Abstract:
An electrical switching apparatus includes a housing, separable contacts within said housing, an operating mechanism structured to open and close said separable contacts, and a trip assembly cooperating with said operating mechanism to trip open said separable contacts in response to a trip signal. The trip assembly includes a sensor structured to sense current flowing through said separable contacts, a number of breakpoint trip mechanisms providing a number of breakpoint trips in response to a number of time-current functions of said sensed current over a number of ranges of predetermined values of said sensed current, and a mechanism structured to provide said trip signal responsive to said number of breakpoint trips. A method of operating the electrical switching apparatus and a power distribution system incorporating the electrical switching apparatus are also provided.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention is directed to electrical switching apparatus and, more particularly, to circuit interrupters, such as circuit breakers. The invention is also directed to power distribution systems and methods employing electrical switching apparatus.  
         [0003]     2. Background Information  
         [0004]     Electrical switching apparatus include, for example, circuit switching devices and circuit interrupters such as circuit breakers, contactors, motor starters, motor controllers, and other load controllers. Circuit breakers are generally old and well known in the art. Examples of circuit breakers are disclosed in U.S. Pat. Nos. 4,751,606 and 5,341,191. Such circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition.  
         [0005]     Molded case circuit breakers, for example, include separable contacts (e.g., a pair per each phase) which may be operated automatically in response to an overcurrent condition. The separable contacts may also be operated manually by way of a handle disposed on the outside of the circuit breaker. Typically, such circuit breakers include an operating mechanism, which rapidly opens and closes the separable contacts, and a trip assembly, which senses overcurrent conditions. Upon sensing an overcurrent condition, the trip assembly actuates the operating mechanism to a trip position which moves the separable contacts to their open position.  
         [0006]     The trip assembly may employ both a microprocessor and a hardware override circuit to detect an overcurrent condition. In response to an overcurrent condition, the microprocessor and/or the hardware override circuit produce various trip signals which actuate the operating mechanism.  
         [0007]     To coordinate the delay times and performance characteristics of one or more circuit interrupters associated with an electrical distribution system, time-trip curves may be employed. Generally, a time-trip curve is a plot of the desired current response characteristics of the circuit interrupter over time.  FIG. 1 , for example, illustrates a portion of a time-trip curve for a circuit breaker. Typically, the time-trip curve includes various types of overcurrent trip conditions, such as a long delay trip, a short delay trip, an instantaneous trip, and/or a ground fault trip. Each type of overcurrent trip condition may be selectively configured in various manners. For example, the short delay trip may be selectively configured as, without limitation, a fixed time response function and/or an I 2 t response function. For simplicity of illustration, only a fixed time short delay trip-curve function and an instantaneous trip-curve function are shown within the time-trip curves depicted in the figures herein.  
         [0008]     Referring to  FIG. 1 , the current factor (per unit) is shown on the horizontal axis and the time factor (per unit) is shown on the vertical axis. When a current factor between about 10 and 15 persists for a time factor of about 0.1 or longer, a short delay trip condition may exist. Accordingly, the circuit breaker processor generates a short delay trip which actuates the operating mechanism, thereby causing the separable contacts to open.  
         [0009]     At higher current factors, it is desirable to have the circuit breaker trip more quickly. In this example, an instantaneous trip condition may exist when the current factor reaches about 15 or greater. More specifically, the curve moves from a short delay trip portion to the instantaneous trip portion at a current factor of about 15. As a result, the hardware override circuit generates an instantaneous trip which actuates the operating mechanism causing the circuit breaker to trip more quickly for a current factor of 15 or greater. In this example, it takes a time factor of approximately 0.01 for the instantaneous trip to cause the separable contacts to open.  
