Abstract:
A number of surge protection device disconnector designs provide protection to a load over a full range of fault currents provide adequate surge protection as well. The designs quench arcs that may tend to occur as a result of MOV faults, thereby protecting the surrounding components.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to surge protection devices, and in particular to a number of surge protection device disconnector designs.  
       BACKGROUND OF THE INVENTION  
       [0002]     Electrical systems, such as electrical power distribution systems, periodically experience over-voltage conditions, such as transient over-voltage conditions, also called “surges.” Over-voltage conditions are problematic to electrical systems because they may cause damage to the loads, such as electronic devices or other hardware, that are coupled thereto. As a result, surge protection devices (SPDs) have been developed to protect the loads from over-voltages that would otherwise damage the loads. SPDs typically provide such protection by coupling various types of known transient-suppressing elements between the phase, and neutral and/or ground conductors of an electrical power distribution system.  
         [0003]     As is known in the art, transient-suppressing elements, such as metal-oxide varistors (MOVs), silicon avalanche diodes (SADs) and gas tubes, typically assume a high impedance state under normal operating voltages. When the voltage across a transient-suppressing element exceeds a predetermined threshold rating, however, the impedance of the element drops dramatically, essentially short-circuiting the electrical conductors and “shunting” the current associated with the over-voltage through the transient-suppressing element and away from the load.  
         [0004]     MOVs are probably the most commonly used transient-suppressing element. An MOV consists of two plates separated by an insulator, such as a metal oxide, that has a known voltage breakdown characteristic. When the voltage between the two plates reaches a certain level (the voltage breakdown level), the insulator breaks down and conducts current. MOVs, however, have operational limitations that must be taken into account when designing an SPD. Specifically, all MOVs have a maximum surge current rating that, if exceeded, may cause the MOV to fail. An MOV 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 the MOV during each operation: the lower the magnitude, the greater the number of operations necessary to cause failure.  
         [0005]     In light of these limitations, prior art SPDs have been developed that use multiple MOVs in parallel combination such that the MOVs share the total transient current. Each individual MOV in such a configuration only conducts a portion of the total transient current, making it less likely that any individual MOV will exceed its maximum transient current capacity. In addition, an SPD that uses a plurality of parallel MOVs can withstand a greater number of operations because of the lower magnitude of transient current conducted by each individual MOV. If internally fused and sorted by V/I characteristics, a parallel combination of MOVs is advantageous because the failure of any individual MOV will not cause a complete loss of SPD functionality.  
         [0006]     When an MOV fails it initially falls into a low impedance (short) state in which it draws a large steady-state current from the electrical system. This current, if not interrupted, will drive the MOV into a thermal runaway condition, typically resulting in an explosive failure of the MOV (involving, for example, fire, toxic smoke and/or hot particles) and damage to or destruction of the SPD and surrounding components.  
         [0007]     Generally, there have been at least two ways in which SPDs have dealt with the hazards of explosive failure. In the first, older approach, a strong metal enclosure is provided around the SPD. The problem with such enclosures is that, despite the heavy metal walls, the enclosures have been known to rupture, release toxic gas and/or not prevent fire in all instances. Another approach is to employ a disconnector, typically a fuse, in the SPD design to disconnect the SPD from the power system. In particular, fuses are typically employed in series with MOVs, preferably with one fuse being in series with each MOV.  
         [0008]     Under fault conditions, SPDs are faced with high (on the order of 1000 A and higher), medium (on the order of 10-1000 A) and low (on the order of 10 A or less) fault currents which typically last a number of milliseconds. In the ideal situation, an SPD disconnector design will provide adequate protection (i.e., will open the circuit) during all levels of fault current (high, medium and low) and against problematic surge currents (i.e., higher than can be adequately handled by the MOV according to its surge rating) while at the same time being able to withstand (i.e., not open) for certain surge current levels (i.e., those that than can be adequately handled by the MOV). The problem with prior art designs that use fuses to protect against MOV failure is that fuses, while effective in certain particular ranges, are not reliable over the full range of fault currents that may occur. For example, it is known to employ a fuse trace copper conductor on a printed circuit board in series with a single MOV. If fuse traces would be designed to handle relatively high surge and fault currents it they would not be suitable for disconnecting a failing MOV under relatively lower fault current conditions.  
         [0009]     Another problem encountered by prior art SPD designs is due to the fact that it is possible, particularly under TOV conditions, to generate excessive heat in an MOV without causing a series over-current fuse to open. This excessive heat could cause damage to other components that could lead to a chain reaction of failures. Therefore, some form of thermal protection, such as a thermal fuse spring, is desirable to prevent these types of failures.  
