Abstract:
A protection circuit for telecommunications equipment powered remotely through a twisted pair subscriber loop, comprising: a charge storage circuit, coupled to the subscriber loop through an arm switch in parallel with an inrush resistor, for storing charge received from the subscriber loop, the charge storage circuit coupled to the telecommunications equipment for providing the charge thereto; a fault detection circuit for setting a first signal during a period of normal operation of the protection circuit and for resetting the first signal for a predetermined period during a fault; a processor adapted for setting an arm signal during the period of normal operation of the protection circuit; and, an AND logic circuit coupled to the arm switch, fault detection circuit, and processor for comparing the first and arm signals and for setting a third signal for opening and closing the arm switch.

Description:
[0001]     This application claims priority from Canadian Patent Application Number 2,434,111, filed Jun. 30, 2003 and incorporated herein by reference.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to the field of telecommunication networks, and more specifically to systems and methods for the powering and fault protection of electronic equipment where the electronic equipment is connected to one end of a subscriber loop and the power source used to power the electronic equipment is connected to the other end of the subscriber loop.  
       BACKGROUND OF THE INVENTION  
       [0003]     In telecommunications systems, some equipment may be located in a remote location where accessing a power source to power the equipment is neither economical nor desirable due to cost or marketing considerations. In these situations, the remote equipment may be powered from a power source through the same type of subscriber loop twisted pair wires (subscriber loops) that are normally used to deliver telecommunication services from service provider equipment to subscriber premises. In this situation, the remote equipment is said to be “loop powered”. Remote equipment that may be loop powered may include remote terminals (RT), pairgain devices, loop extenders, network interface devices (NID), optical network termination (ONT) equipment, integrated access devices (IAD), and subscriber communication equipment such as a POTS (plain old telephone service) telephone, IP (Internet Protocol) telephone, FAX, set top box, or data modem.  
         [0004]      FIG. 1  is a circuit diagram illustrating a typical environment  100  for loop powered remote equipment  110 . The power source  102  that sources charge to the subscriber loop  104  is typically an earth referenced power supply with an output impedance of typically less than 5 ohms. The subscriber loop  104  is located physically in the external environment and may be subject to faults  106  from lightning  150  and mains power lines  160 . Primary protectors  108  are provided on each wire at each end  103 ,  105  of the subscriber loop  104 . The remote equipment  110  typically consists of a protection circuit  120  which includes protection electronics  170  and also a charge storage circuit  180  with an input impedance of typically less than 5 ohms. Subsequent electronics  130 ,  140  in the remote equipment  110  may include a power supply  130  that typically uses a transformer  190  to isolate the application electronics  140  from the high voltages that may occur on the subscriber loop  104  due to faults  106  and a regulator  191  to develop a stable power supply to power the application electronics  140 .  
         [0005]     Due to their physical location in the external environment, faults  106  may occur between the subscriber loops  104  and foreign potentials. These faults may include lightning  150  induced current and voltage, as well as power cross and induced AC current from mains power wires  160 . For subscriber loops that are used for the delivery of mainstream telecommunications services to subscribers who use, for example, POTS telephones, modems, fax machines, and data modems, a number of systems are known for the protection of subscriber loop electronics that source charge on one end  103  of the subscriber loop  104 , typically in the line card of the service provider, and electronics that sink charge from the other side  105  of the subscriber loop  104 , typically the subscriber location. However, these systems are not applicable to subscriber loops that are used for the loop powering of remote equipment, in that the series impedance of electronics on each end of the subscriber loop utilized for loop powering, typically a few ohms, is considerably lower than the series impedance of the electronics on a subscriber loop used for mainstream applications, typically 100 ohms or more. For example, the currents developed from a lightning strike of 1000 V would be 20 times higher in the loop powered circuits, since the input impedance is 20 times smaller.  
         [0006]     In general, primary protectors  108  are provided on subscriber loops for shunting charge to earth typically when the potentials across the primary protector exceed several hundred volts. All circuits connected to the primary protectors  108  must operate with consideration of the independent behaviour of these primary protectors  108 .  
         [0007]     The power supply  102  that sources charge to one end  103  of a subscriber loop  104  is typically earth referenced, thus protection circuits from faults beyond the primary protectors  108  may be designed using relatively simple circuits, or very often no additional circuits at all, that would shunt the fault energy locally to earth. Such protection circuits may be designed to maintain the connection between the power source  102  and the subscriber loop  104  throughout fault events, however this is not always the case.  
