Abstract:
Disclosed is a system and method for providing power load management in a communication system while limiting cost and space requirements.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This Application claims the benefit of application Ser. No. 60/147,140 of JOSEPH EDWARD SUTHERLAND, GUS CLINT SANDERS, JR., AND PAUL DANIEL QUEEN. JR. filed Aug. 4, 1999 for POWER INTERRUPTER FOR LOAD-SHEDDING, the contents of which are herein incorporated by reference.  
           [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to power load management and, more particularly, to systems and methods of management of loads to conserve power.  
           [0004]    1. Description of Related Art  
           [0005]    Typical design constraints on electronic systems, such as those for communication services, include limited cost and limited space requirements.  
         SUMMARY OF THE INVENTION  
         [0006]    It is an object of the present invention to provide systems and methods for power load management without requiring excessive hardware and space.  
           [0007]    To achieve this and other objects of the present invention, in a first system including a building with a subscriber, and a structure located away from the building, a communication system comprises a first circuit for sending a first signal from the structure to the building; a second circuit for sending a second signal from the structure to the building, while the first circuit sends the first signal; a generator that generates a DC power signal in response to an AC power signal; a battery coupled to the generator and to the first circuit; a detector that detects a condition of the AC power signal, to generate a detector signal; a switch between the battery and the second circuit, the switch being responsive to the detector signal, to decouple the battery from the second circuit while maintaining a coupling of the battery to the first circuit.  
           [0008]    According to another aspect of the present invention there is a communication system for a first system with a building with a subscriber, a structure located away from the building, and a battery. The communication system comprises first sending means for sending a first signal from the structure to the building; second sending means for sending a second signal from the structure to the building, concurrently with the previous means; a conducting path for making a coupling between the battery and the first sending means; means for generating a DC power signal in response to an AC power signal and sending the DC power signal to the battery; means for detecting a condition of the AC power signal, to generate a third signal; and means for selectively decoupling the battery from the second sending means, depending on the third signal, while maintaining the coupling of the battery to the first sending means.  
           [0009]    According to yet another aspect of the present invention there is a method for a system including a building with a subscriber, and a structure located away from the building, and a structure enclosing first and second circuits, a battery coupled to the first circuit. The method comprises sending a first signal from the first circuit to the building; concurrently with the previous step, sending a second signal from the second circuit to the building; generating a DC power signal in response to an AC power signal and sending the DC power signal to the battery; detecting a condition of the AC power signal, to generate a third signal; and selectively decoupling the battery from the second circuit, depending on the third signal, while maintaining the coupling, of the battery to the first circuit.  
           [0010]    According to yet another aspect of the present invention there is a system for operating with a power line, a power node downstream from the power line, an electrical outlet with a housing having an insulating face plate, the face plate having spaced openings, and electrical contacts in alignment with each of the openings in the face plate, the electrical contacts being coupled to the power line. The system comprises a load for dissipating power from the power node; a power distribution path from the power node to the load, the power distribution path including a switch having a control input, the power distribution path excluding the electrical outlet; and a sensor for monitoring a first signal from the electrical outlet, to send a control signal to the control input of the switch.  
           [0011]    According to yet another aspect of the present invention there is a method for system including a power line, a load, and an electrical outlet with a housing having an insulating face plate, the face plate having spaced openings, an electrical contacts in alignment with each of the openings in the face plate, the electrical contacts being coupled to the power line. The method comprises receiving power from the power line through a path including a switch having a control input, and excluding the electrical outlet and dissipating the received power in the load; monitoring a first signal from the electrical outlet, to send a control signal to the control input of the switch. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is diagram of a communication system in accordance with a preferred embodiment of the present invention.  
         [0013]    [0013]FIG. 2 is a view of an outdoor cabinet in the preferred communication system.  
         [0014]    [0014]FIG. 3 is a view of a shelf in the cabinet shown in FIG. 2.  
         [0015]    [0015]FIG. 4 is a diagram of a card in the shelf shown in FIG. 3.  
         [0016]    [0016]FIG. 5 is a block diagram showing some circuitry in cabinet shown in FIG. 2.  