         [0010]     A common hardware override circuit  1  for generating an instantaneous trip signal is illustrated in  FIG. 2 . A current transformer  2  produces a current that is in proportion to the current flowing through the separable contacts  3  of the circuit breaker which are associated with a conductor  9  of a power distribution system. The output of the current transformer  2  is passed through a bridge rectifier circuit  4  and onto the hardware override circuit  1 . The current transformer  2 , bridge rectifier circuit  4 , and hardware override circuit  1  are part of the trip assembly for the circuit breaker. The current output of the bridge rectifier  4  develops a negative voltage across a burden resistor  5  with respect to the hardware override circuit  1  common. When this negative voltage reaches a predetermined value (e.g., when the current factor reaches an instantaneous trip level), a diode  6  becomes forward biased and a zener diode  7  breaks over (i.e., the voltage exceeds the break voltage of the zener diode  7 ). After breaking over, the zener diode  7  begins to conduct which, in turn, causes the voltage at the inverting input (−) of a comparator  8  to immediately drop to a level which is less than a reference voltage, V ref , which is applied to the non-inverting input (+) of the comparator  8 . As a result, the output signal of the comparator  8  changes states, thereby initiating the trip of the circuit breaker by actuating the operating mechanism (not shown) which, in turn, causes the separable contacts  3  to move to their open position.  
         [0011]     For some applications, however, it may be desirable to maintain the short delay trip characteristics while delaying the generation of the instantaneous trip signal at higher current factors. When, for instance, a motor is started, inrush currents may exceed a current factor of about 15 for a time factor equal to or greater than 0.01. As a result, a trip assembly employing a hardware override  1  as shown in  FIG. 2  may sense that an instantaneous trip condition exists and undesirably generate an instantaneous trip which initiates opening of the separable contacts  3 . Accordingly, a need exists for a circuit interrupter which effectively delays generation of the instantaneous trip signal for certain current factors.  
         [0012]     Depending upon its location within an electric power distribution system, an electrical switching apparatus may be referred to as being “upstream” and/or “downstream” of another electrical switching apparatus. For example, an electrical switching apparatus provided for an intermediate bus may be both downstream of an electrical switching apparatus for a main bus which supplies the intermediate bus, and upstream of an electrical switching apparatus for a distribution circuit which branches from the intermediate bus.  
         [0013]     To coordinate the tripping of multiple electrical switching apparatus in a distribution system, some installations employ zone interlocks in which a downstream electrical switching apparatus sensing a fault sends an interlock signal to an upstream electrical switching apparatus. The interlock signal blocks generation of a trip signal by the upstream electrical switching apparatus for a certain amount of time, thereby providing the downstream electrical switching apparatus time to react to the fault. The use of zone interlocks, however, requires additional cabling between, and complicates the operation of, the electrical switching apparatus.  
         [0014]     Accordingly, a need exists for an electrical switching apparatus having improved coordination of high current interruptions for use in a distribution system.  
       SUMMARY OF THE INVENTION  
       [0015]     These needs and others are met by the present invention, which is directed to an electrical switching apparatus comprising a housing, separable contacts within the housing, an operating mechanism structured to open and close the separable contacts, and a trip assembly cooperating with the operating mechanism to trip open the separable contacts in response to a trip signal. The trip assembly comprises a sensor structured to sense current flowing through the separable contacts, a number of breakpoint trip mechanisms providing a number of breakpoint trips in response to a number of time-current functions of the sensed current over a number of ranges of predetermined values of the sensed current, and a mechanism structured to provide the trip signal responsive to the number of breakpoint trips.  
         [0016]     As another aspect of the invention, a power distribution system comprises a first bus having a first electrical switching apparatus associated therewith and a second bus having a second electrical switching apparatus associated therewith, the second bus being upstream of the first bus, wherein the electrical switching apparatus of the second bus is associated with a time-trip curve having a number of breakpoint trip-curve functions.  
         [0017]     As another aspect of the invention, a method for generating a trip signal in an electrical switching apparatus comprises sensing a current flowing through separable contacts of the electrical switching apparatus, and providing a number of breakpoint trips in response to a number of first time-current functions of the sensed current over a number of ranges of first predetermined values of the sensed current.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:  
         [0019]      FIG. 1  is a portion of a time-trip curve for a circuit interrupter.  
         [0020]      FIG. 2  is a schematic diagram of a hardware override circuit for a circuit interrupter.  