         [0008]     Equipment  110  located remotely that is loop powered must sink current from a subscriber loop  104 . However, such equipment cannot be earth referenced and typically prevents the conduction of current to earth for voltages within the primary protector activation voltage range. Furthermore, the energy that enters the remote equipment  110  may be common mode which occurs when a fault influences both wires of the subscriber loop twisted pair  104 , or differential mode which occurs when one of the primary protectors activates prior the other primary protector on the subscriber end  105  of the subscriber loop  104 . Thus, protection circuits for electronics that sink energy from a subscriber loop  104  and that form the power supply of remotely powered electronics equipment  110  are challenging to design effectively.  
         [0009]     For the protection of remote equipment  110 , beyond the primary protectors  108 , existing systems typically use simple electronic circuits  170  that have several drawbacks. For example, fuses may be used, however, these require replacement by a service technician after every fault.  
         [0010]     Thyristors may be used to activate prior to the primary protectors  108  or in coordination with the primary protectors  108 , however as this design method requires the thyristors to conduct most of the energy in the fault event, the thyristors must therefore be quite large. Also, the voltage developed across the thyristors may have large peaks during the fault event that is presented across the subsequent electronics  130 ,  140 , thus the subsequent electronics  130 ,  140  must be over-designed and expensive. Furthermore, standards bodies are now requiring more stringent testing, thus solutions based on thyristors have greater difficulty achieving compliance.  
         [0011]     A relay or solid state switch to isolate the remote electronics  110  may be used that is activated when sensors detect a fault event. However, relays being mechanical are prone to wear and tear. In addition, if a relay or solid state switch is used to disconnect the subsequent electronics  130 ,  140  when a fault occurs and later reconnect to the subscriber loop  104  when the fault is cleared, a service interruption results. These protection circuits are thus expensive to deliver and maintain and result in interruption of service when faults occur.  
         [0012]     Even new generation remote telecommunications equipment that is loop powered with copper subscriber loops, and that relies on optical fiber for all transmission, is subject to disruption of service if the remote equipment  110  is susceptible to outages due to fault conditions that affect the copper subscriber loop (i.e., but not the optical fiber). Such remote equipment  110  may require the addition of battery power if achieving minimum service disruption is an objective. This results in increased costs for capital equipment and maintainance.  
         [0013]     A need therefore exists for an effective power and protection system for loop powered remote equipment. Consequently, it is an object of the present invention to obviate or mitigate at least some of the above mentioned disadvantages.  
       SUMMARY OF THE INVENTION  
       [0014]     According to one aspect of the invention, there is provided a protection circuit for telecommunications equipment powered remotely through a twisted pair subscriber loop. The protection circuit includes: a charge storage circuit, coupled to the subscriber loop through an arm switch in parallel with an inrush resistor, for storing charge received from the subscriber loop, the charge storage circuit coupled to the telecommunications equipment for providing the charge thereto; a fault detection circuit for setting a first signal during a period of normal operation of the protection circuit and for resetting the first signal for a predetermined period during a fault; a processor adapted for setting an arm signal during the period of normal operation of the protection circuit; and, an AND logic circuit coupled to the arm switch, fault detection circuit, and processor for comparing the first and arm signals and for setting a third signal for closing the arm switch during the period of normal operation and for resetting the third signal for opening the arm switch during the predetermined period during the fault; whereby the charge storage circuit continues storing charge through the inrush resistor during the fault.  
         [0015]     Preferably, the fault detection circuit provides the first signal by comparing a voltage measured across the arm switch to predetermined low and high voltages for each of the period of normal operation and the predetermined period during the fault, respectively.  
         [0016]     Preferably, the protection circuit further includes a diode circuit coupled between the arm switch and subscriber loop for half-wave rectifying input voltage and current signals from the subscriber loop.  
         [0017]     Preferably, the protection circuit further includes a suppression circuit coupled between the arm switch and the subscriber loop for suppressing electromagnetic interference (EMI) on the input voltage and current signals.  
         [0018]     Preferably, the suppression circuit includes first and second series inductors coupled to a parallel capacitor.  
         [0019]     Preferably, the protection circuit further includes a connect switch coupled between the charge storage circuit and the telecommunications equipment, the connect switch coupled to and controlled by the processor through a connect signal to disconnect the protection circuit from the telecommunications equipment.  
