Patent Document

BACKGROUND OF THE INVENTION 
     The present invention relates generally to a circuit breaker. More specifically the present invention relates to an arc fault circuit breaker, wherein voltage is sensed across a bimetallic element and processed by current sensing components to detect the existence of an arc fault. 
     Arc fault circuit breakers typically comprise a pair of separable contacts that open (trip) upon sensing an arcing current from line to ground, and/or from line to neutral. Arc fault circuit breakers typically use a differential transformer to measure arcing from line to ground. Detecting arcing from line to neutral is accomplished by detecting rapid changes in load current by measuring voltage drop across a relatively constant resistance, usually a bimetallic element (bimetal). Additionally, during over current conditions (i.e., above rated current) the bimetal heats up and flexes a predetermined distance to engage a primary tripping mechanism and trip the circuit breaker. 
     Components of arc fault circuit breakers are generally assembled into separate compartments as defined by their function. More specifically, mechanical components (e.g., load current carrying and switching components) of each pole are assembled into mechanical compartments, while the current sensing components are assembled into an electronics compartment. In order to connect the compartments, the load current of each pole must be routed from the mechanical compartments into the electronics compartment, through appropriate current sensing devices, and back into the mechanical compartments. Additionally, conductors or sensing lines (e.g., wires connected to the bimetal), must also be routed from the mechanical compartment into the electronics compartment. 
     The bimetal has a dual function. First, it engages the circuit breaker&#39;s primary tripping mechanism to trip the circuit breaker during over current conditions (e.g., above its rated current of 10, 15 or 20 amps). Second, it also detects multiple, instantaneous, high-current arcing (e.g., 70 to 500 amps or more) from line to neutral. 
     For the first function, the bimetal is constructed of a pair of dissimilar metallic strips having different coefficients of expansion. When the bimetal conducts current, the dissimilar metallic strips heat up and expand at different rates, causing the bimetal to flex proportionally to the current conducting through it. The bimetal is calibrated to flex a predetermined distance during over current conditions to engage and activate the tripping mechanism. This, however, requires a relatively large amount of space within an already cramped mechanical compartment to accommodate the free movement of the bimetal. This problem is exacerbated by having too many connections attached to the bimetal which must also be allowed to move freely as the bimetal flexes. Additionally, making too many connections to the bimetal during assembly may bend the bimetal enough to throw it out of calibration. Therefore it is desirable to keep to a minimum, the number of connections to the bimetal. 
     The second function utilizes the relatively constant resistance of the bimetal. The voltage drop across the bimetal, is sensed by sensing lines and processed by circuitry (e.g., a printed circuit board) located in the electronics compartment to detect the arcing. When voltage drops indicative of arcing are detected, the circuitry generates a trip signal to activate the tripping mechanism and trip the circuit breaker. However, voltage drops indicating an arc fault are small and rapid, and can be imitated by electromagnetic interference (EMI) in the sensing lines. If the sensing lines are not properly protected, EMI may cause the sensing circuitry to trip the circuit breaker without the occurrence of arcing (false trip). 
     In order to reduce the effects of EMI on prior art circuit breakers a pair of sensing lines (e.g., wires) are first connected to the printed circuit board at assembly. The lines are then twisted together to offset the effects of EMI before they are routed through appropriate openings into the mechanical compartment, where they are connected across the bimetal. However, the twisting process is labor intensive and problematically adds to the cost of assembly. 
     In an alternative prior art embodiment, a pair of shielded wires (e.g., coaxial cables) are used as sensing lines to reduce the effects of EMI. However, shielded wires are expensive and still require connecting two wires across the bimetal in the cramped mechanical compartment, which can result in disturbing the sensitive calibration of the bimetal. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the invention, an arc fault circuit breaker conducting an electric current to a protected load comprises a pair of separable contacts for interrupting the current to the protected load. A first housing of the circuit breaker has a first compartment enclosing the pair of separable contacts. A second housing of the circuit breaker has a second compartment and a first opening. The second housing is assembled to the first housing to enclose the first compartment. A bimetallic element is disposed within the first compartment and conducts the current therethrough. A stud extends from the bimetallic element into the second compartment through the first opening. A conductor electrically connects to the bimetallic element and is routed into the second compartment through the first opening. The conductor and the stud conduct a voltage signal indicative of the current. A circuit board is disposed within the second compartment, and electrically connects to the conductor and the stud within the second compartment, wherein the circuit board processes the signal. 
