Abstract:
In an electrical distribution system having an arc management system for removal of arcing voltage from feeder circuits triggered at least in part by light signals, a secondary optical detector monitors a downstream branch circuit breaker in the feeder circuit compartment to produce an optical signal for arc detection more accurately reflecting the possibility of an arcing fault.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to electrical distribution equipment and conductors. The invention relates more particularly to controlling the misdetection of arc faults by arc fault detection systems in electrical systems. 
     2. Discussion of the Known Art 
     Switchgear enclosures are commonly employed in electrical power distribution systems for enclosing circuit interrupters and switching equipment associated with the distribution system. Typically, switchgear enclosures are comprised of a number of individual stacked or adjacent compartments, each of the switchgear compartments receiving electrical power from a power source and distributing the electrical power through a feeder circuit to one or more loads. Generally, each of the switchgear compartments includes circuit breakers or other interrupters for breaking electric power in a particular feeder circuit in response to hazardous current overloads in the circuit, or normal switching events. 
     In addition to current overloads, the switchgear enclosure may encounter other hazardous conditions known as arcing faults. Arcing faults occur when electric current “arcs” or flows through ionized gas between conductors, e.g., between two ends of broken or damaged conductors, or between a conductor and ground in the switchgear enclosure. Arcing faults typically result from corroded, worn or aged wiring or insulation, loose connections and electrical stress caused by repeated overloading, lightning strikes, vermin, dropped wrenches during maintenance, etc. Particularly in medium-to-high voltage power distribution systems, the ionized gas associated with arcing faults may be released at pressures and temperatures sufficient to severely damage or destroy the switchgear equipment and/or cause severe burning injuries or death to operating personnel. 
     Switchgear enclosures generally provide arc-resistant metal switchgear compartments, often with a means for venting the gases from the compartments in the event of an arcing fault. These compartments are designed to withstand the pressures and temperatures of the gases associated with an arcing fault and reduce the likelihood or extent of damage to switchgear equipment by preventing the gases from entering adjacent switchgear compartments. Safety to operating personnel is enhanced by channeling and venting the hot gases away from operating personnel. However, because these systems do not eliminate the generation and release of hot gases associated with arcing faults, they do not completely eliminate the risk of injury to operating personnel and/or damage to the switchgear equipment. 
     Both passive and active arc control means are know in the art. Passive means include directed venting of the arc blast energy and gasses out of the cabinet. Other passive means may include reinforcement of the cabinet structure in an effort to withstand the blast. Due to the extremely high power which is generated in an arcing fault, they are typically more damaging, more quickly, than other types of faults and it is therefore desirable to have a system which will respond to these types of faults very quickly to remove the voltage supplying the arc. Limiting arc fault duration through active systems is particularly important in limiting potential damage from the blast. Active means usually include some form of sensing and a switching mechanism to control the current. Of course, the quicker the arc is sensed and controlled the less harm is likely to be done by the arcing event. 
     One method employed for enhancing the safety and durability of switchgear enclosures in the event of arcing faults, as described in U.S. Pat. No. 5,933,308 to Garzon, is to provide arc-resistant metal switchgear compartments with a fast-acting means for grounding or shunting the source bus current at the feeder breaker for the equipment in the event of an arcing fault condition. This action extinguishes the arc almost immediately by removing the voltage sustaining it, and causes a bolted fault right at the feeder breaker, whether said feeder breaker is the main breaker inside the equipment, or a feeder breaker elsewhere in the electrical system, and the feeder breaker then will break the circuit using its normal protective functions. This is done in Garzon by monitoring the rise rate of the source or main bus current and monitoring the light produced by arcing events in each feeder compartment by optical detectors. The current signal and the optical signal are AND&#39;ed together to produce an arcing fault detection signal which activates an arc diverter mechanism within the appropriate time frame. In another example, as described in U.S. Pat. No. 7,499,251 to Byron, arc fault control is done by detection through a comparison of current on the main bus against the current through the feeder lines, whereby a difference in current gives a first detection signal. The first detection signal can be used directly, or AND&#39;ed with other detection signals from optical detectors on the feeder lines, to provide the activation signal for operating the arc diverter. Other known arcing fault sensing circuits may use only optical detectors. Removal of the voltage sustaining the arc in known systems may be by operation of an arc extinguishing mechanism including the feeder breaker, an arc diverter mechanism, or both, which are responsive to the arc fault detection systems. 
