Patent Publication Number: US-2023139908-A1

Title: Electronic trip unit with thermal capacity measurement and display

Description:
BACKGROUND 
     Field 
     The disclosed concept relates generally to circuit interrupters, and in particular, to capturing information about thermal overload events in a circuit interrupter. 
     Background Information 
     Circuit interrupters, such as for example and without limitation, circuit breakers, are typically used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition, a short circuit, or another fault condition, such as an arc fault or a ground fault. Circuit breakers typically include separable contacts. The separable contacts may be operated either manually by way of an operator handle or automatically in response to a detected fault condition. Typically, such circuit breakers include an operating mechanism, which is designed to rapidly open and close the separable contacts, and a trip mechanism, such as a trip unit, which senses a number of fault conditions to trip the breaker automatically. Upon sensing a fault condition, the trip unit causes the operating mechanism to trip open the separable contacts. 
     One category of fault conditions that can cause a circuit breaker trip unit to initiate a trip is an overcurrent thermal overload. Low level currents that are over the limit of the circuit breaker rating are dangerous and can cause insulation breakdowns or fires. These thermal overload conditions are measured by multiplying the square of the overcurrent amperage by the length of time over which the overload occurs to obtain an energy value I 2 t. This I 2 t energy of the thermal overload fault is configured for the application capability and tracked by the circuit breaker trip unit. In addition to providing a mechanism for indicating that a trip was caused by a thermal overload, it is often desirable for a circuit breaker to provide a mechanism for indicating that a thermal overload came close to causing a trip but did not actually cause a trip. Circuit breakers typically include some type of alarm for indicating that a “near-miss” event occurred wherein the breaker came close to tripping due to a thermal overload, however, these alarms are generally binary in nature and only indicate that a near-miss event occurred but do not provide detailed information about how close the breaker came to tripping. This information can be crucial, as the levels of current that trigger a thermal overload alarm can vary widely. For example, a lower thermal overload may not cause a trip for several minutes while a higher thermal overload may cause a trip within seconds. 
     There is thus room for improvement in capturing information about thermal overload events in circuit interrupters. 
     SUMMARY 
     These needs and others are met by embodiments of the disclosed concept in which an electronic trip unit for a circuit interrupter provides information to a user about thermal overload conditions and near-miss tripping events in the circuit interrupter, including how much time remains until a trip will be initiated due to a thermal overload, and what the real-time thermal capacity of the circuit interrupter is after a thermal overload condition ends. 
     In accordance with one aspect of the disclosed concept, an electronic trip unit for a circuit interrupter comprises: a processor and a user interface. The processor includes a timer and is structured to receive an output of a current sensor sensing current flowing through a busbar of the circuit interrupter. The processor is configured to detect a thermal overload condition in the circuit interrupter based on the sensed current, to determine a countdown of how much time remains until a maximum thermal capacity of the circuit interrupter is exceeded after detection of the thermal overload condition, to determine the present thermal capacity of the circuit interrupter upon exiting the thermal overload condition, and to display the countdown and the present thermal capacity on the user interface. The electronic trip unit is configured to initiate a trip of the circuit interrupter if the sensed current exceeds the maximum thermal capacity. 
     In accordance with another aspect of the disclosed concept, a circuit interrupter comprises: a first terminal and a second terminal, a busbar disposed between the first terminal and the second terminal, separable contacts structured to be moveable between a closed position and an open position such that the first and second terminals are electrically disconnected from each other when the separable contacts are in the open position, an operating mechanism structured to open and close the separable contacts, a current sensor configured to sense current flowing through the busbar, and an electronic trip unit structured to actuate the operating mechanism. The electronic trip unit comprises a processor and a user interface. The processor includes a timer and is structured to receive an output of a current sensor sensing current flowing through a busbar of the circuit interrupter. The processor is configured to detect a thermal overload condition in the circuit interrupter based on the sensed current, to determine a countdown of how much time remains until a maximum thermal capacity of the circuit interrupter is exceeded after detection of the thermal overload condition, to determine the present thermal capacity of the circuit interrupter upon exiting the thermal overload condition, and to display the countdown and the present thermal capacity on the user interface. The electronic trip unit is configured to initiate a trip of the circuit interrupter if the sensed current exceeds the maximum thermal capacity. 