         [0010]     In addition, upon a failure of an MOV or its associated series fuse due to an overload condition, the MOV or fuse may disintegrate, causing electrically conductive debris to be dispersed in the vicinity of the MOV or fuse. Thus, the main technical problem in SPD disconnector design is how to control arcing between metal parts inside the SPD because the arcing in combination with the debris may cause short-circuits in any electronic circuitry in the vicinity of the MOV or fuse, including other SPD circuits. This is known as quenching the arc. Currently, arc quenching is accomplished by encapsulating the SPD in an epoxy. Epoxy, however, is very hard and therefore creates hazardous conditions under very high fault currents. Specifically, under very high fault currents, the epoxy may explode catapulting many small bullet like projectile in the general vicinity of the SPD.  
         [0011]     One particular SPD design employing a combination of thermal disconnection and fault current protection is described in U.S. Pat. No. 6,636,409, the disclosure of which is incorporated herein by reference.  FIG. 1A  is a cross-sectional view of the SPD design described in U.S. Pat. No. 6,636,409. As seen in  FIG. 1A , SPD  70  includes an exemplary printed circuit board (PCB)  72  and the combination of an MOV  74 , a thermal fuse spring (TFS)  76  and a fuse trace with solder hole (FTWSH)  78 . As is well known, the MOV  74  includes leads  80  and  82 , which are inserted in respective through holes  84  and  86  of the PCB  72 . The FTWSH  78  includes PCB copper traces  88  and  90  and through hole  92 . The through hole  92  is positioned between the MOV lead  80  and the TFS finger  104 . During wave-soldering of the PCB  72 , the through holes  84 ,  86  and  92  are filled with solder, such as conventional solder  94  (e.g., having a melting temperature of between about 175° C. and about 250° C.) in the through hole  84  and conventional solder  96  (e.g., having a melting temperature of between about 175° C. and about 250° C.) in the through hole  92 . In this manner, the MOV leads  80  and  82  are electrically connected to PCB traces, such as copper traces  90  and  98  on opposite sides of the through hole  84 . Also, the solder  96  fills the through hole  92 . The solder  96  is advantageously employed to shorten the disconnection time of the FTWSH  78  under overcurrent conditions by first melting and, then, hastening the disconnection (e.g., by burning) of one or both of the FTWSH PCB copper traces  88  and  90 .  
         [0012]     Before the wave-soldering process, the TFS  76  is placed on the PCB  72  during a surface mounting (re-flow) process. Preferably, a fusible alloy, such as a suitable low temperature solder, shown at  100  and  102 , is employed at the fingers  104  and  106  of the TFS  76 , in order to hold the TFS  76  in a stretched position. In this manner, a series electrical connection is established from PCB copper trace  108 , to solder  102 , to the finger  106  of the TFS  76  and through such TFS to the finger  104 , to the solder  100 , to the copper trace  88 , to the through hole  92 , to the copper trace  90 , to the through hole  84 , to the MOV lead  80  and, thus, to the MOV  74 .  
         [0013]     During normal operation of the SPD  70 , the leakage current through the TFS  76 , FTWSH  78  and MOV  74  is in the order of several μA and there is no significant temperature increase of the MOV  74 , FTWSH  78  and TFS  76 . However, during abnormal conditions, the temperature of the MOV  74  rises. The heat, shown at  110 , is transferred through the MOV leg  80  and the copper traces  90  and  88  and through hole  92  of the FTWSH  78  to the low temperature solder  100 , which is beneath the finger  104  of the TFS  76 . Once the temperature of the TFS finger  104  reaches a certain temperature, e.g., about 95° C., the solder  100  sufficiently softens or melts, and the  112  of the TFS  76 , which finger is biased toward the opposing finger  114 , moves as shown at  115 , thereby opening the circuit and disconnecting the MOV  74 .  
         [0014]      FIG. 1B  is an isometric view of SPD  70 ′ similar to SPD  70  shown in  FIG. 1A  except that two parallel sets of FTWSHs and MOVs are employed with each TFS. In particular, as shown in  FIG. 1B , SPD  70 ′ includes PCB  72 ′ and the combination of eight MOVs  74 A- 74 H, the TFS unit  76 , and eight FTWSH, such as shown by the FTWSH  78 A and  78 B for the respective MOVs  74 A and  74 B. The MOV  74 A includes the leads  80 A and  82 A, and the MOV  74 B includes two leads (only lead  80 B is shown). The FTWSH  78 A includes PCB copper traces  88 A and  90 A and through hole  92 A, and the FTWSH  78 B includes PCB copper traces  88 B and  90 B and through hole  92 B. The through holes  92 A and  92 B are positioned proximate the respective MOV leads  80 A and  80 B. The TFS finger  104  is electrically connected to both of the traces  88 A and  88 B. In this manner, the TFS unit  76  includes 4 TFS members, each of which is electrically connected to two separate series combinations of a FTWSH and an MOV, with both of those FTWSH-MOV series combinations being electrically connected in parallel.  
