Abstract:
The disclosure describes a surge protection device that makes use of metal oxide varistors (MOVs) to limit the magnitude of electrical surges in single phase or multi-phase power distribution networks. Since MOVs generally fail in a low impedance mode when the electrical ratings of the devices are exceeded, current limiting and thermal limiting devices are connected in series with the MOVs to limit the effect of these failures to the surge protection devices. The physical configuration of the surge protection device limits the effects of a current limiter being activated by providing a barrier between the current limiter and the associated MOV. Filler material is also used to limit the effects of debris or plasma gas from an activated current limiter. Multiple surge protection devices may be connected in parallel to provide increased current handling capability, and visual indication of a failed surge protection device may be provided.

Description:
BACKGROUND 
     The invention relates generally to electrical surge protection devices and, more particularly, to electrical surge protection devices that make use of metal oxide varistors (MOVs), thermal protection elements and current limiting elements. 
     Electrical surges occur in single phase or multi-phase power distribution networks, and may be induced be near-by electromagnetic radiation such as lightning discharges. Electrical surges may also result from equipment connected to the power distribution network being turned on or off. Certain electrical equipment may generate electrical surges in a power distribution network in the course of normal operation. Typical surge generating equipment includes motors, lights, and welding equipment. Generally, larger load currents create greater electrical surges when switched on or off. While circuit breakers protect against excess current conditions, surge protection devices (SPDs) protect against excess voltage conditions. These devices may be located at a service entrance to prevent electrical surges from penetrating a local power distribution network, such as a building or a complex of buildings. They may also be located at a source of electrical surges to prevent distribution of the surges, or at sensitive electrical equipment to prevent electrical surges from entering and damaging this equipment. SPDs are generally connected between the phases of a power distribution system and a neutral and ground potential, but other configurations are sometimes used. Multiple SPDs are often connected in parallel to increase current-handling capability. A means for indicating a failure of one or more SPDs is often associated with these devices. 
     Metal oxide varistors (MOVs) are commonly used in combination with current limiting fuse elements in SPDs. MOVs are two-terminal electrical devices that have a nonlinear voltage-current relationship. At low voltages, a MOV exhibits a high impedance between its two terminals, but at voltages higher than a predetermined limit voltage the impedance rapidly changes to a low impedance. This characteristic is useful as a voltage limiter, because as the voltage across the MOV terminals increases, within the power handling capability of the MOV, the voltage is clamped to the predetermined limit voltage. The predetermined limit voltage is a characteristic dependent on a particular MOV, and is determined by the MOV manufacturing process. 
     An overload condition occurs in an SPD if a sustained current, a high current surge pulse, or multiple repetitive surge pulses, having a voltage sufficiently higher than the predetermined limit voltage to cause conduction, flows through a MOV causing the power capability of the MOV to be exceeded. A sustained overload condition will normally cause the MOV to fail in a short-circuit condition. Without protection, the MOV would become over-heated, cause the circuit to be overloaded, and trip a circuit breaker. This could lead to disintegration of the MOV and other components nearby. To prevent this overload condition, a fuse is normally connected in series with the MOV to limit the maximum current through the MOV. Thus, upon a high current overload condition causing a MOV failure, the series fuse opens to prevent a circuit breaker from tripping, but the surge protection capability is lost. However, it is possible, particularly with repetitive multiple surges pulses, to generate excessive heat in the MOV without causing the series fuse to open. This excessive heat could cause damage to other components that could lead to a chain reaction of failures. Some form of thermal protection is required to prevent these types of failures. To alert maintenance personnel of the failure, many SPDs incorporate visual or audio indication of the failure. In order to increase the power and current-handling capability of an assembly that incorporates SPDs, multiple SPDs are often connected in a parallel configuration. 
     Upon a failure of a 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. This debris may cause short-circuits in any electronic circuitry in the vicinity of the MOV or fuse, including other SPD circuits or a failure indication mechanism. Another possibility is that the destruction of a MOV or fuse, due to an overload condition, may vaporize and create an ionized gas or plasma containing metallic particles. This plasma is a conductive gas and is very invasive. It may also cause short-circuits in electrical and electronic circuits that it comes in contact with. A plasma of this nature could short-circuit a power distribution system and has a potential capability of causing extensive damage and bodily harm to nearby personnel. 
