Abstract:
In the transfer of energy from a source to a load, a power distribution system is configured to detect unsafe conditions that include electrically conducting foreign objects or individuals that have come in contact with exposed conductors in the power distribution system. A responsive signal is generated in a source controller including source terminals. The responsive signal reverses a voltage on the source terminals. With the voltage on the source terminals reversed, a measurement of electrical current flowing through the source terminals is acquired; and the source controller generates signals to electrically disconnect the source from the source terminals if and when the electrical current falls outside of high or low limits indicating that there is a conducting foreign object or living organism making electrical contact with the source or load terminals or a failure in power distribution system hardware.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/292,596, filed 8 Feb. 2016, the entire content of which is incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    This invention relates to power distribution system safety protection devices—for example, power distribution systems with electronic monitoring to detect and disconnect power in the event of an electrical fault or safety hazard, particularly where an individual has come in contact with exposed conductors. This invention is applicable to general power distribution, or more specifically to, e.g., electric vehicle charging, telecommunications or alternative energy power systems. 
       BACKGROUND 
       [0003]    Digital electric power, or digital electricity, can be characterized as any power format where electrical power is distributed in discrete, controllable units of energy. Packet energy transfer (PET) is a new type of digital electric power protocol disclosed in U.S. Pat. No. 8,781,637 (Eaves 2012). 
         [0004]    The primary discerning factor in a digital power transmission system compared to traditional, analog power systems is that the electrical energy is separated into discrete units; and individual units of energy can be associated with analog and/or digital information that can be used for the purposes of optimizing safety, efficiency, resiliency, control or routing. 
         [0005]    As described by Eaves 2012, a source controller and a load controller are connected by power transmission lines. The source controller of Eaves 2012 periodically isolates (disconnects) the power transmission lines from the power source and analyzes, at a minimum, the voltage characteristics present at the source controller terminals directly before and after the lines are isolated. The time period when the power lines are isolated was referred to by Eaves 2012 as the “sample period”, and the time period when the source is connected is referred to as the “transfer period”. The rate of rise and decay of the voltage on the lines before, during and after the sample period reveal if a fault condition is present on the power transmission lines. Measurable faults include, but are not limited to, short circuit, high line resistance or the presence of an individual who has improperly come in contact with the lines. 
         [0006]    Eaves 2012 also describes digital information that may be sent between the source and load controllers over the power transmission lines to further enhance safety or provide general characteristics of the energy transfer, such as total energy or the voltage at the load controller terminals. Since the energy in a PET system is transferred as discrete quantities, or quanta, it can be referred to as “digital power” or “digital electricity”. 
       SUMMARY 
       [0007]    A power distribution system regulates transfer of energy from a source on a source side to a load on a load side, wherein the source and load each include terminals. A source controller on the source side is in communication with and responsive to a source sensor that provides feedback to the source controller that includes at least a signal indicative of electric current through the source terminals. A source switching bridge is electrically coupled with the source controller and is responsive to control signals from the source controller for electrically disconnecting the source from the source terminals and for applying a source voltage in either a forward-polarity or reverse-polarity state relative to the source terminals. A load disconnect device is configured to electrically decouple the load from the load terminal. A logic device is implemented in at least the source controller and configured to place the source switching bridge into a reverse-polarity state and to perform at least one current measurement on the current passing through the source terminals when the source switching bridge is in the reverse-polarity state, wherein a current measurement outside of predetermined high or low limits indicates that there is a foreign object or living organism making contact with the source or load terminals or a failure in the power distribution system, and to electrically disconnect the source from the source terminals if the current measurement falls outside the predetermined high and low limits. 
         [0008]    In the transfer of energy from a source to a load, a power distribution system is configured to detect unsafe conditions that include electrically conducting foreign objects or individuals that have come in contact with exposed conductors in the power distribution system. A responsive signal is generated in a source controller including source terminals. The responsive signal reverses a voltage on the source terminals. With the voltage on the source terminals reversed, a measurement of electrical current flowing through the source terminals is acquired; and the source controller generates signals to electrically disconnect the source from the source terminals if and when the electrical current falls outside of high or low limits indicating that there is a conducting foreign object or living organism making electrical contact with the source or load terminals or a failure in power distribution system hardware. 