     SUMMARY OF THE INVENTION 
     Operation of an arc extinguishing mechanism such as activating an arc diverter device, opening of the equipment feeder breaker, or both, will cause complete interruption in operation of the equipment, and can be quite stressful to the equipment and electrical system. As such it is desirable to avoid any kind of nuisance operation. 
     A quick, economical, mechanism for controlling and extinguishing arc events with a minimum of false activation would be welcome in the art. In known arc fault detection systems various current sensors and light sensors are used inside the cabinet to detect current anomalies and light. 
     Many conventional arc flash detection systems use a single, wide field of view light sensor per feeder circuit compartment to detect light produced by undesirable arcing flash events. For medium voltage equipment, in which most circuit breakers use vacuum bottles to contain the breaking contacts, this is a perfectly practical solution. But most low voltage equipment uses air-break, i.e. not vacuum or gas filled, circuit breakers which can produce a significant amount of light from their arc chutes or vents when they trip, i.e., operate in their ordinary manner to interrupt fault currents. These breaker trip events produce a magnitude of light that can easily be sufficient to trigger conventionally used light detectors for arcing faults and thus initiate a false arcing fault detection when the system should merely let the breaker trip and isolate the feeder circuit rather than shutting down the system. 
     To that end, the present invention in its various aspects and embodiments teaches and provides an arc management system having a light sensor including a primary, wide-field, optical detector and a secondary, narrow field, optical detector monitoring light in proximity to the circuit breaker (hereinafter sometimes merely referred to for convenience as a “breaker”). In some aspects of the invention the secondary light detector may include a fiber optic member aimed at an exhaust vent or other light emitting opening of the breaker case, for detecting and signaling light emissions of the circuit breaker. By subtracting the breaker-specific light emissions from the primary wide field optical detector for arcing fault light emissions, and determining if the subtracted total light signal is sufficient to generate an optical based trip signal to an arc fault detection system; proper control over inadvertent activation of the arc extinguishing operations, i.e. the arc diverter, feeder breaker, or the like, may be obtained. 
     Some aspects of the invention may be particularly suited or adaptable for arc management around a molded case circuit breaker contained within an enclosed low voltage switch gear cabinet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other advantages of the present disclosure will become apparent upon reading the following detailed description and upon reference to the drawings of which: 
         FIG. 1  is a block diagram of an arcing fault protection system for a switchgear enclosure according to one embodiment of the present invention. 
         FIG. 2  is a detail view of an exemplary embodiment of one suitable arrangement of a light sensor having a narrow field secondary light detector according to some aspects of the invention, being placed in proximity to the arc vent of a molded case circuit breaker. 
         FIG. 3  is a schematic illustration of an arc fault detection system which may be used to generate an arcing fault detection signal in the system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Turning now to the drawings and referring first to  FIG. 1 , there is shown in phantom a switchgear enclosure, generally designated by reference numeral  10 , including individual compartments  10   a ,  10   b ,  10   c  and  10   d , collectively  10 , for housing various components of an electrical distribution system  12  and having an arc fault detection system of various components as further explained below. A power source  14 , which may comprise, for example, a utility company power transformer, supplies power for the distribution system  12  through a main circuit  16 . The main circuit  16  is typically routed through an upstream, i.e. a main or feeder, breaker  18 . The arc fault detection system may include a main current sensor  20  such as a toroidal coil provided for monitoring the main circuit  16  for characteristics of arcing faults, as is known in the art. A source bus  22  connected to the main circuit  16  distributes electrical power from the power source  14  to a plurality of feeder circuits  24   a ,  24   b ,  24   c , each of which is routed through one of the switchgear compartments  10   a - c . Each of the feeder circuits, collectively  24 , typically supplies power to one or more loads (not shown) downstream of the switchgear enclosure  10 . It will be appreciated that the number of feeder circuits  24  shown here, as well as the number of switchgear compartments  10   a - d , is exemplary only, and may be varied according to the particular type and/or application of the switchgear enclosure  10 . 