         [0017]    [0017]FIG. 6 is a block diagram emphasizing some of the circuitry shown in FIG. 5.  
         [0018]    [0018]FIG. 7 is a state diagram for describing a process performed by the circuitry shown in FIG. 6.  
         [0019]    [0019]FIG. 8 is a diagram emphasizing a cabling arrangement in the preferred system.  
         [0020]    [0020]FIG. 9 is a diagram showing the cabling arrangement of FIG. 8 in more detail.  
         [0021]    [0021]FIG. 10 shows an electrical outlet of FIG. 8 in more detail. 
     
    
       [0022]    The accompanying drawings which are incorporated in and which constitute a part of this specification, illustrate embodiments of the invention and, together with the description, explain the principles of the invention, and additional advantages thereof. Throughout the drawings, corresponding parts are labeled with corresponding reference numbers.  
       DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    [0023]FIG. 1 shows system  1  in accordance with a preferred embodiment of the present invention. System  1  includes central office  3  managed by a telephone company or other type of communication provider. Central office  3  provides communication services to a plurality of subscribers, in office building  8 ,  10 , and  14 ; and homes  12  and  16 . Central office  3  provides communication services to the subscribers via telephone service link  23 , data service link  28 , remote site  5 , and respective subscriber lines  9 ,  11 ,  13 ,  15 , and  17 . Each subscriber line is a tip and ring twisted pair, including 2 copper wires constituting 2 contiguous current paths between remote site 5 and the building of a subscriber.  
         [0024]    Central office  3  includes circuitry that passes data between DS1 link  28  and service provider networks  20  in the global Internet. Thus, system  1  transfers various services between multiple servers and multiple subscribers.  
         [0025]    Remote site  5  includes digital loop carrier system  22  and access circuitry  25 . In this patent application, the word circuit or circuitry encompass both dedicated analog or digital hardware and programmable hardware, such as a CPU or reconfigurable logic array, in combination with programming data, such as sequentially fetched CPU instructions or programming data for a reconfigurable array.  
         [0026]    Interrupter module  107  selectively supplies power to circuitry  25 , as described in more detail below.  
         [0027]    Access circuitry  25  acts to combine data from networks  20  with an analog, voice band, signal from digital loop carrier system  22 , to send a composite signal to subscribers via the subscriber lines. Circuitry  25  receives and encodes data from networks  20  to generate a discreet multitone technology (DMT) signal, combines the DMT signal with an analog signal from digital loop carrier system  22 , and sends the composite signal over line  11  to a subscriber in office building  10 . Conversely, circuitry  25  receives a composite signal from the subscriber in building  10  via line  11 , filters the composite signal to send a digital signal to networks  20 , and filters the composite signal to send an analog signal to digital loop carrier system  22 .  
         [0028]    The exemplary system  1  is optimized for SONET (Synchronous Optical NETwork) OC3 technologies and standards between networks  20  and central office  3 , and for DS1 (Digital Signal 1) technologies and standards between central office  3  and remote site  5 . Those skilled in the art will understand that the basic architecture of system  1  is applicable to many other technologies and standards.  
         [0029]    [0029]FIG. 2 shows cabinet  102  located at remote site  5 . Cabinet  102  encloses DLC system  22 , access circuitry  25 , and batteries  104 . Cabinet  102  receives AC power from 60 Hz AC power source  118 , via power line  119 . Cabinet  102  includes a door (not shown).  
         [0030]    Batteries  104  supply power during disruptions of AC power source  118 . More specifically, DLC system  22  is powered by a −48V bank of batteries  104  kept on continuous float-charge by chargers  121  powered by AC source  118 , which is the local AC power line. Batteries  104  are sized to run DLC system  22  for a specified time (e.g. 8 hours) during AC power outages in order to maintain lifeline POTS (plain old telephone service), and allow the operating company time to deploy an emergency generator if necessary. Each charger  121  is essentially and AC to DC converter that receives an AC signal from the power line  119  and sends a DC signal toward access circuitry  25 .  