         [0020]     Preferably, the protection circuit further includes an inrush switch coupled in series with the inrush resistor, the inrush switch coupled to and controlled by the processor through an inrush signal to disconnect the inrush resistor.  
         [0021]     Preferably, the charge storage circuit includes: a voltage sensor coupled to the processor for measuring voltage provided to the telecommunications equipment and for providing a corresponding voltage signal to the processor; and, a current sensor coupled to the processor for measuring current provided to the telecommunications equipment and for providing a corresponding current signal to the processor.  
         [0022]     Preferably, the charge storage circuit further includes a series resistor coupled between first and second parallel capacitors.  
         [0023]     Preferably, the voltage sensor measures voltage across the first parallel capacitor and the current sensor measures current by measuring the voltage across the series resistor.  
         [0024]     Preferably, the processor is further adapted to, when the voltage signal exceeds a predetermined maximum voltage: reset the inrush signal to open the inrush switch and disconnect the inrush resistor; and, modulate the arm signal, thereby reducing the voltage provided to the telecommunications equipment.  
         [0025]     Preferably, the arm signal is pulse-width modulated.  
         [0026]     Preferably, the processor is further adapted to reset the connect signal to open the connect switch and disconnect the protection circuit from the telecommunications equipment when at least one of the voltage signal is below a predetermined minimum voltage and the current signal exceeds a predetermined maximum current.  
         [0027]     Preferably, the processor is further adapted to, during a period of initial operation of the protection circuit: reset the arm signal to open the arm switch; reset the connect signal to open the connect switch; and, set the inrush signal to close the inrush switch, thereby allowing the charge storage circuit to charge through the inrush resistor.  
         [0028]     Preferably, the processor is further adapted to, when the voltage signal drops below the predetermined low voltage: set the arm signal to close the arm switch and short out the inrush resistor, thereby allowing the charge storage circuit to charge more rapidly through the arm switch; and, set the connect signal to close the connect switch.  
         [0029]     Preferably, the fault includes a lightning induced current fault, a lightning induced voltage fault, a power cross fault, and induced AC current from mains power wires.  
         [0030]     Preferably, the processor is a microprocessor, a state machine, or a linear circuit.  
         [0031]     According to another aspect of the invention, there is provided a method for protecting telecommunications equipment powered remotely through a twisted pair subscriber loop, comprising: storing charge received from the subscriber loop in a charge storage circuit coupled to the subscriber loop through an arm switch in parallel with an inrush resistor, the charge storage circuit coupled to the telecommunications equipment for providing the charge thereto; setting a first signal with a fault detection circuit during a period of normal operation and resetting the first signal for a predetermined period during a fault; setting an arm signal with a processor during the period of normal operation; and, comparing the first and arm signals with an AND logic circuit coupled to the arm switch, fault detection circuit, and processor to set a third signal for closing the arm switch during the period of normal operation and to reset the third signal for opening the arm switch during the predetermined period during the fault; whereby the charge storage circuit continues storing charge through the inrush resistor during the fault.  
         [0032]     Advantageously, the processor allows for flexibility to optimally configure the protection circuit for various fault events. These fault events are sensed by the processor through its current and voltage analog inputs, the data from which is processed to control the processor&#39;s connect, inrush, and arm digital outputs such that the telecommunications equipment connected to the protection circuit is subject to minimum stress through rapid response to the fault event. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]     Further features and advantages of the embodiments of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:  
         [0034]      FIG. 1  is a circuit diagram illustrating a typical environment for loop powered remote equipment;  
         [0035]      FIG. 2  is a circuit block diagram illustrating a charge storage and protection circuit (CSAPC) in accordance with an embodiment of the invention; and,  
         [0036]      FIG. 3  is a flow chart illustrating operations of modules within a processor for controlling the charge storage and protection circuit (CSAPC) in accordance with an embodiment of the invention. 
     
    
       [0037]     It will be noted that throughout the appended drawings, like features are identified by like reference numerals.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]     In the following description, numerous specific details are set forth to provide a thorough understanding of the invention. However, it is understood that the invention may be practiced without these specific details. In other instances, well-known structures and/or processes have not been described or shown in detail in order not to obscure the invention.  
         [0039]     Referring again to  FIG. 1 , in this environment  100  and in the presence of the illustrated external protection electronics  108 , a number of fault conditions may be presented at the input nodes  122  and  124  of the remote equipment  110 .  
