Patent Publication Number: US-10784061-B2

Title: Dynamic coordination of protection devices in electrical distribution systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/076,304, filed on Mar. 21, 2016, and claims the benefit of U.S. Provisional Patent Application No. 62/143,299, filed on Apr. 6, 2015 and U.S. Provisional Patent Application No. 62/301,948, filed on Mar. 1, 2016. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to electrical distribution systems, protection devices used in electrical distribution systems, and methods and apparatus for dynamically coordinating time-current characteristics of protections devices in electrical distribution systems. 
     BACKGROUND OF THE INVENTION 
     Electrical distribution systems distribute electrical power from an electrical power transmission system to electrical power consumers. To protect and isolate electrical loads from abnormal operating conditions and allow electricians and engineers to safely work on and maintain an electrical distribution system, circuit breakers are deployed at various stages in the distribution system. For example, circuit breakers comprise part of the switchgear that is installed within power distribution stations and substations and are installed in panelboards at or near service drops of commercial buildings and residences. 
     A principal function of a circuit breaker is to protect its load and the electrical conductors in the load circuit from overcurrent conditions. In general, there are two types of overcurrent conditions: an “overload” and a “fault.” The National Electrical Code (NEC) defines an “overload” as: “operation of equipment in excess of normal, full-load rating, or a conductor in excess of rated ampacity that when it persists for a sufficient length of time, would cause damage or dangerous overheating.” A “fault” is defined as “an electrical connection, which is made unintentionally, resulting in an excessive amount of overcurrent.” Faults typically produce much higher currents than do overloads, depending on the fault impedance. A fault with no impedance is referred to as a “short circuit” or a “bolted fault.” 
       FIG. 1  is a simplified one-line drawing of a typical electrical distribution system  100 , illustrating how conventional circuit breakers are deployed in the distribution system. Alternating current (AC) power supplied from the secondary winding of a step-down transformer  102  is connected to a first set of circuit breakers within a main distribution panel (MDP)  104 . The first set of circuit breakers in the MDP  104  includes a main circuit breaker, which provides short-circuit and overload protection to all downstream loads in the system. The remaining circuit breakers in the MDP  104  serve to provide fault and overload protection to loads that are either directly connected to the MDP  104 , such as motor load  106 , or to one or more sub-panelboards  108 , which include “downstream” circuit breakers (and possibly other sub-panelboards) that provide fault and overload protection to additional loads, such as motor load  110  and light load  112 . 
     Conventional circuit breakers have been in widespread use for many years. However, there are various challenges and drawbacks relating to their use. One problem relates to the precision, both in terms of time and current, at which they are capable of responding to faults and other overcurrent conditions and the uncertainty that results due to their lack of precision. Conventional circuit breakers are electromechanical in nature and typically use some sort of spring mechanism to control whether line current is allowed to flow into their load circuits. Unfortunately, due to limitations on the magnetics and mechanical design involved, the time it takes, and the current level at which, a conventional circuit breaker trips in response to a fault can vary, even for a circuit breaker that is selected from a group of breakers having the same type and rating, and even among several circuit breakers of the same type and rating provided by the same manufacturer. The time-current precision of a conventional circuit breaker also tends to degrade and deviate over time, due to aging of its electromechanical components. Because of this variability, circuit breaker manufactures will often provide time-current characteristic data for each type and rating of circuit breaker that they manufacture. The time-current characteristic data of the circuit breaker is typically displayed in a two-dimensional logarithmic plot, such as illustrated in  FIG. 2 , with current on the horizontal axis, time on the vertical axis, and “tripped” and “not tripped” regions separated by an uncertainty band within which the trip status of the circuit breaker is uncertain. 
     In an effort to address the time-current uncertainties of conventional circuit breakers, electricians and engineers will often perform what is known as a “selective coordination study” when designing an electrical distribution system. The selective coordination study is usually performed prior to the electrical distribution system being constructed. The goal of the selective coordination study is to select and map circuit breakers in the distribution system design so that only the closest circuit breaker upstream from a fault or overload condition will trip in response to a fault or overload condition. A successful selective coordination study will help to ensure that only those sections of the electrical distribution system that are downstream from the source of the fault or overload condition are isolated and de-energized, allowing the remaining upstream sections of the distribution system to continue operating, despite the fault or overload condition. 
     A selective coordination study is performed taking into consideration the time-current characteristic data provided by the circuit breaker manufacturers. During the study, circuit breakers of different types and amperage ratings are selected and mapped into the design with the goal of preventing the uncertainty bands of the various circuit breakers from overlapping. Overlapping bands is undesirable since it provides an indication that one or more upstream circuit breakers may unwantedly or prematurely trip in response to a fault or overload condition, instead of a downstream breaker that is closer to the source of the fault or overload condition and which could otherwise fully isolate the fault or overload condition on its own. 
     There are software tools available in the prior art that display the uncertainty bands of the various mapped circuit breakers and which can assist electricians and engineers in performing selective coordination studies. Unfortunately, due to the uncertainty bands present in the time-current characteristics of the various mapped circuit breakers, the electrician or engineer will often determine that it is not possible to prevent one or more of the uncertainty bands from overlapping, as illustrated in  FIG. 3 . In order to address this problem, the circuit breakers must be rearranged and/or replaced with circuit breakers of different types and/or ratings. 
     Not only are selective coordination studies cumbersome to perform and time-consuming, they are also prone to error, particularly since human interpretation is involved. For example, when electrical generators and induction motors are part of the system design, assumptions must be made as to how current from such loads might possibly be injected into a fault when a fault occurs. Those assumptions are not always accurate, and the errors that follow, along with other errors that can take place in the selective coordination study, can be unwittingly translated into the actual construction of the electrical distribution system. Moreover, once a selective coordination study has been completed and the study is implemented in hardware, in practice, little adjustment can be made, except for replacing circuit breakers with other types of circuit breakers. Some conventional circuit breakers include mechanical adjustments, which allow the time-current characteristics of the circuit breakers to be manually adjusted once they have been installed. However, those adjustments are often inadequate at preventing the time-current uncertainty bands of the various circuit breakers from overlapping and upstream breakers end up tripping prematurely or unnecessarily, causing a larger portion of the distribution system to be de-energized than is necessary. 
     BRIEF SUMMARY OF THE INVENTION 
     Methods, systems and apparatus for dynamically coordinating the time-current characteristics of a plurality of intelligently-controlled protection devices (PDs) in an electrical distribution system are disclosed. An exemplary dynamically coordinatable electrical distribution system includes a plurality of intelligently-controlled PDs, a communication and control bus (comm/control) bus, and a central computer. The plurality of intelligently-controlled PDs is configured to protect a plurality of associated electrical loads from faults, developing faults, and other undesired electrical anomalies. Each of the PDs further has electrically adjustable time-current characteristics. The intelligently-controlled PDs are communicatively coupled to the comm/control bus and configured to report current data representative of real-time currents flowing through their respective loads to the central computer, via the comm/control bus. The central computer is configured to communicate with the plurality of PDs over the comm/control bus and dynamically coordinate the time-current characteristics of the plurality of PDs based on the current data it receives from the PDs. 
