected. This level of current must be much 
higher than the circuit's rated current, to avoid tripping on motor 
start-up currents and the like. Accordingly, Franklin's device can only 
detect arcs in short circuits, and cannot detect a contact arc in series 
with a current-limiting load. 
Also of general interest are U.S. Pat. Nos. 5,038,246 to Durivage, 
4,951,170 to Fromm, and 4,402,030 to Moser et al. 
One device currently available is the Ground Fault Interrupter or GFI. 
Typical GFI devices are capable of detecting leakage currents to ground as 
low as several milliamps, and trip an associated circuit breaker in 
response. A GFI very effectively reduces not only the danger of fire due 
to shorts to ground but also protects humans that may be in the electrical 
path. A GFI device is not however capable of monitoring contact arcing as 
discussed herein. 
OBJECTS OF THE INVENTION 
It is therefore an object of the present invention to provide a method for 
monitoring line voltage, load current, or energy radiated from the power 
conductors, whereby persistent arcing that may potentially cause a fire 
may be detected, while noise on the power line from other sources, such as 
electric motors, switch closures, lamp dimmers, or communication systems 
is rejected. 
It is a further object of the present invention to provide specific 
additional and/or alternative methods of arc detection for use in specific 
circumstances that demand particular performance criteria. 
It is a further object of the present invention to provide an inexpensive, 
plug-in device that monitors noise on the power line due to arcing, to 
reliably detect dangerous arcing and to warn the user with both visual and 
audible alarm indications. 
It is a further object of the present invention to provide a 
battery-powered arcing diagnostic device that can detect potentially 
hazardous arcing, without any direct connection to the power wiring, by 
receiving radio-frequency emissions from such arcing, and which records 
and displays various parameters of the arcing to assist in diagnosing the 
source of the arcing. 
It is yet a further object of the present invention to provide an apparatus 
capable of detecting potentially hazardous arcing on a particular circuit, 
and which trips a circuit breaker in response to such detection, 
interrupting current to that circuit. 
SUMMARY OF THE INVENTION 
The arc detector of the present invention monitors either the line voltage, 
the line current, or energy radiated from the power line for the presence 
of high-frequency noise exhibiting certain distinctive patterns which have 
been discovered by the inventor to be characteristic of contact arcing, 
and in this manner produces an output responsive to the presence of 
contact arcing. This output may be used to activate an alarm or to 
interrupt current to the arc. 
Electrical arcing produced by alternating voltage will extinguish each time 
the voltage across the arc drops below a value sufficient to sustain the 
arc, and will re-ignite each time the voltage across the arc exceeds the 
arc's ignition voltage. Therefore, arcs sustained by an alternating power 
source will necessarily extinguish at least twice every full cycle of the 
power source frequency. The period of time when the arc is not conducting 
is hereinafter referred to as the `gap`. It has been observed by the 
present inventor that when this gap is very large (20.degree.-90.degree. 
of the 360.degree. line cycle), the arc will be intermittent and highly 
unstable, often self-extinguishing after a short period of time. It has 
also been observed that as this gap becomes smaller 
(1.degree.-20.degree.), the arc becomes more persistent and under certain 
conditions may become self-sustaining. Once the arcing becomes 
self-sustaining, it may persist for minutes or longer and is capable of 
generating significant amounts of heat. 
During the time the arc is conducting current, it produces wideband, 
high-frequency noise ranging from about 10 KHz to perhaps 1 GHz. During 
the time the arc is not conducting current, i.e., during the gaps, it 
produces no noise. The present inventor has realized that the resulting 
characteristic pattern of high-frequency noise with synchronous gaps is 
unique to arcing and that therefore an algorithm for analyzing repetitive 
patterns in the noise can be used to detect arcing. This basic principle 
is common to both the embodiments of the invention disclosed in the 
"grandparent application" now issued as U.S. Pat. No. 5,223,795 and to 
those disclosed in application Ser. No. 08/035,231, the parent of the 
present continuation-in-part application. 
In the embodiments of the invention disclosed in the grandparent 
application, now U.S. Pat. No. 5,223,795, the preferred frequency band 
(i.e., the band of frequencies within which noise was monitored to detect 
arcing) was selected to be 100 KHz to 1 MHz, because signals in this 
bandwidth traverse household wiring with relatively low attenuation. There 
are a variety of common household devices that produce considerable 
amounts of extraneous noise in this frequency range, and consequently the 
detection algorithms were optimized to reject such interference. 
Since the filing of the grandparent application, the inventor has continued 
to research the physical mechanisms of contact arcing, and has developed 
and tested several more prototypes. During this time he has realized 
improvements to the arc detection circuits and the arc detection 
algorithms, particularly with respect to use of a higher frequency band 
for detection of contact arcing. In the embodiments of the invention 
disclosed in the parent of the present continuation-in-part application, 
Ser. No. 08/035,231, the preferred frequency band is 1 MHz to 10 MHz, 
wherein the nature of the interference from extraneous noise sources is 
somewhat different. Therefore, the detection algorithms disclosed in 
application Ser. No. 08/035,231, while substantially similar to those 
discussed in the parent application, were optimized to better address this 
new frequency band. It should be recognized, however, that operation in 
these and other frequency ranges offer relative advantages in varying 
applications and that both the methods and apparatus disclosed in the 
parent and grandparent applications as well as those first disclosed 
herein may be useful in substantially any frequency range, depending on 
various factors. 
As mentioned above, the nature of the noise from extraneous sources in the 
frequency range from 1 MHz to 10 MHz differs from the extraneous noise 
present in lower frequency ranges. Consequently, the detection algorithms 
must address different constraints. In fact, while the attenuation may be 
slightly higher, there are several advantages to operating in the 1-10 MHz 
range for arc detection in household applications. 
First, extraneous noise in this higher frequency range produced by such 
household devices as lamp dimmers and the like tends to be very short 
lived in this higher frequency range, occurring only at current switching 
transitions and decaying very rapidly. Second, synchronous line noise from 
extraneous sources is minimal; because noise present in this frequency 
range will interfere with radio broadcasts, household appliances and other 
equipment are intentionally designed to minimize such noise. For example, 
devices that provide remote control of appliances and the like by using 
carrier-current transmission in the range of 100-300 KHz (and which 
signals must be rejected by the preferred embodiment disclosed in the 
grandparent application) are tuned to produce almost no noise in the 
higher frequency range of the preferred embodiment of the invention 
disclosed herein. In fact, the most prevalent source of extraneous noise 
in the 1-10 MHz bandwidth is AM and communications-band radio 
transmission; since such signals bear no temporal relationship to the line 
frequency, they are easily rejected according to the invention. Third, the 
relative lack of synchronous high-frequency noise in this higher bandwidth 
allows an improved detection algorithm to reliably detect arcing that, 
while substantially persistent, does not necessarily produce the ideal 
pattern of continuous arcing between the two expected gaps per line cycle 
as discussed in the grandparent application. Finally, signals in this 
frequency range are high enough to traverse the interwinding capacitance 
of transformers, and thereby allow detection of arcing across the phases 
of household wiring and within transformer-powered equipment. Accordingly, 
the first step of the method of the invention is to filter and amplify one 
of the line voltage or the load current, so as to extract high-frequency 
noise in a desired frequency range. 
Noise originating from arcing and thus extracted will exhibit certain 
characteristics. First, the high-frequency noise will be present on both 
the line voltage and the load current whenever the arc is conducting. 
Second, the amplitude of the high-frequency noise will go essentially to 
zero as the arc extinguishes and re-ignites. This will occur each time the 
voltage across the arc goes through zero, i.e., every half-cycle of the 
line frequency, and therefore synchronous gaps in the high-frequency noise 
will be produced. If the load is resistive, the voltage across the arc 
will be in phase with the line voltage and consequently these gaps will 
coincide with the line voltage zero-crossings. If the load is reactive, 
the voltage across the arc (and therefore the gap) may be shifted in phase 
relative to the line voltage by up to plus or minus 90.degree.. Therefore, 
depending on the reactance of the load in series with the arc, the gaps 
may or may not occur at the line voltage zero-crossings. In all cases when 
the arc is conducting, however, there will be gaps in this noise at 
intervals equal in time to 1/2 the line frequency cycle. 
Contact arcing becomes dangerous only when the arcing persists long enough 
to transfer substantial heat to flammable materials in the immediate 
vicinity of the arc and thereby ignite a fire. The amount of time required 
for this to happen is a function of both the power dissipated in the arc 
and the thermal resistance and heat capacity of the surrounding materials. 
Therefore, in order to detect contact arcing that may be dangerous, it is 
desirable, once the high-frequency noise is identified as being 
characteristic of arcing, to require that the arcing substantially persist 
for a period of time, typically 1 second or more, before concluding that 
hazardous arcing exists. 
One further consideration is that if the load is half-wave rectified, the 
load current will flow only on like-polarity half-cycles of the line 
voltage and consequently the arc will conduct only during these same 
half-cycles. In this case, the characteristic pattern of high-frequency 
noise followed by a gap will be exhibited only during alternate 
half-cycles of the line voltage. 
It was accordingly the general method of the invention disclosed in Ser. 
No. 08/035,231 to monitor the line voltage, line current, or energy 
radiated from the power line, for the presence of synchronous gaps in the 
high-frequency noise. If continuous monitoring reveals that the gaps occur 
in a manner indicative of the presence of arcing, for example, if 
synchronous gaps are detected in a given fraction of a half-cycles of the 
power frequency over a time interval of predetermined length, potentially 
dangerous arcing is determined to exist. An output indicating the same may 
then be actuated, or another appropriate control action taken. 
The term "synchronous gaps" in the context of this invention means that the 
time between gaps is an integral multiple of T/2, where T is the period of 
the line voltage. 
The preferred embodiment of the present invention uses real-time software 
and a low-cost microprocessor to detect contact arcing. This approach has 
the advantage that the detection algorithms can be readily optimized by 
modifying the detection software. However, other methods that produce the 
same result can be used, for example, to reduce the costs of the unit. A 
purely analog circuit, implementing conventional integration, threshold 
detection and missing pulse techniques might be produced on a single chip 
for the lowest product cost. Alternatively, a digital signal processing 
(DSP) chip using conventional cross-correlation or auto-correlation 
analysis might be employed, at somewhat higher product cost, to detect 
arcing in more stringent applications. 
Regardless of the signal processing technique chosen, the basic detection 
method of the invention of the parent and grandparent applications is to 
determine whether certain predetermined patterns of gaps characteristic of 
arcing occur in the high-frequency noise synchronized to the power 
waveform, and if so whether these patterns are persistent enough to 
indicate dangerous arcing. 