         [0009]    The apparatus and methods described herein offer an alternative form of PET using the method of periodically reversing the polarity of the transmission lines. Since the most common forms of electrical faults are polarity independent, the method allows for detection of a fault based on the load device being equipped with a uni-directional switch, such as a diode. When the polarity of the transmission lines are reversed, the flow of electrical current is inhibited by the uni-directional switch. If there is a fault on the transmission lines, such as due to a person touching the lines, electrical current will continue to flow into the fault when the transmission lines are reversed and can be detected by the source controller. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a block diagram of an embodiment of the safe power distribution system. 
           [0011]      FIG. 2  is a more detailed block diagram of an embodiment of the source controller. 
           [0012]      FIG. 3  is a diagram of a section of an embodiment of the power distribution system with the switching bridge  7  in a non-conducting state. 
           [0013]      FIG. 4 a    is a diagram of a section of an embodiment of the power distribution system with the switching bridge  7  in a forward-conducting state. 
           [0014]      FIG. 4 b    is a diagram of a section of an embodiment of the power distribution system with the switching bridge  7  in a reverse-conducting state, 
           [0015]      FIG. 5 a    is a diagram of a disconnect device  13  constructed using a diode  39 . 
           [0016]      FIG. 5 b    is a diagram of a disconnect device  13  constructed using a controllable switch  38 . 
           [0017]      FIG. 6  is a diagram of an embodiment of an alternative source controller configuration that includes a modulator/demodulator  48  for communications over power lines. 
           [0018]      FIG. 7  is a diagram of using center-tapped isolation transformers  52 - 55  to combine user data and power on common twisted pair cabling. 
           [0019]      FIG. 8  is a diagram of an alternative load and load disconnect device  13  using a series string of diodes  70 ,  72 ,  74 , and  76 . 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The foregoing and other features and advantages of various aspects of the invention(s) will be apparent from the following, more-particular description of various concepts and specific embodiments within the broader bounds of the invention(s). Various aspects of the subject matter introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the subject matter is not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes. 
         [0021]    Unless otherwise defined, used or characterized herein, terms that are used herein (including technical and scientific terms) are to be interpreted as having a meaning that is consistent with their accepted meaning in the context of the relevant art and are not to be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
         [0022]    The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting of exemplary embodiments. As used herein, singular forms, such as “a” and “an,” are intended to include the plural forms as well, unless the context indicates otherwise. Additionally, the terms, “includes,” “including,” “comprises” and “comprising,” specify the presence of the stated elements or steps but do not preclude the presence or addition of one or more other elements or steps. 
       I. Description of Operation 
       [0023]    A block diagram of an embodiment of the power distribution system is shown in  FIG. 1 . The power distribution system regulates the transfer of energy from a source  1  to a load  3 . Periodically, the source controller  5  operates a control signal  40  to reverse the polarity of the source terminals  31   a  and  31   b  relative to the source  1  using a switching bridge  7  for a predetermined time period, known as the “sample period”. During the sample period, the current sensor  8  is employed to measure the current on the transmission lines. 
         [0024]    The normal resistance between the source terminals  31   a  and  31   b  is represented by R src . In a particular embodiment, R src  has a value greater than 1 million Ohms. During normal conditions, when the polarity of the transmission lines is reversed, the current, as sensed by current sensor  8 , would be less than 1 milliamp for a source voltage of 380V. However, during a cross-line fault, as depicted in  FIG. 1  the resistance of a foreign object  6 , such as a human body or conductive element, is introduced and is represented by R leak . The parallel combination of R src  and R leak  will increase the current sensed by current sensor  8  significantly. If the current exceeds a predetermined maximum value, a fault is registered and the switching bridge  7  will be placed in a non-conducting state by the source controller  5 , where the source  1  is electrically disconnected from the source terminals  31   a  and  31   b.    
         [0025]    If no fault conditions are detected, the switching bridge  7  is again commanded to a forward-polarity state. Energy is then transferred between the source  1  and the load  3  until the next sample period. The conducting period between sample periods is referred to as the “transfer period”. 
         [0026]    An additional check for the in-line fault is where the source and load controllers  5  and  9  acquire their respective terminal voltages at sensing points  34  and  35 , as shown in  FIG. 1  during an energy transfer period. In embodiments incorporating advanced monitoring options, the communication link  11  would be implemented; and the source controller  5  would obtain the load terminal voltage through the communication link  11 . The source controller  5  then calculates the voltage difference between the two measurements. The source controller  5  also acquires the electrical current passing through the source terminals  31   a  and  31   b  using a current sensor  8 . The source controller  5  can now calculate the line resistance between the source and load terminals  31   a/b  and  32   a/b  using Ohm&#39;s law, or the relationship: Resistance=Voltage/Current. The calculated line resistance is compared to a predetermined maximum and minimum value. If the maximum is exceeded, switching bridge  7  will be placed in a non-conducting state and an in-line fault is registered. A line resistance that is lower than expected is also an indication of a hardware failure. 