     Further features and advantages of the invention, including a detailed description of the above-summarized and other exemplary embodiments of the invention, will now be described in detail with respect to the accompanying drawings, in which like reference numbers are used to indicate identical or functionally similar elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a one-line drawing of a typical electrical distribution system, illustrating how conventional circuit breakers are deployed in the distribution system; 
         FIG. 2  is a drawing showing the time-current characteristics of a conventional electromechanical circuit breaker; 
         FIG. 3  is a drawing showing the time-current characteristics of several conventional electromechanical circuit breaker, highlighting how the time-current uncertainty bands of the circuit breakers can overlap, even after completing a selective coordination study; 
         FIG. 4  is a one-line drawing of a dynamically coordinatable electrical distribution system, according to an embodiment of the present invention; 
         FIG. 5  is a drawing that depicts one way in which the intelligently-controlled protection devices (PDs) in the dynamically coordinatable electrical distribution system depicted in  FIG. 4  can be implemented, in accordance with one embodiment of the invention; 
         FIG. 6  is a perspective drawing of the PD depicted in  FIG. 5 , illustrating how the PD can be housed within an enclosure and showing other aspects, elements and features of the PD; 
         FIG. 7  is a drawing showing the time-current characteristics of the PD depicted in  FIGS. 5 and 6 ; 
         FIG. 8  is a drawing that depicts one way in which the PDs in the dynamically coordinatable electrical distribution system depicted in  FIG. 4  can be implemented, in accordance with one embodiment of the invention; 
         FIG. 9  is a functional circuit block diagram of the fault detection and response circuitry used in the sense and drive circuit of the PD in  FIG. 8 ; 
         FIG. 10  is a drawing of a flowchart that illustrates a method that the fault detection and response circuitry of the sense and drive circuit of the PD depicted in  FIG. 8  follows in detecting and responding to faults and developing faults; 
         FIG. 11  is a drawing that illustrates how the PD depicted in  FIG. 8  might possibly be modified to produce an intelligently-controlled PD having a mechanical or electromechanically-controlled circuit breaker; 
         FIG. 12  is a perspective drawing of the PD depicted in  FIG. 8 , illustrating how the PD can be housed within an enclosure and showing other aspects, elements and features of the PD; 
         FIG. 13  is drawing of an exploded view of the PD depicted in  FIG. 8 , highlighting the physical attributes of the air-gap disconnect unit included in the PD and the various components involved in its operation; 
         FIG. 14  is a drawing that illustrates how a plurality of PDs like that depicted in  FIG. 8  can be deployed and configured in a panelboard, according to one embodiment of the invention; 
         FIG. 15  is a drawing showing the salient elements of the central computer used in the dynamically coordinatable electrical distribution system depicted in  FIG. 4 ; 
         FIG. 16  is a drawing showing the time-current characteristics of a PD like that depicted in  FIG. 8 , showing the trip-setting parameters t UPPER , t LOWER , i LT , and i MAX  of the PD; 
         FIG. 17A  is a drawing that shows the time-current characteristics of five PDs before being dynamically coordinated; 
         FIG. 17B  is a drawing that shows the time-current characteristics of the same five PDs depicted in  FIG. 17B , after the PDs have been dynamically coordinated using the methods and apparatus of the present invention; 
         FIG. 18  is a drawing of a flowchart that illustrates a method that the central computer is programmed to follow in dynamically coordinating a plurality of PDs in an electrical distribution system, in accordance with one embodiment of the present invention; 
         FIG. 19  is a drawing that illustrates how the panelboard depicted  FIG. 14  can be housed within a panel box; 
         FIG. 20  is drawing depicting a one-line graphical user interface (GUI) page that is displayed on the display of the central computer (in this case, a touchscreen display of a tablet computer) and that a user can view and interact with; 
         FIG. 21  is a drawing depicting a panel GUI page that is displayed on the display of the central computer (in this case, a touchscreen display of a tablet computer) and that a user can view and interact with; 
         FIG. 22  is a drawing of a flowchart that illustrates a method that the central computer is programmed to follow in allowing the user to update display information being displayed on the displays of the PDs and the panel display; 
         FIG. 23  is a drawing depicting a dynamic coordination GUI page that is displayed on the display of the central computer (in this case, a touchscreen display of a tablet computer) and that a user can interact with to assist in dynamically coordinating a plurality of PDs in an electrical distribution system; and 
         FIG. 24  is a drawing of a flowchart that illustrates a method that the central computer performs when a user is interacting with the dynamic coordination GUI page that is displayed on the display of the central computer (in this case, a touchscreen display of a tablet computer) and that a user can interact with to manually coordinate a plurality of PDs in an electrical distribution system. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 4 , there is shown a one-line drawing of a dynamically coordinatable electrical distribution system  400 , according to an embodiment of the present invention. The dynamically coordinatable electrical distribution system  400  may be deployed in the vicinity of the service drop of a building (e.g., a residence, or commercial building), as part of the switchgear in an industrial complex or electrical distribution station or substation, or, in fact, at any stage, section or facility of an electrical power system where a grouping or hierarchy of circuit breakers is desired or necessary to control distribution of power. As illustrated in  FIG. 4 , the dynamically coordinatable electrical distribution system  400  includes a main distribution panel (MDP)  402  and may further include one or more sub-panelboards  404 . The MDP  402  has a service entrance, through which AC power from an input AC power source, such as may be provided at the output of a step-down transformer  406 , for example, connects to a power bus, power cables, or busbars in the MDP  402 . (It should be mentioned that, although in the description that follows an AC electrical distribution system is assumed, the present invention may also be adapted for use in direct current (DC) electrical distribution systems.) Depending on the application, the input AC power may be 3-phase or 1-phase power. A main circuit breaker  408  in the MDP  402  controls whether the received input AC power can be distributed to the remainder of the system. When the main circuit breaker  408  is OFF (i.e., open) the remainder of the system is de-energized and electrically isolated from the input AC power. When the main circuit breaker  408  is ON (i.e., closed) input AC power is allowed to be distributed to inputs of intelligently-controlled circuit breakers  410  in the MDP  402 . These intelligently controlled circuit breakers  410  are referred to as “intelligently-controlled PDs,” “protection devices,” or most succinctly as “PDs” in the detailed description that follows. The descriptor “intelligently-controlled” is used to highlight the fact that the PDs function, and are of a significantly different construction, than conventional circuit breakers. Note the main circuit breaker  408  may also comprise an intelligently controlled PD. Alternatively, it may comprise a conventional circuit breaker. 
     As shown in  FIG. 4 , AC power from the MDP  402  is distributed, via one or more PDs  410 , to one or more directly-connected loads  412  (depicted in the drawing using black-filled squares) and, if present, to one or more sub-panelboards  404 , each of which further includes its own intelligently-controlled PDs  410  that selectively distribute electrical power downstream to additional loads  412 . 
     The PDs  410  in the MDP  402  and sub-panelboard(s)  404  of the dynamically coordinatable electrical distribution system  400  are further configured so that they are in electrical communication with a communications and control bus (“comm/control bus”)  414 . The comm/control bus  414  may comprise any suitable bus technology, such as, for example, an inter-IC (I2C) bus or controller area network (CAN) bus. As will be explained in further detail below, the comm/control bus  414  provides the ability of the PDs  410  to communicate with, and to be controlled by, a central computer  416 , via a head-end interface  418 . The head-end interface  418  can be implemented in various ways, depending on the type of comm/bus  414  being used and the type of operating system and communication protocol used by the central computer  416 . In one embodiment of the invention, the head-end interface  418  includes an adapter or gateway that allows the central computer  416  to make a wired connection to the head-end interface  418 , for example, using universal serial bus (USB) technology, Ethernet technology, or other wired connection technology. In another embodiment of the invention, the head-end interface  418  includes a wireless transceiver (for example, a Wi-Fi transceiver), which allows a wireless transceiver in the central computer  416  to communicate with the comm/control bus  414  and PDs  410  over a wireless link. As will be discussed in detail below, among other tasks, the central computer  416  serves to analyze current data information collected from the PDs  410 ; compute, set, and adjust, even in real time, the time-current characteristics (e.g., trip current, time-to-trip, and/or amperage ratings) of the various PDs  410 ; and dynamically coordinate the various PDs  410  in the distribution system  400 . 
       FIG. 5  is a drawing that depicts one way in which each of the PDs  410  of the dynamically coordinatable electrical distribution system  400  can be implemented, in accordance with one embodiment of the invention. The exemplary PD  500  comprises a microcontroller  502 , computer-readable media (CRM)  504 ; a solid-state device  506 ; a current sensor  508 ; an AC/DC converter  510 ; user control buttons  512 ; a visual display  514 ; and a maintenance disconnect mechanism  516 . Depending on the design and application, the PD  500  can be a 3-phase device, a 1-phase device, or a DC device. In the case of a 3-phase device, the PD  500  is designed, configured and controlled to measure three current measurements, thereby allowing the system to react to any type of fault, including 3-phase and single-line ground faults. 
     The solid-state device  506  may comprise any suitable controlled solid-state device, such as a silicon-controlled rectifier (SCR), insulated-gate bipolar transistor (IGBT), power metal-oxide-semiconductor field-effect transistor (power MOSFET), etc. 
     The AC/DC converter  510  serves to convert AC power from the input AC line (labeled “Line-IN” in  FIG. 5 ) to DC power for powering the microcontroller  502  and other DC components in the PD  500 . In another embodiment of the PD, a separate and dedicated DC power supply independent of AC line power is used. 
     The microcontroller  502  in the exemplary PD  500  includes one or more input/output ports that allow the PD  500  to connect to the comm/control bus  414 , thereby allowing the central computer  416  to address, identify, communicate with, and control the PD  500 . The microcontroller  502  operates according to computer program instructions stored in the PD&#39;s CRM  504 . The CRM  504  may comprise nonvolatile memory (e.g., flash-memory etc.), a magnetic or optical memory, random access memory (RAM) or any combination of these or other types of computer readable media. The CRM  502  may be entirely external to the microcontroller  502  (as depicted in the  FIG. 5 ) or may be embedded, whole or in part, in the microcontroller  502 . 
