Abstract:
An arc flash detection system includes a sensor for determining and responding to the presence of an arc flash condition in electrical equipment by detecting a pressure rise, rate of pressure rise and/or ultraviolet radiation characteristic of an arc flash, and generating a signal in response thereto; and processing means responsive to said signal for operating a protective system to de-energize the electrical equipment within a period of time of sufficiently short duration to prevent a pressure wave from the arc flash from causing unacceptable darn age to equipment or personnel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/853,3992 filed Oct. 24, 2006, and U.S. Provisional Application No. 60/877,344 flied Dec. 27, 2006, the contents of both of these applications being incorporated herein by reference. 
     
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention is directed to a system for detecting incipient arc flash conditions. 
         [0004]    2. Description of the Related Art 
         [0005]    Arc flash is an extremely dangerous, and sometimes lethal, condition arising in electrical equipment wherein an electrical current short circuits across all air gap between conductors. Arcing can occur because of insulation failure, contacting a test probe to the wrong surface, or because of an accidental slip of a tool, etc. Low voltage (e.g. less than 240 volts) and low amperage (e.g., less than 1,000 amps) circuits present negligible risk of arc flash. However, with electrical circuits operating, for example, at several hundred volts and several thousand amps, the energy radiated by an arc flash can be several megawatts. In an arc flash the air becomes ionized, ad metal components are vaporized and blasted outward. Vaporized metal expands to 67,000 times the volume of solid metal and maintains the arc until the circuit is opened. The energy of the arc can create a plasma fireball at a temperature of 20,000° C., four times the temperature of the surface of the sun, which explodes outward, carrying with it bits of molten metal, loose pieces of equipment, and other debris. Personnel within the blast radius can be blown off their feet, suffer broken bones, and punctured organs. The arc flash can ignite clothing and cause burns almost instantaneously which may take months to heal. The intense ultraviolet radiation (U) from the flash can cause damage to the eye. A single arc flash incident can cause millions of dollars of damage to personnel and equipment within a fraction of a second, in addition to the pain and suffering of personnel injured by the flash, as well as their families. 
         [0006]    One of the ways to mitigate the risk of damage and injury is by the use of protective clothing, by restricting work on energized equipment, and procedures mandated by regulatory agencies. 
         [0007]    However, use of protective gear which fully encloses the personnel makes it difficult to perform maintenance operations. Moreover, maintenance may need to be performed on energized equipment. 
         [0008]    An arc flash is terminated by opening the electric circuit to cut off the energy supply. The longer it takes to open the circuit the more energy and damage is propagated by the arc flash. What is needed is a system and method for detecting incipient arc flash conditions and responding thereto in sufficient time to de-energize the circuit before major damage has occurred. By “incipient arc flash” is meant that the arc flash has not progressed to the level of causing extensive damage. Typically, an arc flash is considered incipient if it has not progressed for more than a few milliseconds. 