         [0015]      FIGS. 1C and 1D  are isometric views of another particular TFS design and another particular SPD design employing that TFS design, respectively, that are described in U.S. Pat. No. 6,636,409.  FIGS. 1C and 1D  show respective un-stretched and stretched thermal fuse springs (TFSs)  166  and  168 , which provide protection of MOVs, such as  170 ,  172 ,  174 ,  176 , on both sides of the TFS  168  of  FIG. 1D . The TFS  168  includes a middle base portion  178 , which has a suitable connection, such as a central opening  180  for a conductive fastener or terminal (not shown), for electrical connection to a phase terminal P. The TFS  168  also includes a plurality of first fingers  182  and a plurality of second fingers  184 . The first fingers  182  are electrically interconnected with corresponding fuse traces  186 ,  188  and surge protection circuits, such as the MOVs  174 ,  176 , respectively, which are electrically connected to a common ground G. The second fingers  184  are electrically interconnected with corresponding fuse traces  190 ,  192  and surge protection circuits, such as the MOVs  170 ,  172 , respectively, which are electrically connected to a common neutral N. The exemplary double-sided TFS  168  is, thus, suitable for plural phase-to-ground (P-G) and plural phase-to-neutral (P-N) connections and, hence, provides a practical and cost effective assembly.  
         [0016]     In this example, the first finger  182 , the fuse trace  186 , and the MOV  174  are electrically interconnected in series between the exemplary phase terminal P and the exemplary ground terminal G. Similarly, the second finger  184 , the fuse trace  190 , and the MOV  170  are electrically interconnected in series between the exemplary phase terminal P and the exemplary neutral terminal N. The three terminals P, N, G are also electrically connected to a suitable power source and to a load.  
         [0017]     While effective, the SPD designs described in U.S. Pat. No. 6,636,409 are faced with many of the problems of prior art SPD designs described herein. For example, the designs, while effective for certain low and high fault current ranges, is generally not effective for medium fault currents. Thus, the designs may not be reliable over the full range of fault currents that may occur. In addition, the designs may not effectively provide arc quenching in the case of an MOV failure.  
         [0018]     Thus, there is room for improvement in the field of SPDs, and in particular in SPD disconnector designs that address each of the problems described above.  
       SUMMARY OF THE INVENTION  
       [0019]     According a first aspect of the present invention, a surge protection device is provided for protecting a load that is connected to at least one voltage source from a power grid. The device includes an overcurrent fuse electrically connected to the at least one voltage source, a thermal fuse spring electrically connected to the overcurrent fuse, a fuse trace electrically connected to the thermal fuse spring, and a transient suppressing element, such as an MOV or SAD, electrically connected to the fuse trace. In addition, the overcurrent fuse, the thermal fuse spring, and the transient suppressing element are electrically connected in series between the at least one voltage source and a neutral or ground connection. This configuration provides protection over the full range of fault currents because the overcurrent fuse, the thermal fuse spring, and the fuse trace have overlapping ranges and one or more of them will open in response to low, medium and high fault currents. In the preferred embodiment, the overcurrent fuse is encapsulated in a non-organic material such as silicone. As shown in  FIG. 2 , the device may include a plurality of thermal fuse springs electrically connected to the overcurrent fuse a plurality of parallel fuse trace combinations and a plurality of transient suppressing elements.  
         [0020]     Another aspect of the invention relates to a surge protection device including a substrate, at least one fuse element provided on the substrate, and a molded polymeric (e.g., plastic) enclosure provided on the substrate over the at least one fuse element, wherein the at least one fuse element is received in an interior of the enclosure. The device further includes an overcurrent fuse electrically connected to the at least one fuse element, wherein the overcurrent fuse is received and held within the interior of the enclosure. In one embodiment, the interior includes at least one outer chamber and an inner chamber, wherein the at least one fuse element is received within the at least one outer chamber and wherein the overcurrent fuse is received and held within the inner chamber. Preferably, the overcurrent fuse is encapsulated in a non-organic material such as silicone. In addition, the at least one fuse element may comprise a thermal fuse spring or a plurality of thermal fuse springs. The enclosure may include a support shelf for supporting a current transformer.  