More specifically, in one preferred embodiment, gap monitoring is performed 
by attempting to detect certain specific `features` of the gap, such as 
the leading edge, the trailing edge, or the width of the gap, and then 
measuring the time intervals between successive occurrences of the 
detected feature. The relative advantages and disadvantages of detecting 
each of these three features are discussed in detail below. If the 
selected feature is found to repeat at least a minimum number of times 
spaced by intervals equal to T, where T is the period of the line voltage, 
a pattern indicative of arcing is determined to exist. If this pattern 
persists long enough, and with few enough interruptions, dangerous arcing 
is determined to be present, and a output signal indicating the same is 
provided. 
An apparatus according to the invention of the parent application Ser. No. 
08/035,231 and performing the method described above using any of the 
three gap identifying Features is described in detail below. The device 
according to the invention for monitoring line voltage alone may be 
configured as a self-contained, plug-in `night-light`, providing both 
visual and audible indications of the presence of contact arcing. In this 
preferred embodiment, the arc detector of the invention may be housed in a 
small, plastic enclosure with a standard, two-prong plug on the back and a 
large illuminated switch on the front. The light in the switch is labeled 
"Check Electrical Wiring". Whenever arcing is detected, if the arcing is 
persistent enough to present a hazard, the light will turn on and remain 
on continuously and the device will beep every 3 seconds until the 
illuminated switch is pressed to reset the device. If not manually reset, 
the device will turn the light back off and re-arm itself after 24 hours. 
Normally the detector will remain inactive. If the detector trips an 
imminent danger exists and the homeowner should call an electrician to 
check the house wiring. 
In another embodiment of apparatus according to the invention of the parent 
application, a circuit monitoring load current is combined with an 
electrically actuated circuit breaker to provide automatic shutoff of 
power when a sufficient amount of contact arcing has been detected to 
indicate a potential fire hazard. In this case, a small LED is provided to 
indicate the presence of arcing before enough arcing has been detected to 
trip the breaker. It also serves to indicate, by remaining illuminated, 
that the breaker tripped due to arcing and not overcurrent. Two preferred 
embodiments are shown; one where the device is built into a dual wall 
outlet in much the same style as known ground fault interrupters, and 
another where the device is built into a circuit breaker for installation 
into a distribution panel. 
In a further embodiment, the detection circuitry is wirelessly coupled to 
the power line, for convenient diagnostic use, and suitable analytic 
outputs are provided. 
The Present Second Continuation-in-part Application 
As indicated above, this application is a continuation-in-part of Ser. No. 
08/035,231, which was itself a continuation-in-part of a prior 
application, now issued as U.S. Pat. No. 5,223,095. As the text just 
preceding this paragraph describes, the methods of arc detection disclosed 
and claimed in those prior applications in effect look for gaps 
synchronized to the line frequency in high-frequency noise due to arcing. 
In essence, the gap detection methods of the two prior applications 
require that the arc be substantially continuous, that is, that 
high-frequency noise be present during much of the waveform, apart from 
gaps around the zero-crossings of the current waveform. In most 
circumstances this detection method will be entirely adequate to protect 
against fires, because under most circumstances the amount of heat 
required to ignite a fire would require that a continuous arc be present. 
Accordingly, the arc detectors in the prior application and issued patent 
are capable of preventing most fires, since they are capable of detecting 
substantially continuous arcing. 
However, there are circumstances wherein it would be desirable to detect 
arcing even before it reaches the substantially continuous level required 
to be detected according to the methods of the parent applications. The 
invention disclosed and claimed in the present continuation-in-part 
application describes further understanding of the arcing phenomenon, 
particularly at the inception of arcing. This new knowledge can be used, 
in turn, to derive more sophisticated arc detection algorithms which are 
capable of detecting arcing long before it becomes even substantially 
continuous. 
According to this present understanding, it is realized that the methods of 
arc detection described in the parent application and the issued patent 
are functional because the gaps are well-defined; that is, the gaps in the 
noise due to arcing have sharp edges. These sharp edges--that is, the very 
abrupt rise and fall in the current or voltage noise waveforms at the 
beginning and ending of arcing--are the result of the physics which 
control arcing, as follows. 
Arcing begins when the voltage across a gap filled with air or another 
ionizable substance is sufficient to impel an electron to escape from its 
atom, thus ionizing the atom. That electron, under the influence of the 
electric field between spaced electrodes exhibiting a potential 
difference, will move from the negative electrode toward the positive 
electrode, and will strike another atom, knocking off another electron. 
These two electrons then are accelerated and strike more atoms, releasing 
more electrons, and so on. The phenomenon of the exponential growth in the 
number of free electrons in the gap is referred to as electron "avalanche 
breakdown." This phenomenon is employed in numerous semiconductor devices 
that require fast switching, as is discussed in detail below. 
In effect, this further understanding allows the inventor to provide an arc 
detector which detects further specific patterns in the high-frequency 
noise unique to arcing. The detection circuitry provided according to this 
improved embodiment of the invention monitors the noise on the powerline 
in order to detect bursts of high-frequency noise, each having 
sharply-defined rise and fall times. Such bursts may be detected by 
differentiating the noise signal; equivalently, bursts may be detected by 
identifying a very high-frequency signal, e.g., greater than 10 MHz and 
possibly up to 100 MHz. If it is desired to detect arcing by looking for 
bursts of noise, typically a minimum burst width will be established to 
distinguish noise due to arcing from extraneous pulses from other sources. 
Further, the widths of the bursts and their spacing may be examined to 
determine whether they are substantially nonuniform, in order that 
high-frequency noise due to arcing can be distinguished from avalanche 
breakdown in semiconductor devices used in lamp dimmers and the like; 
these devices emit bursts which are consistent in length and are 
synchronized to the line frequency. The precise time of occurrence of the 
bursts may also be compared to the zero-crossings of the current waveform, 
as at those points there is insufficient energy available to initiate the 
arc.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a simplified equivalent circuit diagram of a contact arc. The 
line voltage 1 provides a "high" side 2 and neutral 3 conductors. All line 
and arc voltages referred to herein are conveniently measured relative to 
this neutral 3. Typical house wiring, i.e., flat three-conductor cable 
with the center conductor ground, acts as a transmission line with a 
characteristic impedance of about 100.OMEGA.. An inductance 4, indicated 
as L.sub.line, and a capacitance 5, indicated as C.sub.line, represent the 
lumped inductance and capacitance, respectively, of the power distribution 
lines. The load 6 is connected in series with a gap 7 across which the 
contact arc current passes when the arc is formed. When the arc is 
conducting current, the gap 7 has an effective resistance indicated as 
R.sub.arc in FIG. 1. The impedance of the load 6, indicated as X.sub.load, 
can be resistive, capacitive or inductive depending on the type of load. 
Incandescent lights and heating elements are typically resistive. 
Synchronous motors and fluorescent lights are typically inductive. Some 
capacitive-start motors appear capacitive before reaching full speed. 
FIGS. 2(a)-2(e) show various waveforms associated with a power line feeding 
a resistive load through a persistent contact arc as functions of time. 
Trace 8 (FIG. 2(a)) shows the line voltage indicating the presence of 
high-frequency noise 9 throughout the cycle except during the gaps 10 and 
11, that is, when the arc is not conducting. Gaps 10 and 11 are typically 
of similar duration during both half-cycles of the waveform, as shown. The 
amplitude of the noise is exaggerated for purposes of illustration. The 
duration of the gaps is indicated as t.sub.b. 
Trace 12 (FIG. 2(b)) shows the current through the load. The high-frequency 
noise generated by the conducting arc is also present on the current 
waveform, again exaggerated for clarity. Because the load is resistive the 
current 12 is in-phase with the voltage 8 (FIG. 2(a)), and goes to zero 
during the interval t.sub.b when the arc is not conducting. The 
high-frequency noise is again present throughout the cycle except during 
the gaps 13 and 14. 
Trace 15 (FIG. 2(c)) shows the voltage across the arc. Trace 16 illustrates 
the line voltage, i.e., the voltage that would be present across the arc 
if the arc did not fire, and is included to illustrate the timing of the 
ignition and extinction of the arc with respect to the line voltage. 
Beginning at point 17, the voltage 16 across the arc is zero, so the arc 
does not conduct and therefore produces no noise. As voltage 16 rises, it 
reaches a point 18 where the arc ignites and begins conducting current. 
The voltage across the arc 15 does not go to zero when the arc is 
conducting because the now-conducting arc has an impedance, typically on 
the order of several or tens of ohms; consequently a voltage is exhibited 
across the arc. Furthermore, the arc tends to maintain a fairly constant 
voltage independent of the current though it. This is illustrated at 19 
throughout the positive half-cycle and at 21 throughout the negative 
half-cycle. The arc produces high-frequency noise continuously during the 
time it is conducting. At point 20, the voltage across the arc lowers to 
the extinction level of the arc and the arc extinguishes. This process 
repeats on the negative half-cycle 21, and thereafter as long as the arc 
persists. 
Trace 22 (FIG. 2(d)) shows the high-frequency noise extracted from either 
the line voltage 8 (FIG. 2(a)) or the load current 12 (FIG. 2(b)). In this 
case, the noise is generally shown in the higher frequency portion of the 
emitted spectrum, e.g., 1-10 MHz. A simple high-pass filter serves to 
remove the line frequency components. As shown, high-frequency noise is 
present throughout the cycle except during the intervals 23 and 24 when 
the arc is not conducting. During these intervals, or gaps, the high 
frequency noise is substantially attenuated, that is, reduced to the level 
of any background noise, which is normally much lower in amplitude than 
the amplitude of the noise due to arcing, as shown. 
It will be noted that the amplitude of the high-frequency noise is somewhat 
higher immediately preceding and following the gaps, i.e., at points 25 
and 26 respectively. When the arc extinguishes and re-ignites, the load 
current abruptly changes. At relatively low frequencies, e.g. 10 KHz to 1 
MHz, this rapid change in load current on an essentially inductive 
distribution system produces additional high-frequency noise that will 
generally exceed the amplitude of the arc noise. Above 1 MHz, while the 
effect is still observed, there may be other, as yet not understood 
physical phenomena that contribute to its presence. 
Trace 27 (FIG. 2(e)) shows a positive logic signal 27 responsive to the 
presence of high-frequency noise, i.e., signal 27 is high when 
high-frequency noise is present and low when noise is absent, as 
illustrated at points 28 and 29. 
FIGS. 3(a)-3(e) show the same measurements of FIGS. 2(a)-2(e) with respect 
to a purely inductive load powered through a persistent contact arc. FIG. 