       SUMMARY OF THE INVENTION 
       [0009]    An arc flash detection system is provided herein. The arc flash detection system comprises: (a) a sensor for determining and responding to the presence of an are flash condition in electrical equipment by detecting a pressure rise exceeding 0.01 psi or a rate of pressure rise characteristic of arc flash, and/or ultraviolet radiation characteristic of an arc flash, and generating a signal in response thereto; and (b) processing means responsive to said signal for operating a protective system to de-energize the electrical equipment within a predetermined period of time. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Various embodiments of the invention are described herein with reference to the drawings wherein; 
           [0011]      FIG. 1  is a schematic diagram illustrating a system for detecting an arc flash; 
           [0012]      FIG. 2  is a schematic illustration sensor for detecting a pressure differential; 
           [0013]      FIG. 3  is a schematic illustration of an alternative embodiment of an optical sensor for detecting are flash conditions; 
           [0014]      FIG. 4 ) is a schematic diagram of a sensor system for detecting and responding to arc flash conditions; and 
           [0015]      FIG. 5  is a schematic diagram of an alternative embodiment of a sensor system for detecting and responding to arc flash conditions. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    Referring now to  FIG. 1 ) in one embodiment of the invention a system  10  is provided for detecting the pressure increase in the vicinity of an arc flash at a level well below that needed to blow the cover panels off electrical equipment, such as a switch board. The arc flash omits a burst of heat and light radiation which heats the air and thus raises the air pressure. This is followed by a pressure wave traveling at about the speed of sound which can attain pressure levels exceeding 40 to 50 psi, depending upon the power feeding the arc. This can easily amount to tons of pressure on a cabinet door which can rip the door off its hinges. The system  10  is adapted to detect a pressure wave from an arc flash exceeding 0.01 psi above atmospheric, or ambient pressure. The system  10  is adapted to send a signal within 1 to 2 milliseconds to a circuit breaker to cut off power to the circuit. The system includes pressure sensors (listed in column  2  of  FIG. 1  under “Pressure Sensing”); interfaces between the pressure sensors and a signal processor (listed in column  3  under “interface to transducer”); signal processors (listed in column  4  under “signal processing”); testing means (listed in column  5  under “Built in Test”); and interfaces to arc flash suppression means (listed in column  6  under “Interface to World”). 
         [0017]    More particularly, there are several ways to use pressure sensing to detect incipient arc flash. A pressure sensor is used to detect pressure rise inside of an enclosure as a means of protecting equipment and personnel against arc flash. The pressure sensor needs to detect pressure change as low as 0.01 psi to allow reaction before the pressure can build up enough to breach the integrity of the switchboard. 
         [0018]    In a first method arc flash pressure is measured directly with an absolute pressure transducer. However, since barometric pressure is always changing, the absolute pressure sensor will change with the barometric pressure. To detect arc flash the absolute pressure transducer must be coupled with a fast microprocessor which continually measures the pressure. The microprocessor includes a clock and calculates the rate of pressure increase. Barometric pressure changes are slow. However, if the microprocessor detects an increase of pressure at a rate which exceeds a predetermined threshold value, an arc flash is indicated. Typically, a rate of pressure increase (Δpsi/millisecond) of above about 0.05 psi/ins is indicative of all arc flash condition. 
         [0019]    Alternatively, a differential pressure sensor, e.g., a pressure transducer  12  (either digital or analog) or a differential pressure switch  13  can be used to detect the difference between the air pressure inside an enclosure and that outside the enclosure. If the pressure inside the enclosure exceeds the outside pressure by a predetermined threshold (e.g., 0.0 psi) the presence of an arc flash is indicated. Typically, pressure ports are required for the pressure sensor to have access to the air pressure inside the equipment compartment and outside the equipment compartment for comparison. 
         [0020]    An additional embodiment of the invention couples a rate of rise pneumatic circuit with a differential pressure transducer  14  or differential pressure switch  15  to detect pressure rise within an enclosure without reference to outside air pressure. 
         [0021]    Referring now to  FIG. 2 , sensor apparatus  100  for detecting the presence of incipient arc flash includes a housing  101  enclosing an interior chamber  102  and having a first orifice  103  and a second orifice  104 . The housing  101  is mounted to the interior surface  108   a  of a compartment  108  in which electrical equipment is housed. First orifice  103  is funnel shaped with the wide end  103   a  of the orifice  103  opening toward the interior chamber  102  and the narrow end  103   b  of the orifice  103  opening toward the interior of the equipment compartment. The orientation and conical shape of the orifice allow the air inside the chamber  102  to exit faster than air can enter it. This means that in the event of a sudden increase of air pressure inside the equipment compartment but outside housing  101 , there will be a time lag before the air pressure can reach equilibrium inside and outside of chamber  102 . 