         [0021]     A further aspect of the invention relates to a surge protection device that includes a substrate having a slot provided therein, a transient suppressing element, such as an MOV or SAD, provided on the substrate, and a thermal fuse spring provided on the substrate. The thermal fuse spring has a finger having a first end and a second end that is biased toward the first end. The first end of the finger is attached to the substrate, such as through a base forming part of the thermal fuse spring, on a first side of the slot and the second end of the finger is attached to the substrate on a second side of the slot opposite the first side. The second end, when attached to the substrate, is electrically connected to the transient suppressing element. The second end of the finger is preferably attached to the substrate and electrically connected to the first end of the fuse trace by a solder material, wherein when the solder material is caused to melt, the second end of the finger moves toward the first end of the finger over the slot. The slot serves to reduce the likelihood that an arc is generated as the second end of the finger moves toward the first end of the finger, thereby opening the fuse. A fuse trace may be provided on the substrate between the second end of the finger and the transient suppressing element. In one embodiment, the device includes a plurality of transient suppressing elements and a plurality of thermal fuse springs provided on the substrate  
         [0022]     According to yet a further aspect of the invention, a surge protection device is provided that includes a substrate, a transient suppressing element provided on the substrate, a first trace provided on the substrate wherein the first end thereof is electrically connected to the transient suppressing element, such as an MOV or an SAD, and a second trace provided on the substrate. In addition, a wire jumper is attached to and extends above the substrate. The first end of the wire jumper is electrically connected to the second end of the first trace and the second end of the wire jumper is electrically connected to the first end of the second trace. The wire jumper increases the length of the fuse link that is provided in limited space on the surface of the substrate. Preferably, the wire jumper is encased in a non-organic material such as silicone. Also, a portion of a first surface of the substrate that includes the first and second traces and the first and second ends of the wire jumper is preferably covered by a layer of non-organic material such as silicone. Similarly, a second surface of the substrate opposite the first surface is also covered by a layer of the non-organic material. The wire jumper and the first and second traces may be made of the same metallic material, or, alternatively, may be made of different metallic materials such that the melting I 2 t of the wire jumper and the first and second traces are substantially equal. In one embodiment, the second end of the second trace is electrically connected to a finger of a thermal fuse spring provided on the substrate. In another embodiment, a barrier made of dielectric material, such as a polycarbonate material, is provided on the substrate beneath the wire jumper.  
         [0023]     A still further aspect of the invention relates to a surge protection device that includes a substrate, a transient suppressing element, such as an MOV or an SAD, provided on the substrate, and a fuse link provided on the substrate. The fuse link includes a first trace provided on a first surface of the substrate, wherein the first end of the first trace is electrically connected to the transient suppressing element, a second trace provided on a second surface of the substrate opposite the first surface, wherein the first end of the second trace is electrically connected to the second end of the first trace, and a third trace provided on the first surface of the substrate, wherein the first end of the third trace is electrically connected to the second end of the second trace. In one embodiment, the longitudinal axis of each of the first, second and third traces are substantially parallel to one another. In an alternative embodiment, the longitudinal axis of the first trace and the longitudinal axis of the second trace are disposed at a first angle with respect to one another and the longitudinal axis of the second trace and the longitudinal axis of the third trace are disposed at a second angle with respect to one another. In one particular embodiment, the first angle and the second angle are substantially equal to 90 degrees. Preferably, a portion of the first surface of the substrate that includes the first and third traces and a portion of the second surface of the substrate that includes the second trace are each covered by a layer of non-organic material such as silicone. The second end of the third trace may also be electrically connected to a finger of a thermal fuse spring provided on the substrate.  
         [0024]     In still a further aspect of the invention, a surge protection device is provided that includes a substrate, a transient suppressing element, such as an MOV or an SAD, provided on the substrate, a main fuse element provided on the substrate, and a bypass link provided in parallel with the main fuse element that has a fuse link therein that is larger than the main fuse element. The main fuse element and the bypass link are electrically connected to the transient suppressing element. The main fuse element may be, for example, a fuse trace or a thermal fuse spring. The bypass link may also include a second transient suppressing element or a capacitor connected in series with the fuse link. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     The accompanying drawings illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain the principles of the invention. As shown throughout the drawings, like reference numerals designate like or corresponding parts.  