         [0027]    An alternative method to measure in-line resistance without a communications link  11  to the load  3  is where the source controller  5  measures the source terminal voltage at sensing point  34  and measures the electrical current passing through the source terminals  31   a  and  31   b  using the current sensor  8 . The voltage and current samples are made nearly simultaneously during the same energy transfer period. The switching bridge  7  is then placed in a non-conducting state, and the source controller  5  immediately takes another voltage sample at sensing point  34 . The difference in magnitude between the first and second voltage samples is proportional to the line resistance. Explained differently, as the transmission line resistance increases, more voltage is dropped across the length of the line for a given current. Since the voltage on the line capacitor  4  is equal to the source voltage minus the voltage drop on the line, measuring the voltage of the transmission lines without current flowing sets the line voltage drop to zero allowing an independent measurement of the voltage across the line capacitor  4 . Once the voltage of the line capacitor  4  is known, the voltage drop on the transmission lines can be calculated by subtracting the earlier measurement at point  34  made when line current was present. 
         [0028]    Referring to  FIG. 3 , the construction of the switching bridge  7  for the reversal of the transmission line voltage is accomplished in this embodiment using what is well known in the industry as a “full-bridge” converter. Switches  60 ,  61 ,  62 , and  63  would typically be transistors. The transistor type is chosen based on the voltage and current requirements. Industry standard transistors that can be employed include field effect transistors (FETs), integrated gate bipolar transistors (IGBTs) or integrated gate commutated thyristors (IGCTs). The electrical implementation of the control signal  40  for controlling the conduction of the switches in the switching bridge  7  is dependent on the type of transistor but is well known to those skilled in the art of power electronics. 
         [0029]    The switching bridge  7  has three states applicable to the present invention: non-conducting, forward-polarity (causes current to flow from the source  1  to the load  3 ) and reverse-polarity (where no current flows to the load  3  under normal operation).  FIG. 3  depicts the non-conducting state since all four switches  60 - 63  are shown in an open or non-conducting state. In  FIG. 4 a   , switches  60  and  63  are acted on by the control signal  40  to be conducting with the other switches  61  and  62  are in a non-conducting state, putting the switching bridge  7  into the forward-operating state. In  FIG. 4 b   , switches  61  and  62  are acted on by control signal  40  to be conducting with the other switches  60  and  63  are in a non-conducting state, putting the switching bridge  7  into the reverse-polarity state. 
         [0030]    There are a number of industry standard methods for constructing the disconnect device  13  of  FIG. 1 . Referring to  FIG. 5 a   , in cases where it is not necessary for the load controller  9  to have the ability to interrupt power to the load terminals  32   a  and  32   b , internal switch  38  can be constructed using only diode  39  to block the back-flow of electrical current when the switching bridge  7  is in a non-conducting or reverse state. In an embodiment where the load controller  9  is configured to control the action of the disconnect device  13 , an arrangement for a disconnect device  13 , as shown in  FIG. 5 b    can be used. In this arrangement, electrical current is blocked in the negative-to-positive direction by a blocking diode  39 . Current flow in the positive-to-negative direction is controlled by an internal switch  38  according to the application of a control signal  41 . The controllable switch  38  provides the capability for the load controller  9  to interrupt power in cases where an unauthorized source of power has been connected to the load terminals  32   a  and  32   b  or where the source controller  5  malfunctions and can no longer interrupt power from the source  1 . In applications, such as battery charging, uncontrolled overcharging can result in battery damage or fire, thus making a controllable load disconnect switch  38  advantageous. Another advantage of using a controllable switch  38  is to implement what is well known to those skilled in the art as “synchronous” rectification, where the action of the switch  38  is controlled to emulate a diode. This provides the functionality of a diode but with higher efficiency, since a device, such as a field effect transistor, may be employed with lower conduction losses than a diode. 