     The computer program instructions stored in the CRM  504  are addressable by the microcontroller  502  and when fetched and retrieved from the CRM  504  direct: how and when the microcontroller  502  produces a Gating Disable signal to turn OFF PD&#39;s solid-state device  506  and instructions and/or commands that direct how and when the microcontroller  502  reports information (e.g., current and voltage information relating to its load) over the comm/control bus  414  to the central computer  416 . The computer program instructions may further include instructions that allow the microcontroller  502  to monitor and determine current flow direction through the solid-state device  506 . With this capability, specific sections of the electrical distribution system that may be at risk of reverse current flow, for example, as forced by the back-EMF of induction motors and field current failures on electrical generators, can be de-energized when necessary. 
     The computer program instructions stored in the CRM  504  of the PD  500  may further include instructions that direct: how and when the microcontroller  502  reports identification information to the central computer  416  over the comm/control bus  414  (e.g., physical address, PD model name and number, fed-from information, and the name and type of load being protected by the PD  500 ); how the microcontroller  502  responds to activations of the user control buttons  512 ; and/or how and what kind of information is displayed on the PD&#39;s display  514  such as, for example, amperage rating of the PD  500 , real-time load current and voltage information, PD name, PD model number, fed-from information, and/or any other real-time or non-real-time information. Preferably, the display  514  comprises an electronic ink display, which is a display technology that allows the information that is being displayed to continue to be displayed even after power to the display  514  is removed. 
     It should be mentioned that whereas in the exemplary embodiment of the invention described here, each of the various PDs includes its own dedicated microcontroller  502 , a single microcontroller or microprocessor could be alternatively employed to control a plurality of the PDs  500  in a given locale (for example, a plurality of PDs in each panelboard). 
     As shown in  FIG. 6 , the PD  500  also includes: line connection terminals (Line-IN and Line-OUT)  602  and  604  for connecting the PD  500  to the AC input and load; a comm/control bus connector  606  that connects the PD  500  to the comm/control bus  414 ; a faceplate  608  with cut-outs for receiving the user control buttons  512 , which may include ON and OFF buttons  610  (e.g., green-colored ON button and red-colored OFF button or, alternatively, red-colored ON button and green-colored OFF button) that power-up and power-down the PD  500  and a RESET button (not shown); indicator lights (for example light-emitting diodes (LEDs)  612 , which preferably emit light of different colors for indicating the ON, OFF and TRIP status of the PD  500 ; an optional audible alarm; a cut-out through which the electronic ink display  514  can be viewed; and a maintenance-disconnect tab or latching mechanism  516  that allows electricians to remove the faceplate  608  so that troubleshooting and maintenance can be performed. In one embodiment of the invention, the maintenance-disconnect mechanism  516  is designed so that when the faceplate  608  is removed electrical power is isolated, thereby protecting electricians and anyone else who may come in contact with the PD  500  with the faceplate  608  removed from electrical hazards, and ensuring compliance with lockout/tagout (LOTO) procedures, which may be required by electrical codes. 
     Because the PD  500  employs the solid-state device  506 , it is able to detect and respond to faults, impending faults and other electrical anomalies much more rapidly than is possible if a conventional electromechanical circuit breaker was to be used. The solid-state device  506  has the inherent ability to change states (i.e., to be turned ON and OFF) in a matter of microseconds. By employing the solid-state device  506 , the PD  500  is therefore able to isolate faults and developing faults over a thousand times faster than is possible using a conventional electromechanical circuit breaker, which typically take several milliseconds to respond to and isolate faults and developing faults. 
     In addition to having the ability to isolate faults and developing faults nearly instantaneously, another significant benefit provided by the PD  500  is that its time-current characteristics are much more precise than are the time-current characteristics of conventional electromechanical circuit breakers. Solid-state devices can be manufactured repeatedly to have nearly identical operating characteristics. This repeatability-in-manufacturing capability significantly reduces variability from one solid-state device to another and, consequently, the variability from one PD  500  to another. The current conducted by the solid-state device  506  can also be rapidly controlled and with a much higher degree of precision than is possible in conventional electromechanical circuit breakers. These attributes result in the PD  500  having a time-current characteristic data profile that is represented by a single line, as illustrated in  FIG. 7 . In contrast, and was explained above in reference to  FIG. 2 , conventional electromechanical circuit breakers of the same type and rating, and even of the same type and rating provided by the same manufacturer, have time and current characteristics that tend to vary with a high degree of variability, resulting in uncertainty bands in their time-current characteristics. (Compare  FIG. 7  to  FIG. 2 .) 
       FIG. 8  is a drawing depicting another way in which the PDs  410  of the dynamically coordinatable electrical distribution system  400  can be implemented, in accordance with another embodiment of the invention. The PD  800  is similar to the PD described in co-pending and commonly assigned U.S. Patent Application No. 62/301,948, which is incorporated herein by reference. Like the PD  500  described above in reference to  FIGS. 5 and 6 , the PD  800  includes a microcontroller  802 ; computer-readable media (CRM)  804 ; a solid-state device  806 ; a current sensor  808 ; a DC power source (not shown); user control buttons  810 ; and a display  814 . However, unlike the PD  500 , the PD  800  further includes a sense and drive circuit  816 , which controls the ON/OFF status of the PD&#39;s  800 &#39;s solid-state device  806  (rather than relying on the microcontroller to perform that task) and an air-gap disconnect unit  818 , which is connected in series with the solid-state device  806 , between the Line-IN terminal and line-in input of the solid-state device  806 . The various components of the PD  800  operate similar to the PD  500  described above, except that the sense and drive circuit  816  is employed to detect the occurrence of faults and developing faults and generate the Gating Disable signal to turn the solid-state device  806  OFF when conditions warrant, rather than directly by the microcontroller (as in the PD  500 ). Another difference between the PD  500  and the PD  800  is that the PD  800  includes the air-gap disconnect unit  818 , which as explained in detail below adds an additional level of isolation capability not provided by the PD  500 . 
       FIG. 9  is a functional circuit block diagram of the fault detection and response circuitry  900  used in the sense and drive circuit  816  of the PD  800 , in accordance with one exemplary embodiment of the invention. The fault detection and response circuitry  900  comprises: a differentiator  402 ; first, second and third high/low comparators  904 ,  906 ,  908 ; an AND logic gate  910 ; and an OR gate  912 . The various electrical components of the fault detection and response circuitry  900  are preferably mounted on printed circuit board (PCB), which may be the same PCB upon which the microcontroller  802  is included or may be a separate PCB dedicated for the sense and drive circuit  816 . 
     The fault detection and response circuitry  900  serves to determine whether a sudden increase in current being drawn by the PD&#39;s load circuit is due to a load being brought online or is due to a fault or developing fault. This function is important since it avoids the solid-state device  806  from being turned OFF unnecessarily when the sudden increase in current is due to a load being brought online and not because of fault or developing fault. The fault detection and response circuitry  900  is also capable of distinguishing between resistive and inductive loads and determining whether a sudden increase in current corresponds to an inrush current of an inductive load when being brought online or may be the result of a developing fault.  FIG. 10  is a flowchart that illustrates a method  1000  that the fault detection and response circuitry  900  of the sense and drive circuit  816  follows in performing these various functions. First, at step  1002 , the fault detection and response circuitry  900  receives a sense current i SENSE  from the PD&#39;s current sensor  808 . The sense current i SENSE  represents the real-time line current being drawn by the load circuit that the PD  800  is serving to protect. At decision  1004  the first high/low comparator  904  in the fault detection and response circuitry  900  determines whether the received sense current i SENSE  has exceeded an “instant-trip threshold current” i MAX . The instant-trip threshold current i MAX  establishes the absolute maximum current that the PD  800  will allow to flow into the load circuit, under any circumstance. If the current being drawn into the PD&#39;s load circuit (as represented by the sense current i SENSE ) ever exceeds the instant-trip threshold current i MAX , the first high/low comparator  904  produces a logic HIGH output, which after passing through the OR logic gate  912  will quickly turn the PD&#39;s solid-state device  806  OFF, as indicated by step  1014  in the method. The time it takes to turn the solid-state device  806  OFF is limited only by the propagation delay through the first high/low comparator  904  and the reaction time of the solid-state device  806  in switching from an ON state to an OFF state. The word “instant” is used here to indicate that this time will be on the order of a few microseconds or even less. Immediately after, or as soon as the solid-state device  806  is being directed to turn OFF, at step  1016  in the method  1000  the PD&#39;s microcontroller  802  will send an electrical pulse to a solenoid in the air-gap disconnect unit  818  of the PD  800  (see  FIG. 8 ). The purpose and function of the air-gap disconnect unit  818  will be described in detail below. 