         [0026]      FIG. 1A  is a cross-sectional view of a prior art SPD design;  
         [0027]      FIG. 1B  is an isometric view of a prior art SPD design similar to the design of  FIG. 1 ;  
         [0028]      FIG. 1C  is an isometric view of a prior art TFS design;  
         [0029]      FIG. 1D  is an isometric view of another prior art SPD design that employs the TFS design of  FIG. 1C ;  
         [0030]      FIG. 2  is a block diagram of an SPDD design according to an aspect of the present invention;  
         [0031]      FIGS. 3A, 3B  and  3 C are top plan, cross sectional and isometric views of a fuse holder according to a further aspect of the present invention;  
         [0032]      FIG. 3D  is an isometric view of an overcurrent fuse and a pair of terminals forming a part of the fuse holder shown in  FIGS. 3A-3C ;  
         [0033]      FIG. 4  is a top plan view of a printed circuit board including an SPDD having a number of the fuse holders of  FIGS. 3A-3C  provided thereon;  
         [0034]      FIG. 5A  is a side view, in partial cross-section, of an SPDD according to a further aspect of the present invention;  
         [0035]      FIGS. 5B and 5C  are isometric views of a portion of the SPDD shown in  FIG. 5A ;  
         [0036]      FIG. 6A  is a side view, in partial cross-section, and  FIG. 6B  is an isometric view of a portion of an SPDD according to yet a further aspect of the present invention;  
         [0037]      FIG. 7A  is a side view, in partial cross-section, and  FIG. 7B  is an isometric view of a portion of an alternate embodiment of an SPDD according to yet a further aspect of the present invention;  
         [0038]      FIGS. 8A, 8B  and  8 C are schematic diagrams of various different embodiments of a fuse link according to still yet a further aspect of the present invention; and  
         [0039]      FIGS. 9A-9F  are schematic diagrams of a number of different configurations for controlling/mitigating the voltage on a fuse when it operates according to yet a further aspect of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]      FIG. 2  is a schematic diagram of a surge protection device disconnector (SPDD)  200  according to an aspect of the present invention. SPDD  200  includes an overcurrent fuse  205  that is electrically connected to a phase conductor  210  of the electrical distribution system.  
         [0041]     The SPDD  200  includes a first branch  215 A and a second branch  215 B which are identical to one another. Preferably, the first and second branches  215 A and  215 B are each similar in structure to the SPD′  70  shown in  FIG. 1B  and described above or the SPD shown in  FIG. 1D . For clarity, only the first branch  215 A will be described, but it will be understood that the second branch  215 B includes identical components. The first branch  215 A includes thermal fuse springs (TFS)  220 A,  220 B,  220 C and  220 D in the form shown in either  FIG. 1A  or  FIGS. 1C and 1D  which are each electrically connected to the overcurrent fuse  205  as seen in  FIG. 2 . The thermal fuse springs (TFS)  220 A,  220 B,  220 C and  220 D are preferably part of a TFS unit such as the TFS unit  76  shown in  FIG. 1B  or the TFS units  166 ,  168  shown in  FIGS. 1C and 1D . Each TFS  220 A- 220 D will open due to heat that is generated in the MOVs  230 A- 230 H (described below) of the first branch  215 A and will effectively function in the low and medium fault current range, meaning that it will open in the presence of such fault currents. The first branch  215 A also includes fuse traces  225 A- 225 H each in the form of fuse trace with solder hole (FTWSH)  78  shown in  FIG. 1A  and described above. As seen in  FIG. 2 , each TFS  220 - 220 D is connected to a respective pair of the fuse traces  225 A- 225 H in the manner shown in  FIGS. 1A and 1B  or  FIG. 1D . Each of the fuse traces  225 A- 225 H will effectively function in the medium fault current range, meaning that it will open in the presence of such fault currents, but will not open in the presence of certain low range fault currents. Finally, the first branch  215 A includes MOVs  230 A- 230 H, each connected to a respective one of the fuse traces  225 A- 225 H in the manner shown in  FIGS. 1A and 1B  or  FIG. 1D . Furthermore, one of the leads of each MOV  230 A- 230 H is connected to a neutral or ground conductor  235  of the electrical distribution system. As will be appreciated, other transient suppressing elements, such as SADs, may be substituted for the MOVs  230 A- 230 H. Thus, as will be appreciated, according to an aspect of the present invention, the SPDD  200  shown in  FIG. 2  will provide protection over the entire range of fault currents, i.e., it will provide protection against high, medium and low fault currents. The operating ranges of each of the three types of fuses (overcurrent, TFS and fuse trace) will vary with system voltage and time (phase angle) of the fault. Also, the ranges of the three types of fuses are preferably overlapping, meaning that under certain conditions, two or even three of the fuse types might respond (open) at the same time to a particular fault condition. In particular, the operating range of the overcurrent fuse preferably overlaps the operating range of the fuse terraces, and the operating range of the fuse traces preferably overlaps the operating range of the TFSs. For example, one TFS finger and one fuse trace might open (disconnect) the circuitry at fault conditions of 100 A fault current and a duration of 5 s.  