3(a) shows the line voltage 30, FIG. 3(b) the load current 33, FIG. 3(c) 
the arc voltage 34, FIG. 3(d) the high-frequency noise 36, and FIG. 3(e) 
the logic signal 39 responsive to detection of high-frequency noise. As 
can be seen, the difference between these traces for an inductive load, 
and the traces illustrated in FIGS. 2(a)-2(e) for a resistive load, is 
that the gaps in the high-frequency noise 31 and 32 occur about 90.degree. 
later in the voltage waveform. This is because the voltage across the arc 
34 (FIG. 3(c)) is delayed by the inductance of the load. As can be seen, 
the inductance of the load also increases the noise generated when the arc 
extinguishes at point 37 and re-ignites at point 38. The position of the 
gaps when a contact arc is in series with a reactive load is thus 
displaced from the line voltage zero-crossings, but otherwise is 
essentially similar to the noise due to arcing in a circuit with a 
resistive load. If the load were purely capacitive, similar characteristic 
traces would show that the gaps lead the zero-crossings by 90.degree.. In 
practice, the load will be somewhere between these limits, thereby 
producing gaps in the range of .+-.90.degree. from each zero crossing. 
In both FIGS. 2 and 3, the voltage at which the arc ignites depends on the 
size of the gap, the physical condition of the electrode surface, the 
temperature and the environmental conditions in the gap. Given that the 
sinusoidal waveform completes one full cycle every 360.degree. the voltage 
V at any time can be expressed in degrees, i.e., V=V.sub.peak Sin .theta., 
where V.sub.peak is the peak voltage reached and .theta. is the number of 
degrees from the zero-crossing. Arcs that ignite between 
.theta.=60.degree.-90.degree. (145-167 actual V on a 118 V (RMS) line) 
tend to be highly intermittent and unstable because the gap is large. Arcs 
that ignite in the 20.degree.-60.degree. (57-145 V) range are still 
generally intermittent, tending to occur in short bursts and rapidly 
self-extinguishing. Arcs in the 1.degree.-20.degree. (3-57 V) range may, 
under certain conditions, persist and become self-sustaining. Arcs in this 
range emit an audible hiss and may develop extremely high temperatures in 
the surrounding materials. Thus arcs in the 1.degree.-20.degree. range are 
particularly dangerous. 
According to the general method of the present invention, either the line 
voltage or the load current is monitored and the high-frequency noise 
extracted. The noise thus extracted is then monitored for the presence of 
gaps conforming to one of several "patterns" or "features" that occur at 
intervals equal to an integral multiple of one-half the line voltage 
period T. Commonly spacing of the gaps at intervals of T is required as a 
condition of detection of arcing. In one embodiment of the arcing 
detection algorithm, a "signature" potentially indicative of contact 
arcing is determined to exist if a predetermined number of gaps are found 
to occur spaced by successive intervals of length T. If the signature 
persists long enough, and/or meets certain further qualifying conditions, 
potentially dangerous arcing is determined to exist. 
More specifically, the high-frequency noise is first monitored for specific 
gap features to determine if qualifying gap patterns are present. Three 
specific gap features and the relative advantages and disadvantages of the 
use of each to determine the presence of gaps indicative of arcing are 
discussed below. 
The first and preferred gap feature, referred to as Feature 1, is 
characteristic of the leading edge of the gap. Feature 1 consists of an 
interval t.sub.a, wherein high-frequency noise is substantially present, 
followed immediately by an interval t.sub.b of some minimum length, during 
which high-frequency noise is substantially absent. See FIG. 4(b), 
discussed in detail below. Typical values for t.sub.a and t.sub.b are 2 ms 
and 0.2 ms respectively. With the high-frequency noise typically 
persisting throughout the each half-cycle and gaps typically 1-2 ms long, 
these values are much shorter than those exhibited in well-formed arcing 
patterns, i.e., as exhibited by potentially dangerous arcs. In fact, these 
values effectively define a feature of the gap, namely the leading edge, 
rather than the whole gap. Consequently widely ranging gap widths and 
intermittent high-frequency noise can still be detected. Qualifying 
patterns are determined to be synchronous to the power waveform if the 
time from the start of an interval t.sub.b in any one feature to the start 
of interval t.sub.b in any subsequent feature is very nearly equal to the 
period T of the power waveform. 
Again, it will be appreciated that "synchronous" in this context does not 
require that gaps be detected at any specific point on successive cycles 
of the power waveform, merely that the gaps be separated by intervals 
equal to integral multiples of the period T/2 of the half-cycle; 
preferably, for reasons discussed fully herein, gaps may be determined to 
be portions of signatures indicative of arcing if they are spaced by 
intervals equal to T, the period of the entire cycle of the power 
waveform, as measured to within a tolerance value Tol. 
It will be appreciated that if the arcing is intermittent, multiple Feature 
1 events can occur within each half-cycle, that is, there may be plural 
gaps in the noise. However, since the line voltage goes through zero only 
once each half-cycle, only one of these features will consistently repeat 
at the same temporal position, cycle after cycle, corresponding to 
extinction of the arc at the zero-crossing of the voltage. Other Feature 1 
events that may occur are random in nature, generated by the arc 
extinguishing for other physical reasons during the time the voltage 
across the arc is relatively high. 
It will also be noted that the synchronicity requirement for a Feature 1 
event to qualify as a pattern is determined by measuring the interval from 
the start of one gap to the start of the next. The present inventor has 
discovered that this point, when the arc extinguishes due to the arc 
voltage declining below the sustaining voltage, occurs at more precise and 
repeatable intervals than the point when the arc re-ignites. This is 
likely because the physical arc conditions are established and stabilized 
when the arc voltage is high and therefore respond in a highly repeatable 
fashion as the arc voltage declines. When the arc re-ignites, on the other 
hand, the localized surface temperatures have cooled slightly and, 
electrons may jump between slightly different areas on the contact surface 
before re-establishing the arc, thus varying the arc re-ignition time 
somewhat from cycle to cycle. 
The second gap feature that may be employed for gap identification, 
referred to as Feature 2, is characteristic of the trailing edge of the 
gap. A Feature 2 event consists of an interval t.sub.b, wherein 
high-frequency noise is substantially absent, followed immediately by an 
interval t.sub.c wherein high-frequency noise is substantially present. 
See FIG. 4(c). Typical values for t.sub.b and t.sub.c are 0.2 ms and 2 ms 
respectively. Again, Feature 2 detects a feature of the gap, that is, its 
trailing edge, rather than the whole gap. Consequently widely ranging gap 
widths and intermittent high-frequency noise can still be detected. 
Qualifying features are determined to be synchronous to the power waveform 
if the time from the start of an interval t.sub.b in any one feature to 
the start of interval t.sub.b in any subsequent feature is substantially 
equal to an integral multiple of T/2, or preferably to T. 
Accordingly, the principal difference between Feature 1 and Feature 2 is 
that the former measures the interval between leading edges of the gaps 
for testing synchronicity of successive gaps, while the latter measures 
the interval between trailing edges of the gaps. Although the present 
inventor has observed that the point at which the arcs extinguish is 
generally more accurate than the point at which the arcs re-ignite, there 
may be circumstances where the reverse is true, and in this case Feature 2 
would be the preferable detection technique. 
Feature 3 is a combination of Features 1 and 2. A Feature 3 event consists 
of an interval t.sub.a, wherein high-frequency noise is substantially 
present, followed immediately by an interval t.sub.b, wherein 
high-frequency noise is substantially absent (the gap), followed 
immediately by an interval t.sub.c wherein high-frequency noise is again 
substantially present. See FIG. 4(d). Qualifying features are determined 
to be synchronous to the line if the time from the start of an interval 
t.sub.b in any one feature to the start of interval t.sub.b in any 
subsequent feature is very nearly equal to an integral multiple of T/2 or, 
preferably, to T. Feature 3 requires that high-frequency noise be present 
on both sides of the gap and therefore establishes limits on the width of 
the gap. This, in turn, makes the detection more selective because the gap 
width must fall within these limits before qualifying as Feature 3. 
Employment of Feature 3 can, however, prevent the algorithm from detecting 
arcing from loads that are half-wave rectified, since in some cases one 
edge of the gap may not be clearly defined by noise. 
Accordingly, the first step in detecting a signature possibly indicative of 
arcing is to monitor the noise to identify gaps by a specific feature of 
each gap, and to then determine whether this feature repeats at intervals 
equal to integral multiples of T/2. In the preferred embodiment, as 
illustrated in the following examples, the detection interval is T. 
Examples of logic signals corresponding to detection of gaps identified by 
the three different gap features discussed above are illustrated in FIGS. 
4(a)-4(d). The three logical states used in FIGS. 4(b)-4(d) to represent 
the presence or absence of noise are explained in the legend (FIG. 4(a)). 
A high logic signal 53 indicates the presence of high-frequency noise, 
i.e., the logic signal is high when the amplitude of the monitored 
high-frequency noise exceeds some threshold level, e.g., during interval 
t.sub.a at 57 in FIG. 4(b). A low logic signal 54 indicates that the 
high-frequency noise is substantially attenuated, i.e., that the amplitude 
of the high-frequency noise is below a predetermined threshold level, as 
exemplified by period t.sub.b at 58 in FIG. 4(b). The hatched pattern 55 
indicates periods during which the amplitude of the high-frequency noise 
does not matter, i.e., that the output of the arc detector is not 
responsive to presence or absence of high-frequency noise during this 
interval, as exemplified at 56 in FIG. 4(b). 
Referring now to FIG. 4(b), two successive instances of gaps identified 
responsive to detection of Feature 1 are illustrated. For a gap to be 
identified by detection according to Feature 1, the high-frequency noise 
signal must be present during the interval t.sub.a, and absent during the 
successive interval t.sub.b. To qualify as a signature indicative of 
arcing, successive identifications of gaps conforming to Feature 1 must be 
spaced such that the interval from the start of interval t.sub.b to the 
start of the next interval t.sub.b is equal to T.+-.Tol, where Tol is the 
specified tolerance on measurement of the interval between successive 
gaps. Accordingly, Feature 1 detection amounts to measurement of the time 
interval between successive leading edges of the gap. 
FIG. 4(c) shows two successive instances of gaps conforming to Feature 2. 
To qualify as a gap pursuant to Feature 2, the high-frequency noise signal 
must be absent during the interval t.sub.b, and present during the 
interval t.sub.c. To qualify as a signature indicative of arcing, 
successive instances of Feature 2 must be spaced such that the interval of 
time from the end of interval t.sub.b to the end of the next interval 
t.sub.b is equal to T.+-.Tol. Thus Feature 2 detection amounts to 
measurement of the spacing in time between successive trailing edges of 
gaps. 