         [0031]    AC power outlet  112  is on interior wall  108  of cabinet  102 . AC power outlet  112  is UL Approved, meaning that AC power outlet  112  conforms to a standard of Underwriters Laboratories Inc. (UL). AC outlet  112  is for powering craft equipment. AC outlet  112  includes 2 sockets each having a left contact  14 , a right contact  116 , and a ground contact  117 . Each of contacts  114 ,  116 , and  117  is in an aperture defined by AC power outlet  112 .  
         [0032]    Node  109  is common to the output of charger  121  and the outputs of batteries  104 . As represented in FIG. 2, the power input of DLC system  22  is connected to node  109 .  
         [0033]    The power input of interrupter module  107  is connected to node  109 . The output of interrupter module  107  is connected to the power input of access circuitry  25 . Module  107  is removably connectable to AC outlet  112  via plug  111 . Plug  111  includes a body  105  composed of an insulating material, and a left conducting prong  113  for contacting left contact  114 , a right prong  115  for connecting with right contact  116 , and a ground prong  120  for contacting ground contact  117 . Interrupter module  107  selectively supplies power to circuitry  25 , depending on the signal from AC source  118 , as sensed through plug  111 .  
         [0034]    More specifically, detector  123  is coupled to left prong  113  and to right prong  115 . Detector  123  detects a voltage difference between contacts  114  and contacts  116  (detects a voltage across contacts  114  and contacts  116 ), by detecting a voltage difference between left prong  113  and right prong  115 . In other words, detector  123  monitors AC power line  119  by receiving a signal through contact  114 . Detector  123  detects a voltage difference between contact  114  and another node. In the preferred embodiment, the other node is contact  116 .  
         [0035]    One or more shelves  30 , housing access circuitry  25 , coexist in cabinet  102  with one or more digital loop carrier systems (DLCs)  22 , providing lifeline POTS service. DLC  22  is a digital transmission system for subscriber loop plant. DLC  22  multiplexes many subscriber voice channels onto very few wires or onto a single fiber pair. More specifically, digital loop carrier system  22  may concentrate individual voice lines to T1 lines, cellular antenna sites, PBXs.  
         [0036]    [0036]FIG. 3 shows compact shelf  30  supporting access circuitry  25  in remote site  5 . Shelf  30  houses low pass filter cards (LPFCs)  70 - 75 , and line termination cards  50 - 55  (LTs) for communication with subscribers.  
         [0037]    Referring to FIGS. 3 and 5, network termination cards  36  and  37  (NTs) interface with DS1 I/O circuitry  8  leading to DS1 line  28 . Alarm-craft interface card  45  collects alarm information from circuitry  25 , displays the alarm information locally, and sends the alarm information to other systems. Shelf  30  can accommodate either 1 or 2 NTs, depending on whether redundancy is required. Each LT includes 4 subscriber lines.  
         [0038]    Shelf  30  is essentially a mechanical backplane mechanically supporting signal busses  35 ,  31 ,  38 , and  39 . Each of busses  35 ,  31 ,  38 , and  39  includes a plurality of parallel data lines and a plurality of control lines.  
         [0039]    Each of cards  36 ,  37 ,  45 ,  50 - 55 , and  70 - 75  connects to the mechanical backplane via a respective backplane connector  18 , such as connector  18  of card  50  shown in FIG. 4. Each backplane connector  18  includes a plastic, insulating housing  93  enclosing and supporting a plurality of parallel conductors  94  for sending signals between a card and the backplane. For each of cards  36 ,  37 , and  50 - 55 , the conductors are for receiving power from node  110 , which is the output of interrupter module  107 . For each of cards  36 ,  37 , and  50 - 55 , the conductors are also for sending signals between the card and busses  35 ,  31 ,  38 , and  39 . For example, the conductors inside connector  18  of NT card  37  allow card  37  to sends signals to downstream bus  35  and receive signals from upstream bus  38 . The conductors in connector  18  of LT card  51  allow LT card  51  to receive signals from bus  35  and bus  31 , and to send signals to bus  38  and bus  39 .  