     It should also be emphasized that the various steps and decisions in the method  1000  represented in the flowchart in  FIG. 10  are not necessarily performed in the order shown. Additionally, because various of the operations performed by the fault detection and response circuitry  900  are performed continuously or simultaneously, the various steps and decisions in the flowchart should not be viewed as necessarily being a timed sequence of events. For example, although decision  1004  appears as a discrete step in a sequence of steps and decisions, decision  1004  is actually performed continuously. So is step  1002  and possibly other steps and decisions in the method  1000 . 
     The differentiator  902  in the fault detection and response circuitry  900  serves to differentiate the sense current i SENSE  it receives from the PD&#39;s current sensor  808  and produce a differentiated sense current di SENSE /dt. This step in the method  1000  is indicated by step  1006  in the flowchart. The differentiated sense current di SENSE /dt is the rate of change of the sense current i SENSE  and is used by the fault detection and response circuitry  900  to determine whether a sudden change in sense current i SENSE  is due to a resistive load being brought online or is representative of a developing fault. Because the line current and sense current i SENSE  are AC signals, with positive and negative half cycles, and since a sudden increase in line current (as represented by the sense current i SENSE ) can possibly occur during either positive or negative half cycles, the differentiator  902  differentiates both positive and negative half cycles of the sense current i SENSE . In this way the fault detection and response circuitry  900  can determine whether a fault may be developing during both positive and negative half cycles of the line current. 
     The second and third high/low comparators  906  and  908  and AND logic gate  910  are the components of the fault detection and response circuitry  900  that determine whether a sudden change in sense current i SENSE  is due to a resistive load being brought online or is representative of a developing fault. As alluded to above, the ability to make this distinction is important since it avoids the solid-state device  806  of the PD  800  from being turned OFF unnecessarily or prematurely in the event that a sudden increase in current is due to a resistive load being brought online and not because of an impending fault. As part of making this determination, at decision  1008  in the method  1000 , the third high/low comparator  908  compares the differentiated sense current di SENSE /dt to a predetermined maximum rate of change in current di/dt_max. If at decision  1008  the differentiated sense current di SENSE /dt is determined to exceed the maximum rate of change in current di/dt_max, the third high/low comparator  908  produces a logic HIGH output. The logic HIGH output provides an indication that a fault may be (though not necessarily) developing in the PD&#39;s load circuit. On the other, if at decision  1008  it is determined that the differentiated sense current di SENSE /dt is less than the maximum rate of change in current di/dt_max, the output of the third high/low comparator  908  remains at a logic LOW. 
     It should be emphasized the fault detection and response circuitry  900  will continue to compare the sense current i SENSE  to the instant-trip threshold current i MAX  (at decision  1004 ), regardless of the value of the differentiated sense current di SENSE /dt. As explained above, the first high/low comparator  904  and OR logic gate  912  will direct the solid-state device  806  to immediately turn OFF (at step  1014  in the flowchart) if the sense current i SENSE  ever rises to a level that exceeds the instant-trip threshold current i MAX . In other words, even if it is determined that the differentiated sense current di SENSE /dt is less than the maximum rate of change in current di/dt_max at decision  1008 , the solid-state device  806  will be turned OFF if i SENSE  ever become greater than i MAX . 
     When a resistive load is being brought online, the current that it draws from the line will be step-like. However, a developing fault will also produce a step-like change in current. Since di SENSE /dt is high both when the resistive load is being brought online and when a fault is developing in the PD&#39;s load circuit, a di SENSE /dt that exceeds di/dt_max is not by itself sufficient to conclude whether a resistive load is being brought online or whether a fault is developing in the PD&#39;s load circuit. However, one significant difference between a developing fault and the a resistive load being brought online is that once the step-like change in current of the resistive load has completed, which will happen very quickly, the magnitude of current that the resistive load draws will level off to some finite value—the specific value depending on the resistance of the load. On the other hand, when a fault is developing, the magnitude of current being drawn from the line will rise and continue to rise to a magnitude that is limited only by the ability of the line to deliver current to the fault. The fault detection and response circuitry  900  exploits this difference by further employing the second high/low comparator  906 . Specifically, as indicated by decision  1010  in the flowchart, the second high/low comparator  906  compares the magnitude of the sense current i SENSE  to the magnitude of a “long-time trip threshold current” i LT . If the current being drawn from the line (as represented by the sense current i SENSE ) rises to a value greater than the long-time trip threshold current i LT , the second high/low comparator  906  produces a logic HIGH output. Accordingly, in a situation where both di SENSE /dt exceeds di/dt_max (a “YES” at decision  1008 ) AND the current being drawn from the line, as represented by the sense current i SENSE , exceeds the long-time trip threshold current i LT  (a “YES” at decision  1010 ), the AND logic gate  910  will generate a logic HIGH output. The logic HIGH output is a true indication that a fault is developing in the PD&#39;s load circuit or that an exceedingly high overload condition is present. Accordingly, once the AND logic gate  910  produces the logic HIGH output, and the logic HIGH output passes through the OR gate  912 , a Gating Disable signal is produced at the output of the fault detection and response circuitry  900 , to quickly turn the solid-state device  806  OFF, as indicated by step  1014  in the flowchart. By turning the solid-state device  806  OFF, the developing fault or exceedingly high overload condition is quickly isolated. On the other hand, even if at decision  1008  it is determined that di SENSE /dt is greater than di/dt_max, so long as it is determined at decision  1010  that the sense current i SENSE  is below the long-time trip threshold current i LT , a conclusion is drawn that the sudden change in sense current i SENSE  (i.e., high di SENSE /dt) is indicative of a resistive load being brought online and the AND logic gate  910  will produce a logic LOW output, thereby allowing the solid-state device  806  to remain ON and the resistive load to be brought online. 
     The fault detection and response circuitry  900  is further capable of distinguishing between resistive and inductive loads and protecting against exceedingly high inrush currents when an inductive load is being brought online. An inductive load will result in a smaller di SENSE /dt when being brought online compared to the near step-like di SENSE /dt that results when a resistive load is being brought online. Accordingly, when the inductive load is being brought online the AND logic gate  910  will not produce a logic LOW output, and so long as the sense current i SENSE  remains below the instant-trip threshold current i MAX  the first high/low comparator  904  will also maintain a logic LOW output as the inductive load is being brought online. However, if the inrush current that the inductive load is drawing while being brought online (or that it draws under any other circumstance) ever exceeds the instant-trip threshold current i MAX , the first high/low comparator  904  will produce a logic HIGH output, which after passing through the OR logic gate  912 , will direct the solid-state device  806  to turn OFF to protect the inductive load and the load circuit wiring from the exceedingly high inrush current. 
     The fault detection and response circuitry  900  in  FIG. 9  provide an entirely hardware solution for detecting and responding to developing faults. A hardware solution is preferred since it provides the fastest way to detect and respond to impending faults. In fact, the fault detection and response circuitry  900  is capable of detecting and isolating developing faults in a matter of a few microseconds, or even less. While a hardware approach is preferred due to the fast detection and reaction capability, a ‘software’ approach could be alternatively used. The PD  500  described above in reference to  FIG. 5  is an example of a software-controlled approach. There, the microcontroller  502  of the PD  500  is programmed and configured to detect and respond to developing faults and the microcontroller generates the Gating Disable that turns the solid-state device  506  OFF when conditions warrant. 
     Although the PD  500  depicted in  FIG. 5  and the PD  800  depicted in  FIG. 8  both utilize a solid-state device to isolate faults and other undesirable overcurrent conditions, it is possible that either PD (the PD  500  or the PD  800 ) could be modified so that it utilizes a mechanical or electromechanical circuit breaker. Although solid-state devices are preferred, controlling a mechanical or electromechanical circuit breaker using sense and drive circuit similar to that described above could possibly allow the mechanical or electromechanical circuit breaker to be controlled more rapidly compared to prior art approaches, and could possibly eliminate, or perhaps at least reduce to some extent, the time-current uncertainties associated with mechanical or electromechanical circuit breakers and/or improve the reaction time and precision at which those types of circuit breakers operate.  FIG. 11  is a drawing that illustrates how the PD  800  might possibly be modified to produce a PD  1100  having a mechanical or electromechanically-controlled circuit breaker  1106 . The PD  1100  includes a microcontroller  1102  programmed to perform functions similar to the microcontroller  802  of the PD  800  and a sense and drive circuit  1104  that controls the opening and closing of the mechanical or electromechanically-controlled circuit breaker  1106 . 