FIG. 4(d) shows two successive instances of gaps conforming to Feature 3, 
wherein both leading and trailing edges of the gaps must conform to the 
stated pattern. To qualify as a gap pursuant to Feature 3, the 
high-frequency noise signal must be present during the interval t.sub.a, 
absent during the interval t.sub.b, and again present during the interval 
t.sub.c. Since Feature 3 specifies both edges of the gap, the range 
allowed for t.sub.b specifies the minimum and maximum width of the gap, 
and therefore is more restrictive than Features 1 and 2. This may be 
desirable to increase the noise rejection capability in some applications. 
The allowable gap width t.sub.b in Feature 3 detection may be adjusted for 
optimum performance in any given application. To qualify as a signature 
possibly indicative of arcing, successive gaps conforming to Feature 3 
must be spaced such that the interval of time from the start of interval 
t.sub.b to the start of the next interval t.sub.b is equal to T.+-.Tol. In 
particular applications, it may be advantageous to measure the interval 
between the end of interval t.sub.b and the end of the next interval 
t.sub.b. 
As noted, regardless of the feature selected for gap identification, the 
gaps may occur anywhere relative to the line frequency, i.e., there is no 
required phase relationship between the gap pattern and the line voltage 
zero-crossings, so that detection takes place regardless of the load 
reactance. If it is desirable to limit the arc detection to a specific 
load reactance, the gap can be further required to appear in a specific 
temporal position relative to the line voltage before determining that an 
arc exists. For example, if the gap is required to exist in the vicinity 
of the zero-crossings of the line voltage, the only arcs that will be 
detected are those in series with a resistive, i.e., non-reactive load. To 
take advantage of this feature, the line voltage should be monitored, 
rather than the current, because the position of the gap on the current 
waveform can vary as a function of other reactive loads on the line. In 
another feature of the invention, the position of the gaps with respect to 
the line frequency waveform may be determined to identify the type of load 
in series with the arc, as an aid in locating the arc. 
In all three cases, successive gaps are preferably required to be detected 
at intervals equal in time to T, rather than T/2. It would also be 
possible to require the gaps to be detected at other integral multiples of 
T/2, as in most cases the gaps are in fact present at intervals of T/2. 
However, there are two distinct advantages to using T instead of T/2. 
First, if the load is half-wave rectified, the high-frequency noise will 
appear only on alternate half-cycles and consequently the gap will repeat 
only once every full cycle, i.e., at intervals of T. Therefore, to enable 
detection of arcing in series with half-wave rectified loads, the interval 
should be set to T. Second, gaps that occur at full-cycle intervals result 
from extinction of the arcing under precisely the same conditions, i.e., 
the same polarity of the arc voltage, and therefore are more repeatable 
and more reliably detected. 
It will be appreciated that certain of the features, e.g., the leading edge 
of the gap, may be spuriously detected due to random high-frequency noise. 
Therefore, to prevent the device from spurious tripping on random noise, 
and according to another aspect of the invention, the detection algorithm 
includes further conditions which must be met prior to the identification 
of arcing, in order to effectively reject random noise events. For 
example, the detection algorithm may require that n identical features 
occur in succession at intervals equal to T. 
The probability of n qualifying features occurring in a row from random 
noise is: 
##EQU1## 
where: .rho.(nHits)=probability that n qualifying features will occur in a 
row 
.epsilon.=number of random events per T 
Tol=interval tolerance (.+-.sec) 
T=line voltage period (sec) 
n=number of successive gap features required 
For example, if we assume an average of 10 random events that exhibit the 
characteristics required of a feature per T (16.66 ms at 60 Hz), a 
tolerance Tol of .+-.100 .mu.s, and set n equal to 3, the probability that 
noise will trigger the device is 0.0017, or approximately one in 570 
cycles. If n is raised to 5, this decreases to one in 39,390. With n set 
to 10, we can expect random noise to qualify as a pattern indicative of 
arcing only one in 1,500,000,000 cycles. 
Once the high-frequency noise on a given line has been determined to be 
characteristic of arcing, that is, when a signature of gaps possibly 
indicative of arcing has been identified, the next step in the method of 
the invention is to determine whether this arcing persists long enough to 
be dangerous, this time being typically on the order of a few seconds. 
This can be accomplished by simply increasing n to a very high value, 
e.g., 100. This approach, however, requires that the arcing persist, 
without interruption, for n cycles in succession. In practice, the 
inventor has found that with persistent arcing across copper contacts, the 
arc may occasionally extinguish for a cycle or two, thereby being 
disqualified as a dangerous arc if n is set too large. This likely results 
from the eventual physical melting and reforming of the contact areas in 
response to the heat generated by the arc. A better approach, therefore, 
is to employ a method whereby a signature of gaps possibly indicative of 
arcing is first determined to exist, e.g. by requiring that n gaps are 
detected in successive cycles, n being chosen to provide reasonable 
rejection of random noise, e.g., n= 5, and then to integrate the number of 
qualified signatures that occur with respect to time until a predetermined 
threshold is reached. 
More generally, arc identification according to the invention requires that 
gaps be detected at intervals equal to integral multiples of T/2 during at 
least a substantial fraction of at least a minimum number of cycles. 
Stated differently, some minimum number of gaps must be detected at 
intervals spaced by T/2 or multiples thereof within a minimum period of 
time. Further conditions may include that a number of gaps be detected at 
successive intervals, for example, of T/2, or of T. 
In one specific embodiment, a simple counter, the `Score` counter, is 
employed to effectively integrate gap detection. When a feature indicative 
of a gap is detected, a timer records the interval of time that has 
elapsed since detection of the prior feature. The timer also analyzes 
previously recorded intervals to determine if there were n previous 
features spaced T seconds apart. If so, a `hit` is said to have occurred 
(i.e., the arcing `signature` has been `recognized`) and the Score counter 
is incremented. If T seconds then elapse without a further `hit`, the 
Score counter is decremented. If the Score counter reaches a predetermined 
count or threshold, the determination is made that dangerous arcing is 
present. In this manner, the algorithm will reliably detect arcing with up 
to one out of every n+1 gaps missing. 
This dual-step method of arc detection offers both high noise immunity and 
the ability to tolerate occasional anomalies in the arcing pattern. The 
first step establishes the high-frequency noise as arcing noise by 
detecting a synchronous pattern or signature of gaps including at least n 
synchronous gaps in succession. The second step effectively integrates the 
signature over time, to determine if the arcing is persistent enough to 
present a hazard. 
It will be appreciated that two signatures, each signature having n 
successive features occurring T seconds apart, can occur simultaneously, 
one on each phase of the cycle, that is, one on the positive half-cycles, 
and one on the negative half-cycles of the power waveform. According to 
one aspect of the invention, these two sets of patterns may be treated 
independently, each incrementing or decrementing the Score counter as they 
occur, quite independent of one another. This has the added advantage that 
the same algorithm will detect arcing on both full-wave loads and 
half-wave rectified loads. If the load is half-wave rectified, gaps will 
occur on one phase only and consequently the Score counter will take twice 
as long to reach the threshold. Since the energy dissipated in the arc on 
a half-wave load is one-half the energy dissipated in the arc on a 
full-wave load, this is the desired result. Similarly, if gaps are 
occasionally missing, this means that the arcing has been momentarily 
interrupted and the trip time is correspondingly longer. 
Selection of the value for the minimum gap length t.sub.b should be made 
responsive to several factors. First, although a particular load type will 
tend to produce very consistent gap widths from cycle to cycle, the gap 
width is highly dependent on both the complex impedance of the load and 
the physical conditions of the arc. In general, persistent arcing has been 
found to produce relatively short gap widths. While intermittent arcing 
tends to produce gap widths ranging from about 20.degree.-90.degree. of 
the line cycle, persistent arcing is nearly always in the range from about 
1.degree. to 20.degree.. It will be noted that Features 1 and 2 do not 
limit the maximum width of the gap, but rather require only that the gap 
be a minimum width that is equal to or greater than t.sub.b. Therefore, if 
t.sub.b is set to 1.degree. (46 .mu.s) the only width requirement for 
either feature is that the gap be greater than 1.degree. long. In 
practice, due to the processing speed capability of the microprocessor 
used, t.sub.b is usually set to several degrees or more. If Feature 3 is 
used, limits must be set on the width t.sub.b of the gaps, generally about 
1.degree.-20.degree. . 
The selection of the value for t.sub.a is again a function of several 
considerations. Under optimum conditions, the high-frequency noise will 
persist during the entire cycle, except during the expected gaps t.sub.b 
when the voltage across the arc drops below the arc sustaining voltage. 
However, in real-world conditions, the signal responsive to high-frequency 
noise may not persist continuously during this time. One problem, 
illustrated in FIGS. 5(a)-5(c), results from the variation of noise 
amplitude across the cycle with respect to the ability to accurately set 
the noise detection threshold. 
Referring now to FIG. 5(a), a high-frequency noise pattern 40 typical of 
persistent arcing is shown. As discussed previously, the noise is higher 
in amplitude just before and after the gaps than between the gaps 62, as 
shown at 60 and 61. If the threshold for detection of gaps in the 
high-frequency noise is optimally set to a relatively low level 63, then 
the resulting logical pattern (FIG. 5(b)) will be nearly ideal. If, 
however, the threshold is set higher, as shown at 64, the noise may 
occasionally dip below the threshold in the middle of the cycle and 
produce logic noise 67 in the output. Therefore, while t.sub.a (or 
t.sub.c) might otherwise be set to T/2 less the maximum expected gap 
width, it is advantageous to set t.sub.a (or t.sub.c) to a smaller value, 
typically about 1/8 the line cycle or 2 ms. In this manner, the 
requirement of the feature is that high-frequency noise exist in the areas 
where it is the highest, just before or just after the gap. 
The present inventor has observed that the "saddle" or "bowtie" variation 
in the amplitude of the noise between successive gaps evident in FIG. 5(a) 
(see also FIGS. 2(a) and 3(a)) is characteristic of noise due to arcing. 
While detection of arcing by monitoring the noise for gaps represents the 
preferred embodiment of the invention (gap detection as described herein 
having a very high signal-to-noise ratio) it is also within the invention 
to detect arcing by monitoring the noise waveform for other such features 
that are synchronous to the fundamental frequency of the power waveform, 
where not excluded by the language of the following claims. 