         [0042]      FIGS. 3A, 3B ,  3 C and  3 D show a number of different views of a fuse holder  240  according to a further aspect of the present invention. As described in greater detail below, the fuse holder  240  is designed to prevent various types of arcing associated with an SPDD design such as SPDD  200 . The fuse holder  240  includes a molded enclosure  245  preferably having a square cross-section. The molded enclosure  245  is made of a polymeric material such as polycarbonate or another plastic material. The enclosure  245  includes external walls  250 A,  250 B,  250 C and  250 D and internal walls  255 A and  255 B. The external walls  250 B and  250 D and the internal walls  255 A and  255 B together form outer chambers  260 A and  260 B therebetween. In addition, the internal walls  255 A and  255 B define a generally rectangular inner chamber  265  including top and bottom apertures  270 A and  270 B. The fuse holder  240  also includes first and second terminals  275 A and  275 B ( FIG. 3D ) for holding and making electrical connections to the overcurrent fuse  205 . The first and second terminals  275 A and  275 B each include a threaded terminal connector  280 A and  280 B for making electrical connections thereto. As seen in  FIGS. 3B and 3C , the first and second terminals  275 A and  275 B having the overcurrent fuse  205  held therein are adapted to be received and held within the inner chamber  265 . In this configuration, the threaded terminal connector  280 A extends through the aperture  270 A and the threaded terminal connector  280 B extends through the aperture  270 B. As also seen in  FIGS. 3B and 3C , the fuse holder  240  is adapted to be placed over the TFSs  220 A- 220 D in the first and second branches  215 A and  215 B of the SPDD  200  shown in  FIG. 2 , in which case the TFSs  220 A- 220 D are received within the outer chambers  260 A and  260 B. Similarly, the fuse holder  240  may be placed over the TFS units  166 ,  168  shown in  FIGS. 1C and 1D . Finally, the enclosure  245  includes a shelf  285  for holding an annular current transformer  290  which measures the current flowing through the overcurrent fuse  205 .  
         [0043]     It has been found during testing that the surface of the overcurrent fuse  205 , if exposed to air, burns due to heat generated inside the overcurrent fuse  205  Thus, in the preferred embodiment of the fuse holder  240 , the overcurrent fuse  205  is embedded in a non-organic material such as, without limitation, silicone. The non-organic material (e.g., silicone) provides a cooling effect for the body of the overcurrent fuse  205  during medium fault current conditions. By increasing thermal mass of the SPDD  200 , the non-organic material does not change the response time of the overcurrent fuse  205  under high fault current conditions, but does keep the temperature of the surface of the overcurrent fuse  205  down during medium fault current conditions. In addition, typical, off the shelf fuses that may be used for overcurrent fuse  205  are not designed to operate for fault currents below a certain level, and may explode or burn in that region. With the benefit of the cooling effect of the non-organic material as described above, the minimum fault current at which the overcurrent fuse  205  will operate can be reduced, thus allowing the overcurrent fuse  205  to be effective over a greater range.  
         [0044]      FIG. 4  is a schematic diagram showing four fuse holders  240  mounted on a PCB  300  having a ground bus bar  305  and a neutral bus bar  310 . The fuse holders  240  prevent arcing between the different metal parts, namely the TFSs  220 A- 220 D and the ground and neutral bus bars. As will be appreciated, these metal parts are connected to different voltage potentials isolated by air. The air provides sufficient dielectric strength under normal conditions, but when contaminated by metal-oxide dust, ionized gas and/or plasma, such as in the case of an explosive failure of an MOV, the spacing between the parts might not be large enough to prevent severe arcing, which tends to occur in the areas shown by the arrows in  FIG. 4 . Placing barriers between these parts, in the form of the fuse holders  240 , prevents the arcing.  
         [0045]     Thus, the fuse holder  240  performs at least the following functions: (i) holding the overcurrent fuse  205 , (ii) holding the current transformer  290  ( FIG. 3A ), (iii) preventing arcing from one TFS to another, and (iv) preventing arcing from a TFS to a ground and/or neutral bus bar ( FIG. 4 ).  
         [0046]      FIGS. 5A, 5B  and  5 C show portions of an SPDD  315  according to a further aspect of the present invention. The SPDD  315  is similar to the SPDs  70  and  70 ′ shown in  FIGS. 1A and 1B  and the SPD shown in  FIG. 1D  in that it includes a PCB  320  having a number of MOVs  325 , a number of fuse traces (with solder holes)  330 , and thermal fuse springs (TFSs)  335  having fingers  340  having ends  345  provided/mounted thereon. As described in connection with  FIGS. 1A and 1B , each of the ends  345  of the finger  340  is connected to a respective fuse trace  330  by a fusible alloy, such as a suitable low temperature solder, in order to hold each TFS  335  in a stretched position. In this manner, a series electrical connection is established from each MOV  325  to a respective fuse trace  330  to a respective TFS  335 . In the preferred embodiment (shown in  FIG. 5A -C), the TFSs  335  are part of a TFS unit in the form shown in  FIGS. 1C and 1D . In that type of configuration, the TFS unit includes a base  347  that is attached to the PCB  320  by mechanical means, wherein an inner end of each of the fingers  340  (the end opposite the end  345 ) is attached to the base  347 .  