     In accordance with another aspect of the disclosed concept, a method of informing a user of a circuit interrupter that a thermal overload condition is present in the circuit interrupter comprises: providing a current sensor and an electronic trip unit, the current sensor being structured to sense current flowing through a busbar of the circuit interrupter, and the electronic trip unit comprising a user interface and a processor. The processor comprises a timer and is structured to receive an output of the current sensor. The method further comprises: detecting, with the processor, a thermal overload condition in the circuit interrupter based on the sensed current; determining, with the processor, after detection of the thermal overload condition, a countdown of how much time remains until a maximum thermal capacity of the circuit interrupter is exceeded; determining, with the processor, upon the current decreasing such that the circuit interrupter exits the thermal overload condition, the present thermal capacity of the circuit interrupter; and displaying the countdown and the present thermal capacity on the user interface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full understanding of the disclosed concept can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
         FIG.  1    is a schematic diagram of a circuit interrupter including a thermal overload detector in accordance with an example embodiment of the disclosed concept; 
         FIG.  2    is an illustrative example of a trip curve that can be used by a thermal overload detector of the circuit interrupter shown in  FIG.  1    in accordance with an example embodiment of the disclosed concept; 
         FIG.  3 A  shows a user interface displaying an example of thermal overload metrics that are provided to a user of the circuit interrupter shown in  FIG.  1   , in accordance with an example embodiment of the disclosed concept; 
         FIG.  3 B  shows the user interface shown in  FIG.  3 B  displaying another example of thermal overload metrics that are provided to a user of the circuit interrupter, in accordance with an example embodiment of the disclosed concept; and 
         FIG.  4    is a is a flow chart of a method for providing detailed information about thermal overload and near-miss tripping events to a user of a circuit interrupter in accordance with an example embodiment of the disclosed concept. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Directional phrases used herein, such as, for example, left, right, front, back, top, bottom and derivatives thereof, relate to the orientation of the elements shown in the drawings and are not limiting upon the claims unless expressly recited therein. 
     As used herein, the singular form of “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. 
     As used herein, the statement that two or more parts or components are “coupled” shall mean that the parts are joined or operate together either directly or indirectly, i.e., through one or more intermediate parts or components, so long as a link occurs. As used herein, “directly coupled” means that two elements are directly in contact with each other. As used herein, “fixedly coupled” or “fixed” means that two components are coupled so as to move as one while maintaining a constant orientation relative to each other. As used herein, “movably coupled” means that two components are coupled so as to allow at least one of the components to move in a manner such that the orientation of the at least one component relative to the other component changes. 
     As employed herein, the term “number” shall mean one or an integer greater than one (i.e., a plurality). 
     As employed herein, the term “processor” shall mean a programmable analog and/or digital device that can store, retrieve and process data; a controller; a control circuit; a computer; a workstation; a personal computer; a microprocessor; a microcontroller; a microcomputer; a central processing unit; a mainframe computer; a mini-computer; a server; a networked processor; or any suitable processing device or apparatus. 
       FIG.  1    is a schematic diagram of a circuit interrupter  10  in accordance with an example embodiment of the disclosed concept. The circuit interrupter  10  includes a first terminal  11 , a second terminal  12 , a line conductor  14  connecting the first terminal  11  and second terminal  12 , separable contacts  16 , and an operating mechanism  18 . The line conductor  14  may be comprised of one or more busbars. The separable contacts  16  are disposed along the line conductor  14  such that tripping open the separable contacts  16  stops current from flowing through the line conductor  14  from the first terminal  11  to the second terminal  12 . The operating mechanism  18  is structured to trip open the separable contacts  16 . 
     The circuit interrupter  10  also includes a current sensor  20  structured and disposed to sense current flowing through the line conductor  14  (i.e., the busbars of the line conductor  14 ). However, it will be appreciated that the current sensor  20  may also be employed to sense current flowing through a neutral conductor without departing from the scope of the disclosed concept. The circuit interrupter  10  further includes an electronic trip unit  22  with a processor  24 . Processor  24  may comprise, for example and without limitation, a microprocessor. The processor  24  includes a thermal overload detection module  26  with a timer  28 , and is structured to receive the output of the current sensor  20  and to detect faults in the circuit interrupter  10  based on the sensed current. In response to detecting a fault, the electronic trip unit  22  is structured to cause the operating mechanism  18  to trip open the separable contacts  16 . The thermal overload detection module  26  encompasses software and/or firmware instructions for executing overload detection functions, as detailed herein with respect to the remaining figures. 