         [0040]    Each of cards  36 ,  37 ,  45 ,  50 - 55 , and  70 - 75  is removably connected to the mechanical backplane.  
         [0041]    More details about shelves, such as compact shelf  30 , are disclosed in connection with a “RAM (Remote ADSL Mux)” in U.S. patent application Ser. No. 08/891,145 by RICHARD M. CZERWIEC, JOSEPH E. SUTHERLAND, PETER M. L. SCHEPERS, GEERT A. E. VAN WONTERGHEM, MARLIN V. SIMMERING, EDUARD C. M. BOEYKENS, CHRIS VAN DER AUWERA, PETER A. R. VAN ROMPU, KURT PYNAERT, DANIEL A. C. VERLY, GILBERT A. F. VAN CAMPENHOUT, RICHARD H. BAILEY, ROBERT N. L. PESCHI, DIRK M. J. VAN AKEN, EMMANUEL F. BOROWSKI, PETER P. F. REUSENS, HERMAN L. R. VERBUEKEN, FRANK RYCKEBUSCH, KOEN A. G. DE WULF filed Jul. 10, 1997 for TELECOMMUNICATIONS SYSTEM FOR PROVIDING BOTH NARROWBAND AND BROADBAND SERVICES TO SUBSCRIBERS; SUBSCRIBER EQUIPMENT; A SHELF THEREFOR; A REPLACEABLE LOWPASS FILTER UNIT; LINE TERMINATION EQUIPMENT; NETWORK TERMINATION EQUIPMENT; AND A TELECOMMUNICATIONS RACK WITH A PLURALITY, the contents of which is herein incorporated by reference.  
         [0042]    The RAM cited in the previous paragraph, is also described in European Patent Application No. 98401239.3 by RICHARD M. CZERWIEC, JOSEPH E. SUTHERLAND, PETER M. L. SCHEPERS, GEERT A. E. VAN WONTERGHEM, MARLIN V. SIMMERING, EDUARD C. M. BOEYKENS, CHRIS VAN DER AUWERA, PETER A. R. VAN ROMPU, KURT PYNAERT, DANIEL A. C. VERLY, GILBERT A. F. VAN CAMPENHOUT, RICHARD H. BAILEY, ROBERT N. L. PESCHI, DIRK M. J. VAN AKEN, EMMANUEL F. BOROWSKI, PETER P. F. REUSENS, HERMAN L. R. VERBUEKEN, FRANK RYCKEBUSCH, KOEN A. G. DE WULF, filed May 25, 1998 for a TELECOMMUNICATIONS SYSTEM FOR PROVIDING BOTH NARROWBAND AND BROADBAND SERVICES TO SUBSCRIBERS; SUBSCRIBER EQUIPMENT; A SHELF THEREFOR; A REPLACEABLE LOWPASS FILTER UNIT; LINE TERMINATION EQUIPMENT; NETWORK TERMINATION EQUIPMENT; AND A TELECOMMUNICATIONS RACK WITH A PLURALITY. The contents of European Patent Application No. 98401239.3 are herein incorporated by reference.  
         [0043]    [0043]FIG. 5 is a block diagram emphasizing some signal paths in the preferred system. In the example immediately following, NT  37  includes a DS1 port in an active mode and NT  36  includes a DS1 port in a standby mode. Referring FIGS. 3 and 5, each LT has an associated LPF card (LPFC). For example, bus  88  includes 4 pairs of conductors, a pair for each subscriber, between LT  50  and LPFC  70 . Bus  89  includes 4 pairs of conductors between LT  51  and LPFC  71 . Bus  90  includes 4 pairs of conductors between LT  52  and LPFC  72 . Bus  91  includes  4  pairs of conductors between LT  53  and LPFC  73 .  
         [0044]    An LPFC includes any filtering circuitry provided to the subscriber lines. For example each LPFC includes a respective low pass filters (LPFs)  92  between the subscriber lines and DLC  22 .  