     Therefore, there is a need for a compact SPD for use in power distribution systems that is capable of limiting the voltage amplitude of a surge pulse, will open-circuit if a predetermined current limit is exceeded, will open-circuit if the MOV power dissipation creates a temperature that exceeds a predetermined thermal threshold, and will provide a means whereby any conductive debris or plasma gas is contained within a confined region where further damage cannot be propagated. There is also a need to configure multiple SPDs in parallel in order to increase current handling capability. Visual display of a failed SPD is also needed so that maintenance personnel will be alerted to a failed condition. 
     SUMMARY 
     The present invention is directed to a compact device that satisfies these needs. The present invention provides a compact means for limiting the voltage amplitude of a surge pulse through the use of a MOV. A series fuse element is provided that limits the maximum current through the MOV. A series thermal limiting element is also provided that limits the maximum temperature in the vicinity of the MOV. Suitable barriers and filler materials are provided to limit the extent of debris or plasma gas. Multiple SPDs may be configured in a parallel configuration and visual indication of a failed SPD may be provided. 
     A device having features of the present invention is a surge protection device for a power distribution network that comprises a current limiter connected between a first input terminal and a bridge terminal, the first input terminal connecting to the power distribution network, a thermal protector connected between the bridge terminal and a central terminal, a metal oxide varistor connected between the central terminal and a second input terminal, the second input terminal connecting to the power distribution network, a current sense resistor connected between the bridge terminal and the central terminal, and a thermal sense resistor connected between the central terminal and an indicator terminal. The current limiter may comprise a perforated silver ribbon, a strand of silver wire, multiple strands of silver wire, a silver ribbon, a copper ribbon, or a perforated copper ribbon. The current limiter may be enclosed in a fuse tube. The thermal protector may comprise a device selected from the group consisting of a low melting point alloy wire, a lead-indium alloy wire, a lead-antimony alloy wire, and a thermal cutout device. The thermal protector may be positioned in close proximity with the metal oxide varistor. The current sense resistor may be replaced by a current sense capacitor and the thermal sense resistor may be replaced by a thermal sense capacitor. A bridge may be positioned between the current limiter and the metal oxide varistor for providing an isolating barrier. The device may include a housing for containing the surge protection device, sand for filling void spaces within the housing, and potting material for sealing the housing. A failure indicator circuit may be connected to the indicator terminal. The configuration of the current sense resistor and the thermal sense resistor supplies a signal at the indicator terminal that provides a distinction between a thermal protector open circuit and a current limiter open circuit. The failure indicator circuit may comprise a summing resistor connected between the indicator terminal and a ground, a rectifier having an anode connected to the indicator terminal and a cathode connected to a comparator circuit first input terminal, a capacitor and resistor parallel circuit connected between the rectifier cathode and the ground, a zener diode having a cathode connected to the rectifier anode and an anode connected to ground, a comparator circuit second input terminal connected to ground, and a comparator circuit output connected to a visual indicator. The comparator may be replaced by a microprocessor. Multiple surge protection devices may be connected to a multi-phase power distribution system. 
     In an alternate embodiment of the invention, a surge protection circuit for a power distribution network comprises a plurality of identical circuits, each circuit comprising a current limiter connected between a first input terminal and a bridge terminal, a thermal protector connected between the bridge terminal and a central terminal, a metal oxide varistor connected between the central terminal and a second input terminal, a current sense resistor connected between the bridge terminal and the central terminal, and a thermal sense resistor connected between the central terminal and an indicator terminal, wherein the first input terminals of each identical circuit are connected together and connect to the power distribution network, the second input terminals of each identical circuit are connected together and connect to the power distribution network, and the indicators terminals are connected together. Each current limiter may comprise a perforated silver ribbon, a strand of silver wire, multiple strands of silver wire, a silver ribbon, a copper ribbon, or a perforated copper ribbon. Each current limiter may be enclosed in a fuse tube. Each thermal protector may comprises a device selected from the group consisting of a low melting point alloy wire, a lead-indium alloy wire, a lead-antimony alloy wire, and a thermal cutout device. Each thermal protector may be positioned in close proximity with the metal oxide varistor in the same circuit. Each current sense resistor may be replaced by a current sense capacitor and each thermal sense resistor may be replaced by a thermal sense capacitor. A bridge may be positioned between each current limiter and each metal oxide varistor for providing an isolating barrier. The invention may further comprise a housing for containing the surge protection device, sand for filling void spaces within the housing, and potting material for sealing the housing. A failure indicator circuit may be connected to the indicator terminal. The configuration of the current sense resistors and the thermal sense resistors supplies a signal at the indicator terminal that provides a distinction between thermal protector open circuits and current limiter open circuits. The failure indicator circuit may comprise a failure detection circuit, a comparator, and a visual indicator. The comparator may be replaced by a microprocessor. Multiple surge protection devices may be connected to a multi-phase power distribution system. 