         [0031]    The transistor type used for the internal switch  38  is chosen based on the voltage and current requirements. Industry standard transistors that can be used include FETs, IGBTs or IGCTs. The electrical implementation of the control signal  41  for controlling the conduction of the internal switch  38  is dependent on the type of transistor but is well known to those skilled in the art of power electronics. 
         [0032]    As shown in  FIG. 2 , the source controller  5  includes a microprocessor  20 , communication drivers  17  and  22  and signal-conditioning circuits  24 ,  26 , and  28 . The load controller  9  of  FIG. 1  is similar in construction to the source controller  5  but is configured with different operating software to perform the functions described in the operation sequence section, below. Referring to  FIG. 1 , before beginning operation, self-check and initialization steps are performed in steps (a), (b) and (c). The switching bridge  7  and the disconnect device  13  (if using a controllable switch  38 ) are commanded to remain in an open (non-conducting) state during initialization. 
       II. Operational Sequence 
       [0033]    Referring to  FIG. 1 , the source controller  5  verifies that the source voltage at point  33  is within a predetermined expected value and that there is no current flowing in the source power conductors, as reported by the current sensor  8 . The source controller  5  also performs a built-in testing algorithm, as is typical in the industry, to verify that its hardware and firmware are functioning properly.
       a) If the embodiment incorporates advanced monitoring options, a communication check is performed by the source controller  5  through the communication link  11  to the load controller  9 . For distribution systems that provide secured energy transfer, the source controller  5  will expect a digital verification code that matches a predetermined value to ensure that the source and load equipment are electrically compatible and authorized to receive power before energy transfer is initiated. For example, a verification code may be utilized for applications where the energy is being purchased. The source controller  5  sends a request (e.g. via an electronic communication) via the communication link  11  to the load controller  9  asking it for its status. The load controller  9  responds (e.g. via another electronic communication) with the value of voltage and current on its conductors and any fault codes. The source controller  5  verifies that the load voltage is within a predetermined value and that there is no current flowing in the load power conductors (indicating a possible failed source disconnect, failed current sensors or other hardware problem). The load controller  9  also performs built-in testing algorithms, as is typical in the industry, to verify that its hardware and firmware are functioning properly. If there is no fault registered, the sequence progresses to step (c). If a fault is registered, the sequence is repeated starting at step (a).   b) The source controller  5  makes another measurement of the source voltage at point  33  to determine the duration of the transfer period, where energy will be transferred from the source  1  to the load  3 . The higher the source voltage, the higher the potential fault current; and, hence, the shorter the transfer period. The source voltage measurement is applied to an internal table or function in the processor  20  of the source controller  5  to determine a safe duration value for the transfer period. The use of a variable transfer period is not required for the operation of the disclosed process but can make energy transfer more efficient and less prone to false alarms, since the number of measurements can be maximized and the amount of switching instances can be minimized according the length of the period. The alternative is to maintain a fixed duration transfer period that is configured for the highest possible source voltage and for the worst-case safety conditions.   c) Following the determination of the transfer period, the sample period begins. The source controller  5  acts to place the switching bridge  7  into the reverse-polarity state. If the load circuit  51  incorporates a controllable disconnect switch  38 , the load controller  9  senses any reversal in current or decreases in voltage on the transmission lines when the voltage reverses and immediately opens the disconnect switch  13 . No action is necessary from the load controller  9  if it is employing a diode  39  to perform the disconnect in the disconnect device  13 .   d) Immediately after reversal, the source controller  5  measures the transmission line current using the current sensor  8 . If the current value exceeds a predetermined maximum, a hardware fault is registered, and the switching bridge  7  is placed into the non-conducting state. The sequence skips to step (h). If there is no fault registered, the operational sequence continues to step (f).   