         [0040]     When lightning  150  strikes in close proximity to the subscriber loop  104 , current is induced in the subscriber loop  104  which may develop a potential difference between nodes  122  and  124  that exceeds the activation voltage of the primary protectors  108 . Due to the independent nature of the primary protectors  108 , one protector may change from open circuit to closed circuit before the second protector does resulting in a differential voltage appearing across terminals  122  and  124 . These faults may have either polarity. This event will be referred to as a “lightning event” in the following. A lightning event typically lasts tens of microseconds.  
         [0041]     Similarly, lightning  150  may strike in close proximity to the subscriber loop  104  and both primary protectors  108  on the wires connected to terminals  122  and  124  may activate at approximately the same time. In this event, the fault potential is experienced on both nodes  122 ,  124  simultaneously and with the same polarity. This type of fault may be referred to as a “common mode potential fault event”.  
         [0042]     Due to the close proximity of power lines  160  to the subscriber loop  104 , currents typically as high as 100 mA may be induced in the subscriber loop  104  and that will appear common mode on both wires at each end  103 ,  105  of the subscriber loop  104 . However, once they find a current path, these currents will generally not create sufficient potential to activate the protectors  108 . This type of fault, which may be referred to as a “common mode induced current fault”, may occur indefinitely during normal operation.  
         [0043]     Similarly due to the close proximity of power lines  160  to the subscriber loop  104 , physical events may occur that result in the power line  160  connecting to the subscriber loop  104 , typically through some resistance due to physical aspects of the fault connection. This type of fault may be referred to as a “power cross event” and may be sustained for long periods of time until the physical fault connection is removed and repaired.  
         [0044]      FIG. 2  is a circuit block diagram illustrating a charge storage and protection circuit (CSAPC)  200  in accordance with an embodiment of the invention. The CSAPC  200  is designed to replace the protection circuit  120  shown in  FIG. 1 . The CSAPC  200  includes a electromagnetic interference (EMI) suppression circuit  250 , a diode circuit  210 , a fault detection circuit  230 , an arm switch  224 , an inrush resistor  222 , an inrush switch  220 , an “AND” logic circuit  236 , a charge storage circuit  260 , a connect switch  270 , and a processor  280 . The function of the components of the CSAPC  200  will now be described.  
         [0045]     Circuit  250  provides EMI suppression. Input diode  210  serves to block all reverse current from the subscriber loop  104  that attempts to enter at node  204  ( 124  in  FIG. 1 ) and leave at node  202  ( 122  in  FIG. 1 ). Switch  220  and resistor  222 , which is typically 400 ohms, serve to limit inrush current, and switch  224  is activated after the inrush event is completed.  
         [0046]     Charge storage circuit  260  consists of capacitor  262  and capacitor  266  coupled through resistor  264 , which is typically 0.5 ohms.  
         [0047]     Lightning protection is provided generally by circuit  230 . Linear circuit  232  presents the voltage sensed across switch  224  to the negative input  235  of voltage comparator  234 . Threshold selection circuit  240  outputs one of two voltages to the positive input  236  of voltage comparator  234 , either “Vlow”  301  when switch  242  is on, which occurs whenever switch  224  is on, or “Vhigh”  302  when switch  246  in on, which occurs whenever switch  224  is off, as inverted by inverter  244 .  
         [0048]     Delay circuit  241  implements the change in threshold output from circuit  240  a short time after a change in state of switch  224 , typically 10 μs. When the positive input  236  to comparator  234  is greater in potential than the negative input  235 , its output is on, which turns on switch  224  depending on the state of AND gate  236 .  
         [0049]     Switch  270  connects the charge storage circuit  260  to the subsequent electronics  130 ,  140  connected to nodes  206  ( 195  in  FIG. 1 ) and  208  ( 196  in  FIG. 1 ).  
         [0050]     Linear circuit  282  presents the voltage sensed across resistor  264  to the processor  280  via the processor&#39;s “Current”  303  analog input, the voltage being proportional to the current through resistor  264 . Linear circuit  284  presents the voltage sensed across capacitor  262  to the processor  280  via the processor&#39;s “Voltage”  304  analog input.  