     Another embodiment of the present invention is a method of fabricating a surge protection device for a power distribution network, comprising connecting a current limiter between a first input terminal and a bridge terminal, the first input terminal being connected to the power distribution network, connecting a current limiter between a first input terminal and a bridge terminal, the first input terminal being connected to the power distribution network, connecting a thermal protector between the bridge terminal and a central terminal, connecting a metal oxide varistor between the central terminal and a second input terminal, the second input terminal being connected to the power distribution network, connecting a current sense resistor between the bridge terminal and the central terminal, and connecting a thermal sense resistor between the central terminal and an indicator terminal. The current limiter may comprise a perforated silver ribbon enclosed within a fuse tube. The thermal protector may be positioned in close proximity with the metal oxide varistor. A bridge may be positioned between the current limiter and the metal oxide varistor for providing an isolating barrier. The embodiment may further comprise positioning the connected circuit components within a housing, filling the housing with sand, and sealing the housing with potting material. The method may further comprise connecting a failure indicator circuit to the indicator terminal. The current sense resistor may be replaced by a current sense capacitor and the thermal sense resistor may be replaced by a thermal sense capacitor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and adbantages of the present invention will become better understood with regard to the following description, appended claim, and accompanying drawings where: 
     FIG. 1 shows a connection diagram of surge protection devices for a wye-configured power distribution network; 
     FIG. 2 shows a typical equipment configuration of the surge protection devices for a wye-configured power distribution network; 
     FIG. 3A shows a typical component configuration of a single-circuit surge protection device, FIG. 3B shows a schematic diagram of a single-circuit surge protection device using sensing resistors, FIG. 3C shows an enclosed module of a single-circuit surge protection device, and FIG. 3D shows a schematic diagram of a single-circuit surge protection device using sensing capacitors; 
     FIG. 4A shows a typical component configuration of a multiple-circuit surge protection device, FIG. 4B shows a schematic diagram of a multiple-circuit surge protection device using sensing resistors, FIG. 4C shows an enclosed module of a multiple-circuit surge protection device, and FIG. 4D shows a schematic diagram of a multiple-circuit surge protection device using sensing capacitors; 
     FIG. 5A shows a typical perforated ribbon configuration of a current limiter, and FIG. 5B shows a typical perforated ribbon configuration of a current limiter enclosed within a fuse tube; and 
     FIG. 6A shows a schematic diagram of a multiple-circuit surge protection device connected to circuitry and comparator for failure indication, and FIG. 6B show an alternative connection to a microprocessor. 
    
    
     DETAILED DESCRIPTION 
     Turning now to FIG. 1, FIG. 1 shows a connection diagram of surge protection devices  10  for a wye-configured power distribution network. Seven surge protection devices  200  are required for a wye-configured power distribution network. A similar configuration of six surge protection devices would be required for a delta-configured power distribution network. A surge protection device  200  connects between each of three phases  110 ,  120 ,  130  and neutral  140 , and between each of three phases  110 ,  120 ,  130  and ground  150 , as shown. A surge protection device  200  also connects between neutral  140  and ground  150 . A failure indication device  300  is associated with and connects to each surge protection device  200 . 
     Turning now to FIG. 2, FIG. 2 shows a typical equipment configuration of the surge protection devices  20  for a wye-configured power distribution network. Seven surge protection devices  200  are mounted on a printed circuit board  170 . Connectors are provided for phase A  112 , phase B  122 , phase C  132 , neutral  142 , and ground  152 . Not shown are the failure indication devices that receive signals through the connector  160 . 