     Like the PD  500  described above in reference to  FIGS. 5 and 6 , the various components of the PD  800  depicted in  FIG. 8  are preferably housed in an enclosure, such as illustrated in  FIG. 12 . The enclosure includes a front face  1202  with cut-outs for the PD&#39;s  800 &#39;s ON and OFF buttons  810 ; a cut-out for the electronic ink display  814 ; and a cut-out for an air-gap disconnect RESET button  1204 , the purpose of which will be described next. 
       FIG. 13  is an exploded view of the PD  800  without the electronics (microcontroller  802 , sense and drive circuit  816 , and solid-state device  806 ) shown. This exploded view of the PD  800  highlights the physical attributes of the air-gap disconnect unit  818  (see  FIG. 8 ) and the various components involved in its operation, including the RESET button  1204 . As shown in the drawing, the PD  800  is housed in an enclosure that includes a front enclosure member  1302 , through which cut-outs for the ON/OFF buttons  810  (see  FIG. 12 ), air-gap disconnect reset button  1204 , and display  814  are made; a mid enclosure member  1304 ; and a bottom enclosure member  1306 . A solenoid  1308 , which forms the actuating component of the air-gap disconnect unit  818 , and associated holding member  1310  are mounted next to one another on a mounting plate  1312 , with the holding member  1310  designed to fit under the L-shaped holders  1314  and the solenoid  1308  mounted alongside on solenoid mounts  1316 . The solenoid  1308  includes a plunger  1318 , which under normal operating conditions (e.g., in the absence of a fault, developing fault, or other unacceptable overcurrent condition) remains retracted in the solenoid housing. The holding member  1310  is configured to slide in a direction parallel to the direction that the plunger  1318  travels, and includes a tab  1320  at one end. The tab  1320  has a size and dimensions that allows it to fit inside a slot  1322  formed through a central section of a connector blade holster  1324 . During normal operating conditions, when power is being distributed to the connected load and no fault or other undesired overcurrent condition is present or developing in the load circuit, the tab  1320  of the holding member  1310  remains positioned in the slot  1322  formed through the connector blade holster  1324 . With the tab  1320  positioned in the slot  1322 , the holding member  1310  serves to hold electrically conductive male connector blades  1326  in corresponding electrically conductive receptacles  1328  of a female line-to-load connector  1330  and prevent holster retraction springs  1332  from pulling the connector blade holster  1324  and attached male connector blades  1326  out of the receptacles  1328 . By holding the electrically conductive male connector blades  1326  in the electrically conductive receptacles  1328 , line current is allowed to flow to the load (so long as the solid-state device  806  is also ON). However, upon the sense and drive circuit  816  sensing and reporting to the microcontroller  802  that a fault or exceedingly high and unacceptable overcurrent condition is present or developing in the load circuit, the microcontroller  802  responds by transmitting an electrical pulse to the solenoid  1308 . The electrical pulse causes the solenoid  1308  to eject its plunger  1318 . The holding member  1310  is attached to the plunger  1318 . Accordingly, when the plunger  1318  is ejected from the solenoid housing, the tab  1320  of the holding member  1310  is removed from the slot  1322  in the connector blade holster  1324 . Once the tab  1320  has been removed from the slot  1322 , the retraction springs  1332  are able to lift the connector blade holster  1324 , pulling the attached electrically conductive male connector blades  1326  out of the electrically conductive receptacles  1328  of the female line-to-load connector  1330 . Pulling the male connector blades  1326  out of the receptacles  1328  results in the formation of an air gap, which serves to fully isolate the load from whatever fault or other hazard is developing or is present. Because the air gap is in series with the solid-state device  806 , the air gap also prevents any leakage current that might otherwise flow through solid-state device  806  from flowing into the load circuit. 
     It should be pointed out that the PD  800  depicted in  FIG. 8  is an example of a three-phase PD. Accordingly, there are three male connector blades  1326  attached to the bottom of the connector blade holster  1324  and three corresponding receptacles  1328  formed in the female line-to-load connector  1330 . In a single-phase PD, only a single male connector blade  1326  and corresponding single female receptacle  1328  would be needed to create the air gap. It should also be pointed out that the sense and drive circuit  816  described above in reference to  FIG. 8  is an example of a sense and drive circuit  816  designed for use in a single-phase PD. In the case of a three-phase PD, the sense and drive circuit  816  could be modified for use in a three-phase PD, thereby allowing the modified sense and drive circuit to react to any type of fault or undesired overload condition, including three-phase and single-line ground faults. 
     During the air-gap disconnect process the air-gap-disconnect RESET button  1204  is forced out of (i.e., pops out of) the front enclosure member  1302  by a compression spring  1334 . The air-gap-disconnect RESET button  1204  has a hole  1336 , through which a maintenance or service worker can insert a padlock or other locking device to complete a lockout-tagout (LOTO) safety procedure. Completing the LOTO safety procedure ensures that the PD  800  will not be accidentally reset by the maintenance or service worker and will not be inadvertently reset by other persons unaware of the hazard. Once the hazard has been cleared by the maintenance or service worker, the padlock or other locking device can then be removed and the PD  800  can be reset by pressing the air-gap-disconnect RESET button  1204  back into the enclosure. Pushing the air-gap-disconnect RESET button  1204  back into the enclosure forces the electrically conductive male connector blades  1326  to be reinserted into the electrically conductive receptacles  1328  of the female line-to-load connector  1330  and allows the tab  1320  at the end of the holding member  1310  to be reinserted into the slot  1322  in the connector blade holster  1324 . Note that the solenoid  1308  has an internal spring that pulls the plunger  1318  back into the solenoid housing shortly after it has been ejected and the air-gap has been formed. Since the holding member  1310  is also attached to the plunger  1318 , the tab  1320  at the end of the holding member  1310 , when the plunger  1318  is pulled back into the solenoid housing, the holding member  1310  is also pulled back to it normal operating condition position, with the tab  1320  reinserted back into the slot  1322  of the connector blade holster  1324 . With the tab  1320  reinserted back into the slot  1322 , the holding member  1310  is then able to once again hold the male connector blades  1326  in the receptacles  1328  of the female line-to-load connector  1330  without the retraction springs  1332  pulling the connector blade holster  1324  and attached male connector blades  1326  out of the receptacles  1328 . The holding member  1310  will then continue to hold the male connector blades  1326  in the receptacles  1328  until the air-gap disconnect process is once again activated. 
     In the description above, the air-gap disconnect process is activated automatically upon the sense and drive circuit  816  determining that a fault or other potentially harmful overcurrent condition is present or developing in the load circuit. The PD  800  also provides the ability for a person to manually activate the air-gap disconnect process. This manual control is provided by the OFF button, which is electrically connected to the microcontroller  802 . When a person presses the OFF button, the microcontroller  802  responds by sending an electrical pulse to the solenoid  1308  to activate the air-gap disconnect process. 
     It should be pointed out that because the real-time load current information sensed by the currents sensor  808  in the PD  800  is sent to the PD&#39;s microcontroller  802  and not just to the sense and drive circuit  816 , the air-gap disconnect unit  818  can still be activated even if the solid-state device  806  should ever fail and even if any component in the fault detection and response circuitry  900  of the sense and drive circuit  816  ever fails. This ability to activate the air-gap disconnect unit  818  independent of the operational status of the solid-state device  806  and independent of the operational status of the fault detection and response circuitry  900  provides a “fail-safe.” 
       FIG. 14  is a drawing that illustrates how a plurality of the PDs  800  depicted in  FIG. 8  can be deployed and configured in a panelboard  1400 , such as, for example, the MDP  402  or one of the sub-panelboards  404  in the dynamically coordinatable electrical distribution system  400  described above in reference to  FIG. 4 . A power distribution backplane with busbars and/or other electrical conductors are configured to receive AC power from the service drop (e.g., 208 to 600 VAC) and distribute the received AC power to the various PDs  800  in the panelboard  1400 . The PDs  800  then distribute the AC power they receive to their respective loads and electrically isolate their respective loads from the AC power they receive when conditions warrant, in the manner described above. The PDs  800  are also electrically connected to the network comm/control bus  414 , so that they can communicate with and be controlled by the central computer  416  over the comm/control bus  414 , via the head-end interface  418 . Note that the head-end interface  418  may include a wired adapter (for example, a USB-CAN bus adapter if the comm/control bus adapter is a CAN bus) or a USB-comm/bus bus dongle that allows the central computer  416  to make a wired connection to the head-end interface  418  and communicate and control the PDs  800  in the panelboard  1400 . Alternatively (or additionally) the central computer  416  and head-end interface  418  can be equipped with wireless transceivers (e.g., Wi-Fi transceivers), thereby allowing the central computer  416  to communicate with the comm/control bus  414  and PDs  800  over a wireless link. The head-end interface  418  may also or alternatively include a wide-area-network capable (WAN-capable) adapter that allows the central computer  416  to communicate with and control the PDs  800  over a wide area network (WAN), such as the Internet or a cellular communications network. With this capability, the central computer  416  can then be situated remotely, if necessary or desired and possibly controlled by a utility company. 