Referring now to the problem of avoiding false indications of arcing due to 
noise from other sources, there are two basic types of extraneous noise 
present on power lines in the 1-10 MHz frequency range. The first is very 
short-lived impulse noise from lamp dimmers, switching power supplies, and 
the like. This type of noise can be rejected simply by making the 
intervals t.sub.a and t.sub.c longer than the maximum impulse noise 
duration. Since typical impulse noise extends no longer than several 
hundred microseconds, and since the typical requirement for t.sub.a or 
t.sub.c is several milliseconds or more, this type of extraneous noise is 
easily rejected. 
The second type of noise commonly found on power lines is interference from 
local AM radio broadcast stations. This noise is particularly apparent 
when the gain of the system is made high enough to detect even distant and 
low-power arcing. While this type of noise may produce occasional 
qualifying patterns through random coincidence, this noise has no temporal 
relationship to the line frequency and therefore will not produce 
repetitive patterns synchronous with the line frequency. Therefore, this 
and other non-line-synchronized forms of extraneous noise are also 
effectively ignored. 
A third type of noise commonly present is due to arcing in electric motors 
with brush contacts. A typical example of this is the common household 
drill. Such motors produce a substantial amount of high-frequency noise in 
the 100 KHz-1 MHz range but much less in the 1 MHz to 10 MHz range. In 
either case, however, the noise does not ever go to zero; it is present 
throughout the cycle. This is likely due to the mechanical momentum of the 
rotating armature and the back EMF produced. In any case, the lack of a 
gap in each half-cycle of the line frequency prevents false triggering of 
the arc detection device according to all three patterns of the present 
method. 
A fourth type of noise sometimes present is communication signals from 
carrier-current transmitters. There are a number of devices on the market 
that provide remote control of appliances by using carrier-current 
transmission techniques to convey information over the power lines. These 
are typically tuned to transmit bursts in the 100-300 KHz range. The 
amplitude of the noise in the 1-10 MHz frequency range is again very low 
because noise in this band would interfere with AM radio broadcasts and is 
therefore intentionally minimized by the manufacturer. 
FIG. 6 shows typical extraneous noise found on household wiring in the 1-10 
MHz range. The modulated waveform 69 is typical of AM radio transmissions 
picked up on household wiring. The amplitude variations, though not 
random, have no temporal relationship to the line frequency and 
consequently will not trip the detector. 
Typical impulse noise, from lamp dimmers and the like, is also shown in 
FIG. 6. Two spikes 70, 71 are illustrated that occur synchronously to the 
line power. Since feature detection according to the invention requires 
high-frequency noise to be detected for a minimum interval (t.sub.a) the 
high-frequency is required to be on is typically 2 ms or more, and since 
such impulse noise spikes are always 0.1 ms or less in duration, these 
spikes do not cause the detector to falsely trip. 
It will be appreciated that the relative danger from fire due to arcing is 
a function of the power dissipated in the arc. The power dissipated in the 
arc represents the rate of heat production and is approximately 
proportional to the load current. The energy (total heat) produced is this 
power multiplied by the time the arc is conducting. While the load current 
is not directly sensed and therefore not directly taken into account by 
the present detection method, the detector of the invention does in fact 
respond more rapidly to high-current arcing, for two distinct reasons. 
First, the amplitude of the high-frequency noise produced by the arcing is 
approximately proportional to the level of current through the arc. 
Therefore, high-current arcing will produce very clear noise signatures 
with a high signal-to-noise ratio leading to rapid and reliable detection. 
Second, the heat produced by the arc lowers the thermionic emission 
threshold at the contacts, enhancing the physical processes that cause the 
arc to become persistent and self-sustaining. Therefore, the higher the 
load current the faster persistent arcing patterns will develop. 
To this point, and relative to the methods discussed so far, it does not 
matter whether the high-frequency noise examined according to the 
invention is extracted from the line voltage or the load current. 
According to the invention, either load current or line voltage (or both) 
may be monitored for arc detection. There are, however, several distinct 
differences between the two approaches, each providing relative advantages 
and disadvantages. As discussed below, radio-frequency energy emitted from 
household wiring may also be similarly monitored. 
The first embodiment of the invention is a plug-in, night-light style 
alarm, monitoring line voltage so that high-frequency noise originating 
anywhere on the line or within most equipment will be detected. This is 
advantageous because the alarm in this embodiment can conveniently monitor 
an entire household by monitoring noise on the plug contacts, i.e., the 
line voltage. 
In the second embodiment of the present invention, the arc detection 
circuit controls a circuit breaker capable of interrupting the current to 
the load when arcing is detected. In this application it is desirable to 
monitor only that noise which originates on the protected circuit. 
Therefore, the current flowing to a particular load is monitored and the 
high-frequency component extracted. The only noise present will be that 
which is generated by sources on that circuit. Also, since the circuit 
breaker is wired in series with the load, a current-carrying conductor is 
available for current monitoring. For these two reasons, the use of 
high-frequency current monitoring is advantageous in this application. 
In yet a third embodiment of the invention, the detection circuit is 
packaged in a battery-powered, wireless detection device that monitors the 
high-frequency emissions from the power wiring through a self-contained 
receiving antenna. This device works best in the higher end of the 
frequency spectrum (1-100 MHz) because a relatively short antenna can be 
used. 
A prototype circuit built according to the invention examined the line 
voltage for high-frequency noise in the range of 5-10 MHz. The detection 
algorithm was implemented in a Microchip Technology, Inc. PIC16C54 
microprocessor and the device was housed in a convenient plug-in 
enclosure. It used Feature 1 detection of gaps, where t.sub.a and t.sub.b 
were set to 2 and 0.2 ms respectively. The interval tolerance was set to 
128 .mu.s and the threshold for the Score integrator (as described in 
detail below) was switchable from 2.sup.0 to 2.sup.8. This prototype's 
response was tested extensively with a variety of loads. The alarm was 
tripped repeatedly and reliably when a 150 watt light bulb anywhere on the 
circuit was powered through a persistent short arc, yet did not trip from 
interference originating from lamp dimmers, carrier current transmitters, 
fluorescent lights, motor speed controllers, load switching, or broadcast 
radio interference. In a further experiment, a variable speed electric 
drill would not trip the alarm even at close range and despite the large 
amount of high-frequency noise generated. However, when the same drill was 
powered through a short arc, the alarm tripped whenever the arc carried 
current. 
FIG. 7 shows a simplified block diagram of an arc detector monitoring noise 
in the line voltage for characteristic patterns of noise according to the 
invention. The detector of FIG. 7 provides both visual and audible 
warnings to the user. 
A standard plug 72 connected to the power line provides both power and 
signal connection to the detector through the high conductor 73, neutral 
conductor 74 and ground 75. An optional switch 76 allows the unit to be 
powered and to sense high-frequency noise either line-to-line or 
line-to-ground. The normal mode of operation is line-to-line but the 
device may perform better line-to-ground in some applications. 
A power supply 77 provides regulated DC voltage V+ at 79 to operate the 
unit. V+ is a convenient low voltage, typically 5 or 12 volts. Circuit 
common is depicted at 78. The processing circuitry, lamp, and audible 
indicator can be made to operate at under One watt total, so that it is 
convenient to configure the power supply as an AC-DC converter using a 
capacitor as the voltage dropping element. It is preferable to use a 
half-wave rectifier so that the line neutral 74 or ground 75, depending on 
the setting of switch 76, can be made circuit common 78. This simplifies 
the signal detection coupling. The Maximum Integrated Products MAX611 AC 
to DC Regulator chip works well in this application, producing 5 V DC at 
up to 150 ma from 120 or 240 VAC input. 
An optional zero-crossing detector 82 functions to produce a narrow pulse 
(10.degree.-100.degree.) at each zero-crossing of the line voltage. 
Zero-crossings are conveniently sensed through a high-value resistor 80. 
Conventional techniques produce an output pulse responsive to each 
zero-crossing. The output of the zero-crossing detector 80 is supplied to 
an input 83 of a microprocessor 92. Microprocessor 92 employs the 
zero-crossing pulse to measure the line frequency and determine the 
correct line period for use in the detection algorithm. In this manner, 
the device can be used in other countries where the line frequency varies 
from 60 Hz. Zero-crossing detection can also be used to relate the 
presence of gaps in the noise to the phase of the line voltage in order to 
make the device selective to arcing in series with particular types of 
loads, i.e., loads with specific reactance. In order to maintain the 
timing information accurately in the presence of high-frequency noise, a 
capacitor 81 can be added to input resistor 80 to produce a low pass 
filter. A single-pole filter response with a corner frequency of about 
100-1000 Hz works well. 
The high-frequency noise is conveniently sampled through a small coupling 
capacitor 84 forming part of a high-pass filter 85, or a tuned band-pass 
filter to remove the line voltage frequency. The noise is then amplified 
by gain-controlled amplifier 86 and detected by detector 87. Detector 87 
functions as a full- or half-wave rectifier to detect the high-frequency 
components and produce a rectified signal responsive to the amplitude of 
the noise. This detector 87 is preferably a biased square-law diode 
detector or a multiplying detector, both of known type, to provide 
sensitivity in the sub-millivolt range. The output of the detector 88 is 
fed to an automatic gain control (AGC) circuit 90 whose output feeds the 
gain-control input 91 on the signal amplifier 86. The AGC circuit is 
desirably configured to produce a substantially constant average noise 
amplitude at point 88 over a range of 60 dB or more of input noise 
amplitude. The resulting gain-controlled noise signal 88 is fed into a 
comparator 89 to compare its amplitude to a predetermined threshold value 
and produce a logic-level signal 95 responsive to the presence of 
high-frequency noise of average amplitude above the threshold value. 
Logic-level signal 95 is supplied to microprocessor 92 for implementation 
of the detection algorithm according to the invention. 
As noted previously, the bandwidth of a contact arc extends from about 10 
KHz to about 1 GHz. The selection of the preferred bandwidth of noise 
analyzed according to the invention to detect arcing is a function of the 
desired performance of the device. Clearly, the frequency band must first 
be high enough to sufficiently reject the 50 or 60 Hz power line 
frequency. Any frequency above about 10 KHz will suffice for this purpose. 
In the frequency range between about 100 KHz and 1 MHz, there is a 
considerable amount of extraneous synchronous line noise from common 
household sources. If noise in this bandwidth is to be analyzed to detect 
the presence of gaps, the detection algorithm must be designed to reject 
noise from these sources. The parent application describes a variety of 
methods whereby such noise can be rejected and arcing in this frequency 
range can be reliably detected. In the frequency range between about 1 MHz 
and 10 MHz, the preferred frequency band for the embodiments of the 
invention disclosed in the present application, such extraneous 
synchronous line noise is substantially absent or very limited in 
duration. However, because the bandwidth of arc noise is so wide, 
virtually any frequency bandwidth can be used if conditions warrant. In 
the prototype, a low-Q tuned input filter with a passband of about 5-10 
MHz was used. 