     In alternative exemplary embodiment of the invention, the circuit breaker comprises a first conductive path disposed on the circuit board. The first conductive path electrically connects to the stud for conducting the voltage signal. A second conductive path disposed on the circuit board electrically connects to the conductor for conducting the voltage signal. The first and second conductive paths run substantially parallel and proximate to each other for a predetermined distance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings wherein like elements are numbered alike in the several Figures: 
     FIG. 1 is a perspective view of a circuit breaker in an exemplary embodiment of the present invention; 
     FIG. 2 is an exploded view of the mechanical compartment of the circuit breaker of FIG. 1; 
     FIG. 3 is an exploded view of the electronics compartment of the circuit breaker of FIG. 1; and 
     FIG. 4 is schematic view of the printed circuit board of the circuit breaker of FIG. 3 in an exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1,  2 , and  3 , an exemplary embodiment of a fully assembled, single pole, arc fault circuit breaker is shown generally at  10 . Circuit breaker  10  comprises a first housing  12 , a second housing  14 , and a cover  16  that are assembled securely together with a plurality of permanent fasteners (not shown). First housing  12  defines a mechanical compartment  24 , having load current carrying and switching components  26  disposed therein (see FIG.  2 ). Second housing  14  defines an electronics compartment  62 , having current sensing components  72  and neutral current carrying components  74  disposed therein (see FIG.  3 ). A load current from a source (not shown) connects to line connection  38  (see FIG.  2 ), and conducts along the current carrying and switching components  26  to load lug  18  for customer connection to a load (not shown). A neutral current from the load connects to neutral lug  20  (see FIG.  3 ), and conducts along the neutral current carrying components  74  to neutral return wire  22  for customer connection to the source. Arc faults are sensed and processed by sensing components  72 . 
     Referring to FIG. 2, the mechanical compartment  24  is shown in detail. First housing  12  is generally rectangular in shape, and formed of electrical insulative material (i.e., plastic). First housing  12  comprises first insulative tab  28 , first rim  30 , and first side wall  32 . First tab  28  protrudes forwardly from the front of first housing  12  adjacent load lug  18  to provide an insulative barrier. First rim  30  extends around the periphery of first side wall  32 . A first rectangular slot  34  is located in rim  30  at the top and rear of first housing  12  and sized to receive pole handle  36 . First side wall  32  and first rim  30  define the mechanical compartment  24  which includes the load current carrying and switching components  26 . The load current carrying and switching components  26  within the mechanical compartment  24  are electrically connected (e.g., welded, bolted, or crimped) to form a load current path. The load current path begins at line connection  38  where the load current enters the mechanical compartment  24 . Line connection  38  includes a lower tab  40  to connect to a source line (not shown), and a fixed contact  42  which extends downwardly from the upper end of line connection  38 . Blade  44  is pivotally engaged to the first housing  12  and pivotally attached to insulated pole handle  36 . A lower end of blade  44  includes a flat contact point  46  which is forcibly biased against contact point  42  to provide electrical continuity for the load current. Pole handle  36  is pivotally attached to first housing  12  and extends outwardly from mechanical compartment  24  into the electronics compartment  62  (see FIG.  3 ). 
     Blade  44  is electrically connected to a bottom end of bimetal element (bimetal)  50  via braided wire  48 . A top end of bimetal  50  is, in turn, electrically connected to L-shaped strap  52 . L-shaped strap  52  comprises a vertical strap body  54  and a horizontal stud extension  56 . Horizontal stud  56  is substantially perpendicular to vertical strap body  54 , and extends outwardly from mechanical compartment  24  into electronics compartment  62  as shown in FIG.  3 . Load terminal  58  also extends outwardly from the mechanical compartment  24  into electronics compartment  62 . Load terminal  58  is, in turn, electrically connected to the load lug  18 . The load current path conducts the load current from the line connection  38 , through contacts  42  and  46 , through blade  44 , braid  48 , bimetal  50 , and L-shaped strap  52 . At this point, the load current path passes out of the mechanical compartment  24  through horizontal strap extension  56 . The load current path returns to the mechanical compartment  24  through load terminal  58  and out through the load lug  18  to the load. When an arc fault is detected, the pole handle  36  pivots clockwise under the force of a tripping mechanism (not shown), causing blade  44  to pivot and separate contact points  42  and  46 , thereby opening the load current path. 
