Abstract:
Improved modular transient voltage surge suppressor apparatus that provide a simple structure for coupling multiple modules are disclosed. In general, such apparatus includes a substrate; a mounting post coupled to and extending substantially perpendicular to the substrate; and a transient voltage surge suppression module, wherein the module includes a non-conductive housing having a surge suppression circuit contained therein, and mounting means coupled to the non-conductive housing, the mounting means comprising a bore therethrough for slidably mounting the transient voltage surge suppression module on the mounting post, the bore having an internal profile corresponding to an external profile of the mounting post.

Description:
CLAIM OF BENEFIT UNDER 35 U.S.C. §119(E) 
     This Application claims the benefit of U.S. Provisional Application No. 60/241,954, filed Oct. 21, 2000. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to transient voltage surge suppression apparatus and, more specifically, to improved modular designs for such apparatus. 
     BACKGROUND OF THE INVENTION 
     For many years, manufacturers of electronic systems have recommended that users take measures to isolate their hardware from transient overvoltages (also called “surges”) that may cause damage to sensitive electronic devices. Transient voltage protection systems (so-called “surge suppressors”) are designed to reduce transient voltages to levels below hardware-damage susceptibility thresholds; providing such protection can be achieved through the use of various types of transient-suppressing elements coupled between the phase, neutral and/or ground conductors of an electrical distribution system. 
     Conventional transient-suppressing elements typically assume a high impedance state under normal operating voltages. When the voltage across a transient-suppressing element exceeds a pre-determined threshold rating, however, the impedance of the element drops dramatically, essentially short-circuiting the electrical conductors and “shunting” the current associated with the transient voltage through the element and thus away from the sensitive electronic hardware to be protected. 
     To be reliable, a transient-suppressing element itself must be capable of handling many typical transient-voltage disturbances without internal degradation. This requirement dictates the use of heavy-duty components designed for the particular transient voltage environment in which such elements are to be used. In environments characterized by high-magnitude or frequently-occurring transients, however, multiple transient-suppressing elements may be required. 
     In many applications, the transient-suppressing elements typically employed are metal-oxide varistors (“MOVs”); silicon avalanche diodes (SADs) and gas tubes are other types of transient-suppressing elements. When designing a system incorporating MOVs it is important to recognize the limitations of such devices, and the effects that the failure of any given MOV may have on the integrity of the total system. All MOV components have a maximum transient current rating; if the rating is exceeded, the MOV may fail. An MOV component may also fail if subjected to repeated operation, even if the maximum transient current rating is never exceeded. The number of repeated operations necessary to cause failure is a function of the magnitude of transient current conducted by an MOV during each operation: the lower the magnitude, the greater the number of operations necessary to cause failure. A designer of transient voltage protection systems must consider these electrical environment factors when selecting the number and type of MOVs to be used in a particular system. Therefore, to design a reliable transient voltage suppression system, a designer must consider both the maximum single-pulse transient current to which the system may be subjected, as well as the possible frequency of transients having lower-level current characteristics. 
     Although individual MOVs have a maximum transient current rating, it is possible to construct a device using multiple MOVs, in parallel combination, such that the MOVs share the total transient current. In this manner, each individual MOV must only conduct a fraction of the total transient current, thereby reducing the probability that any individual MOV will exceed its rated maximum transient current capacity. Furthermore, by using a plurality of individual MOVs, a transient voltage protection system can withstand a greater number of operations because of the lower magnitude of transient current conducted by each individual MOV. 
     When a transient voltage suppression system incorporates multiple MOVS, it is important that the system be designed such that the failure of an individual MOV does not cause a complete loss of system functionality. When an MOV fails, due to either exceeding its maximum transient current rating or frequent operation, it initially falls into a low impedance state, drawing a large steady-state current from the electrical distribution system. This current, if not interrupted, will quickly drive an MOV into thermal runaway, typically resulting in an explosive failure of the MOV. 
     To avoid the explosive failure of MOVs, an appropriately-rated current-limiting element, such as a fuse, should be employed in series with MOVs. If the transient-suppressing device incorporates a plurality of parallel-coupled MOVs, however, a single fuse in series with the parallel combination of MOVs may open-circuit even if only a single MOV fails, resulting in a disconnection of the remaining functional MOVs from the electrical distribution system. Therefore, better-designed systems incorporate individual fuses for each MOV, such that the failure of an individual MOV will result only in the opening of the fuse coupled in series with the failed MOV; the remaining functional MOVs remain connected to the electrical distribution system, via their own fuses, to provide continued transient voltage protection. 