         [0051]     The processor  280 , which may be powered directly from capacitor  262 , is adapted to execute in a timed sequence a process based on linear input voltages Current  303  and Voltage  304  to output voltages at the processor&#39;s digital outputs “Inrush”  305 , “Ann”  306 , and “Connect”  307  to activate switch  220 , switch  224  depending on the state of AND gate  236 , and switch  270 , respectively. The processor  280  may be a microprocessor, a state machine, or a linear circuit. The processor  280  may include memory (not shown) such as RAM, ROM, disk drives, databases, etc. The processor  280  has stored therein data representing sequences of instructions which when executed cause the process described herein to be performed. Of course, the processor  280  may contain additional software and hardware a description of which is not necessary for understanding the invention.  
         [0052]     In particular, the processor  280  includes computer executable programmed instructions for directing the CSAPC  200  to implement the embodiments of the present invention. The programmed instructions may be embodied in one or more software modules (not shown) resident in the memory of the processor  280 . Alternatively, the programmed instructions may be embodied on a computer readable medium (such as a CD disk, floppy disk, memory card, or memory chip) which may be used for transporting the programmed instructions to the memory of the processor  280 . Alternatively, the programmed instructions may be embedded in a computer-readable, signal-bearing medium that is uploaded to a network by a vendor or supplier of the programmed instructions, and this signal-bearing medium may be downloaded through an interface (not shown) to the processor  280  from the network by end users, suppliers, buyers, etc.  
         [0053]     Moreover, the following description does not limit the implementation of the invention to any particular computer programming language. The present invention may be implemented in any computer programming language provided that the operating system (OS) of the processor  280  provides the facilities that may support the requirements of the present invention. A preferred embodiment is implemented in the C or C++ computer programming language (or other computer programming languages such as assembler). Any limitations presented would be a result of a particular type of operating system or computer programming language and would not be a limitation of the present invention.  
         [0054]      FIG. 3  is a flow chart illustrating operations  300  of modules within a processor  280  for controlling the charge storage and protection circuit (CSAPC)  200  in accordance with an embodiment of the invention. The initialize operation  310  is the state of operations  300  wherein the CSAPC  200  attempts to settle the charge storage circuit  260  to a stable voltage. The normal operation  312  is the state of operations  300  when the CSAPC  200  is not in the initialize operation  310 .  
         [0055]     With respect to normal operation  312 , “Vmax” is the maximum Voltage (i.e., as measured at input  304  in  FIG. 2 ) that can be tolerated by the subsequent electronics  130 ,  140  connected at nodes  206  and  208  (see  FIG. 2 ) under normal operation. “Vpwm” is a voltage margined several volts below Vmax that would be the Voltage  304  delivered to the subsequent electronics  130 ,  140  at nodes  206  and  208  when in pulse width modulation (PWM) mode, as described below. “Imax” is the load Current (i.e., as measured at input  303  in  FIG. 2 ) that is beyond the expected current range of the subsequent electronics  130 ,  140  at nodes  206  and  208  under normal operation. “Vmin” is the Voltage  304  that is too low to sustain the basic operation or functionality of the subsequent electronics  130 ,  140  at nodes  206  and  208 .  
         [0056]     The “PWM mode” is the state of normal operation  312  for actively preventing charge from entering the charge storage circuit  260  (see  FIG. 2 ) such that Voltage  304  does not rise higher than Vpwm. The “Linear mode” is the state of normal operation  312  when not in PWM mode.  
         [0057]     When in PWM mode, operation  330  selects operation  331  to execute PWM. That is, the Voltage  304  is regulated through PWM of the incoming fault waveform. Operation  332  then detects when PWM is no longer needed, such that operation  330  may select operation  333  and enter the Linear mode.  
         [0058]     Operation  320  detects if Voltage  304  exceeds Vmax, and selects to enable the PWM mode. Operation  340  detects conditions which result when too little charge is entering the charge storage circuit  260  or too much charge is being drawn from the charge storage circuit  260  such that Current  303  exceeds Imax or Voltage  304  is less than Vmin, respectively, and selects operation  341  to reduce operating conditions or operation  342  to power down and enter the initialize operation  310 .  
         [0059]     Referring to  FIG. 1 , current supplied from the power source  102  on the power source end  103  of the subscriber loop  104  is normally DC (direct current) in nature, which is required to enter node  122  and leave node  124  on the remote equipment end  105  of the subscriber loop  104 . The voltage from node  122  to node  124  may vary. The voltage delivered by the power source  102  on the power source end  103  of the subscriber loop  104  may be referred to as “Vsupply”  107  (see  FIG. 1 ), and may range from 10 V to 200 V. The current load of the subsequent electronics  130 ,  140  may be referred to as “Iload”  109  (see  FIG. 1 ) The resistance of a subscriber loop  104 , which may be referred to as “Rloop”  101  (see  FIG. 1 ), may vary from one loop to the next and between 0 ohms to 2000 ohms. Depending on the application, the maximum Rloop  101  that can be tolerated for a given Vsupply  107  and Iload  109  may require the use of several parallel subscriber loops.  