         [0022]    Second orifice  104  is larger and has a substantially uniform cross section. A differential pressure sensor  105 , which can be a differential pressure transducer or differential pressure switch, is mounted at the exterior end of the second orifice  104 . A first side  105   a  of the differential pressure sensor  105  is at the pressure of air inside the chamber  102 . A second side  105   b  of the differential pressure sensor  105  is at the pressure of air inside the equipment compartment  108  but outside the housing  101 . 
         [0023]    Slow changes of barometric pressure do not cause a significant pressure differential because the air pressure inside the chamber  102  has time to equalize with the air pressure in the equipment compartment. However, in the event of an arc flash, air pressure inside the equipment compartment  105  will exceed the air pressure inside chamber  102 . Thus the differential pressure sensor  105  will detect a pressure difference. If the pressure difference exceeds a predetermined threshold (e.g., 0.01 psi) an arc flash is indicated. 
         [0024]    The advantage of the sensor apparatus  100  is that is can be mounted inside the equipment cabinet  108 . An air pressure port through the side wail of the equipment cabinet is not required because the pressure differential is not measured with respect to ambient air pressure outside of the equipment cabinet. 
         [0025]    Referring again to  FIG. 1 , as can be seen, various means of pressure sensing are listed in column  2 . Likewise various interface means may be employed to transmit signals from the pressure sensing device to a signal processor. If the pressure sensor is an analog device, an analog to digital converter  16 , or an analog to frequency converter  17  may be employed. Alternatively, if the sensor is a digital device or frequency generating device the signal may be conducted directly to a signal processor such as by direct digital transmission  18  or direct frequency transmission  19 . 
         [0026]    In yet another alternative, the pressure sensor can effect contact closure  20  of a pressure switch  25 . 
         [0027]    In yet another embodiment as listed in column  4  of  FIG. 1 , the signal processing can be performed by any of several methods or types of equipment: a microprocessor  21 , a differential amplifier  22 , by level detection  23 , a Schmitt trigger  24 , pressure switch  25  or a comparator  26 . In the case of  21   a  microprocessor is used to rapidly sample the time varying analog output from the analog pressure transducer  11 ,  12 , or  14 . This may be done by the use of an analog to digital converter in the microprocessor or via a separate analog to digital interface circuit such as  16 ,  17 , or  19 . Each reading is compared to the previous readings. If the reading exceeds the ordained threshold the presence of an arc flash is recognized and the microprocessor sends an appropriate signal to open the appropriate circuit breaker. 
         [0028]    Item  22  uses a differential amplifier to determine when the analog output voltage from the pressure transducer exceeds a threshold value. A reference voltage is placed on one input to a differential amplifiers and the analog voltage output from the arc flash pressure sensor is placed on the other input. Anytime the arc flash voltage exceeds the reference voltage the differential amplifier will greatly amplify the difference and produce a signal that can be interfaced to open the appropriate circuit breaker. 
         [0029]    Alternatively, if the transducer or its inter-face circuit produces an output whose frequency varies with pressure then a digital circuit can be constructed to produce and output, whenever the output frequency exceeds a threshold level  23 . 
         [0030]    Alternatively, if the pressure transducer is a pressure switch the output may contain considerable noise that would produce erratic performance. The pressure switch output would be connected to a Schmitt Trigger circuit that will output a single output transition once the pressure switch begins to close and will filter out any contact bounce in the pressure switch. 
         [0031]    A comparator  26  encompasses any means by which an unknown signal from a transducer, be it an analog or frequency signal, is compared to a reference signal such that an output is produced when the unknown signal exceeds that of the reference signal. 
         [0032]    Optionally, the output from a pressure switch can be connected directly to a digital input of a microprocessor  21  and the microprocessor allowed to sample the input at a high rate of speed. Once the microprocessor determines that the switch has remained closed for a predetermined time the microprocessor would open the proper circuit breakers. Additionally, the pressure switch  13  or  15  could have their contact closure  20  directly interfaced  25  to the breaker via analog  29 , or digital  30 ,  31  means. 