     Referring now to  FIG.  2   , a graph of an overcurrent trip curve  30  is shown. The thermal overload detector  26  of circuit interrupter  10  is configured to determine, in accordance with a trip curve such as trip curve  30 , how long an overcurrent condition should be permitted to persist before the electronic trip unit  22  initiates a trip. The trip curve  30  plots time (t) against amperes squared (I 2 ) and depicts how quickly a trip will be initiated at various overcurrent levels. It should be noted that trip curve  30  is plotted logarithmically on both the x- and y-axes. The x-axis depicts current levels that are expressed as multiples of the current rating of circuit interrupter  10  such that, for a current rating of R, each increment on the x-axis can be expressed as nR, wherein n is an integer. The y-axis denotes the amount of time that has elapsed since the current flowing through the circuit interrupter  10  has reached a given amperage. 
     Still referring to  FIG.  2    and trip curve  30 , three different types of data points reflecting events captured by a thermal overload detector  26  are displayed on the graph, as noted in the legend. The three types of data points displayed are: overload no trip (referred to hereinafter as “overload”), short delay fault no trip (referred to hereinafter as “short delay fault”), and trip. It should be noted that any values falling below trip curve  30  are indicative of current levels and durations that do not cause the trip unit  22  to initiate a trip, and that any values occurring above trip curve  30  are indicative of current levels and durations that do cause the trip unit  22  to initiate a trip. It will be appreciated that it is often desirable for circuit interrupters such as circuit interrupter  10  to have either or both short delay and long delay settings activated so that transient overcurrent conditions do not cause the circuit interrupter to trip, and the presence of both overload and short delay fault data points in  FIG.  2    indicates activation of both short delay and long delay settings. 
     It will be further appreciated that relatively lower overcurrent conditions can be permitted to persist for a longer period of time before initiating a trip, and that relatively higher overcurrent conditions should only be permitted to persist for a short period of time before initiating a trip. The relatively lower overcurrent conditions that can persist for a longer period of time are referred to as overload, and the relatively higher overcurrent conditions that should only persist for a shorter period of time are referred to as short delay faults. The left-hand portion of trip curve  30  as denoted by reference number  32  is the region in which overload faults occur, as data points falling under the trip curve  30  in this region have lower amperage values and correspond to more time having elapsed relative to the right-hand side of the curve  30 . The right-hand portion of trip curve  30  denoted by reference number  34  is the region in which short delay faults occur, as data points falling under the trip curve  30  in this region have higher amperage values and correspond to less time having elapsed relative to the left-hand side of the curve  30 . The innovations of the present disclosure are directed toward activity occurring in the overload region  32  rather than in the short delay fault region  34 . 
     Continuing to refer to  FIG.  2   , as previously stated, the thermal overload detector  26  of circuit interrupter  10  is configured to determine, in accordance with a trip curve such as trip curve  30 , how long an overcurrent condition should be permitted to persist before the electronic trip unit  22  initiates a trip. It is expected that the thermal energy of current levels occurring above trip curve  30  may cause components of the circuit interrupter  10  to melt and/or catch on fire within a relatively short amount of time, which is why the trip data points in  FIG.  2    occur just above the trip curve  30  in both the overload region  32  and the short delay fault region  34 . Current levels below the trip curve  30  are said to be at or below the maximum thermal capacity of the circuit interrupter  10  (it will be appreciated that current levels falling just below the trip curve are considered to be at or near maximum capacity), and current levels above the trip curve  30  are said to exceed the maximum thermal capacity of the circuit interrupter  10 . 
     Any level of current considered high enough to necessitate monitoring is referred to as a pickup level. Current that reaches the magnitude of a pickup level triggers the timer  28  of thermal overload detector  26 . For each given level of current within the area under the trip curve  30  in  FIG.  2   , the given current level can continue to flow for a predetermined length of time (in accordance with the trip curve  30 ), as monitored by timer  28 , before the trip unit  22  initiates a trip. The timer  28  is configured to run for as long as the current remains at or above the pickup level. The length of time that a given pickup level of current can flow encompasses a tolerance level, as denoted by the thickness T of curve  30 . For example and without limitation, if a pickup current of 300 A should generally only be allowed to flow for 100 seconds before the trip unit  22  initiates a trip, for a chosen tolerance level of ±10%, a current of 300 A may cause a trip after flowing for as little as 90 seconds (90% of 100 s) or could flow for as long as 110 seconds (110% of 100 s) before causing a trip, depending on what other factors the trip unit  22  is programmed to take into account before initiating a trip. 
     Still referring to  FIG.  2   , the trip unit  22  determines the thermal energy of the current flowing through the circuit interrupter  10  for current values within the overload region  32  using the following thermal energy formula: 
         K=I   2   t   (1)
 