         [0060]     Referring to  FIG. 2 , protection diode  210  results in only a marginal decrease of the supplied voltage, less than one volt, at the subscriber loop interface at nodes  202  and  204  but serves several purposes. First, it prevents the CSAPC  200  from being subject to reverse potentials at nodes  202  and  204 , if the subscriber loop  104  is connected backwards. Further, diode  210  serves to reduce fault energy by approximately half, on average, as the diode blocks all faults that cause the current in the subscriber loop  104  to reverse. Thus, any faults that generate a potential at node  204  that is more positive than the potential at node  202  are blocked by diode  210  and are thus rendered benign. Third, during a fault event where the current in the subscriber loop  104  is AC (alternating current) in nature, diode  210  half-wave rectifies the current and in essence supplies additional charge into the CSAPC  200  that may be used to power the remote equipment  110  during such a fault. As such, it is an advantage of the present invention that the remote equipment  110  can be normally powered directly from a mains AC power source located near the remote equipment  110  rather than through the powered subscriber loop  104 , noting that mains AC power sources are generally readily available and convenient.  
         [0061]     Upon connection of the remote equipment  110  to the powered subscriber loop  104 , or when processor  280  begins its initialize operation  310 , Inrush  305  (see  FIG. 2 ) is set to on which turns on switch  220 , Arm  306  is set to off which turns off switch  224  through AND gate  236 , and Connect  307  is set to off which turns off switch  270 .  
         [0062]     Vhigh  302  is set to be approximately 5 V greater than the voltage settled across switch  224  at the end of a lightning event, which depends on the application selection of Vsupply  107 , maximum Iload  109 , which determines the maximum Rloop  101  allowable, and the resistance of resistor  222 . For example, for Vsupply  107 =190 V, Rloop  101 =600 ohms, and resistor  222  selected to be 400 ohms, Vhigh  302  would be 75 V.  
         [0063]     Vlow  301  is set a few volts higher than the worst case maximum of two voltages. The first voltage is the voltage developed across switch  224  which corresponds to the desired current flowing through switch  224  in a lightning event at which point switch  224  is to be turned off. For example, if the on-impedance of switch  224  is selected to be 1.25 ohms when the maximum desired current flows through switch  224  is set at 5 amps, the first voltage is 6.25 V. The second voltage is the maximum decay across the charge storage device  260  for one cycle of the AC power fail fault frequency under maximum Iload  109 . For example, if the total capacitance  262 ,  266  of the charge storage circuit  260  is 500 μF and the maximum Iload  109  is 700 mA, the decay in one cycle at 60 Hz is 6 V. Thus, for the first and second voltages of this example, Vlow  301  would be set to approximately 10 V (i.e., a few volts higher than 6.25 V).  
         [0064]     The power-up or initialize operation  310  of the processor  280  and CSAPC  200  will now be described. When the CSAPC  200  is first powered, current from the subscriber loop  104  initially flows to storage capacitor  262  through switch  220  and resistor  222 , which serves to limit the inrush current. As capacitor  262  charges, capacitor  266  charges virtually simultaneously through resistor  264 . As capacitor  262  charges and the potential across it increases, the potential across switch  224  decreases proportionally until it is lower than Vlow  301 , such that comparator  234  remains on whether it is comparing the voltage across switch  224  to either threshold Vlow  301  or Vhigh  302 . During the inrush event, the processor  280  monitors the rate of change of Voltage  304  to determine when the inrush event has sufficiently settled to ensure that the voltage across switch  224  is less than Vlow  301 , at which point Arm  306  and Connect  307  are set to on. With comparitor  234  on and Arm  306  on, switch  224  turns on through AND gate  236  which effectively provides a parallel current path that shorts out resistor  222 , thus connecting the subscriber loop interface nodes  202  and  204  to charge storage circuit  260 , which more rapidly fully charges the charge storage circuit  260 . With Connect  307  on, switch  270  is on which connects the fully charged storage circuit  260  to the subsequent electronics  130 ,  140  at nodes  206  and  208 . Operation  310  then transfers to normal operation  312 .  