         [0021]      FIG. 3  is a block diagram in schematic form of circuit interrupter according to the present invention.  
         [0022]      FIG. 4  is a schematic diagram of a hardware override circuit for the circuit interrupter of  FIG. 3  according to one embodiment of the present invention.  
         [0023]      FIG. 5  is a portion of a time-trip curve for a circuit interrupter employing the hardware override circuit of  FIG. 4 .  
         [0024]      FIG. 6  is a schematic diagram of a hardware override circuit for the circuit interrupter of  FIG. 3  according to an alternative embodiment of the present invention.  
         [0025]      FIG. 7  is a portion of a time-trip curve for a circuit interrupter employing the hardware override circuit of  FIG. 6 .  
         [0026]      FIG. 8  is a block diagram in schematic form of an electric power distribution system employing a number of circuit interrupters according to the present invention.  
         [0027]      FIG. 9  is a portion of a time-trip curve for a circuit interrupter shown in  FIG. 8 .  
         [0028]      FIG. 10  is a portion of a time-trip curve for a circuit interrupter shown in  FIG. 8 .  
         [0029]      FIG. 11  shows the time-trip curves from  FIGS. 5, 9 , and  10  superimposed on a single graph.  
         [0030]      FIG. 12  illustrates a portion of a time-trip curve for one of each different type of circuit interrupter shown in  FIG. 8  according to another embodiment of the present invention.  
         [0031]      FIG. 13  illustrates a portion of a time-trip curve for one of each different type of circuit interrupter shown in  FIG. 8  according to another embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]     As employed herein, the term “number” shall mean one or more than one, and the singular form of “a”, “an”, and “the” include plural referents unless the context clearly indicates otherwise.  
         [0033]     The statement that two or more parts are “connected” or “coupled” together shall mean, as employed herein, that the parts are joined together either directly or joined together through one or more intermediate parts. Further, the statement that two or more parts are “attached” shall mean that the parts are joined together directly.  
         [0034]     As employed herein, the statement that a part is “electrically interconnected with” one or more other parts shall mean that the parts are directly electrically connected together or are electrically connected together through one or more electrical conductors or generally electrically conductive intermediate parts. Further, as employed herein, the statement that a part is “electrically connected to” one or more other parts shall mean that the parts are directly electrically connected together or are electrically connected together through one or more electrical conductors.  
         [0035]     A block diagram in schematic form of a molded case circuit breaker  10  is shown in  FIG. 3 . The circuit breaker  10  includes a housing  10 A in which a number of separable contacts  11  (e.g., a pair per each phase or power line) are contained. As shown in  FIG. 3 , separable contacts  11  are associated with a conductor  9  of a power distribution system (only one phase of which is illustrated). The separable contacts  11  may be operated automatically in response to an overcurrent condition. The separable contacts  11  may also be operated manually by way of a handle (not shown) disposed on the outside of the circuit breaker  10 . Typically, such circuit breakers  10  include an operating mechanism  12 , which rapidly opens and closes the separable contacts  11 , and a trip assembly  13 , which senses overcurrent conditions. Upon sensing an overcurrent condition, the trip assembly  13  actuates the operating mechanism  12  to a trip position which moves the separable contacts  11  to their open position.  
         [0036]     In the current embodiment, trip assembly  13  employs both a microprocessor  14  and a hardware override circuit  15  to detect an overcurrent condition and/or to actuate the operating mechanism  12 . As shown in  FIG. 3 , the trip assembly  13  includes a number of sensors such as, and without limitation, a current transformer  17  which provides, to a rectifier circuit  16 , a current proportional to the current flowing in conductor  9 . The output of the rectifier circuit  16  is provided to both the microprocessor  14  and the hardware override circuit  15 . In response to an overcurrent condition, the microprocessor  14  and/or the hardware override circuit  15  produce various trip signals (e.g., short delay trip; long delay trip; instantaneous trip; breakpoint trip; etc.) which are provided to a trip device  18 . The trip device  18 , in turn, actuates the operating mechanism  12 .  