     Bimetal  50  has a dual function. It engages and activates the primary tripping mechanism (not shown) for tripping the circuit breaker  10  during over current conditions (e.g., above the circuit breaker&#39;s rated current of 10 amps 15 amps or 20 amps). By utilizing the different expansion rates of its bimetal construction, the bimetal is calibrated to flex a predetermined distance at the circuit breaker&#39;s rated current. Once the rated current is exceeded, any additional flexing of the bimetal will engage and activate the tripping mechanism of the circuit breaker. Additionally, bimetal  50  provides relatively constant resistance in series with the current path. Therefore, the voltage drop across the bimetal is indicative of the current in the current path. Arcing from line to neutral results in rapid current changes (e.g., 70 to 500 amps peak) in the current path, which can be sensed as rapidly changing voltage across the bimetal. 
     Detecting arc faults from line to neutral is accomplished by sensing the rapidly changing voltage across the bimetal  50 . The voltage sensed is by electrically connecting (e.g., welding) a single wire (sense line or conductor)  60  from the bottom end of bimetal  50  to the current sensing components  72  in the electronics compartment  62 . Additionally, the top end of bimetal  50  is connected to the current sensing components  72  through the horizontal stud extension  56  to provide a return path for the voltage signal. Advantageously, by utilizing stud extension  56 , the number of sensing lines welded to the bimetal is reduced to a single line  60 , as opposed to a pair of lines in prior art circuit breakers. This significantly reduces the number of connections made to the bimetal during assembly and, consequently, the risk of bending the bimetal and disturbing its sensitive calibration. Also, by reducing the number of connections to the bimetal, the problem of having to accommodate the free movement of the connections as the bimetal flexes is correspondingly reduced. 
     Referring to FIG. 3, the electronics compartment  62  is shown in detail. Second housing  14  is generally rectangular in shape and formed of electrical insulative material, i.e., plastic. Second housing  14  comprises second insulative tab  64 , second rim  66 , and second side wall  68 . Second tab  64  protrudes forwardly from the front of second housing  14  adjacent neutral lug  20  to provide an insulative barrier. Second rim  66  extends around the periphery of second side wall  68 . A second rectangular slot  70  is located in rim  66  and cooperates with slot  34  to receive and secure pole handle  36  when housings  12  and  14  are assembled together. Second side wall  68  and second rim  66  define the electronics compartment  62  which includes the current sensing components  72  and the neutral current carrying components  74 . The second housing  14  is assembled securely against first housing  12  with a plurality of permanent fasteners (not shown). When secured against first housing  12 , second housing  14  encloses mechanical compartment  24  and insulates and secures load lug  18  between tabs  28  and  64 . 
     Second side wall  68  of second housing  14  includes rectangular through holes  76  and  78  and circular through hole  80  to provide openings in the second housing  14  to permit the load terminal  58 , horizontal stud  56  and wire  60  respectively, to extend through to the electronics compartment  62 . The load current path is completed by electrically connecting stud  56  and load terminal  58  to the respective ends of the wire connector  82 . 
     Current sensing components  72  comprise circuit board  84 , which is electrically connected to solenoid  86 , current sensing transformer  90 , and optional current sensing transformer  92 . Printed circuit board  84  is connected across the bimetal  50  by connecting, e.g., welding, square post  94  of printed circuit board  84  to wire connector  82  proximate the electrical connection between wire connector  82  and stud  56 . Additionally, wire  60  from the bottom end of bimetal  50  is connected (e.g., welded) to stake  96  on printed circuit board  84 . When an arc fault occurs from line to neutral, voltage across bimetal  50  changes rapidly. These rapid voltage changes are sensed by wire  60  and stud  56 , which are connected across bimetal  50 . Upon receiving the signals from wire  60  and stud  56 , circuit board  84  amplifies and processes the voltage signal, and provides a trip signal to a solenoid  86  to trip the arc fault circuit breaker  10 . 
     As more particularly discussed hereinafter, conductive paths (traces)  104 ,  105  and  106  on circuit board  84  (as shown in FIG. 4) receive the voltage signal to be processed by circuit board  84 . Traces  104  and  106  are run substantially parallel and proximate to each other. This significantly reduces the effects of EMI on the voltage signals from bimetal  50 , and prevents false trips. Unlike prior art circuit breakers, circuit board  84  advantageously eliminates the requirement to use expensive twisted or shielded (e.g., coaxial) wires to reduce EMI. 