         [0065]     It is an advantage of the present invention that the charge storage circuit  260  is located on the subscriber loop side of the protection switch  270  that serves to disconnect the subsequent electronics  130 ,  140  of the remote equipment  110  from the subscriber loop  104  only when necessary in a fault event, thus providing that the charge storage circuit  260  may continue to be connected to the subscriber loop  104  during fault events, and as such will collect charge from all or a portion of the fault event for the purpose of powering the remote equipment  110 . This allows the remote equipment  110 , in most cases, to remain in service throughout a fault event, as opposed to known systems in which the intent is to disconnect the fault event energy from the remote equipment  110 .  
         [0066]     It is a further advantage of the present invention that the presence of the processor  280  allows for flexibility to optimally configure the CSAPC  200  for various fault events. These fault events are sensed by the processor  280  through its Current  303  and Voltage  304  analog inputs, the data from which is processed to control the Connect  307 , Inrush  305 , and Arm  306  digital outputs of the processor  280  such that the subsequent electronics  130 ,  140  are subject to minimum stress through rapid response to the fault event. As such, the CSAPC  200  and subsequent electronic components  130 ,  140  in the remote equipment  110  need not be oversized or made more rugged than required to deliver their function under normal operation, which reduces the cost of the remote equipment  110 . Furthermore, this optimization maximizes the probability of avoiding service interruptions due to fault events. Moreover, by implementing the operations  300  in software modules stored in the memory of the processor  280 , rather than as a hardware implemented state machine, for example, ease and flexibility of adapting to present and future powering, fault requirements, and features are facilitated.  
         [0067]     In the event of a lightning fault where node  202  is more positive than  204 , the industry recognized model transient event typically has a rise time to peak voltage and further fall time to half the peak voltage of either 10 μs to 1000V and 1000 μs further to 500 V, or 2 μs to 2500 V and 10 μs to 1250 V, respectively. This transient speed typically exceeds the cumulative response time of sensors, processor algorithms, and subsequent switching events, mitigating the need for a local independent circuit to deal with the fault. In the present invention, circuit  230  is configured to deal with such faults. As lightning current flows through switch  224 , a voltage develops across it proportional to its on-impedance. When this potential exceeds Vlow  301 , comparator  234  turns off, and turns off switch  224  through AND gate  236 . The lightning current continues to flow safely through switch  220  and resistor  222  into the charge storage circuit  260 , thus adding more charge to the charge storage circuit  260  to power the subsequent electronics  130 ,  140  during the fault. With switch  224  off, threshold circuit  240  outputs Vhigh  302  to comparator  234 . After switch  224  turns off, the fault increases to its maximum voltage as seen at nodes  202  and  204 , then decays until the voltage across switch  224  decreases to a voltage dependant on Vsupply  107 , Rloop  101 , Iload  109 , and the resistance of resistor  222  which under all application conditions settles below Vhigh  302 . At that time, comparator  234  turns on which turns on switch  224  through AND gate  236 . After which, the potential across switch  224  falls below Vlow  301  more rapidly than the delay  241  which prevents switching in the threshold selection circuit to Vlow  301 , such that the comparitor  234  remains on and thus switch  224  remains on through AND gate  236  throughout the termination of the lightning fault event, and the circuit  200  continues under normal operation  312 .  
         [0068]     In the event of a common mode potential fault, both nodes  202  and  204  will simultaneously reach a potential at the activation voltage of the primary protectors  108 , which is typically several hundred volts. In the loop powered remote equipment  110 , it is required that all interfaces to earth potential block voltage to a level greater than the worst case activation voltage of the primary protectors  108 , typically 1000 V. Thus as all voltages developed in the remote equipment  110  are below the voltage blocked at all interfaces to earth potential, this fault it rendered benign.  
         [0069]     In the event of common mode induced current fault, the potential at the powered end  103  of the subscriber loop  104  is tied to earth typically through a few ohms resistance. Thus, a significant potential does not develop and power supply  102  is expected to continue in normal operation indefinitely in the presence of this fault. In the loop powered remote equipment  110 , the common mode induced current fault currents, which typically do not exceed 100 mA per subscriber loop wire, are blocked with respect to earth potential at all interfaces and thus do not flow into the CSAPC  200 . As a result, they must flow through the subscriber loop resistance, typically 1000 ohms maximum per wire, to the powered end  103  of the subscriber loop  104  which is tied to earth. In this case, the common mode potential developed at nodes  202  and  204  reaches a maximum positive or negative voltage of approximately 100 V, which will not activate the primary protectors  108 , nor does it exceed the voltage barrier to earth which is typically 1000 V, and thus this fault is rendered benign.  