The power wiring in most buildings in the United States is flat 
three-conductor cable with the center conductor ground. Romex and Amerflex 
are commercial examples of this type of cable. While these cables are well 
specified in the 100-300 KHz range, little information is available on the 
attenuation characteristics in the higher frequency range. The attenuation 
across a typical home was found to vary between about 10 and about 80 dB, 
depending on the distance between the arc and the detector, the loads 
present in the circuit, and on whether the load and the detector were on 
the same phase. 
The gain required for the present invention to work well over an entire 
household or business is on the order of 40 to 80 dB. With a gain of 70 
dB, the device appears to work well across both phases in one test 
installation. The amplifier must behave well when clipping and provide 
quick turn-off time to avoid extending the width of high-amplitude pulses. 
Clipping diodes at the input and a limiter or automatic gain control 
device can be incorporated into the amplifier to accomplish these goals. 
The microprocessor 92 is preferably a low-cost, single-chip processor with 
integral ROM and RAM operating at 1-20 MHz. Microprocessor 92 drives one 
or more alarm indicator lamps indicated at 94 via control line 98, drives 
an audio annunciator 97 via control line 96, and accepts input from the 
user via switch 98. The tasks microprocessor 92 must perform depend on the 
precise variations used on the method of the invention. 
More specifically, the microprocessor, by monitoring the logic-level signal 
indicative of the presence or absence of high-frequency noise, can detect 
gaps conforming to any of the Features disclosed in the parent 
application, additional detectable patterns in the noise characteristic of 
arcing, or the patterns discussed in issued U.S. Pat. No. 5,223,795, and 
can determine whether the selected Feature or pattern is synchronous to 
the power waveform and persists sufficiently long to satisfy the 
conditions for determining that arcing exists. As discussed above, in the 
preferred embodiment of the parent application one of several Features is 
selected; if the microprocessor detects the selected Feature at 
synchronous intervals during more than a predetermined fraction of the 
cycles of the power waveform, a signature indicative of possible arcing is 
considered to have been detected; and if the signature persists for an 
appropriate period of time, arcing is considered to have been detected and 
an appropriate control action taken. 
In one convenient implementation of the invention as described in the 
parent application, the microprocessor saves the time at which each 
Feature is detected in a circular buffer, typically eight registers long, 
together with a `Hit` value saved in a corresponding set of eight 
registers. As each subsequent Feature is similarly detected, the 
microprocessor compares the new time to previous times stored in the 
circular buffer to determine whether a Feature had been detected T seconds 
prior to the time of detection of the present Feature, within a tolerance 
Tol. If so, a `Hit` is said to have occurred and the `Hit` register for 
that position in the buffer is incremented; if not, the Hit register for 
that position is cleared to zero. In this manner, the Hit registers, one 
for each position of the circular buffer, contain the number of successive 
Hits that have occurred at various relative positions along the waveform. 
If any of the Hit registers reaches n, the number of successive Hits 
required for a signature, the Score counter is incremented and the Hit 
register is reset to 1. If one or more line cycles go by without any Hits, 
the Score counter is decremented. If the value stored by the Score counter 
reaches a predetermined value, arcing is determined to have occurred. 
In essence, the microprocessor carries out all these tasks; programming a 
microprocessor to do so is within the skill of the art. The microprocessor 
might alternatively be programmed to otherwise detect appropriate 
sequences of detection of patterns of high-frequency noise characteristic 
of arcing, for example, integrating the number of synchronous gaps 
detected during a predetermined interval as disclosed in the parent 
application, or in other equivalent ways, specifically including 
monitoring of the high-frequency noise on the power line for synchronous 
occurrence of the `bowtie` or `saddle` pattern discussed above in 
connection with FIG. 5(a). Randomness and bandwidth tests as described in 
the issued U.S. Pat. No. 5,223,795 can also be incorporated, to further 
differentiate noise due to arcing from other sources of high-frequency 
noise. 
In a further enhancement, the microprocessor can measure the phase of the 
gaps in the noise relative to the zero-crossings of the power waveform, in 
order to identify the load in series with the arc as inductive, resistive 
or capacitive. A indication responsive to this determination may be 
provided to the user in locating the arcing. 
FIG. 8 shows an embodiment of the arc detector of the invention configured 
as a diagnostic device, e.g., for use by a repairman to quantify and 
locate intermittent arcing somewhere within a household or the like before 
the arcing reaches dangerous levels. In this embodiment, radio-frequency 
energy responsive to the noise on the power waveform is coupled to the 
detector via an antenna 99. Typically, the detector in this embodiment may 
be successively disposed in the vicinity of various likely sources of 
arcing, or may be placed in the vicinity of a suspected source of arcing 
and left for a period of time to record arcing events. The detector may 
also be plugged into the power distribution wiring to establish the timing 
of the waveform for load type identification, as discussed above. The 
detection circuit employed in the embodiment of FIG. 8 is substantially 
similar to the circuit described in FIG. 7, as indicated by use of the 
identical reference numerals. For convenient hand-held use, the detector 
may be powered by a battery 103 connected between circuit V+ at 104 and 
common at 105. A keypad 101 and display 102 are provided to enable the 
user to access internal registers of the microprocessor storing relevant 
characteristics of the detected arcing, such as the nature of the 
load--resistive, capacitive, or inductive--the length and timing of the 
arcing detected, and the like. The gain control -5 feedback signal may be 
converted to a digital value by an analog-to-digital converter 100 and 
supplied to the microprocessor 92 for further analysis, e.g., to allow 
display of the relative amplitude of the arcing noise with respect to 
background noise on the power line. 
FIG. 9 shows a simplified block diagram of an arc detector according to the 
invention, in this embodiment sensing the current for arc detection and 
interrupting current flow to the load when arcing is detected. This 
apparatus employs the same basic detection circuit described in FIG. 7, 
but is configured to provide power to a load and to interrupt current to 
the load in the event that a predetermined level of arcing occurs. 
As shown in FIG. 9, the main power lines, consisting of high 107, neutral 
108 and ground 106 conductors, provide power to the load through a switch 
element 111 controlled by a solenoid 113 of a conventional circuit breaker 
(i.e., also providing overcurrent protection) to high, neutral, and ground 
conductors 112,116, and 117, respectively. The high line is routed through 
a current transformer 109 having a pass band of 1-10 MHz, that monitors 
current to the load. A zero-detection circuit is used to monitor the line 
voltage if it is desired to have the same device operable in systems 
having differing line frequencies, as in other countries. The current 
transformer 109 monitors only current flowing to the load, thereby 
isolating the arc detection circuit from arcs generated on the individual 
circuit protected by breaker 113. 
The detection circuit employed in the embodiment of FIG. 9 is substantially 
similar to the circuit described in FIG. 7, as indicated by use of the 
identical reference numerals. A conventional electrically-actuated circuit 
breaker with an actuating coil 113 driven by a control line 114 from the 
microprocessor controls a circuit breaker contact 111 disposed in-line 
between the high conductor 107 and the load, in order to interrupt power 
flow to the load when the microprocessor 92 determines that an arc of 
sufficient magnitude and duration has occurred, or if an overcurrent 
occurs. 
The detector circuit of FIGS. 7, 8, and 9 and the discussion to this point 
refer to a detector operating on one phase of a 220 VAC split-phase power 
line, as is commonly found in residences. It should be understood that the 
present invention may be applied to power systems with any voltage and 
phase configuration. All that is required is to provide a detector across 
each phase to be monitored. 
FIGS. 10(a) and (b) show respectively side and front elevational views of 
one suitable packaging approach for housing the circuit shown in FIG. 7. A 
plastic injection-molded case 119 houses the complete detector. A 
combination indicator/switch 122 serves to both indicate the presence of 
an arc and to reset the light when the unit is in the hold mode. 
Referring now to the side view illustrated in FIG. 10(a), the package is 
designed to be plugged directly into a conventional 3-prong wall outlet 
and is self-supporting on the plug terminals 118. These terminals, or 
other terminal types for 2-prong domestic or international applications, 
are mounted or molded directly into the plastic package 119. A legend 121 
on the face 120 of the unit warns the user to seek assistance if arcing is 
detected. 
FIG. 11 shows a front view of one possible package for the radio-coupled 
version of the detector of the invention as optimized for diagnostic use. 
The circuit of FIG. 8 is disposed in a case 123 having a folding, 
telescoping antenna 124 for radiative coupling of energy from the 
waveform. A text display 125 provides various data useful in determining 
the source of arcing, as indicated. Barograph displays 127, 128 compare 
the relative levels of the noise examined for the presence of arcing and 
the background noise on the line. Pushbuttons 129 provide suitable control 
options. It will be apparent that many other arrangements are within the 
scope of the invention. As noted, the diagnostic version of the detector 
may further comprise a plug-in connection for establishing the relation of 
the arc gaps to the zero-crossings of the line power, allowing 
identification of the type of load. 
FIGS. 12(a) and (b) show side and front views, respectively, of one 
suitable packaging approach for housing the current-monitoring circuit of 
FIG. 9 in a dual-outlet enclosure similar to commonly available ground 
fault interrupters. A molded plastic case 132 houses the entire assembly. 
Screw terminals 131 and 133, together with two further terminals on the 
other side and a ground terminal, serve as connections to attach the line, 
load and ground conductors. A metal bezel 130 fits around the case 132 and 
serves to mount the device in conventional outlet boxes. Two outlets 135, 
138 provide the load connections. A test switch 136 will manually trip the 
circuit breaker when pressed to test for proper operation of the device. 
An LED 139 indicates that contact arcing has occurred sometime in the last 
24-48 hours. If the LED 139 is ON, it can be reset by pressing a reset 
switch 137; if the LED lights, indicating the arc has been detected again, 
the user is warned to locate and cure the arcing condition. 
FIGS. 13(a) and (b) show side and front views, respectively, of another 
suitable packaging approach for housing the current-monitoring circuit of 
FIG. 9 in a conventional circuit breaker package. A molded plastic case 
140 houses the entire assembly. The circuit breaker package accesses the 
High side of the line through an integral contact on the back of the 
package (not shown). As Neutral or Ground is also required for the circuit 
to operate, an additional wire 141 is provided for connection thereof. 