         [0047]     As described above, under certain conditions, a TFS  335  will trip. Specifically, under certain conditions the solder at the end  345  of the finger  340  of a TFS  335  will melt and release the finger  340  from its stretched position back to a tripped position (where the end  345  of the finger  340  is closer to the base  347  as seen in  FIG. 5C ). Normally, during such tripping an arc would be formed between the finger  340  and the PCB  320  (this is the case with the SPDs  70  and  70 ′ shown in  FIGS. 1A and 1   b  and the SPD shown in  FIG. 1D ). While the end of the finger  340  moves toward the base  347 , the arc is dragged along the PCB  320 . Such an arc may cause other, bigger arcs between other metal parts. The arcing is exacerbated due to the fact that carbon traces (or traces from other contaminants) are typically left on the PCB when a fuse opens. However, according to an aspect of the present invention, such arcing is prevented by providing a slot  350  in the PCB  320  beneath each TFS  335  in between the end of the finger  340  and the base  347 . This is the case because the slot  350  increases the dialectic strength (by a factor of approximately 2) of the space between the base  347  and the portion of the PCB  320  to which the end  345  of the finger  340  was soldered, thereby significantly reducing arcing conditions. In the preferred embodiment, the slot is about 0.2 inches wide.  
         [0048]      FIGS. 6A and 6B  show portions of an alternative embodiment of the SPDD  315  shown in  FIGS. 5A, 5B  and  5 C, designated as SPDD  315 ′, that includes a modified fuse trace  330 ′. As stated elsewhere herein, the fuse traces have to open at fault currents in the medium range (e.g., 10 A-1000 A). For such a fault range, usually only one MOV  325  is failing at a time, which means only one fuse trace at a time has to operate. In order to clear the fault current properly at a high voltage (e.g., on the order of 600 Vac), the fuse trace has to have a certain length, and optimally should be as long as possible. Thus, according to an aspect of the present invention, fuse trace  330 ′ includes a mechanism for increasing the length thereof (as compared to fuse trace  330 ). In particular, a wire jumper  355  preferably encased in a silicone tube (or a tube made of another non-organic material) is provided between and electrically connected to a first trace  360 A provided on the PCB  320  at one end thereof and a second trace  360 B provided on the PCB  320  at the opposite end thereof. The first trace  360 A is also electrically connected to the MOV  325  in a manner described elsewhere herein, and the second trace  360 B is electrically connected to the end  345  of the finger  340  of the TFS  335  by the low temperature solder. Preferably, the wire jumper  355  is electrically connected to the first and second traces  360 A and  360 B my means of solder holes provided in the PCB  320 . In addition, as seen in  FIGS. 6A and 6B , the first and second traces  360 A and  360 B, the ends of the wire jumper, the leads of the MOV  325 , and the top and bottom of the adjacent portions of the PCB  320  are covered by a silicone layer  365  (or a layer of another suitable non-organic material). The silicone tube surrounding the wire jumper  355  helps to quench arcing during the fuse opening period and the silicone layer  365  helps to quench arcing over the surfaces of the PCB  320  from one via through-hole to another via through-hole by preventing contaminants form being released into the air. The total cross-section of the first and second traces  360 A and  360 B and the wire jumper  355  may be made of the same metal. Alternatively, the first and second traces  360 A and  360 B and the wire jumper  355  may be made of different metals by suitably selecting cross-sections that will make the melting I 2 t of the elements substantially the same.  
         [0049]      FIGS. 7A and 7B  show portions of an alternative embodiment of the SPDD  315 ′, designated as SPDD  315 ″. The SPDD  315 ″ differs from the SPDD  315 ′ in that it further includes a barrier  370  provided below the wire jumper  355 . The barrier  370  is made of a dielectric material having a dielectric strength higher than air, such as a polycarbonate material like Lexan. The barrier  370  is preferably an elongated, rectangular shaped element inserted into a slot provided in the PCB  320 . The barrier  370  provides further isolation between the ends of the wire jumper  355  and the first and second traces  360 A and  360 B, thereby helping to prevent arcing. In addition, although both SPDD 315 ′ and SPDD  315 ″ are shown as having the slot  350 , it will be appreciated that the wire jumper  355  and silicone layer  365  may be used in embodiments that do not include the slot  350 .  