         [0037]     It should be noted that the particular arrangement of devices illustrated in  FIG. 3  is exemplary and other arrangements, within in the scope of the invention, are contemplated. For example, although discussed as separate components, it is contemplated that the hardware override circuit  15  and the microprocessor  14  may be implemented as a single device, such as and without limitation, an integrated circuit. Also, one or more power lines or phases may be employed.  
         [0038]     A schematic of the hardware override circuit  15  according to one embodiment of the present invention is illustrated in  FIG. 4 . As discussed above in conjunction with  FIG. 3 , the current transformer  17  produces a current in proportion to the current flowing through separable contacts  11 . The output of the current transformer  17  is supplied to bridge rectifier circuit  16 . The current output of the bridge rectifier circuit  16  is provided to the hardware override circuit  15 , where it develops a negative voltage across a burden resistor  19  with respect to the hardware override circuit  15  common. If the magnitude of this negative voltage is large enough, diode  20  becomes forward biased.  
         [0039]     The hardware override circuit  15  employs an array  30  having zener diode  21 , zener diode  21   a , and resistor  22 . In the exemplary embodiment, zener diode  21  is electrically connected in parallel with the series combination of zener diode  21   a  and resistor  22 . Furthermore, in the exemplary embodiment, the break over voltage for zener diode  21  is greater than the break over voltage of zener diode  21   a . Accordingly, if the negative voltage developed at the burden resistor  19  reaches a first predetermined value, diode  20  becomes forward biased and zener diode  21   a  breaks over (i.e., the voltage exceeds the break voltage of the zener diode  21   a ). Referring briefly to  FIG. 5 , which shows an exemplary portion of a time-trip curve for the hardware override circuit  15  of  FIG. 4 , diode  21   a  breaks over at a current factor of about 15 to 20.  
         [0040]     As a result of diode  21  a breaking over, current flows through resistor  22  which, in conjunction with capacitor  23 , delays generation of a trip signal output by comparator  24 . As seen in  FIG. 5 , the delay in this example is a time factor of about 0.06. More specifically, resistor  22  and capacitor  23  provide an RC time constant which increases the amount of time that it takes for the voltage at the inverting input (−) of comparator  24  to drop below a reference voltage, V ref  (applied at the non-inverting input (+) of comparator  24 ). Once the voltage at the inverting input (−) of comparator  24  drops below V ref , however, the output signal of the comparator  24 , which is provided to the trip device  18  (as shown in  FIG. 3 ), changes states; thereby initiating the trip of the circuit breaker  10 .  
         [0041]     If the negative voltage at burden resistor  19  reaches a second predetermined value, zener diode  21  breaks over (i.e., the voltage exceeds the break voltage of the zener diode  21 ). Referring briefly to  FIG. 5 , diode  21  breaks over at a current factor of about 20 or greater. After breaking over, the zener diode  21  begins to conduct which, in turn, causes the voltage at the inverting input (−) of a comparator  24  to immediately drop to a level which is less than reference voltage, V ref . As a result, the output signal of the comparator  24 , which is provided to the trip device  18  (shown in  FIG. 3 ), changes states thereby initiating the trip of the circuit breaker  10  (i.e., within a time factor of about 0.01).  
         [0042]     It should be noted that the particular arrangement of devices illustrated in  FIG. 4  is exemplary and other arrangements, within in the scope of the invention, are contemplated. For example, diode  20  may be omitted from the hardware override circuit  15  while remaining within the scope of the present invention.  
         [0043]     Referring to  FIG. 5 , the portion of the time-trip curve between the fixed time short delay trip-curve function and the instantaneous trip-curve function is referred to herein as a “breakpoint trip-curve function”. The components of the trip assembly  13  ( FIG. 3 ) which provide a breakpoint trip may be referred to as a “breakpoint trip mechanism”. In the embodiment illustrated in  FIG. 4 , for example and without limitation, zener diode  21  a, resistor  22 , capacitor  23 , and comparator  24  of hardware override circuit  15  form a breakpoint trip mechanism.  