Screw terminal 148 provides High to the load. In this version, an internal 
interruption relay operates both as a standard current-operated circuit 
breaker and a voltage-actuated relay driven by the arc detector. The 
handle 142 can be manually actuated in the same manner as a conventional 
circuit breaker. A test switch 147 will manually trip the circuit breaker 
when pressed to test for proper operation of the device. An LED 145 
indicates that contact arcing has occurred sometime in the last 24-48 
hours. If the LED 145 is ON, it can be reset by pressing the Reset switch 
146. 
The Present Second Continuation-in-Part Application 
As indicated above, this application is a continuation-in-part of Ser. No. 
08/035,231, filed 22 Mar. 1993, which was itself a continuation-in-part of 
a prior application now issued as U.S. Pat. No. 5,223,795. In the parent 
application (essentially the entire specification of which is reproduced 
above) and the issued patent, emphasis was placed on detecting arcs by 
monitoring high-frequency noise on the current or voltage waveform for 
gaps synchronous to the waveform. That approach requires "substantially 
continuous" noise between the gaps--that is, there must be enough noise so 
that the gaps can be readily detected. This method works very well in 
preventing most fires, in that the chance of fire is small until an arc 
has developed that is substantially continuous. However, there are 
circumstances--e.g., explosive environments and the like--in which it 
would be highly desirable to be able to detect arcs which are 
intermittent, and in particular, which are not yet consistent enough to 
present the regular series of gaps detected by the parent application and 
the issued patent. 
The capability of detecting intermittent arcs not exhibiting regular 
patterns of gaps in substantially continuous noise, while nonetheless 
distinguishing arcing from other sources of noise, is provided by the 
invention of the present second continuation-in-part application. 
This new invention is based on further physical understanding of the 
process of arcing. Accordingly, a brief theoretical discussion precedes 
explanation of the subject matter of the present second 
continuation-in-part application. 
As indicated above, when arcing occurs, the random motion of charged 
particles in the arc generates broadband high-frequency noise. As 
discussed in the parent application and the issued U.S. Pat. No. 
5,223,795, the arc will extinguish for a short period each time the 
voltage across the arc falls below the arc sustaining voltage and will 
reignite each time it again rises above the ignition voltage. This 
produces synchronous gaps in the high-frequency noise. If the arcing is 
substantially continuous, the resulting pattern of gaps is clear and can 
be used to discern arcing from other noise that might be present on the 
line. If, however, the arcing is intermittent and does not persist for 
substantial periods between the gaps, the pattern becomes less obvious and 
more difficult to discern. There are applications where it would be useful 
to be able to detect even this intermittent form of arcing and either 
provide an indication that arcing exists or interrupt the current to the 
arc. Therefore, the purpose of this second continuation-in-part 
application is to further explain the characteristics of high-frequency 
noise from electrical arcing and disclose additional methods of and 
apparatus for detecting such arcing. 
Arcing occurs when the voltage across a gap filled with air or another 
ionizable material is sufficiently high to ionize a chain of atoms across 
the gap, causing electrons to flow across the gap. The physical initiation 
and extinction of electrical arcs occurs extremely rapidly. This is 
because the voltage breakdown mechanism that initiates the arc is actually 
a process of electron avalanche that occurs as the material (air, for 
example) between the electrodes is ionized, forming a conductive path. 
When even a single electron gains enough energy to bridge the gap between 
the electrodes, it will (probability=1) collide with an atom of the air, 
knock an electron out, and thereby ionize the atom. (Most of the ions 
produced in air are probably actually negative ions, i.e., an additional 
electron is accepted by the ionized atom. However, electron flow, rather 
than "hole flow," is described herein for ease of understanding.) At this 
point, there are two electrons, each of which will collide with atoms and 
ionize them. Then there will be four electrons, and so on. In effect, a 
single electron moving across the gap rapidly grows to a large number of 
electrons moving across the gap, the number of electrons increasing in an 
exponential fashion. This is referred to as electron "avalanche 
breakdown." The exponential increase in the number of electrons flowing, 
i.e., the current, produces a very fast rise in the current. The rise in 
the current can be detected and used to distinguish arcing. 
The predominant characteristic of intermittent, low-voltage arcing is that 
it is, in general terms, an "all-or-nothing" process, i.e., it occurs in 
discrete, well-defined bursts. This is a result of positive hysteresis 
inherent in the arcing process, i.e., once an electron gets over the 
potential barrier, it forms a conductive path that in turn lowers the 
barrier. Thus, once initiated, the process is self-enhancing until thermal 
or other effects come into play and disrupt the flow of current. 
Several points are worth mentioning before proceeding. First, the erratic 
jumps of electrons between individual atoms in the air is the mechanism 
that produces the broad spectrum of noise observed to be generated by 
arcing. Because the specific path length and direction of each individual 
electron are random, so are the electric and magnetic fields produced 
thereby. Consequently, the detected signal exhibits a wide bandwidth and a 
noisy frequency spectrum. Secondly, the process of electron avalanche 
breakdown may occur in a gas (e.g., air) or any other nonconducting or 
semiconducting material. In a semiconductor (including semiconductive 
oxide layers formed on switch contacts and the like), charge carriers can 
acquire enough energy to make electron-hole pairs by impact ionization. 
Avalanche breakdown in semiconductors is a useful mechanism that has been 
widely applied. In fact, a common means of producing pulses having very 
fast rise times is to drive a semiconductor into a triggered-avalanche 
mode. Pulse rise times of tens of picoseconds or less can be readily 
achieved in this manner. Thyristors, including SCRs and TRIACs as commonly 
used in lamp dimmers, also work on this principle of avalanche 
multiplication. A low-energy trigger pulse is used to initiate 
avalanche-multiplication of the electron-hole pairs, very rapidly "turning 
on" the device. 
In terms of arc detection, the fact that the rise in current at the 
initiation of the arc is inherently extremely rapid can be used by a 
detection mechanism to distinguish this current from other currents 
resulting from extraneous noise sources. The actual rise time will be a 
function of the various capacitances present across the junction. Since 
capacitance is proportional to the area of the conducting surfaces and 
inversely proportional to the separation, a gas is perhaps the least 
capacitive (least area, largest separation) and therefore fastest physical 
structure available. 
Arc extinction also appears to be fast, although the mechanism is somewhat 
different. At the time of filing the present continuation-in-part 
application, it is the inventor's belief that arc extinction is actually 
the functional reverse of avalanche breakdown. At the end of the arc, just 
before it extinguishes, a path of ionized atoms exists. When the field 
drops to below the level that can support continued current flow (i.e., in 
the vicinity of the zero-crossings of the sinusoidal current waveform), 
these ions will absorb the electrons remaining in the conduction path and 
thereby rapidly extinguish the arc. It appears likely to the inventor that 
this process is somewhat slower than avalanche ignition--because a 
pre-existing arc will consist of a column of ionized atoms, such that it 
will take some time for all conductive paths to become electrically 
neutral--but no tests to determine whether this is the case have yet been 
carried out. 
In the parent application and issued patent identified above, the 
inventor's recognition of the significance of the fast initiation and 
extinction of the arc current was apparent, in that the arc detectors 
disclosed and claimed therein detected arcs by identifying sharply-defined 
gaps in the high frequency noise due to arcing occurring at each 
zero-crossing of the waveform. However, it now appears to the inventor 
that the abrupt change in current at the start and/or end of each burst of 
arcing is itself enough to reliably identify the noise as having 
originated from an arc. To thus identify arcs would be advantageous: 
detection of gaps synchronous to the waveform requires that some minimum 
amount of noise present, such that a chance of fire would be present. 
Detection of intermittent arcing responsive to analysis of a minimum-noise 
avalanche effect would greatly reduce this possibility, would increase the 
detection sensitivity of the instrument, and would be practicable, 
assuming no other noise source were encountered capable of generating such 
rapid changes in the line current. 
Initial testing (see below) indicates that lamp dimmers, the most commonly 
found source of fast rise time noise, produce impulses with a rise time of 
about 1 .mu.s. Since lamp dimmers themselves employ avalanche-mode devices 
as above, it is either the internal capacitance of the device or an 
external snubber that slows down the change in current. An in-line spark 
on a 150 watt light bulb, on the other hand, produces an initial rise time 
of less than 5 ns (5 ns being the minimum response time of the 
instrumentation used for these tests) each time the arc re-ignites. Thus, 
it is likely that this difference in noise current rise time--nearly three 
orders of magnitude--can be used to differentiate arc noise from other 
extraneous noise. 
Tests performed prior to filing this application consisted of measuring the 
high-frequency current on the high side of the line feeding one of two 
loads, a 150 watt halogen floor lamp with an internal dimmer or a standard 
150 watt light bulb through a carbon spark gap. The length of the cords 
between the measurement and the load was about 10 feet in both cases. FIG. 
14 shows the result. In FIG. 14, trace 150 is the impulse noise from the 
lamp dimmer, and trace 151 is the high-frequency noise produced by the arc 
in series with the 150 watt light bulb. 
It will be apparent from FIG. 14 that an initial pulse is present at the 
beginning of the noise burst exhibited by trace that is much higher in 
amplitude than the noise itself. The initial rise time of the arc noise 
was measured to be 10 ns, indicating frequency components on the order of 
100 MHz. The rise time of the dimmer impulse, on the other hand, is about 
1 .mu.s indicating frequency components no higher than about 1 MHz. 
Clearly, if burst rise times or frequencies in these ranges can be 
monitored (and if there are no noise sources commonly found which emit 
such bursts), this characteristic can be used to distinguish arcing from 
such other sources of noise. 
The present inventor has found no reference reporting the RF impedance of 
conventional household power lines providing results above 30 MHz. 
However, the results of measurements reported in J. R. Nicholson et al, 
"RF Impedance of Power Lines and Line Impedance Stabilization Networks in 
Conducted Interference Measurements", IEEE Transactions on Electromagnetic 
Compatibility (May 1973) and reproduced in FIG. 15, show that the maximum 
average impedance, approximately 100.OMEGA., occurs at 1 MHz and above, 
and remains approximately constant at least up to 30 MHz. This impedance 
of approximately 100.OMEGA. at high frequencies is more than sufficient 
for good transmission. Accordingly, it is feasible to monitor arcing in 
household power lines by examining the noise for frequency components in 
excess of 10 MHz, and possibly up to and above 100 MHz. While standing 
wave effects become substantial at frequencies much above 10 MHz, the 
effect being unpredictable cancellation and reinforcement of particular 
frequencies, i.e., of coherent signals, the bandwidth of the noise due to 
arcing is broad and therefore the total response of the detector of the 
invention will be little affected. 