         [0045]    NT  37  receives Asynchronous Transfer Mode (ATM) cells from DS1 line  28 , via circuitry  8 , and sends the cells over downstream bus  35 . Each ATM cell includes a pair of identifiers: a Virtual Path Identifier (VPI) and a Virtual Channel Identifier (VCI). Each LT recognizes a set of VPI/VCI pairs (addresses) as identifying a cell destined for one or more subscribers connected to the LT. For example, LT  52  recognizes a set of 1 or more VPI/VCI addresses as identifying a cell destined for a subscriber in building  14 . Upon recognizing such a cell, LT  52  generates a DMT signal encoding the cell, and sends the signal to LPFC  72 . LPFC  72  combines the DMT signal with an analog signal from DLC  22 , to send a composite signal to the subscriber in building  14 , via line  15 .  
         [0046]    When a subscriber wishes to send data to service provider networks  20 , the subscriber modem encodes the data in a DMT signal and sends the DMT signal over, a subscriber line. This DMT signal passes from one of the LPFCs, to a high pass filter in an LT car, to send a digital signal to NT  37  via upstream bus  38 .  
         [0047]    Thus, NT card  37 , downstream bus  35 , and upstream bus  38  act to provide the subscribers with access to service provider networks  20 . During this time, NT card  36 , downstream bus  31 , and upstream bus  39  are in a standby mode in case NT  37 , bus  35 , or bus  38  should malfunction.  
         [0048]    Interrupter module  107  receives DC power on node  109  and selectively passes the DC power to-access circuitry  25  via power node  110 , depending on a detected condition of a signal on power line  119 .  
         [0049]    [0049]FIG. 6 is a block diagram emphasizing interrupter module  107  in more detail. Module  107  is a small unit including a voltage detector  123  for detecting a power outage of AC power source  118 , and timers  125  including an outage timer for measuring the duration of the outage and a recovery timer for measuring a duration of power restoration after an outage. Drivers  127  are responsive to timers  125 . Drivers  127  command relay  136  to open or close via relay control inputs  140 , thereby selectively connecting node  109  to node  110 .  
         [0050]    Converter  129  converts the −48 volt power signal from batteries  104  to a voltage level usable by detector  123 , timers  124 , and drivers  127 .  
         [0051]    Relays  137  and  138  are provided in case cabinet  102  contains multiple battery systems and multiple shelves  30 . In other words, interrupter  107  can interrupt power to multiple shelves  30 , each fed from a separate cabinet power bus, for compatibility with distributed cabinet power practice.  
         [0052]    Outlet  135 , represented in FIG. 6, is a pass-thru grounded AC power outlet, to functionally replace the outlet occupied by plug  111  of module  107 . Outlet  135  has the same structure as outlet  112 . When plug  111  is engaged with one of the sockets of outlet  112 , both sockets of outlet  135  are coupled to power line  119 .  
         [0053]    [0053]FIG. 7 is a state diagram describing the position of relay contacts  136 ,  137 , and  138 . Interrupter module  107  interrupts DC power input to access circuitry  25  in response to a local AC power outage persisting for more than a specified length of time, t1. When detector  123  detects an AC outage, timers  125  start an outage timing process. If AC power is restored before the time period t1 has elapsed, timers  125  reset the timing process, and any subsequent outage starts the timer process again from 0. Once the outage has persisted for the required time t1, drivers  127  command relays  136 ,  137 , and  138  to open, thereby interrupting battery power to access circuitry  25 . The time t1 may be several minutes, for example.  
         [0054]    When detector  123  detects that AC power has been restored, timers  125  start a restoration timing process. When AC power has been restored without further interruption for a specified length of time t2 drivers  127  command relays  136 ,  137 , and  138  to close. thereby restoring power to access circuitry  25 .  
         [0055]    Module  107  includes a light emitting diode (LED)  132  for visual status indication. Referring to FIG. 7, in state  1  (AC present, DC not interrupted) drivers  127  cause LED  132  to be continuous green. In state  2  (AC outage, DC not yet interrupted ) drivers  127  cause LED  132  to be flashing green. In state  3  (AC outage, DC interrupted) drivers  127  cause LED  132  to be continuous red. In state  4  (AC restored, but DC still interrupted) drivers  127  cause LED  132  to be flashing red.  