     Turning now to FIG. 3, FIG. 3A shows a typical component configuration of a single-circuit surge protection device  30 , FIG. 3B shows a schematic diagram  32  of a single-circuit surge protection device using sensing resistors, and FIG. 3D shows a schematic diagram  35  of a single-circuit surge protection device using sensing capacitors. The following description applies to FIGS. 3A,  3 B, and  3 D. The circuit components are mounted on a printed circuit board  310  having connector pins  312  positioned on a side opposite the components for connecting to another printed circuit board. A current limiter component  330  connects between a first input terminal  320  and a bridge terminal  324  located on an upper portion of a bridge  340 . The current limiter component  330  limits the maximum current through a metal oxide varistor  360 . The bridge  340  is positioned in a perpendicular relationship with the printed circuit board  310 . The bridge  340  is positioned between the current limiter  330  and the metal oxide varistor  360  to provide an isolating barrier between the two components, and to provide a support means to extend the length of the current limiting component  330  for extinguishing any electrical arcing in the current limiting component  330 . A thermal protector component  350  connects between the bridge terminal  324  and a central terminal  370 . The thermal protector component  350  is positioned in close proximity to the metal oxide varistor  360  and open-circuits upon reaching a predetermined temperature in order to prevent disintegration of the metal oxide varistor  360  from excessive self-heating. A current sense resistor  371  also connects between the bridge terminal  324  and the central terminal  370 . A current sense capacitor,  381  shown in FIG. 3D, could also be used in place of the current sense resistor  371  shown in FIG.  3 B. The metal oxide varistor  360  connects between the central terminal  370  and a second input terminal  322 . The metal oxide varistor  360  is typically between a 10-millimeter and an 80-millimeter device. A thermal sense resistor  372  connects between the central terminal  370  and an indicator terminal  328 . A thermal sense capacitor,  382  shown in FIG. 3D, could also be used in place of the thermal sense resistor  372  shown in FIG. 3B. A failure-indicating device may be connected to the indicator terminal  328 . The current sense resistor  371  or current sense capacitor  381  and the thermal sense resistor  372  or thermal sense capacitor  382  are configured to provide a distinguishing indication between a current limiter component  330  open-circuit and a thermal protector component  350  open-circuit. The current limiter component  330  may be a single strand of silver wire, multiple strands of silver wire, silver ribbon, perforated silver ribbon, copper ribbon, or perforated copper ribbon. A fuse tube may also enclose the current limiter component  330 , as shown in FIG. 5B, in order to increase the current limiting capability of the current limiter component  330 . The thermal protector component  350  is a low melting point alloy wire, such as lead-antimony alloy wire or lead-indium alloy wire. The thermal protector  350  may also be a commercially available thermal cutout device. A power distribution network connects to the first input terminal  320  and the second input terminal  322 . 
     FIG. 3C shows an enclosed module  34  containing a single-circuit surge protection device  30 . The structure  30  shown in FIG. 3A is positioned within a housing  390  and the housing  390  is filled with sand  392 . Sand  392  is used to position the thermal protector component  350  and to thermally couple it to the metal oxide varistor  360 . The sand  392  and the bridge  340  provide isolation between the current limiter component  330  and the other components, including the thermal protector component  350  and the metal oxide varistor  360 . Sand  392  is also required to enable interruption by the current limiter component of high fault currents without dispersement of conductive plasma gasses. 
     Turning now to FIG. 4, FIG. 4A shows a typical component configuration  40  of a multiple-circuit surge protection device, FIG. 4B shows a schematic diagram  42  of a multiple-circuit surge protection device using sensing resistors, and FIG. 4D shows a schematic diagram  45  of a multiple-circuit surge protection device using sensing capacitors. It is understood by those having ordinary skill in the relevant art that any number of surge protection circuits may be connected in a parallel configuration to achieve a predetermined current handling capability, the number not being limited to one as shown in FIG. 3 or four shown in FIG.  4 . The following description applies to FIGS. 4A,  4 B and  4 D. The circuit components are mounted on a printed circuit board  410  having connector pins  412  positioned on a side opposite the components for connecting to another printed circuit board. Current limiter components  430  connect between a first input terminal  420  and bridge terminals  424  located on an upper portion of a bridge  440 . The current limiter components  430  limit the maximum current through metal oxide varistors  460 . The bridge  440  is positioned in a perpendicular relationship with the printed circuit board  410 . The bridge  440  is positioned between the current limiters  430  and the metal oxide varistors  460  to provide an isolating barrier between the two sets of components, and to provide a support means to extend the length of the current limiting components  430  for extinguishing any electrical arcing in the current limiting components  430 . Thermal protector components  450  connect between the bridge terminals  424  and central terminals  470 . The thermal protector components  450  are positioned in close proximity to the metal oxide varistors  460 , and open-circuit upon reaching a predetermined temperature in order to prevent disintegration of the associated metal oxide varistor  460  from excessive self-heating. Current sense resistors  471  also connect between the bridge terminals  424  and the central terminals  470 . Current sense capacitors.  481  shown in FIG. 4D, could also be used in place of the current sense resistors  471  shown in FIG.  4 B. The metal oxide varistors  460  connect between the central terminals  470  and a second input terminal  422 . The metal oxide varistors  460  are typically between a 10-millimeter and an 80-millimeter device. Thermal sense resistors  472  connect between the central terminals  470  and an indicator terminal  428 . Thermal sense capacitors,  482  shown in FIG. 4D, could also be used in place of the thermal sense resistors  472  shown in FIG. 4B. A failure-indicating device may be connected to the indicator terminal  428 . The current sense resistors  471  or current sense capacitors  481  and the thermal sense resistors  472  or thermal sense capacitors  482  are configured to provide a distinguishing indication between current limiter components  430  open-circuit and thermal protector components  450  open-circuit. The current limiter components  430  may be a single strand of silver wire, multiple strands of silver wire, silver ribbon, perforated silver ribbon, copper ribbon, or perforated copper ribbon. A fuse tube may also enclose the current limiter component  430  as shown in FIG. 5B, in order to increase the current limiting capability of the current limiter component  430 . The thermal protector components  450  are a low melting point alloy wires, such as lead-antimony alloy wire or lead-indium alloy wire. The thermal protector  450  may also be a commercially available thermal cutout device. A power distribution network connects to the first input terminal  420  and the second input terminal  422  of the surge protection device  40 . 