     In the prior art, there are transient suppression circuits that incorporate a plurality of parallel-coupled MOVs with an individual fuse provided for overcurrent protection of the MOVs. U.S. Pat. No. 5,153,806 to Corey teaches the use of a single fuse to protect a plurality of MOVs, as well as an alarm circuit for indicating when the fuse has open-circuited. Similarly, U.S. Pat. No. 4,271,466 to Comstock teaches the use of a single fuse in series with a plurality of MOVs, as well as a light-emitting diode (“LED”), coupled in parallel with the fuse, to emit light when the fuse is blown. The deficiencies of these types of circuits is that the failure of a single MOV can cause the fuse to fail whereby the remaining functional MOVs are decoupled from the circuit; i.e., the remaining functional MOVs are disconnected from the electrical distribution system and thus cannot provide continued protection from transient voltages. 
     There are also a limited number of transient suppression devices that employ multiple over-current limiting elements with multiple parallel-coupled MOVs or other transient suppression devices. Such devices known in the prior art, however, typically employ a bare fusible element mounted on the printed circuit board on which the MOVs are mounted. When an MOV associated with a particular fusible element fails, the fusible element typically open circuits. The open-circuiting of a fusible element is often accompanied by electrical arcing, which is particularly true in the area of transient suppression devices because of the large voltages and currents usually present when a suppression device fails. Because of the close proximity of the bare fusible elements, the electrical arcing of one fusible element can result in the destruction of adjacent elements, thereby decoupling remaining functional MOVs from the circuit and further limiting the remaining suppression capacity of the device. 
     The inadequacy of the prior art is that the failure of a single MOV component may cause a current-limiting element, such as a fuse, in series with a plurality of parallel-coupled MOVs to open-circuit, thus eliminating all transient voltage suppression capability of the parallel-coupled MOVs. In prior art circuits that have employed multiple current-limiting elements with multiple parallel-coupled MOVs (or other transient suppression devices), the failure of a current-limiting element can cause electrical arcing that can result in the destruction of adjacent current-limiting elements, or MOVs, thus resulting in further degradation of the suppression capacity of the circuit. Therefore, there is a need in the art for improved apparatus for providing over-current protection to a plurality of parallel-coupled transient-suppression devices; such improved apparatus preferably reduce, or eliminate, the possibility of failures due to electrical-arcing. 
     As described supra, it is known in the prior art to provide multiple MOVs, in parallel combination, such that the MOVs share the total transient current. Furthermore, such circuits can be housed in individual modules, and multiple modules can be coupled in parallel to increase the surge capacity of the device. Examples of prior art modular devices are disclosed by Ryan, et al. in U.S. Pat. Nos. 5,701,227, 5,953,193, 5,966,282, and U.S. Pat. No. 5,969,932, incorporated herein by reference. A particular inadequacy of such prior art modular devices, however, is the manner in which the modules are coupled together, which requires each module in a stack of modules to be independently coupled to each adjacent module. This manner of assembly increases not only the number of physical parts, but also the assembly time, as well as the disassembly time required to repair or replace a failed module. Accordingly, there is a further need in the art for improved modular structures for housing transient voltage suppression circuits. 
     SUMMARY OF THE INVENTION 
     To address certain above-described deficiencies of the prior art, the present invention provides improved modular transient voltage surge suppressor apparatus that provide a simple structure for coupling multiple modules. In general, such apparatus includes a substrate; a mounting post coupled to and extending substantially perpendicular to the substrate; and a transient voltage surge suppression module, wherein the module includes a non-conductive housing having a surge suppression circuit contained therein, and mounting means coupled to the non-conductive housing, the mounting means comprising a bore therethrough for slidably mounting the transient voltage surge suppression module on the mounting post, the bore having an internal profile corresponding to an external profile of the mounting post. 