         [0033]    Optionally, a built-in testing means listed in column  5  such as a shunt calibration resistor  27  or a parallel electrical contact  28  can be employed. Most pressures transducers that produce an analog voltage output are based upon the use of positioning several sensing elements in the well known Wheatstone bridge configuration. The strain on a diaphragm produces a change in resistance in one or more of the four legs of the Wheatstone bridge. This resistance change is proportional to pressure. If a fixed resistance of a known value is electrically connected in parallel (shunt a portion of the circuit) with one or more of the legs of the bridge it will cause the transducer to produce an output signal identical to a known pressure. This test connection is easy to accomplish via a microprocessor. It will allow testing and calibration of the transducer and its interface and is called shunt calibration  27 . Most analog pressure transducers can be manufactured with a shunt calibration feature. 
         [0034]    Pressure switches contain a diaphragm that moves against a spring, closing a contact and producing a single go/no-go output. Tins output is not amenable to shunt calibration. If this case a remotely operated electrical contact  28  is placed in parallel to the pressure switch contact. While this does not test the pressure switch, it does test the sensor interface and signal processing. 
         [0035]    Finally, an interface such as by analog voltage (0-5V, 1-5V, etc.) or current transmission (4-20 ma, etc.)  29 , digital (RS232, 1(485, contact closure, Ethernet, etc.)  30 , or wireless (e.g., 802.11, radio frequency, Infra red, etc.) transmission  31 , can be employed for communication with control equipment which effects de-energizing of the electric circuit feeding power to the arc flash. For example, circuit breakers can be used to open the circuit. 
         [0036]    The response time of these sensors must be less that a millisecond to detect the phenomena of interest well before any damage occurs. However, the system must be sufficiently selective, for example by optionally including delay features) to distinguish arc flash in various background conditions. For instance, the pressure sensing system must ignore any pressure surges created by the blast from the arc chutes of an air circuit breaker (typically less than 40 ms) and yet operate correctly in the presence of an arc flash. Switchboards containing vacuum breakers do not require such a delay. The larger an arc flash the faster the pressure rise and each of these pressures sensing techniques operates faster with the creation of a larger arc. Therefore the system must incorporate adjustable delay features to allow for various field conditions and applications. 
         [0037]    Referring now to  FIGS. 3 ,  4  and  5  an alternative embodiment of the invention detects incipient arc flash by detection of characteristic wavelengths of ultraviolet (UV) radiation. This embodiment is operational in frill sunlight and can be used, for example, to detect arcing during daytime and outdoors. 
         [0038]    Although the sun produces UV radiation, the shorter wavelengths are largely attenuated by the ozone layer of the atmosphere. Moreover, the light from an arc flash has a higher percentage of short wavelength UV radiation (i.e., about 2000 Å to about 2950 Å) and different spectral characteristics as described below. A sensor adapted to detect short wave UV light is useful to indicate arc flash. More particularly, the sensor  200  of the invention is adapted to be responsive to the wavelengths of light characteristic of vaporized copper which has strong spectral emission lines at about 325 nm. This feature is strongly indicative of an arc flash, which consumes copper (e.g., from copper contacts or wire), as opposed to sunlight, corona flames, incandescent, fluorescent or other sources of light which do not. 
         [0039]    The arc moves rapidly, causing its light intensity to vary with time. If the photo diode signal path contains a high pass filter that rejects relatively slow changes in light due to clouds or ambient artificial lights (i.e., the 120 Hz flicker of fluorescent lights) then only light from the are will be amplified. Thus, one can use the DC amplification of the 325 nm light or one can use the AC amplification of the high pass filtered signal to discriminate the arc signal from ambient signals. Alternatively, one can combine both techniques for additional protection from false signals due to changes in the ambient light conditions. 