       FIG. 15  is a drawing that shows the salient elements of the central computer  416 , which may comprise a server, desktop computer, laptop computer, tablet computer, smartphone, or any other type of computing device. As shown in the drawing, the central computer  416  includes a microprocessor  1502 ; computer readable memory (CRM)  1504 ; an optional human-machine interface (HMI)  1506 , through which a user can interact with central computer  416 ; an optional display  1508 ; and a storage device  1510  (e.g., a magnetic hard drive or a solid-state drive) that may be configured to store, among other things, current and voltage information associated with the PDs  410  (e.g., trip-setting parameters for the PDs  410 , historical and/or heuristically-derived time-current information and characteristics of the PDs  414 , the electrical distribution system in which the PDs  414  of the system  400  are deployed, etc.). 
     The non-transitory CRM  1504  of the central computer  416  is configured to store computer program instructions that direct how the microprocessor  1502  of the central computer  416  operates. These computer program instructions may include, but are not limited to: instructions that direct how and when microprocessor  1502  communicates with the PDs over the comm/control bus  414 , via the head-end interface  418 ; instructions that direct how and when the microprocessor  1502  receives or fetches current and/or voltage information; instructions that control how and when the microprocessor analyzes current and/or voltage information received from the PDs and historical and/or heuristic current and/or voltage information retrieved from storage  1510 ; instructions that direct how the microprocessor  1502  calculates trip-setting parameters for the PDs to adapt to; and instructions that determine how and when, and under what circumstances, the microprocessor  1502  transmits updated trip-setting parameters to the PDs, in order to dynamically coordinate the PDs in the system  400 . Some or all of these operations can be performed in real time, and in most circumstances without disrupting the general operation of the distribution system  400 . The real-time capability not only affords the ability to adjust, control and optimize, in real time, the trip settings of the PDs, it also completely eliminates the need for pre-planned or ad hoc selective coordination studies. It also allows higher-level zones or sections of the distribution system  400  that may be operationally important or which may be susceptible or sensitive to sudden increases in current to be closely monitored and dynamically adjusted, if necessary or desired. 
     How the central computer  416  operates to dynamically coordinate PDs in the electrical distribution system  400  will now be described. Before describing the various operations that the central computer  416  performs in dynamically coordinating the PDs, reference is first made to  FIG. 16 .  FIG. 16  is a drawing shows the time-current characteristic of a PD (assuming a PD constructed like the PD  800  depicted in  FIG. 8 ). The “upper” short-time trip time threshold t UPPER  in the time-current characteristics establishes how long the PD  800  will tolerate a load current higher than the long-time trip threshold current i LT  before the fault detection and response circuitry  900  of the sense and drive circuit  816  (see  FIG. 9 ) produces a Gating Disable signal to turn OFF the PD&#39;s  800 &#39;s solid-state device  806 . The “lower” short-time trip time threshold t LOWER  establishes how long the PD  800  will tolerate a load current just below the instant-trip threshold current i MAX . Some or all of these “trip-setting parameters,” t UPPER , t LOWER , i LT , and i MAX  for one or more of the PDs  800  (and also possibly the current rating of one or more of the PDs  800  are transmitted to the microcontrollers  802  of the PDs  800  (over the comm/control bus  414 , via the head-end interface  418 , prior to or during the dynamic coordination process described in reference to  FIG. 18  below. 
     To better understand what the dynamic coordination of the PDs entails, reference is also made to  FIGS. 17A and 17B , which show the time-current characteristics of PDs (labeled “1” through “5”) involved in a dynamic coordination.  FIG. 17A  shows the time-current characteristics of five PDs labeled “1” through “5” before the coordination, and  FIG. 17B  shows the time-current characteristics of the PDs “1” through “5” after the coordination has been completed. Prior to the coordination being performed ( FIG. 17A ), the PD  800  labeled with a “1” (which may correspond to the main PD  408  in the MDP  402  in  FIG. 4 , for example) is seen to have time-current characteristics that are too close to the time-current characteristics of the PD  800  labeled with a “2.” Additionally, the time-current characteristics of the PDs  800  labeled “4” and “5” are seen to overlap. Both of these conditions are non-optimal since either can result in one or more of the PDs  800  not tripping when it/they should or can result in one or more of the PDs  800  tripping prematurely when it/they should not. For example the instant-trip threshold current i MAX  and long-time trip threshold current i LT  settings of the PD  800  labeled with a “1” are both likely too low. Additionally, the instant-trip threshold current i MAX  and long-time trip threshold current i LT  settings of the PD  800  labeled with a “4” are also too low and/or the instant-trip threshold current i MAX  and long-time trip threshold current i LT  settings of the PD  800  labeled with a “5” are too high. 
     Now that the goal of the dynamic coordination process has been described, reference is made to  FIG. 18 , which is a flowchart that illustrates one exemplary method  1880  that the microprocessor  1502  of the central computer  416  is programmed to follow in dynamically coordinating a plurality of PDs in an electrical distribution system, in accordance with one embodiment of the invention. First, at step  1802  the central computer  416  receives real-time sensed current data (and possibly measured line voltage information) from one or more of the PDs  800  over the comm/control bus  414 , via the head-end interface  418 . At step  1804  the central computer  416  then analyzes the received sensed current data, measured voltage data, and possibly retrieves historical and/or heuristic (i.e., non-real-time) current and/or voltage information stored in the central computer&#39;s storage  1510 . After analyzing the received sensed current data and possible other date retrieved from the storage  1510 , at decision  1806  the central computer  416  determines whether there is a coordination problem or other non-optimal coordination among the various PDs  800  that are being coordinated. If a non-optimal coordination is determined not to be present, at decision  1808  it is determined whether to continue or end the method  1800 . If it is determined that the method  1800  should continue, the method  1800  loops back to step  1802 . Otherwise, the method  1800  ends. If the central computer  416  determines that the PDs  800  are not optimally coordinated at decision  1806 , at decision  1810  the central computer  416  determines whether the non-optimal coordination might be correctable. If the central computer  416  concludes that the non-optimal coordination is not correctable, at step  1812  the central computer  416  alerts the system user or overseer that it is not able to correct the problem and the method  1800  ends. However, if the central computer  416  determines that the non-optimal coordination might be corrected (“YES” at decision  1810 ), at step  1814  the central computer  416  computes new trip-setting parameters for one or more of the PDs  800 . Next, at step  1816  the central computer  416  transmits the new trip-setting parameters to one or more of the PDs  800  (over the comm/control bus  414  and via the head-end interface  418 ), commanding the one or more PDs  800  to adjust to the newly-computed trip-setting parameters. Once the PDs  800  have adjusted to the new trip-setting parameters, the central computer  416  then performs a system check at step  1818  to determine whether the PDs  800  have been properly coordinated (such as in  FIG. 17B  above). If at decision  1820  the central computer  416  determines that the PDs  800  have been properly coordinated, at step  1822  the central computer  416  notifies the system user or overseer that the coordination has been successfully completed. As indicated by decision  1824 , the method  1800  may then end or it may loop back to step  1802  so that the central computer  416  can monitor the system and re-coordinate in the event that a non-optimal coordination subsequently arises. If at decision  1820  the central computer  416  determines that the coordination was unsuccessful, at decision  1826  it is determined whether to continue with another attempt to coordinate. If “NO,” at step  1828  the central computer  416  alerts the system user or overseer that the coordination could not be completed. On the other hand, if the central computer  416  determines at decision  1826  that further adjustment of the trip-setting parameters might possibly help to optimize the coordination among the PDs  800 , at step  1830  the central computer performs further analysis and computes new trip-setting parameters for one or more of the PDs  800  once again, and at step  1832  commands the one or more PDs  800  to adjust to the new trip-setting parameters. Once the PDs  800  have adjusted to the new trip-setting parameters, the central computer  416  performs a system check at step  1834  to determine whether the PDs  800  have been properly coordinated. If at decision  1836  it is determined that the coordination has been successfully completed, at step  1838  the central computer  416  notifies the system user or overseer of the successful coordination. Next, as indicated by decision  1840 , the method  1800  may then be terminated or may loop back to step  1802  so that the central computer  416  can monitor the system and re-coordinate in the event that a non-optimal coordination arises in the future. On the other hand, if at decision  1836  the central computer  416  determines that the coordination was unsuccessful, at decision  1842  it is determined whether to continue with another attempt to coordinate. If “NO,” at step  1844  the central computer  416  alerts the system user or overseer that the coordination could not be completed and the method  1800  ends. On the other hand, if the central computer  416  determines at decision  1842  that further adjustment of the trip-setting parameters might possibly help to optimize the coordination among the PDs  800 , the method continues once again at step  1830 . The central computer  416  may then make further attempts to optimize the coordination. If after several attempts, the coordination is determined not to be possible the system user or overseer is notified of the inability to complete the coordination and the method  1800  ends. 