     In a specific exemplary embodiment illustrated and described hereinafter, such apparatus includes a substrate; first and second mounting posts coupled to and extending substantially perpendicular to the substrate; and a transient voltage surge suppression module mounted thereon. The transient voltage surge suppression module includes a non-conductive housing having a surge suppression circuit contained therein, and first and second electrically-conductive buses mechanically coupled to the non-conductive housing and electrically coupled to first and second terminals of the surge suppression circuit, respectively. The first and second electrically-conductive buses each include a bore therethrough for slidably mounting the transient voltage surge suppression module on the first and second mounting posts, respectively; the bores have an internal profile corresponding to an external profile of the mounting posts. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject matter of the claims recited hereinafter. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a schematic of an exemplary transient-voltage suppression circuit; 
     FIG. 2 illustrates an isometric view of an exemplary module for housing the transient-voltage suppression circuit illustrated in FIG. 1; 
     FIG. 3 illustrates an isometric view of the internal structure of the exemplary module; 
     FIG. 4 illustrates an isometric view of the transient-voltage suppression circuit illustrated in FIG. 1 adapted to fit the internal structure of the exemplary module; 
     FIG. 5 illustrates an isometric view of the internal structure of the exemplary module, including therein the transient-voltage suppression circuit illustrated in FIG. 4; 
     FIG. 6 illustrates a top view of the internal structure of the exemplary module, including therein the transient-voltage suppression circuit illustrated in FIG. 4; 
     FIG. 7 illustrates an isometric view of a structure for mounting a single exemplary module (per mode of protection) to a mounting substrate; 
     FIG. 8 illustrates an isometric view of a structure for mounting two exemplary modules (per mode of protection) to a mounting substrate; 
     FIG. 9 illustrates an isometric view of a structure for mounting three exemplary modules (per mode of protection) to a mounting substrate; 
     FIGS. 10-A and  10 -B illustrate side views of an exemplary physical structure for mounting and interconnecting multiple modules, while ensuring that all electrical path lengths through each module are equalized; and 
     FIG. 11 illustrates an exploded isometric of a structure for interconnecting status ports between adjacent stacked modules. 
    
    
     DETAILED DESCRIPTION 
     Referring initially to FIG. 1, illustrated is an exemplary transient-voltage suppression circuit  100 . The transient-voltage suppression circuit  100  includes a plurality of parallel-coupled circuits, generally designated  110 , each of which includes a current-limiting element  111  and a transient-suppressing element  112 . Those skilled in the art will readily appreciate that the transient-voltage suppression circuit  100  may have any desired number of the parallel-coupled circuits  110 , and that the total transient-suppressing capacity of the transient-voltage suppression circuit  100  is a function of the number of parallel-coupled circuits  110 . 
     In the exemplary transient-voltage suppression circuit  100 , the current-limiting elements  111  are fuses, or thermal cutoffs, and the transient-suppressing elements  112 , which are each coupled in series with a thermal cutoff  111 , are metal oxide varistors (“MOV”). Each series-coupled thermal cutoff  111  and MOV  112  is coupled between a bus  120  and a bus  130 . The bus  120  is couplable to a first electrical conductor of a power distribution system (not shown) via terminal  125 , and the bus  130  is couplable to a second electrical conductor of the power distribution system via terminal  135 ; the first and second electrical conductors may be, for example, a phase and neutral conductor (or phase and ground conductor), respectively. An electrical load (not shown) to be protected by the transient-voltage suppression circuit  100  would also be coupled to the first and second electrical conductors. When exposed to a transient voltage occurring between the electrical conductors of a power distribution system to which transient-voltage suppression circuit  100  is coupled, the impedance of each MOV  112  changes by many orders of magnitude from a substantially high-impedance state to a very low impedance state, i.e., a highly conductive state, thereby “shunting” the current associated with the transient voltage through the MOV and thus away from the sensitive electronic hardware to be protected. Thus, the MOVs can be electrically connected in parallel between electrical conductors of a power distribution system to provide protection from transient voltages to an electrical load also coupled to the electrical conductors. 
     As those skilled in the art understand, when an MOV is subjected to a transient voltage beyond its peak current/energy rating, it initially fails in a short-circuit mode. An MOV may also fail when operated at a steady-state voltage well beyond its nominal voltage rating, or if subjected to repeated operations due to transient voltages having associated current levels below the peak current/energy rating for the MOV. When an MOV fails in the short-circuit mode, the current through the MOV becomes limited mainly by the source impedance of the power distribution system to which the MOV is coupled. Consequently, a large amount of energy can be introduced into the MOV, causing the MOV to become very hot, which can result in mechanical rupture of the MOV package accompanied by expulsion of package material; this failure mode may be prevented by proper selection of a current-limiting element that “clears” the fault. The current-limiting element  111  is preferably selected to interrupt the fault current that is caused to flow through the MOV  112  (as well as the current-limiting element) due to the failure of the MOV. 