In the parent application and issued U.S. Pat. No. 5,223,795 referred to 
above, bandwidth of arc noise is recognized as a key element of the 
detection methods. The present improvement, however, recognizes further 
that intermittent arcing will always occur in well-defined bursts and that 
the initial rate of change of the current, that is, the burst rise time, 
will be extremely high, producing frequency components at 10 MHz and 
above. 
A second inherent characteristic of intermittent arc-generated noise is 
that, as the arcing becomes more and more persistent, the bursts of noise 
produced will tend to be nonuniform in duration and spacing until the 
arcing becomes substantially continuous, i.e., until the bursts fill the 
temporal interval between the gaps always present at the zero-crossings of 
the current waveform. Nonuniformity of the bursts is particularly the case 
with intermittent arcing on relatively low-voltage systems, e.g., 120 volt 
residential systems. The reason is generally as follows. 
Arcing will occur between any pair of opposed conductors--e.g., between a 
worn plug contact and the corresponding contact on the receptacle--when 
the electric field strength is high enough to allow electrons to escape 
the thermionic barrier of the contact material. In a low-voltage system, 
e.g., 120 volts, the necessary field strength can only occur if the gap is 
small, on the order of a few thousandths of an inch or less. Because of 
minute surface irregularities at the contact junction, the first arc will 
occur at that asperity that is closest to the other contact, i.e., where 
the field strength is the highest. The instant an arc jumps this gap, the 
field strength across the surface of the contacts drops dramatically due 
to the low initial resistance of the arc. The resulting field strength is 
not sufficient to initiate an arc at another site and consequently the 
majority of the current will attempt to flow along the path of the first 
site. The extreme current constriction at this point rapidly melts the 
metal and either causes a stable metallic bridge to form, thereby 
establishing contact, or causes the site to "explode", thereby breaking 
the connection. 
If a connection is made having a conductive area sufficient to handle the 
current flow, arcing ceases. If the connection breaks, the field strength 
immediately rises again and another arc forms at a new location on the 
contact surface. This opportunistic arc site meandering is highly chaotic 
and occurs very rapidly. Only when a stable bridge is formed or when a new 
site is no longer available does the arcing event cease. Thus, the 
duration of the entire arcing event is dependent on extremely rapid, 
random physical changes occurring at the contact interfaces, and therefore 
itself tends to be random. 
A third characteristic of intermittent arc-generated noise is that the 
bursts of noise will generally not be synchronous to the line frequency, 
i.e., they will occur at nonuniform intervals. While there may be some 
tendency for an arc to re-ignite in the same vicinity on the voltage 
waveform as it did the previous cycle, e.g., at the peak, again the 
microscopic processes at the contact junction are so energetic and locally 
destructive that the precise point of re-ignition will vary considerably 
from cycle to cycle. This is a particularly useful quality in terms of 
discriminating arc noise from extraneous noise because other sources of 
impulse noise, e.g., dimmers, are necessarily synchronous to the line 
frequency. 
The fourth and final characteristic of intermittent arc-generated noise is 
that arc bursts will not occur in the immediate vicinity of the current 
zero-crossings, i.e., the gaps as described in the parent application and 
issued U.S. Pat. No. 5,223,795 referred to above will always be present. 
Accordingly, the preferred method of detecting intermittent contact arcing 
according to the present continuation-in-part application consists of 
determining if the noise exhibits one or more of the following 
quantifiable characteristics: 
1. The noise occurs in sharply defined bursts. 
2. The rate of change of the current is above a predetermined level. 
3. Significant high-frequency components (e.g., &gt;10 MHz) are present. 
4. The width of successive noise bursts exhibits nonuniformity. 
5. The intervals between successive bursts exhibit nonuniformity. 
Furthermore, possible arcing events can be disqualified if they are 
determined to take place in the "gaps", i.e., in the vicinity of the 
zero-crossings of the current waveform. 
In the preferred embodiment of the present invention, a microprocessor is 
used to qualify these features and determine if intermittent arcing is 
present. The circuit is essentially identical to that described in the 
parent application, e.g., as shown in FIG. 9. The microprocessor can 
simultaneously or alternatingly carry out the methods for detecting 
substantially continuous arcing disclosed and claimed in the parent 
application and issued U.S. Pat. No. 5,223,795. 
The determinations of whether the noise occurs in sharply defined bursts, 
whether the rate of change of the current exceeds a predetermined level, 
or whether significant high frequency components are present--that is, 
testing for characteristics 1-3 above--can each be made based on the rate 
of change of the current; that is, these characteristics are substantially 
equivalent. 
To measure the rate of change of current, the current sensing transformer 
can be made to respond to the derivative of the current by simply not 
loading its secondary winding. 
Current transformers operate by magnetically coupling the power line 
conductor to be monitored to a secondary coil driving a load resistor, R. 
A time-varying current I.sub.p flowing in the power line conductor 
produces a flux .phi. through the coil, and this flux in turn induces a 
voltage across it. The resulting current in the load resistor, I.sub.L, is 
governed by the following differential equation. 
##EQU2## 
In a standard current transformer, the load resistor R is kept small, so 
that: 
##EQU3## 
When this is the case, the current through the load resistor will be 
approximately proportional to the flux and therefore the primary current: 
##EQU4## 
If, however, the load resistor R is made large, then: 
##EQU5## 
and the current through the load resistor will now be approximately 
proportional to the rate of change of the flux and therefore the rate of 
change of the primary current: 
##EQU6## 
Thus, by simply operating the current transformer with a high load 
resistance, the output can be made to respond to the derivative of the 
primary current. A current transformer used in this manner is often 
referred to as a Rogowski coil (see Pellinen et al, "Rogowski coil for 
measuring fast, high-level pulsed currents", Rev. Sci. Inst. 51(11), 
November 1980), and has found wide application for measuring currents in 
high-current, particle physics. The Rogowski coil uses an air core to 
prevent saturation but the equations apply to any core material. 
When the derivative of the current has thus been determined, the 
microprocessor can compare the derivative of the current to a 
predetermined level and provide a signal that arcing may possibly be 
present when that derivative exceeds the predetermined level. 
The signal that an arcing event may possibly have been detected is then 
preferably processed according to one or more further tests to distinguish 
the possible arcing event from other sources of bursts of high-frequency 
noise which may exist on the line. Several different tests may be 
performed. For example, as indicated above, intermittent arcing exhibits a 
characteristic of nonuniformity in that the bursts are irregular in 
length, are spaced irregularly, and are not synchronous to the power 
waveform; at the same time, intermittent arcing can only take place when 
the electric field available is sufficient to cause ionization to take 
place as described above. Therefore if a high-frequency noise burst is 
detected in the vicinity of the zero-crossing of the current waveform, it 
can be determined that an arcing event was not, in fact, detected. 
Accordingly, measurements of the width of successive noise bursts and the 
intervals between successive bursts (or both) may be used to provide 
further discrimination, that is, to further determine whether the bursts 
are in fact due to intermittent arcing on the power line. Sources of high 
frequency noise generated by lamp dimmers and the like will necessarily be 
synchronous to the line frequency, so that if bursts of noise are detected 
that are not synchronized to the line frequency, this may be taken as an 
indication that arcing has occurred. However, in the presently preferred 
embodiment these tests are employed only after the bursts of 
high-frequency noise have already been determined to exhibit a very fast 
rate of change of the current, which would in any case disqualify 
high-frequency noise due to lamp dimmers (so far as now known), as 
discussed above. 
The timing of the bursts can also be compared to the zero-crossings of the 
current waveform, in order that high-frequency noise bursts in the near 
vicinity of zero-crossings of the current waveform can be determined to be 
due to some cause other than arcing, again for reasons discussed above. 
It should be realized that, although the method of detection of 
intermittent arcing of the invention is also capable of detecting 
continuous arcing as disclosed in the parent application and issued patent 
referred to above, the present invention may also be provided in a 
practical device employing both methods. 
The methods of detecting arcing by detecting rapid rates of change of 
current provided by the present invention may also be employed as the 
control element of a circuit breaker that responds instantly to the 
initial spark produced during a "hard" short, i.e., a short produced by a 
low resistance connection being placed directly across the power line. In 
this case, an intense arc may occur at the instant of contact but 
extinguish as the connection is made. If the initial arc is energetic 
enough, molten metal may be expelled from the connection thereby causing a 
fire hazard. Since the duration of the arc may be much shorter than a line 
frequency cycle, detection cannot be done using gap analysis methods. 
However, the initial rate of change of the current will be extremely high 
due to the low source impedance; this fact can be used to indicate the 
presence of the short, and to accelerate the tripping of the breaker. 
FIG. 16(a) shows a line frequency view of the current 153 produced by a 
direct short across a 120 volt residential power line together with an 
expanded view (FIG. 16(b)) of the initial high-frequency current pulse 
produced by the arc. The initial rise time of the current 154 is on the 
order of 10 ns or less. It should be noted that the bandwidth of the 
current sensor used falls off above 10 MHz, and that the transformer 
probably saturates at about 10 A, as can be seen by its attenuated 
response, and therefore the actual rise time may be much higher. 
It should be understood that while a distinction is made herein between 
line fault arcing and contact arcing for purposes of clarity, the arc 
detector of the present invention may be applicable to detection of arcs 
due to both conditions. For example, when a relatively high-resistance 
line fault occurs, it may exhibit substantially the same characteristics 
as described for contact faults. 
It should also be understood that while this invention specifically 
addresses prevention of electrical fires in households, offices and the 
like, the same methods are generally applicable to a wide range of 
applications where it is desirable to detect the presence of contact 
arcing. For example, it may be desirable to monitor long utility power 
distribution lines continuously for arcing due to breaks or 
high-resistance shorts and, if detected, provide an alarm or interrupt 
current to the line. Another useful application would be to monitor the 
windings of large electric motors or generators for the presence of 
arcing. The present invention is applicable to the detection of arcing in 
any alternating-current power system. 
Furthermore, while the current document makes reference to 115 V, 60 Hz 
mains power as standard in North America, the present invention will work 
at any line voltage or frequency. 
Finally, it will be appreciated that although the preferred embodiments of 
this invention discussed in detail herein each use a microprocessor to 
analyze high-frequency noise patterns to determine whether arcing exists, 
the same basic detection methods can be implemented in a variety of 
different ways, including analog circuitry without a microprocessor, 
circuitry using a digital signal processor, or in other ways. 
Inasmuch as the present invention is subject to many variations, 
modification and changes in detail, it is intended that all subject matter 
discussed above or shown in the accompanying drawings be interpreted as 
illustrative only and not be taken in a limiting sense.