         [0056]    Audible AC-outage indicator  130  may be a buzzer such as piezo transducer, for example. Drivers  127  activate indicator  130  during states  2  and  3  (AC outage). This feature alerts local craft in case they inadvertently unplug module  107 , while looking for an AC-outlet for tools or test equipment, for example.  
         [0057]    Timers  125  and drivers  127  cutoff indicator  130  after time period, t3, and clear this cutoff condition upon transition to state  1 .  
         [0058]    [0058]FIG. 10 shows outlet  112  in more detail. Outlet  112  includes an upper socket body  160  composed of an electrically insulating material, and lower socket body  161  composed of an electrically insulating material. Socket body  160  defines a left aperture  162 , a right aperture  163 , and a round aperture  164 . Left contact  114  is inside of left aperture  162 , right contact  116  is inside right aperture  163 , and ground contact  117  is inside round aperture  164 .  
         [0059]    Socket body  165  is composed of an insulating material. Socket body  165  defines left aperture  165 , right aperture  166 , and round aperture  167 . Left contact  114  is inside left aperture  165 , right contact  116  is inside right aperture  166 , and ground contact  117  is inside round aperture  167 .  
         [0060]    In summary, node  109  is downstream from power line  119 . Outlet  112  includes housing  126  with socket body  160 . Body  160  defines apertures  162 ,  163 , and  164  having a certain spacing relative to each other. Outlet  112  includes electrical contact  114  in alignment with aperture  162 , and contact  116  in alignment with aperture  163 . Contacts  114  and  116  are electrically coupled to power line  119 . Access circuitry  25  is essentially a load for dissipating power from node  109 . A power distribution path from the node  109  to access circuitry  25  includes relay  136  having a control input  140  responsive to voltage detector  123  and timers  125 , via drivers  127 . This power distribution path excludes electrical outlet  112 . Detector  112  is a type of sensor that monitors a signal from outlet  112 , via plug  111 , to generate a control signal for control input  140  of relay  136 .  
         [0061]    Contacts  113 ,  115 , and  120  extend from insulating body  105  of plug  111 . Contacts  113 ,  115 , and  120  have a spacing corresponding the spacing of aperatures  162 ,  163 , and  164 .  
         [0062]    [0062]FIG. 8 is a diagram emphasizing some cabling in the preferred system. Module  107  includes male connector  141  and female connector  142  of interrupter cable  143 . Connector  141  connects directly to connector  147 , to connect node  109  with interrupter  107 . Connector  142  connects with connector  139  to connect node  110  to access circuitry  25 .  
         [0063]    [0063]FIG. 9 shows the cabling arrangement of FIG. 8 in more detail. Male connector  141  includes a plastic, insulating housing  152  enclosing and supporting internal wires  146  and  151 . Internal wire  146  is for transferring DC power from cable  134 , via connector  147 ; to interrupter  107 , via wire  149  in cable  143 . Wires  146  and  149  are part of node  109 .  
         [0064]    Internal wire  151  is for transferring DC power from interrupter  107 , via wire  155  in cable  143 ; to access circuitry  25 , via inter-connector wire  157  and female connector  14 . Wires  153 ,  155 , and  157  are part of node  110 .  
         [0065]    “Y” splitters allow daisy-chaining of multiple shelves  30  off a single set of interrupter  107  terminals.  
         [0066]    The power plug may a “pig-tail” type, as shown in FIG. 8, or may be mounted directly on the housing  106  of interrupter module  107 . DC power connections are via via screw terminals, covered for safety. To minimize cable clutter, multiple sets of terminals may be provided on housing  106  of module  107 , rather than by “Y” splitters.  
         [0067]    In any event, a “Y” splitter physical design is presently preferred at the shelf  30  end of the cable between each shelf  30  and module  107  to electrically insert module  107  in the access circuitry  25  power path, thereby allowing easy addition of module  107  to existing installations, and avoiding substantial change in basic installation procedures or cabling.  