     In many conventional transient-voltage suppression circuits, a bare fusible element, such as an uninsulated copper wire, is often used as a current-limiting element in series with MOV transient suppressing elements. The bare fusible elements are typically mounted on a printed circuit board to which the MOVs are also mounted. It has been recognized that when such bare fusible elements are mounted in close proximity, the electrical arcing resulting from the open-circuiting of one fusible element can cause damage to other adjacent fusible elements, as well as other adjacent electrical components. The damage caused to an adjacent fusible element may cause that element to open-circuit, thereby eliminating an additional MOV from the circuit and degrading the overall transient suppression capacity of the circuit. Furthermore, the electrical arcing of a fusible element can cause arc “tracking” on the circuit board; the electrical arcing results in carbon deposition on the circuit board, thus forming a conductive path, or “track,” which helps to sustain the electrical arc and prevent clearing of the fault. In circuits that employ a thermal couple as a current-limiting element, the heat generated by a failed, or failing MOV, can interfere with the desired operation of the thermal couple. These types of problems, and others, are addressed by certain inventions disclosed herein. 
     Turning now to FIG. 2, illustrated is an isometric view of an exemplary module  200  in accordance with principles of an invention disclosed herein; the module  200  can house, for example, the transient-voltage suppression circuit  100  illustrated in FIG.  1 . Module  200  includes a body  210  having a lid  220  secured thereto by screws  230 . The body  210  has opposing sidewalls  211   a ,  211   b  (hidden), opposing endwalls  212   a ,  212   b  (hidden), and a bottom  213  (hidden) that form a substantially rectangular enclosure. The body  210  and lid  220  are preferably constructed from a non-conductive material. 
     At either end of body  210  are electrically-conductive bus portions  240   a ,  240   b ; the bus portions  240   a ,  240   b  each include an electrically-conductive tab (not shown), described infra, that passes through the respective endwalls  212   a ,  212   b  for coupling to an electrical circuit housed within module  200 . The bus portions  240   a ,  240   b  can be machined, for example, from solid copper or brass. In the exemplary embodiment, the bus portions  240   a ,  240   b  each have a substantially square cross-section and extend from a location proximate the lid  220  to the bottom  213  of enclosure  200 . At either end of bus portions  240   a ,  240   b  are substantially flat opposing faces, or contact surfaces,  241   a  and  241   b  (hidden). Extending longitudinally through each bus portion  240   a ,  240   b  are bores  242   a ,  242   b , respectively. As described hereinafter, the bores  242   a ,  242   b  provide a means for one or more modules  200  to be slidably-mounted in a stacked arrangement. In certain embodiments, it can be desirable to “key” the module  200  such that it can only be mounted in a particular orientation. In the exemplary embodiment, module  200  is keyed by including a channel  243  that extends along bore  242   a ; the channel  243  corresponds to a pin on one of the two required mounting posts (described infra), such that the module  200  can only be mounted in a desired position. In an assembled device containing one or more modules  200  (as described more fully infra), the contact surfaces  241   b  can engage, or mate against, either a surface of a mounting substrate, such as printed circuit board (PCB), or a contact surface  241   a  of an adjacent module  200  in a stack of such modules. When two or more modules  200  are stacked, the bus portions  240   a ,  240   b  of each module thereby form a bus structure that provides electrical conductivity from module to module. 
     Turning now to FIG. 3 (with continuing reference to FIG.  1 ), illustrated is an isometric view of the internal structure of the exemplary module  200 , in accordance with principles of an invention disclosed herein. As noted previously, a failure of an MOV can result in electrical arcing and the generation of tremendous heat that can undesirably affect the operation of an associated current-limiting element. The exemplary internal structure of module  200  illustrated in FIG. 3 addresses this problem. As illustrated in FIG. 3, module  200  includes an internal wall structure including internal opposing sidewalls  311   a ,  311   b , and internal opposing endwalls  312   a ,  312   b ; each of the internal walls extends upwardly from the bottom  213  of module  200 . According to the principles of an invention disclosed herein, the internal walls divide the internal compartment of module  200  into at least first and second chambers  320 ,  321 ; i.e., the chamber  320  is intermediate to the external and internal walls, and the chamber  321  is formed within the internal walls. Preferably, the lid  220  includes a groove  340  that engages the upper edges of internal opposing sidewalls  311   a ,  311   b , and internal opposing endwalls  312   a ,  312   b  when coupled to the body  210 ; the groove  340  can serve to further isolate the first and second chambers  320 ,  321 . 
     As previously noted, the bus portions  240   a ,  240   b  each include an electrically-conductive tab that passes through the respective endwalls  212   a ,  212   b for coupling to an electrical circuit housed within module  200 . As illustrated in FIG. 3, bus portion  240   a  has a tab  351   a , and bus portion  240   b  has a tab  351   b . Each tab includes a threaded hole  352  (one shown) for coupling to bus bars associated with an electrical circuit mounted in the module  200  (described more fully with reference to FIGS. 4,  5  and  6 , infra). 