     FIG. 4C shows an enclosed module  44  of a multiple-circuit surge protection device  40 . The structure  40  shown in FIG. 4A is positioned within a housing  490  and the housing  490  is filled with sand  492 . Sand  492  is used to position the thermal protector components  450  and to thermally couple these components to the metal oxide varistors  460 . The sand  492  and the bridge  440  provide isolation between the current limiter component  430  and the other components, including the thermal protector devices  450  and the metal oxide varistors  460 . Sand  492  is also required to enable interruption by the current limiter component  430  of high fault currents without dispersement of conductive plasma gasses. 
     Turning now to FIG. 5, FIG. 5A shows a typical perforated ribbon configuration of a current limiter  50 , and FIG. 5B shows a typical perforated ribbon configuration of a current limiter enclosed within a fuse tube  52 . The dimensions associated with the silver ribbon  500  may vary, depending upon the application. However for a typical application, the dimensions of the silver ribbon  500  may be 0.0075 inches thick and 0.150 inches wide, with 0.093-inch diameter holes  510  spaced on 0.250-inch centers. FIG. 5B shows the perforated ribbon  500  enclosed within a fuse tube  520 . The fuse tube  520  is filled with sand  530 . This configuration is capable of interrupting a current in excess of 200,000 amperes. 
     Turning now to FIG. 6, FIG. 6A shows a schematic diagram  60  of a multiple-circuit surge protection device connected to failure detection circuitry, a comparator, and a failure indicator. FIG. 6B show an alternative connection  62  comprising the failure detection circuitry and a microprocessor. The multiple-circuit surge protection device  42  is the same circuit previously described and shown in FIG.  4 B. For brevity, the description of the circuit  42  is not repeated here. A single phase  610  of a power distribution network is shown connected to the input terminals  612 ,  614  of the surge protection device  42 . The indicator terminal  616  of the surge protection device  42  is shown connected to a current summing resistor  620 . A voltage developed across the current summing resister  620  is rectified by a diode  630 , filtered by a capacitor  640  and resistor  660 , and applied to the input terminals  672 ,  674  of a comparator  670 . A zener diode  650  limits the maximum voltage excursion at the input terminals  672 ,  674  of the comparator  670  to a safe maximum operating level. If any of the current limiter components or the thermal protector components of the surge suppression device  42  open-circuit, the current through the summing resistor  620  will decrease, resulting in a decrease in the quiescent voltage at the input terminals  672 ,  674  of the comparator  670 . The comparator  670  detects this change in voltage, indicative of a failure of a circuit component in the surge protection device  42 , and activates an indicator  680  connected to an output  676  of the comparator  670 . In this manner, multiple levels of failures may be detected in the surge protection device  42 . FIG. 6B shows an alternate embodiment  62 , comprising a microprocessor  690  having input terminals  692 ,  694  connected to the summing and filter circuit described above. The microprocessor is capable of converting the rectified input signal from the summing resistor  620  into a digital representation of the failure signal. In this manner, multiple levels of failures in the surge protection device  42  may be detected and distinguished by the microprocessor for notification to maintenance personnel. 
     Although the present invention has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the embodiments herein.