     In the exemplary method  1800  described above, the central computer  416  is programmed so that it performs the dynamic coordination method  1800  automatically, upon determining that the PDs in the electrical distribution system  400  are not optimally coordinated. The central computer  416  can also be programmed to perform the dynamic coordination method  1800  independent of the operational status of the electrical distribution system, for example, in accordance with a predetermined preventative maintenance schedule. In this manner, the central computer  416  can maintain optimal coordination among the PDs at all times and re-coordinate when necessary, for example to adapt the coordination to changing load conditions. 
     The central computer  416  can also (or alternatively) be programmed so that it performs the dynamic coordination method  1800  in response to a command received by a user (e.g., a command entered through the HMI  1506  of the central computer  416  by an electrician or other technician) or in response to a command received from the system overseer, who or which may be an electrical utility or other organization or person having the legal authority to initiate the dynamic coordination method  1800 . As will be explained below, the central computer  416  can also (or alternatively) be programmed so that a user of the central computer  416  can assist in the coordination and manually adjust or override the trip-setting parameters of PDs being coordinated. 
       FIG. 19  is a drawing that illustrates how the panelboard  1400  depicted  FIG. 14  can be housed within a panel box  1902 . In one embodiment of the invention the panel box  1902  includes a door  1904  with a window  1906  and a handle or latch  1908  that is used to open and close the door  1904  to access the panelboard  1400 , reset tripped PDs  800 , and perform maintenance and troubleshooting. The head-end interface  418  between which the comm/control bus  414  and central computer  416  are interfaced may be located inside the panel box  1502 , outside the panel box  1902 , or at some remote location. Preferably, the head-end interface  418  is located near the panel box  1902 , however, so that the central computer  416  can be easily connected to and interfaced with the comm/control bus  414  (e.g., using a USB connector and cable in a situation where the head-end interface  418  includes a USB-comm/control bus adapter) or wirelessly (e.g., in a situation where the central computer  416  and head-end interface  418  are both equipped with wireless transceivers (e.g., Wi-Fi transceivers)). 
     In one embodiment of the invention the panelboard  1400  is further equipped with a panel display module that is configured so that it is in electrical communication with the comm/control bus  414 . As illustrated in  FIG. 19 , the panel display module includes a panel display  1910 , which may be: positioned so that it can be displayed through a cut-out in the front face of the panel box  1902  (as in  FIG. 19 ), located inside the panel box  1902  (e.g., so that it can be viewed through the panel box door window  1906 ), or mounted outside the panel box  1902  (e.g., affixed to an exterior wall of the panel box  1902 ). Like the PD displays  514  and  814  of the PDs  500  and  800  described above (see  FIGS. 5 and 8 ), the panel display  1910  is preferably an electronic ink display, so that even when power is removed from the panel display  1901  the information that it displays continues to be displayed. The panel display  1910  may be configured to display any relevant information (real-time or non-real-time) descriptive of the panelboard  1400 . For example, in  FIG. 19  the panel display  1910  is shown to be displaying the name of the panelboard  1400  (“Atom Panel  1 ”), the panelboard from which it is fed power (“MDP”), the incoming line voltage (“208/120V”), and the maximum current (“225 A”) that the panelboard  1400  is able to supply to the various loads connected to the panelboard  1400 . 
     As alluded to above, the CRM  1504  of the central computer  416  (see  FIG. 15 ) may be configured to store computer program instructions that allow a user of the central computer  416  (e.g., an electrician, engineer or other technician) to interact with the electrical distribution system and its PDs, for example, the panelboard  1400  and the PDs  800  in the panel box  1902 . Providing this user-interactive capability allows the user to manually enter, control and even override the trip-setting parameters computed by the central computer  416 . In one embodiment of the invention, this user-interactive capability is provided in the form of a graphical user interface (GUI). In accordance with this embodiment of the invention, the computer program instructions stored in the CRM  1504  of the central computer  416  include instructions that direct the microprocessor  1502  of the central computer  416  how to generate one or more GUI pages that are displayed on the central computer&#39;s display  1508 . Preferably, the display  1508  is equipped with touchscreen technology, which enables the user of the central computer  416  to interact with the GUI pages by touching the screen of the display  1508  or using a stylus. Using simple or multi-touch gesture using one or more fingers, the user can scroll, zoom, input information, etc. and control what GUI pages and content are being displayed on the display  1508 . The GUI and display  1508  could alternatively (or additionally) be configured so that the user can interact with the GUI pages and content using a mouse, touchpad, or other non-touchscreen input device. To facilitate user-interactivity, the GUI pages preferably include icons and widgets, such as radio buttons, sliders, spinners, drop-down lists, menus, combo and text boxes, scrollbars, etc. 
       FIG. 20  is a drawing depicting how one of the GUI pages generated by the central computer  416  (which in this case comprises a tablet computer) may comprise a one-line GUI page  2002  that displays the panelboards in an electrical distribution system. The one-line GUI page shows that the electrical distribution system that the tablet computer is connected to (via the comm/control bus  414 ) comprises an MDP (GUI element  2004  labeled “Panel MDP”) fed from an electrical utility (GUI element  2006 ) and a downstream sub-panelboard (GUI element  2008  labeled “Panel HVAC”). The MDP and sub-panelboard GUI elements  2004  and  2008  further display the line voltages and maximum current that the MDP and sub-panelboard are able to supply to their respective loads. 
     Each of the MDP and sub-panelboard GUI elements  2004  and  2008  in the one-line GUI page shown in  FIG. 20  may be a user-interactive button, which the user of the central computer  416  can touch to open a panel GUI page showing how various PDs  800  in the selected MDP or sub-panelboard are configured.  FIG. 21  illustrates, for example, a panel GUI page  2102  that is generated by the central computer  416  and displayed on the central computer&#39;s display  1508  after the user has touched the sub-panelboard GUI element  2088  in the one-line GUI page  2002 . In addition to displaying images  2104  of the various PDs configured in the selected sub-panelboard, the panel GUI page  2102  includes load-name labels that identify the loads being protected by the various PDs in the sub-panelboard. Images of what is presently being displayed on the PDs displays (e.g., PD display  814  in  FIG. 12 ) and on the panel display  1910  (see  FIG. 19 ) may also be displayed in the panel GUI page  2102 . 