         [0044]     Additionally, the components of the trip assembly  13  which produce an instantaneous trip may be referred to as an “instantaneous trip mechanism”. For example, and without limitation, zener diode  21  and comparator  24  of hardware override circuit  15  form an instantaneous trip mechanism. Furthermore, the components of the trip assembly  13  which produce a short delay trip, may be referred to as a “short delay trip mechanism”. For example, and without limitation, processor  14  forms a short delay trip mechanism. Furthermore, the components of the trip assembly  13  which produce a long delay trip may be referred to as a “long delay trip mechanism”. For example, and without limitation, processor  14  forms a long delay trip mechanism. A number of components of trip assembly  13  may be structured to produce a trip signal (e.g., electrical and/or mechanical) for actuating the operating mechanism  12  in response to one or more of the short delay trip, the breakpoint trip(s), and/or the instantaneous trip. For example, and without limitation, trip device  18  ( FIG. 3 ) is structured to produce this trip signal in the present embodiment.  
         [0045]     Although the instant embodiment is discussed in conjunction with a short delay trip-curve function and an instantaneous trip-curve function, it should be apparent that the breakpoint trip-curve function of the present invention may be used with any trip-curve functions, alone or in combination. For example, and without limitation, it is contemplated that a circuit breaker may employ a time-trip curve having, in addition to a breakpoint trip-curve portion, any one or more of a long delay trip portion, a short delay trip portion, an instantaneous trip portion, and/or a ground fault trip portion. Furthermore, it is contemplated that one or more of these trip-curve functions may be implemented using various (e.g., thermal; magnetic; instantaneous; etc.) devices while remaining within the scope of the present invention.  
         [0046]     Returning to  FIG. 4 , it should be noted that the amount of delay between when zener diode  21   a  breaks over and the output of comparator  24  changes state is selectable by changing the value of resistor  22  and/or the value of capacitor  23 . For example, a circuit breaker  10 ′ ( FIG. 8 ) having a breakpoint trip-curve function ( FIG. 9 ) with a current factor of between about 15 and 20 and a time factor of about 0.04 may be obtained by reducing the resistance of the resistor  22  and/or reducing the capacitance of capacitor  23 . Likewise, a circuit breaker  10 ″ ( FIG. 8 ) having breakpoint trip-curve function ( FIG. 10 ) with a current factor of between about 15 and 20 and a time factor of about 0.02 may be obtained by further reducing the resistance of the resistor  22  and/or the capacitance of capacitor  23 . As will be discussed below in conjunction with  FIG. 8 , these circuit breakers (i.e.,  10 ,  10 ′,  10 ″), each having different overcurrent/time trip characteristics, may be used to coordinate high current interruptions in a power distribution system.  
         [0047]     It should further be noted that the time-trip curve may contain (and the hardware override circuit  15  structured to implement) more than one breakpoint trip-curve function.  FIG. 6 , for example, is a schematic diagram of a hardware override circuit  15 ′ for a circuit breaker (not shown) having more than one breakpoint trip-curve function (as shown in  FIG. 7 ) according to another embodiment of the present invention. Hardware override circuit  15 ′ may be used, for example and without limitation, to coordinate switching apparatus (not shown) in a power distribution system (not shown).  
         [0048]     Hardware override circuit  15 ′ has an array  30 ′ having components which form portions of two breakpoint trip mechanisms. The first breakpoint trip mechanism includes, without limitation, zener diode  21   a ′, resistor  22 ′, capacitor  23 ′, and comparator  24 ′; the second breakpoint trip mechanism includes, without limitation, zener diode  21   b , resistor  22   a , capacitor  23 ′, and comparator  24 ′. In this embodiment, zener diode  21 ′ is electrically connected in parallel with the series combination of zener diode  21   a ′ and resistor  22 ′, and in parallel with series combination of zener diode  21   b  and resistor  22   a.  Although shown with two breakpoint trip mechanisms, any number may be employed while remaining within the scope of the present invention.  