     In the exemplary embodiment illustrated in FIG. 3, the internal sidewalls  311   a ,  311   b  include a series of slits, generally designated  313 , along an upper edge of the walls proximate the plane in which the lid  220  occupies when coupled to the body  210 . These slits  313  can function as passageways for electrical leads intermediate to electrical components housed within the separate chambers  320 ,  321 . For example, for the circuit  100  illustrated in FIG. 1, the MOVs  112  can be housed within chamber  321 , while the current-limiting elements  111  coupled in series with the MOVS can be housed within chamber  320 ; the electrical lead that couples each MOV  112  to its associated current-limiting element  111  can be routed through a slit  313 , whereby the MOVs  112  are isolated within chamber  321  from the current-limiting elements  111  within chamber  320 . 
     As also shown in FIG. 3, internal endwall  312   a  extends from sidewall  211   a  to sidewall  211   b , whereby a third chamber  322  is formed within module  200 ; i.e., chamber  322  is bounded by a portion of sidewalls  211   a ,  211   b , endwall  212   a , and internal endwall  312   a . This third chamber  322  can be used, for example, to isolate other electronic circuitry from, for example, the MOVs disposed in chamber  320  and the current-limiting elements disposed in chamber  321 . For example, monitoring circuitry can be provided to indicate the operational status of one or more of the MOVs or current-limiting elements. The isolation of such status circuitry can be very important because if the status circuitry is not properly insulated from the electrical arcing and/or heat associated with the failure of an MOV or current-limiting element, the status circuitry itself can be damaged and fail to properly provide a failure indication. The status circuitry can, for example, provide an external visual indication of a failure, such as by illuminating (or extinguishing) a light emitting diode (LED)  350  provided external to module  200 . Those skilled in the art are familiar with various monitoring circuits suitable for transient voltage suppression circuits; see, for example, U.S. Pat. No. 5,914,662, issued to Roger S. Burleigh, which is commonly assigned with the instant application and incorporated herein by reference. 
     Turning now to FIG. 4 (with continuing reference to FIGS.  1  and  3 ), illustrated is an exemplary physical structure of the transient-voltage suppression circuit  100 , illustrated in FIG. 1, adapted to fit the internal structure of the exemplary module  200 . The MOVs  412  (corresponding to the MOVs  112  of FIG. 1) are centrally arranged to be housed within chamber  321  of module  200 . A first terminal  413  of each MOV  412  is coupled to a first bus bar  420 . The first bus bar  420  includes a hole  421  at one end through which a screw (not shown) can be inserted to couple the first bus bar  420  to tab  351   a  associated with bus portion  240   a . The first bus bar  420  can be, for example, solid copper or brass; alternatively, the first bus bar  420  can be a PCB having appropriate circuit traces to electrically couple each of the first terminals  413 . 
     A second terminal  414  of each MOV  412  is coupled to a first terminal  415  of a corresponding current-limiting element  411 ; the terminals can be coupled, for example, by soldering. A second terminal  416  of each current-limiting element  411  is coupled to a second bus bar  430 . In the exemplary embodiment, second bus bar  430  is constructed from separate bus bar portions  430   a ,  430   b  and  430   c  that are joined by coupling means  431 ; such coupling means can be, for example, a rivet or a bolt and nut. The second bus bar  430  (or bus bar portions  430   a ,  430   b ,  430   c ) can be, for example, solid copper or brass. Alternatively, bus bar portions  430   a  and  430   c  can each be a PCB having appropriate circuit traces to electrically couple each of the second terminals  416  of current-limiting elements  411 , and the bus bar portion  430   b  can be a solid conductor. The bus bar portion  430   b  includes a tab  432  having a hole  433  through which a screw (not shown) can be inserted to couple the second bus bar  430  to tab  351   b  associated with bus portion  240   b  (see FIG.  3 ). 