e) Following the sample period, the source controller  5  acts to put the switching bridge  7  into a forward-polarity state. If the load circuit  51  incorporates a controllable disconnect switch  38 , the load controller  9  will sense the rapid increase in voltage across the capacitor  4  measured by a voltage sensor at point  35  and immediately close the disconnect switch  38 . No action is necessary if the load circuit  51  uses a diode  39  as a disconnect switch. Both controllers  5  and  9  continue to measure voltage and current at their respective terminals  31   a  and  31   b  and  32   a  and  32   b.      f) The source controller  5  calculates the line resistance from the voltage and current samples acquired in steps (c), (d), and (e), using one of the two methods described herein. If there is no serial communication employed between the source and load controllers  5  an  9 , the source controller  5  adds a small period where the switching bridge  7  is placed in a non-conducting state directly after the forward-conducting state in the transfer period. The difference in voltage at point  34  of  FIG. 1  before and immediately after the switching bridge  7  is placed in the non-conducting state is divided by the current to calculate line resistance. If the source and load controller  5  and  9  are equipped with serial communications, the source controller  5  can request the load voltage reading from the load controller  9  to calculate the voltage difference between the source side and the load side. Dividing the voltage difference by current returns a value for line resistance. If the line resistance is greater than a predetermined maximum value, an in-line fault is registered by the source controller  5 . A calculated line resistance less than a predetermined minimum value is indicative of a hardware failure. If a fault is registered, the source controller  5  immediately places the switching bridge  7  into a non-conducting state and proceeds to step (h). If there are no faults registered, the operational sequence repeats, starting at step (c).   g) The power distribution system is in a faulted state due to an in-line fault, cross-line fault or hardware failure. In particular embodiments, the system will allow configuration of either an automatic reset or manual reset from a faulted state. If the system is configured for manual reset, the switching bridge  7  will remain in a non-conducting state until an outside system or operator initiates a restart. The system will then restart the operational sequence from step (a). If the system is configured for automatic restart, then a delay period is executed by the source controller  5  to limit stress on equipment or personnel that may still be in contact with the power distribution conductors. In particular embodiments, the period is from 1 to 60 seconds. The system then restarts the operational sequence from step (a). For an additional level of safety, mechanical contactors may be included in series with the switching bridge  7  and/or with the disconnect switch  38  to act as redundant disconnects in the event that either the switching bridge  7  or the disconnect switch  38  has malfunctioned.       
 
       III. Summary, Ramifications and Scope 
       [0041]    The source controller  5  and the load controller  9  can include a logic device, such as a microprocessor, microcontroller, programmable logic device or other suitable digital circuitry for executing the control algorithm. The load controller  9  may take the form of a simple sensor node that collects data relevant to the load side of the system. It does not necessarily require a microprocessor. 
         [0042]    The controllers  5  and  9  can be computing devices and the systems and methods of this disclosure can be implemented in a computing system environment. Examples of well-known computing system environments and components thereof that may be suitable for use with the systems and methods include, but are not limited to, personal computers, server computers, hand-held or laptop devices, tablet devices, smart phones, multiprocessor systems, microprocessor-based systems, set top boxes, programmable consumer electronics, network PCs, minicomputers, mainframe computers, distributed computing environments that include any of the above systems or devices, and the like. Typical computing system environments and their operations and components are described in many existing patents (e.g., U.S. Pat. No. 7,191,467, owned by Microsoft Corp.). 
         [0043]    The methods may be carried out via non-transitory computer-executable instructions, such as program modules. Generally, program modules include routines, programs, objects, components, data structures, and so forth, that perform particular tasks or implement particular types of data. The methods may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote computer storage media including memory storage devices. 
         [0044]    The processes and functions described herein can be non-transitorially stored in the form of software instructions in the computer. Components of the computer may include, but are not limited to, a computer processor, a computer storage medium serving as memory, and a system bus that couples various system components including the memory to the computer processor. The system bus can be of any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. 
         [0045]    The computer typically includes one or more a variety of computer-readable media accessible by the processor and including both volatile and nonvolatile media and removable and non-removable media. By way of example, computer-readable media can comprise computer-storage media and communication media. 
         [0046]    The computer storage media can store the software and data in a non-transitory state and includes both volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of software and data, such as computer-readable instructions, data structures, program modules or other data. Computer-storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to store the desired information and that can accessed and executed by the processor. 
         [0047]    The memory includes computer-storage media in the form of volatile and/or nonvolatile memory such as read only memory (ROM) and random access memory (RAM). A basic input/output system (BIOS), containing the basic routines that help to transfer information between elements within the computer, such as during start-up, is typically stored in the ROM. The RAM typically contains data and/or program modules that are immediately accessible to and/or presently being operated on by the processor. 