         [0050]     Due to space limitations present in prior are SPDD designs (i.e., the limited space available on the PCBs), the fuse traces used therein have typically been very short. However, the shorter the fuse trace, the more likely that arcing will occur when the fuse trace opens. Thus, according to a further aspect of the present invention, a number of fuse link designs are provided which serve to increase the effective length of a fuse trace by providing traces on both sides of a PCB.  FIG. 8A  is a schematic diagram of a first embodiment of a fuse link  375  according to an aspect of the present invention. The fuse link  375  includes a first conductive trace (e.g., a copper trace)  380 A provided on a bottom surface of a PCB (not shown), a second conductive trace (e.g., a copper trace)  380 B provided on a top surface of a PCB (not shown), and a third conductive trace (e.g., a copper trace)  380 C provided on the bottom surface of a PCB (not shown). The first conductive trace  380 A is electrically connected the second conductive trace  380 B through solder filled via  385 A and the second conductive trace  380 B is electrically connected the third conductive trace  380 C through solder filled via  385 B. In addition, solder filled via  385 C is provided to enable the first conductive trace  380 A to be electrically connected to the lead of an MOV (not shown), such as MOV  325  shown in  FIGS. 5A-5C ,  6 A and  6 B, and  7 A and  7 B, and solder filled via  385 D is provided to enable the third conductive trace  380 A to be electrically connected to the finger of a TFS, such as TFS  335  shown in  FIGS. 5A-5C ,  6 A and  6 B, and  7 A, through, for example, an additional trace and solder.  FIG. 8B  is a schematic diagram of a second embodiment of the fuse link, designated at  375 ′, according to a further aspect of the present invention. In the embodiment shown in  FIG. 8B , the fuse traces  380 A- 380 F are alternately provided on the top and bottom sides of the PCB (not shown) in a manner such that the longitudinal axis of adjacent fuses traces  380 A- 380 F are angled with respect to one another. In the preferred embodiment shown in  FIG. 8B , the angle is about 90°, although other angles are possible. This is in contrast to the first embodiment shown in  FIG. 8A  in which the longitudinal axis of adjacent fuses traces  380 A- 380 C are substantially parallel to one another. According to a further, preferred aspect of the present invention shown in  FIG. 8C , the fuse link  375  (or  375 ′) includes a first layer of silicone  390 A (or another suitable non-organic material) that covers the vias and the fuse traces on the top surface of the PCB and a second layer of silicone  390 B (or another suitable non-organic material) that covers the vias and the fuse traces on the bottom surface of the PCB, which layers  390 A and  390 B have arc extinguishing properties. Another advantage of the fuse links  375  and  375 ′ is that they have multiple break points. The multiple break points (gaps) will result in a number of smaller arcs (each fuse gap will arc) as opposed to a single large arc (when only one gap is present). As will be appreciated, it is easier to extinguish multiple small arcs as opposed to a single large arc.  
         [0051]     Moreover, it is known that the voltage across the terminals of a fuse rises when the fuse opens under fault conditions such as those described herein. It is therefore desirable to keep that voltage as low as possible in order to prevent arcing between the fuse terminals due to the dielectric breakdown of the air between the terminals.  FIGS. 9A-9F  show a number of different configurations for controlling/mitigating the voltage on a fuse when it operates (opens) according to yet a further aspect of the present invention.  FIG. 9A  shows a first embodiment of an arrangement for mitigating voltage that includes a main fuse link  400  and a bypass link having bypass fuse link  405  connected in parallel therewith. The main fuse link  400  may be any fuse link, such as, without limitation, the fuse traces  225 A-H, the fuse traces  330  and  330 ′, and the fuse links  375  and  375 ′ described herein. The main fuse link  400  is smaller than the bypass fuse link  405 , which functions to redirect the fault current once the main fuse link  400  opens. The term smaller as used herein means that the main fuse link  400  has a smaller impedance than and/or a lower rms fault current rating than the bypass fuse link  400 . As a result, instead of arcing through the remains of the open (melted) main fuse link  400 , the fault current will flow through the relatively larger bypass fuse link  405 . Because the bypass fuse link  405  is larger, it is easier to control arcing once it opens.  FIG. 9C  shows an alternate arrangement employing the bypass fuse link  405  wherein the main fuse link  400  is replaced with a thermal fuse spring  410 , such as TFS  335  described herein. The principles of operation remain the same.  FIGS. 9B and 9D  show still further alternative arrangements (using a main fuse link  400  and a thermal fuse spring, respectively) wherein a low-clamping (i.e., 10-20% of the nominal voltage) MOV  415  is provided in series with the bypass fuse link  405  in the bypass link. The low-clamping MOV  415  will clamp and conduct the re-directed fault current once the main fuse link  400  or thermal fuse spring  410  operates. The bypass fuse link  405  can have a longer length than the main fuse link  400  and will have a much smaller arc than the main fuse link arc, which means that the clearing time will be shorter. The arc for the by-pass fuse link  405  will be smaller because the by-pass fuse link  405  has a higher impedance, better heat dissipation (due to its longer length) and a smaller voltage drop. Finally,  FIGS. 9E and 9F  show still further alternative arrangements (using a main fuse link  400  and a thermal fuse spring, respectively) wherein a capacitor  420  is provided in series with the bypass fuse link  405  in the bypass link. In this case, the bypass fuse link  405  has a low impedance (in the milliohms range) at higher arc frequencies (in the range from 10 th  harmonics to RF).  
         [0052]     While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, deletions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.