     Turning now to FIG. 5 (with continuing reference to FIGS. 2,  3  and  4 ), illustrated is an isometric view of the internal structure of the exemplary module  200 , including therein the transient-voltage suppression circuit  400  illustrated in FIG.  4 . As previously described, and as can be seen in FIG. 4, the slits  313  function as passageways for the electrical leads (or terminals) intermediate to the MOVs housed within chamber  321 , and the current-limiting elements housed within chamber  320 . In this exemplary embodiment, the second terminal  414  of each MOV  412  is bent to pass through a slit  313  into the chamber  320 ; within chamber  320 , the second terminal  414  of each MOV  412  is soldered to the first terminal  415  of a corresponding current-limiting element  411 . The first bus bar  420  is electrically and mechanically coupled to the tab  351   a  associated with bus portion  240   a  by a screw  552 , and the second bus bar  430  is electrically and mechanically coupled to the tab  351   b  associated with bus portion  240   b  by a screw (hidden; see FIG.  6 ). 
     Turning now to FIG. 6, (with continuing reference to FIGS. 2,  3  and  4 ), illustrated is a top view of the internal structure of the exemplary module  200 , including therein the transient-voltage suppression circuit  400  illustrated in FIG. 4 (this figure provides details not readily seen in FIGS.  4  and  5 ). As can be seen readily in this figure, the MOVs  412  are all located within chamber  321 , while the current-limiting elements  411  are all located within chamber  320 . The common first terminals  413  of each MOV  412  are electrically and mechanically coupled to first bus bar  420 , which is electrically and mechanically coupled to tab  351   a  of bus portion  240   a  by a screw  552 . Similarly, the second terminals  416  of each current-limiting element  411  are electrically and mechanically coupled to second bus bar  430  (comprised of bus bar portions  430   a ,  430   b  and  430   c ), and the tab  432  of second bus bar  430  is electrically and mechanically coupled to tab  351   b  of bus portion  240   b  by a screw  553 . In a preferred embodiment, the chambers  320 ,  321  and  322  are filled with arc-quenching desiccated sand prior to sealing module  200  by securing lid  220 . 
     Now, turning to FIG. 7, illustrated is an isometric view of an exemplary structure  700  for mounting a single module  200  (per mode of protection) to a mounting substrate  710 , which can be, for example, a printed circuit board (PCB). Mounting posts  720   a ,  720   b , which can be internally threaded, are secured perpendicularly to the substrate  710  by bolts  730  (one shown) that pass through substrate  710 . The mounting posts  720   a ,  720   b  are disposed at a distance corresponding to the distance between bores  242   a ,  242   b  of bus portions  240   a ,  240   b , respectively, of module  200 . The mounting posts  720   a ,  720   b  have an external diameter substantially equal to the internal diameter of bores  242   a ,  242   b , and provide a means for module  200  to be slidably-mounted thereon. In certain embodiments, it can be desirable to “key” the module  200  such that it can only be mounted within a device in a particular orientation. In the exemplary embodiment, module  200  is keyed by including a channel  243  that extends along bore  242   a ; the channel  243  corresponds to a pin  721  on mounting post  720   a , such that the module  200  can only be mounted in a desired position. Once module  200  is slid onto mounting posts  720   a ,  720   b , it is secured in place by bolts  750   a ,  750   b , which screw into the mounting posts. Preferably, the mounting posts  720   a ,  720   b  have a length slightly less than the length of bus portions  240   a ,  240   b , respectively; the difference in length allows for the module  200  to be securely compressed against the substrate  710  when bolts  750   a ,  750   b  are tightened. 
     As described supra, module  200  houses an electrical circuit, such as transient voltage suppression circuit  100  that is to be coupled between two electrical conductors, such as phase and neutral, phase and ground, or neutral and ground conductors. To accomplish this, means are provided to couple the bus portions  240   a ,  240   b  to the desired conductors. In one embodiment, this can be accomplished by providing electrical circuit traces, or “contact pads,”  711   a ,  711   b , on PCB  710 . The contact pads  711   a ,  711   b  are electrically coupled to contact surfaces  241   b  (hidden) at the lower ends of bus portions  240   a ,  240   b  when module  200  is slid onto mounting posts  720   a ,  720   b  and seated against PCB  710 . Alternatively, or in combination with contact pads  711   a ,  711   b , electrical conductor coupling means can be provided proximate the contact surfaces  241   a  at the upper ends of bus portions  240   a ,  240   b . For example, the coupling means can be conventional compression lugs  740   a ,  740   b . The compression lugs  740   a ,  740   b  have mounting holes  741   a ,  741   b , respectively, through which bolts  750   a ,  750   b  pass before being screwed into the mounting posts  720   a ,  720   b , thereby securing the compression lugs mechanically, and electrically coupling them to the contact surfaces  241   a ,  241   b  at the upper ends of bus portions  240   a ,  240   b.    