         [0048]    The computer may also include other removable/non-removable, volatile/nonvolatile computer-storage media, such as (a) a hard disk drive that reads from or writes to non-removable, nonvolatile magnetic media; (b) a magnetic disk drive that reads from or writes to a removable, nonvolatile magnetic disk; and (c) an optical disk drive that reads from or writes to a removable, nonvolatile optical disk such as a CD ROM or other optical medium. The computer-storage medium can be coupled with the system bus by a communication interface, wherein the interface can include, e.g., electrically conductive wires and/or fiber-optic pathways for transmitting digital or optical signals between components. Other removable/non-removable, volatile/nonvolatile computer storage media that can be used in the exemplary operating environment include magnetic tape cassettes, flash memory cards, digital versatile disks, digital video tape, solid state RAM, solid state ROM, and the like. 
         [0049]    The drives and their associated computer-storage media provide storage of computer-readable instructions, data structures, program modules and other data for the computer. For example, a hard disk drive inside or external to the computer can store an operating system, application programs, and program data. 
         [0050]    The source and load controllers  5  and  9  can be used to meter energy transfer and communicate the information back to the user or to a remote location. For example, the disclosed methods and system can be implemented on a public charging station for electric vehicles and can be utilized to send electricity consumption data back to a central credit card processor. The transfer of information can be through an outside communication link  15 , as depicted in  FIG. 1 . A user can also be credited for electricity that is transferred from his electric vehicle and sold to the power grid. The outside communication link  15  can also be used to transfer other operational information. For example, an electric vehicle can have contacts under its chassis that drop down to make connection to a charging plate embedded in a road surface. The communication link  15  can transfer proximity information indicating that the car is over the charging plate. The information can inhibit energizing the charger plate unless the car is properly positioned. 
         [0051]    The source switching bridge  7  can be supplemented by the addition of an electromechanical relay or “contactor” providing a redundant method to disconnect the source  1  from the source terminals so as to provide a back-up in the case of a failure of the source switching bridge  7 . The load disconnect device  13  can be supplemented by an electromechanical relay or contactor in the same fashion. The electromechanical contactor activation coils can be powered by what is known to those skilled in the art as a “watchdog circuit”. The watchdog circuit continually communicates with the source or load controllers  5  and  9 ; otherwise, the contactor will automatically open, providing a fail-safe measure against “frozen” software or damaged circuitry in the controllers  5  and  9 . 
         [0052]    Referring to  FIG. 3 , one or both of switches  61  and  62  can be low-current, high-resistance switches or can include a current-limiting series-resistor. This is due to the fact that these switches are active only during the reverse-conducting state of the switching bridge  7 , where minimum current is expected to flow under normal conditions. If a fault is experienced that does draw significant current, much of the source voltage will be dropped across the switch  61 / 62  or series-resistor. Thus, an indicator of fault current can be determined by simply measuring the voltage at point  34  of  FIG. 1  and registering a fault if the voltage is more than a predetermined value less than the source voltage. 
         [0053]    An alternative embodiment of a combined load and load disconnect device  13  is shown in  FIG. 8 . In this embodiment, a series string of uni-directional switching devices—in this case, light emitting diodes  70 ,  72 ,  74 , and  76 —are combined in parallel with resistors  71 ,  73 ,  75 , and  77 . When the switching bridge  7  is in the forward-polarity state, current flows freely through the diodes  70 ,  72 ,  74 , and  76 . When the switching bridge  7  is in the reverse-polarity state, electric current is blocked by the diodes  70 ,  72 ,  74 , and  76 ; but a limited amount is allowed to flow in parallel resistors  71 ,  73 ,  75  and,  77 . If there is a fault within the load disconnect device  13 , represented by fault resistor (R fault)  78 , the parallel combination of R fault, fault  78  and resistors  71 ,  73 ,  75 , and  77  will cause an increase in current during the time when the switching bridge  7  is in the reverse-polarity state. If the current exceeds a predetermined value, a fault is registered and the source controller  5  acts to place the switching bridge  7  into a non-conducting state to disconnect the source  1  from the source terminals  31   a  and  31   b . Alternatively, a measurable decrease in current can indicate a damaged load disconnect device  13  and the source controller  5  would again register a fault and place the switching bridge  7  into a non-conducting state. 