         [0049]     In the exemplary embodiment, the break over voltage for zener diode  21 ′ is greater than the break over voltage of zener diode  21   a ′ which voltage, in turn, is greater than the break over voltage of zener diode  21   b.  Additionally, the resistance value of resistor  22 ′ is less than the resistance value of resistor  22   a.    
         [0050]     Accordingly, if the negative voltage at burden resistor  19 ′ reaches a first predetermined value, diode  20 ′ becomes forward biased and zener diode  21   b  breaks over (i.e., the voltage exceeds the break voltage of the zener diode  21   b ). Referring briefly to  FIG. 7 , which is an exemplary portion of a time-trip curve for the hardware override circuit  15 ′ as shown in  FIG. 6 , diode  21   b  breaks over at a current factor of about 15 to 20. As a result of diode  21   b  breaking over, current flows through resistor  22   a  which, in conjunction with capacitor  23 ′, delays the voltage drop at the inverting input of comparator  24 ′ (and thus, delays generation of the trip signal). As seen in  FIG. 7 , the delay in this example is a time factor of about 0.05. Once the voltage at the inverting input (−) of comparator  24 ′ drops below V ref , however, the output signal of the comparator  24 ′ changes states thereby initiating the trip of the circuit breaker (not shown).  
         [0051]     If the negative voltage at burden resistor  19 ′ reaches a second predetermined value, zener diode  21   a ′ breaks over (i.e., the voltage exceeds the break voltage of the zener diode  21   a ′). Referring briefly to  FIG. 7 , diode  21   a ′ breaks over at a current factor of about 20 to 25. As a result of diode  21   a ′ breaking over, current flows through resistor  22 ′ which, in conjunction with capacitor  23 ′, delays generation of a trip signal. As discussed above, resistor  22 ′ has a resistance value that is less than the resistance value of resistor  22   a.  Thus, as seen in  FIG. 7 , the delay caused by resistor  22 ′ and capacitor  23 ′ (a time factor of about 0.03 in this example) is less than the delay caused by resistor  22   a  and capacitor  23 ′. Once the voltage at the inverting input (−) of comparator  24 ′ does drop below V ref , however, the output signal of the comparator  24 ′ changes states thereby initiating the trip of the circuit breaker.  
         [0052]     If the negative voltage at burden resistor  19 ′ reaches a third predetermined value, zener diode  21 ′ breaks over (i.e., the voltage exceeds the break voltage of the zener diode  21 ′). Referring briefly to  FIG. 7 , diode  21 ′ breaks over at a current factor of about 25 or greater. After breaking over, the zener diode  21 ′ begins to conduct which, in turn, causes the voltage at the inverting input (−) of a comparator  24 ′ to immediately drop to a level which is less than a reference voltage, V ref . As a result, the output signal of the comparator  24 ′ changes states thereby initiating the trip of the circuit breaker (i.e., within a time factor of about 0.01).  
         [0053]     It should be noted that the particular arrangement of devices illustrated in  FIG. 6  is exemplary and other arrangements, within in the scope of the invention, are contemplated. For example, diode  20 ′ may be omitted from the hardware override circuit  15 ′ while remaining within the scope of the present invention  
         [0054]      FIG. 8  illustrates a typical electric power distribution system  25  having a bus  26  which provides power to a number of other buses  27   a ,  27   b  which, in turn, energize a number of other buses  28   a - 28   e . For clarity, and without limitation, the bus  26  is referred to herein as the “main bus”; the buses  27   a - 27   b  are referred to as “intermediate buses”; and the buses  28   a - 28   e  are referred to as “distribution circuits”.  
         [0055]     Distribution circuits  28   a - 28   e  provide power to a number of load devices  29   a  - 29   e.  Often, power transformers (not shown) step down the voltage at various points in the distribution system  25 . An electrical switching apparatus is provided for the main bus  26  (i.e., circuit breaker  10 ), for at least some, if not all, of the intermediate busses  27   a - 27   b  (i.e., circuit breakers  10 ′), and for at least some, if not all, of the distribution circuits  28   a - 28   e  (i.e., circuit breakers  10 ′).  