     The switchgear enclosure  10  typically includes switching and monitoring equipment associated with the respective feeder circuits  24 . For example, in the embodiment shown in  FIG. 1 , the switchgear enclosure  10  includes a plurality of circuit interrupters, here shown as circuit breakers  26   a,b,c  distributed among the compartments  10   a,b,c . Also included are the components of an arc fault detection system including a plurality of light sensors  28   a,b,c  distributed among the compartments. These light sensors, collectively  28 , will each include primary optical detectors  61  such as a typical single, wide field of view detector, to detect light produced by undesirable arc flash, and secondary optical detectors  63  for monitoring the branch circuit breakers  26 . In one embodiment, the primary and secondary optical detectors are contained within a single unit along with associated electronics for combining their signals and determination of a threshold for an optical output signal, as shown in  FIG. 2 , which may provide for ease of manufacture, placement, and control. 
     As shown in the exemplary first compartment  10   a , a secondary optical detector  63 , monitors the breaker  26 . It will be understood that each feeder circuit compartment may have the arrangement described herein for the first compartment. Referring also to the detail view of  FIG. 2 , the circuit breaker  26  may for example be a three phase molded case circuit breaker, provided for interrupting, i.e. breaking, electric power in the respective feeder circuits  24  in response to current overloads. Preferably a fiber optic line  53  is held by a bracket  49  in proximity to an exhaust vent  51  in the case  25  of the circuit breaker  26 , for transmitting arcing light from a trip event occurring within the circuit breaker  26 . The fiber optic line  53  will transmit the light to the secondary detector  63 , thereby placing the detectors  28  out of the path of the arc gases and debris exiting the breaker case  25 . It will be appreciated that either or both optical detectors may be combined with fiber optics if such implementation is desired by the designer. 
     Referencing  FIG. 3 , the secondary detector  63  then generates a signal on its output line  29  to a differential amplifier  47 . The differential amplifier  47  accepts and combines input  57  from the primary optical detector  61  and the input  29  from the secondary optical detector  63  and subtracts the breaker trip event light component of the secondary optical detector from the arcing light component of primary optical detector. The differential amplifier  47  outputs the combined, i.e. subtracted, total light signal  45  to output electronics  48  for a threshold determination; via comparator or the like, and if the threshold is reached, a high output signal is sent from output electronics  48  to the triggering AND gate  40  of the arc fault detection system. 
     The light signal subtraction can be done in any number of conventional ways such as the differential amplifier illustrated, logic gates, a digital controller or the like. Preferably the present light sensor may determine a threshold for outputting an optical signal inside the detector  28  such as by output electronics  48  including a comparator for the differential amplifier signal to a reference level, which can be adjustable, and additional electronics for producing a suitable signal to the AND gate. The output electronics may be suitably configured in a variety of ways by the designer, keeping in mind that speed of operation for arc extinguishment is a primary concern. 
     The optical signal  45  is thus a decision made inside the light sensor  28  that sufficient light has been detected to warrant a trip or actuation of the arc extinguisher. The optical signal  45  may be used separately or may then be ANDed with the current detection signal  27 , as shown, and if both signals are active, the arc extinguisher  65  will be commanded to operate. In accordance with  FIG. 3 , the arc fault detection system will trigger an arc extinguishing mechanism  65 , which, upon receipt of an arcing fault detection signal  34 , quickly removes voltage from all downstream circuits, thereby extinguishing arcing fault currents which may have occurred on any of the feeder circuits  24  before they are permitted to generate gases at dangerous pressures and/or temperatures. The arc diverters  32  and breakers  18  may be referred to herein for convenience either individually or collectively as an “arc extinguisher” when serving that function. The various types of arc extinguisher apparatuses may be used individually or together as indicated by the dotted control line off of line  34  to the arc diverter  32 . The selection of types and operation of the extinguishing apparatus and their location are not central to the invention and are left to the choice of the designer. In some embodiments, for example, the arcing fault currents can be extinguished by the arc diverter  32  shorting the phases together in less than about 4 milliseconds, effectively eliminating the generation of dangerous gases associated with the arcing fault. In some embodiments the feeder breaker may be signaled to trip immediately without waiting for its ordinary overcurrent trip conditions. 
     The disclosed arrangement is intended to minimize the chance that shutdown of the system will occur due to “false” signals because it is unlikely that false signals will be detected by both the current sensor  20  and a light sensor  28 . It will be appreciated that the present system of light sensors with separate optical detectors will work equally well with arc fault detection systems that are triggered only by optical signals. 
     Having thus described a system of control for arc management systems for electrical systems; it will be appreciated that many variations thereon will occur to the artisan upon an understanding of the present invention, which is therefore to be limited only by the appended claims.