         [0054]    The data communication link  11  and/or external communication link  15  can be implemented using various methods and protocols well known to those skilled in the art. Communication hardware and protocols can include RS-232, RS-485, CAN bus, Firewire and others. The communication link  11  can be established using copper conductors, fiber optics or wirelessly over any area of the electromagnetic spectrum allowed by regulators, such as the Federal Communications Commission (FCC), as set forth in Part 18 of the FCC rules—for example, the 2.4 GHz, 3.6 GHz, 4.9 GHz, 5 GHz, and 5.9 GHz frequencies allocated for WiFi or the 915 Mhz frequency allocated for ZigBee. Wireless communication can be established using any of a number of protocols well known to those skilled in the art, including Wi-Fi, ZigBee, IRDa, Wi-Max and others. The data communication link  11  and/or external communication link  15  of  FIG. 1  can be what is referred to by those skilled in the art as “communication over power lines”, or “communication or power line carrier” (PLC), also known as “power line digital subscriber line” (PDSL), “mains communication”, or “broadband over power lines” (BPL). Referring to the revised source controller of  FIG. 6 , communication signals generated by a microprocessor  20  are superimposed on the source terminals  31   a  and  31   b  using a modulator/demodulator  48 . The hardware and software methods of the modulator/demodulator  48  are well known to those skilled in the art. Although the source controller  5  is used as an example, an identical implementation of the modulator/demodulator  48  can be contained in the load controller  9 , allowing bidirectional communication between the source and load controllers  5  and  9 . The transmitting side, either the source  1  or load  3 , combines the communication signals with the power waveform on the source or load terminals  31   a  and  31   b  or  32   a  and  32   b . The receiving side, either the source  1  or the load  3 , would then separate the communication signals from the power waveform. 
         [0055]    To allow simultaneous power transfer and user-data communications, the system can be configured as depicted in  FIG. 7 . In one example, a CAT 5 communication cable is used to transfer ethernet data between an end-user&#39;s computer and an ethernet switch; and the same cable conductors can be used to provide 400-600 Watts of power to the computer, itself, using the methods described herein. Referring to  FIG. 7 , the source circuitry  50  can include all of the source components; or, referring to  FIG. 1 , the source circuitry  50  can include the source  1 , the source controller  5 , the switching bridge  7 , and all related source components. The load circuitry  51 , shown in  FIG. 7 , can represent all of the load components; or, referring to  FIG. 1 , the load circuitry  51  can include the load  3 , the load controller  9 , the load disconnect device  13 , and all related load components. The output conductors of the source circuitry  50  are applied to the center tap points of isolation transformers  52  and  54  on the source side of the configuration. Corresponding center tap points on isolation transformers  53  and  55  are on the load side of the configuration and are electrically connected to center points on transformers  52  and  54  through the transformer windings. On the source side, ethernet data can be applied to the windings of transformers  52  and  54  that are electrically isolated from the center-tapped side using a balanced conductor pair configuration that is well known to those in the industry. On the load side, the pairs are picked up on the corresponding windings of the transformers  53  and  55  that are electrically isolated from the center-tapped side containing the power. Because the power is essentially direct current, it passes through the transformers  52  and  54  on the source side to the load side without causing magnetic excitation and, therefore, does not corrupt the data that is also resident on the communication lines. The described hardware configuration of center-tapped transformers is commonly used in the industry for implementing power over ethernet (PoE) as is described in PoE standard IEEE-802.3a. PoE does not have the safety features, described herein, and is therefore limited to approximately 48V to avoid the possibility of electrical shock. 
         [0056]    Thus the scope of the disclosed invention should be determined by the appended claims and their legal equivalents, rather than the examples given. In describing embodiments of the invention, specific terminology is used for the sake of clarity. For the purpose of description, specific terms are intended to at least include technical and functional equivalents that operate in a similar manner to accomplish a similar result. Additionally, in some instances where a particular embodiment of the invention includes a plurality of system elements or method steps, those elements or steps may be replaced with a single element or step; likewise, a single element or step may be replaced with a plurality of elements or steps that serve the same purpose. Moreover, while this invention has been shown and described with references to particular embodiments thereof, those skilled in the art will understand that various substitutions and alterations in form and details may be made therein without departing from the scope of the invention. Further still, other aspects, functions and advantages are also within the scope of the invention; and all embodiments of the invention need not necessarily achieve all of the advantages or possess all of the characteristics described above. Additionally, steps, elements and features discussed herein in connection with one embodiment can likewise be used in conjunction with other embodiments. Still further, the components, steps and features identified in the Background section are integral to this disclosure and can be used in conjunction with or substituted for components and steps described elsewhere in the disclosure within the scope of the invention. In method claims, where stages are recited in a particular order—with or without sequenced prefacing characters added for ease of reference—the stages are not to be interpreted as being temporally limited to the order in which they are recited unless otherwise specified or implied by the terms and phrasing.