     Turning now to FIG. 8, illustrated is an isometric view of an exemplary structure  800  for mounting two exemplary modules (per mode of protection)  200   a ,  200   b  to a mounting substrate  710 . The exemplary structure  800  is identical to structure  700 , with the single exception that mounting posts  820   a ,  820   b  have a length substantially equal to the combined length of two bus portions  240   a , such that two modules  200   a ,  200   b  can be slid thereon. In this embodiment, the modules  200   a ,  200   b are electrically coupled, in parallel, through the surface contact of the contact surfaces  241   a  (one shown; one hidden), at the upper ends of the bus portions  240   a ,  240   b  of module  200   a  with the contact surfaces  241   b  (hidden) at the lower ends of the bus portions  240   a ,  240   b  of module  200   b . Thus, when modules  200   a  and  200   b  are stacked, the bus portions  240   a ,  240   b  of each module form a bus structure that provides electrical conductivity from module to module. Preferably, the mounting posts  820   a ,  820   b  have a length slightly less than the combined lengths of two bus portions  240   a (and  240   b ); the difference in length allows for the modules  200   a ,  200   b  to be securely compressed against the substrate  710  when bolts  750   a ,  750   b are tightened, while also ensuring good electrical contact between the contact surfaces  241   a  and  241   b  of bus portions  240   a ,  240   b  of the adjacent modules  200   a ,  200   b , respectively. 
     Turning now to FIG. 9, illustrated is an isometric view of an exemplary structure  900  for mounting three exemplary modules (per mode of protection)  200   a ,  200   b , and  200   c  to a mounting substrate  710 . The exemplary structure  900  is identical to structure  700  (and  800 ), with the single exception that mounting posts  920   a ,  920   b  have a length substantially equal to (or slightly less than) the combined length of three bus portions  240   a , such that three modules  200   a ,  200   b  and  200   c  can be slid thereon. Those skilled in the art will recognize that the principles described herein disclose a novel structural approach to mounting any number of modules  200 . The novel structure is particularly advantageous for the parallel coupling of transient voltage suppression circuits, because it does not require any additional hardware to mount each additional module, which simplifies both manufacture and disassembly for the repair or replacement of a module if its internal circuitry fails. For example, if module  200   a  fails, it is only necessary to 1) remove bolts  750   a ,  750   b , 2) slide modules  200   c ,  200   b  and  200   a  off of mounting posts  920   a ,  920   b , 3) replace module  200   a  with a functional module, slide modules  200   a ,  200   b  and  200   c  back onto mounting posts  920   a ,  920   b , and 4) secure bolts  750   a ,  750   b.    
     Although the exemplary structures  700 ,  800  and  900  are characterized by modules  200  having bus portions  240   a ,  240   b  that provide both the mechanical and electrical means for coupling multiple modules, the principles of the present invention are not so limited. The main principle of this invention is the providing of one or more mounting posts, tracks, channels, or similar structures onto which one or more modules can be slidably-mounted; the electrical coupling of the modules is not necessarily provided by the same mechanical means. For example, electrical contact plates could be provided on the top and bottom of each module for electrical coupling to an adjacent module (or substrate), while a separate mechanical structure (or structures) can be provided for slidable engagement with one or more mounting posts, tracks, channels, or similar structures. Thus, the mechanical and electrical coupling features of the present invention are separable, without departing from the principles disclosed herein. 
     As described supra with reference to FIG. 1, multiple MOVs can be coupled in parallel combination such that the MOVs share the total current associated with a transient voltage. In this manner, each individual MOV must only conduct a fraction of the total transient current, thereby reducing the probability that any individual MOV will exceed its rated maximum transient current capacity. As also described supra, a circuit of parallel-coupled MOVs, such as circuit  100 , can be enclosed in a module  200 , and multiple modules can then be coupled in parallel. Although the teachings of the prior art have recognized that multiple modules can be coupled in parallel, the prior art has failed to recognize that the manner in which the modules are coupled can have an impact on the capability of an individual module to provide its full transient-suppressing capacity; i.e., the prior art structures for coupling multiple transient suppressing modules yield systems having a transient suppressing capacity less than the sum of the suppressing capacities of each module. 