     In accordance with one embodiment of the invention, the GUI computer program instructions stored in the CRM  1504  of the central computer  416  further include user-interactive instructions that provide the user of the central computer  416  the ability to change the information that is displayed by the electronic ink displays of the PDs (e.g., PD display  814  in  FIG. 12 ) and/or the information that is being displayed by the electronic ink panel display  1910  (see  FIG. 19 ).  FIG. 22  is a flowchart that illustrates a method  2200  that the central computer  416  is programmed to follow in allowing the user to update this display information. Note that only salient steps in the method  2200  are presented in the flowchart, and the various steps and decisions in the flowchart are not necessarily performed in the order shown or as a sequence of discrete events. For example, some of the steps and decisions may be performed continuously and some of the steps and decisions may be performed simultaneously. First, at step  2202  the central computer  416  directs the display  1508  of the central computer to display the panel GUI page  2102  ( FIG. 21 ) to the user. Next, at decision  2204  the central computer  416  determines whether the user has entered a command indicating that the user wishes to update information being displayed on the panel display  1910 . If the central computer  416  determines that the user has input a command to update the information being displayed by the panel display  1910 , at step  2206  the central computer  416  then receives updated panel display information from the user. The updated panel display information may be entered by the user using a physical keyboard, if the central computer  416  is equipped with a physical keyboard. Alternatively, the panel display element  2106  in the panel GUI page  2102  (see  FIG. 21 ) can be programmed to serve as a user-interactive button, which when touched by the user opens up a user-interactive text box and virtual keyboard that the user can interact with to enter the user&#39;s desired panel display information. After receiving the updated panel display information from the user, the central computer  416  responds at step  2208  by refreshing and updating the panel display  1910  accordingly. At decision  2210  the central computer  416  then determines whether the user has entered a command indicating that the user wishes to update information being displayed by the electronic ink display(s) of one or more of the PDs. If the central computer  416  determines that the user has input a command to update the information being displayed by the electronic ink displays of one or more of the PDs, at step  2212  the central computer  416  then receives the updated PD display information from the user. Again, the updated PD display information may be entered by the user using a physical keyboard, if the central computer  416  is equipped with a physical keyboard. Alternatively, the PD images  2104  displayed on the panel GUI page  2102  (see  FIG. 21 ) can be programmed to serve as user-interactive buttons, which when touched by the user opens up a user-interactive text box and virtual keyboard that the user can interact with to enter the updated PD display information for the one or more PDs. Finally, after receiving the updated PD display information, at step  2214  the central computer communicates the updated PD display information to the appropriate microcontrollers  802  of the PDs (over the comm/control bus and via the head-end interface  418 ), so that the microcontrollers  802  can then update their electronic ink displays accordingly. (Note if the names of any of the loads of any of the PD electronic displays have been updated, the central computer  416  automatically updates the load-name labels next to the corresponding images  2104  of the PDs in the panel GUI page (see  FIG. 21 ).) 
     In the exemplary dynamic coordination method  1900  described above (see  FIG. 19  and accompanying description), the central computer  416  is programmed so that it dynamically coordinates PDs in an electrical distribution system automatically, i.e., without the need for any user assistance. In some circumstances, for example, if the central computer  416  is unable to complete a successful coordination, it may be desirable or necessary for an electrician, engineer or other technician to manually coordinate the PDs or override the trip-setting parameters that the central computer  416  computes, in order to complete a successful coordination. To support this ‘user-assisted’ dynamic coordination, the GUI program instructions stored in the central computer&#39;s CRM  1504  and executed by the microprocessor  1502  of the central computer  416  may further include instructions that direct the central computer  416  to generate and display a dynamic coordination GUI page  2302 , such as illustrated in  FIG. 23 . The dynamic coordination GUI page  2302  preferably includes a time-current coordination overlay  2304  that displays the time-current characteristics of the PDs being manually coordinated. In the exemplary dynamic coordination overlay  2304  shown in  FIG. 23 , time-current characteristics of five PDs are shown. The five PDs are labeled “1,” “2,” “3,” “4” and “5.” However, in a preferred embodiment, each of the time-current characteristic lines has a unique color, so that they are distinguished by different colors rather than by numbers, and the legend in the coordination overlay  2304  that identifies the time-current characteristic lines also uses corresponding and matching colors. Each of the time-current characteristic lines also serves as a user-interactive button, which when touched by the user causes the central computer  416  to display the trip-setting parameters of the selected PD on the dynamic coordination GUI page  2302 , including the long-time trip threshold long-time trip threshold current i LT , short-time trip time threshold t UPPER , and instant-trip threshold current i MAX  (see  FIG. 16 ). In the snapshot of the dynamic coordination GUI page  2302  shown in  FIG. 23 , the PD that is selected is PD #2, which is the main PD in a panelboard having the name “Panel HVAC,” as shown in the text box with the label “Name=” in the upper left corner of the dynamic coordination GUI page  2302 . The dynamic coordination GUI page  2302  further includes sliders  2306  that the user can touch and slide to manually change the long-time trip threshold current i LT , short-time trip time threshold t UPPER , and instant-trip threshold current i MAX  of the selected PD. (Although not shown, a slider can also be included for adjusting the short-time trip time threshold t LOWER .) Further displayed on the dynamic coordination GUI page  2302  is the current rating of the selected PD. As shown, the PD that is currently selected has a current rating of 100 A. Note that the Name and Rating of the selected PD can also be changed by entering the desired change in Name and/or Rating in the text boxes with the “Name=” and “Rating=” labels. Also included in the dynamic coordination GUI page  2302  is a table  2308  that displays real-time metering information of the selected PD, including real-time voltage, real-time amperage, real-time kilowatts and real-time temperature. Finally, the dynamic coordination GUI page  2302  includes user-interactive “CLOSE,” “OPEN,” “COORDINATION” and “APPLY” buttons. The “OPEN” and “CLOSE” buttons are user-interactive buttons that allow the user to remotely deactivate (i.e., turn OFF) the solid-state device of the selected PD (to “open” the selected PD&#39;s load circuit) and remotely activate (i.e., turn ON) the solid-state device of the selected PD (to “close” the selected PD&#39;s load circuit). The “APPLY” button is a user-interactive button that is used by the user to direct the central computer to apply any changes the user has entered through the dynamic coordination GUI page  2302 , including any changes made to the trip-setting parameters of a selected PD via the sliders  2306 , any changes the user has entered in the “Name=” and “Rating=” text boxes of a selected PD, etc. Finally, the “COORDINATE” button is a user-interactive button that is used by the user to request the central computer to coordinate the various PDs represented in the coordination overlay  2304 , once the user has completed individually adjusting the trip-setting parameters of one or more of the PDs. 
       FIG. 24  is a flowchart that illustrates a method  2400  the central computer  416  performs when a user is interacting with the dynamic coordination GUI page  2302  to manually coordinate a plurality of PDs. Note that the steps and decisions represented in the flowchart are not necessarily performed in the order shown, and some steps and decisions may be performed continuously or simultaneously, as will be appreciated by those of ordinary skill in the art. At step  2402  in the method  2400  the central computer  416  receives real-time sense current data (and possibly also real-time voltage information) from the PDs that are being coordinated. After receiving the current and/or voltage information from the PDs, at step  2404  the central computer generates and displays the dynamic coordination GUI page  2302  on its display  1508 , including the time-current characteristics of all PDs that are going to be manually coordinated and some or all of the other elements and controls of the exemplary dynamic coordination GUI page  2302  depicted in  FIG. 23 . (Note that in displaying the time-current characteristics the trip-setting parameters of the various PDs are also received from the PDs or are retrieved from the storage unit  1510 .) Next, at step  2406  the central computer  416  receives a command from the user indicating that the user has selected one of the PDs for adjustment. Responding to the user command, at step  2408  the central computer  416  then updates the dynamic coordination GUI page  2302  so that the Name, Rating, trip-setting parameters of the selected PD (long-time trip threshold long-time trip threshold current i LT , short-time trip time threshold t UPPER , and instant-trip threshold current i MAX ), and real-time metering table  2308  are displayed. Next, at step  2410  the central computer  2410  receives updated trip-setting parameters from the user, which the user inputs by adjusting the sliders  2306  of the selected PD and commands the central computer to accept by touching the “APPLY” button. At decision  2412  the central computer then determines whether the user has selected another PD to adjust. If “YES” the method  2400  loops back to step  2406  where the central computer  416  waits to receive PD selection and trip-setting parameter adjustments for other of the PDs in the coordination overlay  2304 . After all trip-setting adjustments have been input by the user, the user then touches the “COORDINATE” button, requesting that the central computer  416  coordinate the PDs accordingly. After receiving the COORDINATION request, at step  2414  the central computer  416  determines at decision  2416  whether the user&#39;s coordination request might pose a hazard or other problem. If a hazard or other problem might possibly occur if the coordination is made as directed by the user, at step  2418  the coordination request is denied, and at step  2420  the central computer presents an error message on the dynamic coordination GUI page  2302  (and possibly alerts the user with an audible warning) that the coordination request could not be accepted. If, on the other hand, the central computer  416  determines that the coordination request is acceptable, at step  2422  the central computer  416  transmits (over the comm/control bus  414  and via the head-end interface  418 ) the user-set trip-setting parameters to the PDs that are being coordinated. Finally, after the PDs have adjusted to the user-set trip-setting parameters, at step  2444  the central computer  416  updates the coordination overlay  2304  image on the dynamic coordination GUI page  2302  and displays a message to the user that the coordination has been successfully completed. 
     While various embodiments of the present invention have been described, they have been presented by way of example and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail may be made to the exemplary embodiments without departing from the true spirit and scope of the invention. Accordingly, the scope of the invention should not be limited by the specifics of the exemplary embodiments but, instead, should be determined by the appended claims, including the full scope of equivalents to which such claims are entitled.