         [0056]     Depending upon their location within the distribution system  25 , an electrical switching apparatus may be referred to as being “upstream” and/or “downstream” of another electrical switching apparatus. For example, the circuit breaker  10 ′ associated with intermediate bus  27   a  is both downstream of circuit breaker  10  for main bus  26  and upstream of the circuit breakers  10 ″ associated with distribution circuits  29   a - 29   c.    
         [0057]     Each circuit breaker  10 ,  10 ′,  10 ″ has its own overcurrent/time trip characteristic for responding to faults in the distribution system  25 . These overcurrent/time trip characteristics are coordinated through a hierarchical arrangement in order that only the closest protection device above the fault trips to minimize the interruption to service in the distribution system  25 .  
         [0058]     As discussed above, circuit breaker  10  has a breakpoint trip-curve function ( FIG. 5 ) with a current factor of between about 15 and 20 and a time factor of about 0.06; circuit breaker  10 ′ has a breakpoint trip-curve function ( FIG. 9 ) with a current factor of between about 15 and 20 and a time factor of about 0.04; and circuit breaker  10 ″ has a breakpoint trip-curve function ( FIG. 10 ) with a current factor of between about 15 and 20 and a time factor of about 0.02.  FIG. 11  illustrates the time-trip curve of circuit breaker  10  ( FIG. 5 ), the time-trip curve of circuit breaker  10 ′ ( FIG. 9 ), and the time-trip curve of circuit breaker  10 ′ ( FIG. 10 ) superimposed on a single graph.  
         [0059]     Referring to  FIG. 8 , should load  29   a  develop a high current fault, for example, it is desirable to trip only the circuit breaker  10 ″ associated with distribution circuit  28   a  (to which load  29   a  is electrically connected) without tripping the circuit breaker  10 ′ associated with intermediate bus  27   a  and/or circuit breaker  10  associated with main bus  26 . For instance, should the fault at load  29   a  have a current factor of  18 , circuit breaker  10 ″ will trip within a time factor of 0.02. This affords circuit breaker  10 ″ the opportunity to clear the fault associated with load  29   a  without circuit breaker  10 ′ and/or circuit breaker  10  (which have time factors of 0.04 and 0.06, respectively) tripping. By only tripping the circuit breaker  10 ″ associated with distribution circuit  28   a , the power supplied to the other branches (e.g., distribution circuits  28   b - 28   c ) fed by the bus  27   a , as well as the power supplied to the intermediate bus  27   b  fed by the main bus  26 , remain uninterrupted. Although not shown, power distribution system  25  may employ a number of circuit breakers at least one of which has a plurality of breakpoint trip mechanisms.  
         [0060]     Additionally, the power distribution system  25  may employ a number of circuit breakers which have the same breakpoint delay time factors, but different pick-up current factors.  FIG. 12 , for example, illustrates the time-trip curves of three circuit breakers ( 10   a ,  10   b ,  10   c ) superimposed on a single graph. As shown in  FIG. 12 , each of these circuit breakers ( 10   a ,  10   b ,  10   c ) have a breakpoint delay time factor of approximately 0.01, however, circuit breaker  10   c  picks-up at a current factor of between about 12 and 17, circuit breaker  10   b  picks-up at a current factor of between about 17 and 22, and circuit breaker  10   a  picks-up at a current factor of between about 22 and 30.  
         [0061]     Further the power distribution system  25  may employ a number of circuit breakers which have the different breakpoint delay time factors and different pick-up current factors.  FIG. 13 , for example, illustrates the time-trip curves of three circuit breakers ( 10   a ′,  10   b ′,  10   c ′) superimposed on a single graph. As shown in  FIG. 13 , circuit breaker  10   c ′ has a breakpoint delay time factor of approximately 0.02 and picks-up at a current factor of between about 12 and 17, circuit breaker  10   b ′ has a breakpoint delay time factor of approximately 0.04 and picks-up at a current factor of between about 17 and 22, and circuit breaker  10   a ′ has a breakpoint delay time factor of approximately 0.06 and picks-up at a current factor of between about 22 and 30.  
         [0062]     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.