         [0070]     In the event of power cross fault, or when powered from a local AC mains power source, AC current enters the remote equipment  110  and is half-wave rectified by input diode  210 . In the instance that the AC current arrives when the remote equipment  110  is running in normal operation  312 , charge from the AC current is added to charge storage circuit  260 , and Voltage  304  will rise. Depending on the charge contributed by the AC current, Voltage  304  may not rise above Vmax to require any action, and the processor  280  will continue in linear mode. If Voltage  304  rises above Vmax as detected by operation  320 , PWM mode is enabled.  
         [0071]     PWM mode is implemented by operation  331  by pulse width modulating Arm  306  to control the state of switch  224  through AND gate  236  to regulate Voltage  304  at Vpwm while the CSAPC  200  is under load by the subsequent electronics  130 ,  140 . Inductors  252  and  254  of the EMI circuit  250  serve to minimize the current peaks of the regulation. Inrush  305  is turned from on to off. The PWM operation  331  phase locks to the half-wave rectified waveform, using Voltage  304  and Current  303 , and turns Arm  306  on during the half-period when the waveform is blocked by diode  210  to reduce noise and to modulate the turn-off timing of Arm  306  to occur sometime during the half-period when the waveform is not blocked by diode  210 , i.e., when charge is being added to the charge storage circuit  260 . The maximum decay of the charge storage circuit  260  must be less than a margined amount below Vlow  301  in order to ensure that comparator  234  remains on constantly during the PWM operation  331 , in order for switch  224  to activate through AND gate  236  when Arm  306  is turned on, and to ensure circuit  230  will respond in the event of lightning while in PWM mode.  
         [0072]     In the example given above, if the total capacitance  262 ,  266  of the charge storage circuit  260  is 500 μF and the maximum Iload  109  is 700 mA, the decay in one cycle at an AC current frequency of 60 Hz is 6 V, in which case the selection of Vlow  301  at 10 V would ensure proper operation. As such, the operation of the remote equipment  110  may continue indefinitely in the presence of added AC current. When the fault is removed, or for example the AC current is reduced, the additional charge added to charge storage circuit  260  will be reduced, Voltage  304  will decrease, the pulse width modulation will no longer be required as detected by process  332 , and operations  300  will return to Linear mode by operation  333 .  
         [0073]     Due to a fault event, the equipment  102  providing the power at the powered end  103  of the subscriber loop  104  may activate its own protection circuitry and disconnect from the loop  104 . In this case, the operation of the remote equipment  110  would have to be sustained solely from the charge delivered by the fault, which may not be sufficient, in which case Voltage  304  would decrease. Also, it may occur that under normal operation of the remote equipment  110 , excessive functionality or services are enabled, or an equipment failure occurs, or a fault occurs within the subsequent electronics  130 ,  140 , in which case Current  303  would increase. According to another aspect of the invention, operation  340  can determine if Current  303  exceeds Imax, or if Voltage  304  is below voltage Vmin, in which case operation  341  is executed to reduce in graduated fashion the operating conditions experienced by the subsequent electronics  130 ,  140 . Such a change in operating conditions may include the dynamic reduction (e.g., minimization) in services provided by the remote equipment  110 . If the changes in operating conditions cannot be reduced further, operation  342  is selected to power down the remote equipment  110  and then proceed to enter the Initialize operation  310 .  
         [0074]     It is a further advantage of the invention that the subscriber loop  104  used to loop power the remote equipment  110  may also be utilized to carry transmission information to the remote equipment  110  using a transmission protocol including, for example, the POTS frequency band, ISDN, DSL, or other protocols.  
         [0075]     While aspects of the invention are primarily discussed as a process or method, a person of ordinary skill in the art understands that the apparatus discussed above with reference to a CSAPC  200  and processor  280  may be programmed to enable the practice of the method of the invention. Moreover, an article of manufacture for use with a CSAPC  200  and processor  280 , such as a pre-recorded storage device or other similar computer readable medium including program instructions recorded thereon may direct the CSAPC  200  and processor  280  to facilitate the practice of the method of the invention. It is understood that such apparatus and articles of manufacture also come within the scope of the invention.  
         [0076]     The embodiments of the invention described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.