         [0040]    Referring now to  FIG. 3 , an arc sensor  2001  which is “solar blind” is schematically illustrated. By “solar blind” it is meant that the sensor can operate without interference from sunlight. The sensor  200  includes a hermetically sealed housing  201  enclosing an interior space. The housing  201  can be of metal or plastic fabrication as long as it is opaque. A lens or window  203  admits light to the interior, UV filter  204  transmits only UV radiation with a wavelength centered at 325 nm+/−5 nm (i.e., maximum transmission is at about 325 nm n) and ranging from about 300 to about 350 nm. A filter suitable for use in the invention is available from Newport Corporation, Irvine, Calif. By using a filter selectively transmissive of the wavelengths of light characteristic of vaporized copper, are flash is distinguished from other sources of light. 
         [0041]    The sensor  200  includes a silicon or silicon carbide photodiode  205  positioned to receive light transmitted through lens  203  and filter  204  and to respond thereto by generating an electrical signal which is transmitted to a circuit board  210  having amplifier(s)  209  and other electronic components. Photodiode  205  is preferably a UV enhanced silicon or a silicon carbide photodiode. Pins  202  are for mechanically and electrically connecting sensor  200  to an arc flash detection system. 
         [0042]    Sensor  200  preferably includes a built-in test mechanism (BIT) to confirm that the sensor is functioning. The BIT mechanism includes one or more light emitting diodes (LEDs)  207  which, upon remote command, emit a light beam  206  which is reflected off of the inside of housing  201  and or filter  204  into the photodiode  205 . This excites the photodiode so as to provide confirmation that the sensor is operational. 
         [0043]    Referring now to  FIG. 4  a typical circuit diagram is illustrated for an arc flash detection system  300 . The sensor circuitry  310  includes means for gain and response time adjustment including capacitor C 1  and resistor R 4 . R 1  is used to set the operating current for the LED. R 2  and R 3  are used to set the sensitivity and dynamic range of the photo diode. C 2  is optional and is present when the circuit is used in an AC amplification mode. C 2  is not present when the circuit is used in a DC amplification mode. Appropriate values for the electrical components are readily determined by those skilled in the art. 
         [0044]    The analog signals from the sensor circuit  310  are sent via lines S- 1  and S- 2  to an arc response control unit  320 . Additional sensors  330  can also be included in the system  300 . Output  0 - 1  from the are response control unit  320  can be sent to arc flash warning systems such as alarms, flashing lights, sirens etc. Output  0 - 2  from the arc response control unit  320  can be used for activating protective systems such as circuit breakers. Power is sent to the are response control unit via line P- 1  and to the sensor circuit  310  via line P- 2 . 
         [0045]    Referring now to  FIG. 5 , an alternative arc flash sensor system  400  is illustrated wherein the se sensor circuit  410  provides output  0 - 3  directly to the circuit breakers to open the circuit breaker in the event of an incipient arc flash. This output can take the form of an analog signal, a logic level signal, a contact closure, or a solid state relay. 
         [0046]    The embodiments of the arc flash detection system herein are adapted to detect an arc flash within less than 1 millisecond from the instant of initiation, and to respond thereto within 1 or 2 milliseconds of said detection by sending a signal for de-energizing of the electrical circuit, such that the arc flash is suppressed as quickly as the circuit breakers permit (typically within about 35 milliseconds from initiation). 
         [0047]    The pressure-responsive sensor (e.g., sensor  100 ) and the optical sensor (e.g., sensor  200 ) can be combined in a parallel arrangement in a system such that activation of either sensor will activate the system to open the circuit breaker(s). This arrangement provides greater back-up for arc flash detection. Alternatively, the sensors  100  and  200  can be combined in series in a system such that both must be activated before the system opens the circuit breaker(s). This arrangement provides added protection against erroneous tripping of the breaker(s) by false positive readings. 
         [0048]    While the above description contains many specifics, these specifics are to be considered as exemplification of various embodiments of the invention and not as limitations. Those skilled in the art will envision other embodiments within the scope and spirit of the invention as defined by the claims appended hereto.