     As illustrated in the transient-voltage suppression circuit  100  of FIG. 1, and the exemplary physical structure  400  of FIG. 4, the buses  120  and  130  (corresponding to bus bar  420  and  430 , respectively) are physically opposed such that the electrical path length through all MOVs  112  are equal. The equal electrical path lengths ensure that all MOVs  112  will share the current associated with a transient voltage in substantially equal parts. For example, if ten parallel-coupled circuits  110  are provided, one tenth of the transient current will flow through each MOV  112 . In prior art systems that have coupled multiple modules in parallel, however, the sharing of the transient current between MOVs in different modules has not been ensured. For example, in the prior art modular device disclosed in U.S. Pat. No. 5,701,227,the phase and neutral (or ground) conductors are both coupled to connections directly proximate the bottom module in a stack of modules. The modules that occupy positions above the lowest module will therefore have electrical path lengths through their internal components (e.g., MOVs) that are longer than the electrical path length through the lowest module and, therefore, the MOVs in the upper module(s) will not equally share a transient current with the MOVs in the lowest module. 
     Turning now to FIG. 10, illustrated is a side view of an exemplary physical structure for mounting and interconnecting multiple modules, while ensuring that all electrical path lengths through each module are equalized. As previously described, two modules  200   a  and  200   b  can be mounted in a stacked orientation, whereby the internal circuits are coupled in parallel electrically by the bus portions  240   a  and  240   b  of each module. As shown in FIG. 10, a first electrical conductor coupling means  1040   a , such as a compression lug, is coupled proximate the lower contact surface  241   a  of bus portion  240   b  associated with module  200   a , while a second electrical conductor coupling means  1040   b , such as a compression lug, is coupled proximate the upper contact surface  241   a  of bus portion  240   a  associated with module  200   b , whereby the electrical path lengths  1000   a  and  1000   b  through modules  200   a ,  200   b , respectively, are of substantially equal length. Thus, each MOV in module  200   a  will share equally any transient current with each MOV in module  200   b . Those skilled in the art will recognize that the exemplary structures  700 ,  800  and  900  can be readily adapted to provide such current sharing between all modules. 
     Another problem in the prior art is how to monitor the status of multiple modules. In some prior art systems, independent monitoring circuits are provided in each module. The disadvantages of this approach are that a greater number of components must be housed within a module, and thus the size of a module must be increased, as well as adding additional cost to the system. In some prior art systems, monitoring conductors from each module are routed to an external monitoring circuit. The disadvantages of this approach are that adequate free space must be provided between modules in a stack, and/or between adjacent stacks of modules, to route the monitoring conductors to the monitoring circuit, thus increasing the size of the system, as well as an increase in the amount of labor necessary to assemble a system. FIG. 11 illustrates an exploded isometric of an exemplary structure for interconnecting status interfaces between adjacent stacked modules that overcomes these disadvantages of the prior art. 
     As illustrated in FIG. 11, two modules  200   a  and  200   b  are stacked according to the principles disclosed supra. To accommodate the communication of module status information between modules and/or other circuitry coupled to the modules via the mounting substrate, each module is provided with status ports for coupling status information between modules and/or the substrate. In the exemplary embodiment illustrated in FIG. 11, each module  200   a ,  200   b  includes an upper status port  221  in the lid  220 , and a lower status port (hidden) in the bottom  213  of body  210 . The upper status port  221  and lower status port can provide electrical connections from internal monitoring circuitry within a module to internal monitoring circuitry within each adjacent module, or simply provide a means of coupling monitoring signal points from within each module to external monitoring circuitry. 
     In one embodiment, a status interconnector  1110  is provided to couple the upper status port  221  of module  200   a  to the lower status port (hidden) of module  200   b . The exemplary status interconnector  1110  includes a non-conductive central body  1111  through which two electrical pin conductors  1112 ,  1113  pass. The first ends  1112   a  and  1113   a  of each pin conductor  1112 ,  1113 , respectively, are receivable by the upper status port  221  of module  200   a ; the second ends  1112   b  and  1113   b  of each pin conductor  1112 ,  1113 , respectively, are receivable by the lower status port (hidden) of module  200   b . As shown in FIG. 7, a status connector  760  can also be provided on substrate  710  to couple to the lower status port (hidden) on module  200   a . Thus, all modules in a stack of modules can be easily interconnected for status monitoring purposes without the need for routing any external conductors, which allows adjacent stacks of modules to be closely packed together. Although illustrated as a separable component, those skilled in the art will recognize that status interconnector  1110 , or a similar structure, can be integrated with each module; e.g., the lower status port of each module  220  can provide one or more electrical pin conductors to be received in the upper status port  221  of an adjacent module  220  (or substrate  710 ). Furthermore, the status interconnector  1110  can include any number of electrical pin conductors as required for a particular status monitoring circuit. 
     From the foregoing detailed description, it is apparent that the present application discloses improved modular structures for housing transient voltage suppression circuits. Although the present invention and its advantages have been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.