     Solenoid  86  comprises trip rod  88  for engaging the trip mechanism (not shown) to pivot the pole handle  36  in response to the trip signal, and provides the means to trip the circuit breaker  10  under arc fault conditions. That is, when an arc fault is sensed, circuit board  84  generates a trip signal to actuate solenoid  86 , which extends the trip rod  88  to activate the trip mechanism which pivots pole handle  36 . The pole handle  36  pivots, which in turn pivots blade  44  to separate contacts  42  and  46  and thereby opens the load current path. 
     The neutral current carrying components  74  within the electronics compartment  62  are electrically connected (e.g., welded, bolted, or crimped) to form a neutral current path for the neutral current. The neutral current path begins at neutral lug  20  where the neutral current enters the electronics compartment  62 . Neutral lug  20  secures the neutral lead connected to the load (not shown) against neutral terminal  98  to provide electrical continuity thereto. Neutral terminal  98  is electrically connected to neutral return wire  22  via copper braid  100 . Insulated sleeve  102  surrounds a portion of copper braid  100  and provides electrical insulation between copper braid  100  and sense line  60 . Copper braid  100  is routed through the center of sensing transformer  90  such that the flow of the neutral current through the center of transformer  90  is in the opposite direction of the flow of the load current through lead  82 . 
     Both the copper braid  100  of the neutral current path, and wire connector  82  of the load current path are routed through the current sensing transformer  90  to sense fault currents from line to ground as is well known. This is accomplished by routing the flow of the neutral current through the sensing transformer  90  in the opposite direction to the flow of the load current. The total current flow through sensing transformer  90  thus cancels unless an external ground fault current is caused by arcing from line to ground. The resulting differential current, sensed by sensing transformer  90 , is indicative of the ground fault current and is processed by circuit board  84 . Arcing from line to ground is thereby detected. 
     Optional oscillating current transformer  92  is used for ground fault applications where a method is needed to detect improper wiring by the customer (e.g., the neutral current path is wired backwards). Copper braid  100  of the neutral current path is routed through the optional oscillating current transformer  92 . The resulting signal, injected by oscillating current transformer  92  and sensed by current sensing transformer  90 , is indicative of the neutral current resulting from improper wiring, and is processed by circuit board  84 . 
     Referring to FIGS. 3 and 4, a detailed schematic of the conductive paths (traces)  104 ,  105  and  106  on circuit board  84  are shown in FIG.  4 . Wire  60  from the bottom end of bimetal  50  is connected to stake  96 . The voltage signal from the bimetal  50  travels through the stake  96  onto circuit board  84 . Once on the circuit board  84 , the signal travels along the conductive path formed by traces  105  and  106 . Trace  105  (shown as a dotted line) is located on the opposite side of board  84  relative to trace  106 , and connects stake  96  to trace  106  at through-hole  107 . Trace  105  is located on the opposite side of board  84  to avoid contact with other components (not shown). Substantially parallel and proximate to trace  106  is trace  104 , which provides the return path for the voltage signal back through square post  94 . Stud  56  is welded directly to square post  94  and acts as a grounding conductor to carry the voltage signal back to the top end of bimetal  50  through L shaped strap  52  (shown in FIG.  1 ). Preferably, traces  104  and  106  are proximate to each other by a distance ranging from 0.8 mm to 1 mm, and run substantially parallel to each other to their points of termination. By placing traces  104  and  106  substantially parallel and proximate to each other, the effective coupling area (antenna) of traces  104  and  106  is minimized and, therefore, the possibility of EMI coupling is substantially reduced. Additionally, stud  56  further reduces the possibility of EMI coupling by eliminating a wire that would act as an antenna for the input signal. This significantly reduces the possibility of generating false trip signals due to EMI coupling. Advantageously, this eliminates the need to use expensive shielded wire, e.g., coaxial cable, or time consuming twisted pair wire to connect printed circuit board  84  to bimetal  50 . Therefore, the time and cost of assembly is significantly reduced from that of the prior art. 
     While the exemplary embodiment of the conductive paths on the circuit board  84  are shown as traces, one skilled in the art would recognize that the invention can apply to other conductive paths as well, e.g., embedded wires. While the exemplary embodiment of arc fault circuit breaker  10  is shown as a single pole circuit breaker, one skilled in the art would recognize that the invention can apply to multi-pole circuit breakers as well (e.g., two or three pole). 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments failings within the scope of the appended claims.

Technology Category: h