     wherein I is current in amperes, t is time in seconds, and K is a value directly proportional to the thermal energy of the current. A non-limiting illustrative example of how thermal overload detector  26  uses the thermal energy formula (1) to determine whether a thermal overload condition exists is now provided. In this non-limiting example, the current rating of circuit interrupter  10  is 100 A, the slope of trip curve  30  within the overload region  32  is chosen to be set at 6 times the current rating, and the user of circuit interrupter  10  chooses to set a time delay of 20 seconds for this particular overload condition (i.e. chooses to allow current that is 6 times the magnitude of the rated current of 100 A to persist for up to 20 seconds before the trip unit  22  initiates a trip). Applying formula (1), the thermal energy factor K is determined to be 7,200,000 A 2 s: 
     
       
      
       K=I 
       2 
       t  
      
     
         K =(6*100  A ) 2 *(20 s ) 
         K= 7,200,000  A   2   s    
     The amount of time that other overload currents can persist for this particular set of conditions in this example is then determined based on the K value of 7,200,000 A 2 s. For instance, an overload of 200 A could persist for 180 seconds based on the K factor of 7,200,000 A 2 s: 
     
       
      
       K=I 
       2 
       t  
      
     
       7,200,000  A   2   s =(200 A ) 2   *t    
         t= 180  s    
     As the preceding example demonstrates, the value of the K factor changes in accordance with the length of the overload delay chosen by the user and the slope of the trip curve  30  in the overload region  32 . It should be noted that, because the user of the circuit interrupter is able to choose the length of an overload delay, not only does the processor  24  use the trip curve  30  to determine how long a pickup current can safely flow, but prior to the circuit interrupter  10  being put into service, the processor  24  actually chooses the specific trip curve  30  or generates the values for the specific trip curve  30  that corresponds to the time delay chosen by the user. In an exemplary embodiment of the disclosed concept, the circuit interrupter  10  is configured to provide the user with a discrete number of preset overload delays to choose from, and it will be appreciated that, for each of the preset delays, the processor  24  can simply be programmed to store the specific trip curve  30  corresponding to the preset delay. However, it will be appreciated that the circuit interrupter  10  can alternatively be configured to allow the user to choose the length of the overload delay within a continuous range of delay lengths, and that the processor  24  can be configured to generate the values of the trip curve  30  after the user has chosen the overload delay length (since the number of possible overload delay lengths would be much higher and storing so many corresponding trip curves  30  may not be an optimal use of memory). In addition, it will be appreciated that the circuit interrupter can also be structured to allow the user to determine the maximum magnitude of current that can flow during an overload, and that the slope of trip curve  30  in the overload region  32  would be adjusted correspondingly. 
     Referring now to  FIGS.  3 A and  3 B , the circuit interrupter  10  comprises a user interface  40  configured to be in electrical communication with processor  24  and to display various metrics associated with thermal overload events and compiled by thermal overload detector  26 . The user interface  40  can, for example and without limitation, be included in the electronic trip unit  22 . In a first non-limiting example, if the current flowing through the circuit interrupter  10  has reached a pickup level, the processor  24  is configured to use the user interface  40  to display a countdown indicating how much time remains until the thermal capacity of the circuit interrupter will be exceeded (as determined by the I 2 t energy calculated for the overload event) and the trip unit  22  will initiate a trip (if the current does not decrease below a pickup level before the countdown ends), as shown in  FIG.  3 A . In a second non-limiting example, after a pickup level current has exited pickup, i.e. decreased below a pickup level before causing trip unit  22  to initiate a trip, the processor  24  is configured to use the user interface  40  to display the present thermal capacity of the circuit interrupter  10  as a percentage of the maximum thermal capacity, as shown in  FIG.  3 B . It will be appreciated that the processor  24  is configured to continually update the present thermal capacity as the current level continues to decrease or otherwise change after exiting thermal overload. The levels of current just beneath the trip curve  30  are considered to represent the maximum thermal capacity of the circuit interrupter  10 . Knowing the present thermal capacity of the circuit interrupter  10  is valuable for a user because thermal capacity signifies the ability or lack thereof of the circuit interrupter  10  to handle another overload event. This is particularly true when the user is testing the trip unit  22 , as the user needs to know after exiting pickup when the unit has fully cooled and is able to handle another overload. 