         [0068]    Of course, certain numerical quantities will be specified depending on the requirements of the system. These quantities include battery voltage range, the outage and recovery time periods: t1 and t2, maximum power dissipated, maximum current controlled (based on maximum access circuitry  25  load).  
         [0069]    It is preferred that the failure-to-trigger rate be such that interrupter  107  fails to interrupt DC power during less than 1% of all actual AC outages.  
         [0070]    It is preferred that the false-triggering rate be such that interrupter  107  interrupts DC power inappropriately (i.e. when there is no AC outage) at a rate that decreases the total cell relay service reliability of the access circuitry  25  traffic by less than about 10%. Based on 22.7 min/yr down time for total cell relay service in cabinet applications, the false-triggering rate objective, above, translates to a FIT (failures in ten thousand hours) rate of about 1400 FITS (downtime of about 0.75 min/yr) for interrupter  107  (based on support of 3 shelves  30 ).  
         [0071]    Module  107  maintains any isolation required between AC &amp; DC inputs including grounds.  
         [0072]    It is preferred that module  107  not falsely trip a ground fault interrupter, if cabinet  102  so equipped, upon installation, removal, or during operation.  
         [0073]    The most economical embodiments may require some custom molding for housing  106  of module  107 , and the cable connector (“Y”) preferably matches that on the load.  
         [0074]    As a general design consideration, it is presently preferred that any malfunction of interrupter  107  tends to leave the access circuitry  25  powered.  
         [0075]    Relays  136 ,  137 , and  138  may be mechanical or may be solid state with no moving parts.  
         [0076]    A set of the LTs share upstream bus  38  using a priority-based, cell grant multiplexing scheme, such as described in U.S. patent application Ser. No. 09/084,750 by PHILIPPE GUILLAUME DOBBELAERE and PASCAL LEFEBVRE, filed May 26, 1998 for a method of prioritized data transmission and data transmission arrangement. The contents of U.S. application Ser. No. 09/084,750 are herein incorporated by reference.  
         [0077]    A priority-based, cell grant multiplexing scheme, is also described in U.S. patent application Ser. No. 09/022,177 by PHILIPPE GUILLAUME DOBBELAERE and GEERT ARTHUR EDITH VAN WONTERGHEM, filed Feb. 11, 1998 for a priority-based access control method and arrangement. The contents of U.S. application Ser. No. 09/022,177 are herein incorporated by reference.  
         [0078]    The priority-based, cell grant multiplexing scheme, cited in the previous paragraph, is also described in European Patent Application No. 97400303.0 by PHILIPPE GUILLAUME DOBBELAERE and GEERT ARTHUR EDITH VAN WONTERGHEM, filed Feb. 11, 1997 for a Priority-based access control method and arrangement. The contents of European Patent Application No. 97400303.0 are herein incorporated by reference.  
         [0079]    Systems and methods of detecting silent failures in redundant circuitry are disclosed U.S. patent application Ser. No. 09/450,714 by RICHARD M. CZERWIEC, JAN DE GROOTE, RICHARD R. RZONCA, MARLIN V. SIMMERING, and GEERT VAN WONTERGHEM filed Nov. 30, 1999 for COMMUNICATION SYSTEM HAVING ENHANCED RELIABILITY, the contents of which is herein incorporated by reference.  
         [0080]    Interrupter  107  of the embodiment described above is an external implementation, in the sense that interrupter  107  is a separate module installed outside the access circuitry  25 . One advantage of this external implementation is that it facilitates control of power to multiple shelves  30 .  
         [0081]    The connection arrangement of the preferred system may be contrasted with the conventional scheme in which a female connector of cable  134  would connect directly to male connector  139  of access circuitry  25 .  
         [0082]    Module  107  including the connector arrangement described above provides a general-purpose method of retrofitting a system to add a power interrupter function with minimal modification to existing hardware. The connector arrangement allows quick insertion of the module  107  into the electrical path to a load, with only a brief downtime.  
         [0083]    According to an alternative embodiment, the interrupter is integrated into access circuitry  25  itself.  
         [0084]    Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or the scope of Applicants&#39; general inventive concept. The invention is defined in the following claims.