     The examples provided in  FIGS.  3 A and  3 B  are illustrative of the benefits provided by the systems and methods disclosed herein, as existing circuit breakers provide limited information about thermal overload conditions and near-miss tripping events, i.e. events in which an overload has occurred but is not great enough cause the breaker to trip. In existing breakers, alarm event captures may be generated during near-miss events, but these captures do not provide detailed information about how close the breaker came to tripping. A snapshot is taken at the time of the alarm, but the duration of the overload and behavior of the current during the near-miss overload is not captured. In these existing systems, when the current through the breaker reaches a pickup level, the pickup is typically indicated by a binary indication system such as a single LED lighting up. With such an indication system, when the LED lights up, the user is only alerted to the fact that an overload has occurred but does receive information regarding how soon tripping will occur. This leaves a significant gap in the information available to the user, as the time remaining until a trip can be anywhere from a few seconds to several minutes, depending on the magnitude of the current. Furthermore, in existing systems, after exiting pickup, a user does not know when the trip unit has fully cooled and is able to handle another overload. 
     Referring again to  FIG.  3 B  and as previously stated, in a simplified sense, the present thermal capacity can be thought of as the thermal capacity associated with the maximum point on the graph of trip curve  30  that the current reached before exiting pickup, i.e. the value of K calculated from the current and time coordinates of the maximum point reached using the thermal capacity formula (1). In a more detailed sense, several factors affect how the processor  24  determines the present thermal capacity, including how much time the thermal overload detector  26  has to sample and process the current readings from the current sensor  20 . Specifically, each cycle of sampling performed by the processor  24  on the measurements provided by the current sensor  20  takes a precise amount of time to complete. The number of current samples used by the processor  24  can be thought of as being collected in a “trip bucket”. The size of the trip bucket is directly related to both the time delay setting chosen by the user for overload conditions and the magnitude of a pickup current. This intuitively makes sense, as setting a longer delay for overload conditions leads to a longer interval of time in which to sample the current while setting a shorter delay accordingly leads to a shorter sampling interval, and a pickup current of lesser magnitude will be able to persist for longer than a current of greater magnitude before a trip is initiated such that the processor can take more samples of the lesser magnitude current (as indicated by the downward slope of the trip curve  30  in the overload region  32 ). Accordingly, the longer the set delay is for overload conditions and the lesser the magnitude is of the pickup current, the more samples of current the processor  24  can take, and the larger the trip bucket is. This means that more data points are available for the calculation of thermal capacity for larger trip buckets than smaller trip buckets. This does not mean that the determinations of thermal capacity made by processor  24  for shorter delay or greater magnitude pickup current events are not accurate, but simply that less data points are used in such determinations. 
       FIG.  4    is a flowchart of a method  100  for informing a user of a circuit interrupter of the details of thermal overload conditions and near-miss tripping events, in accordance with an example embodiment of the disclosed concept. The method of  FIG.  4    may be employed, for example, with the circuit interrupter  10  shown in  FIG.  1    and the user interface  40  shown in  FIGS.  3 A and  3 B , and with trip curves such as trip curve  30  shown in  FIG.  2   , and is described in conjunction with the circuit interrupter  10 , user interface  40 , and trip curve  30  shown in  FIGS.  1 ,  2 ,  3 A, and  3 B . However, it will be appreciated that the method may be employed in other devices as well without departing from the scope of the disclosed concept. 
     The method begins at  101  where the current sensor  20  is provided and disposed around the line conductor busbar  14  of the circuit interrupter  10  in order to sense the current flowing through the busbar  14 . At  102 , the electronic trip unit  22  is provided such that the thermal overload detector  26  is configured to receive the output of the current sensor  20 , and the thermal overload detector  26  is programmed with a number of stored preset overload delays and a corresponding number of trip curves  30  such that each preset delay has an associated trip curve  30 . At  103 , the thermal overload detector  26  detects a thermal overload condition in the circuit interrupter  10  based on the sensed current and in accordance with the trip curve corresponding to the preset delay chosen by the user. At  104 , the thermal overload detector  26  determines the time remaining until the maximum thermal capacity of the circuit interrupter  10  is exceeded and the processor  24  displays a countdown of the time remaining on the user interface  40 . At  105 , after the current decreases from the pickup level, the thermal overload detector  26  determines the present thermal capacity of the circuit interrupter  10  and the processor  24  displays the present thermal capacity on the user interface  40 . 
     While specific embodiments of the disclosed concept 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 disclosed concept which is to be given the full breadth of the claims appended and any and all equivalents thereof.