IE84881B1 - Transient voltage surge suppression - Google Patents

Abstract

ABSTRACT An integrated fuse device (1) comprises a varistor stack (11), a thermal fuse (12), and a current fuse ( 13) within an enclosure (2) having device terminals (3). The vanstor Mack(ll)isconnmnedtothethennalflme(l2)byz1Cutennnufl(20)andisConnaned to the device terminal (3) by steel terminal (10) of much smaller cross-sectional area. Being of Cu material and having a greater cross-sectional area, the terminal (20) connected to the thermal fuse (12) has greater thermal conductivity than the steel terminal (10) to the end cap (3). The thermal fuse ('12) comprises a plurality of links having a melting point to melt with sustained overvoltage, the links having a diameter in the range of 2mm to 3mm. The links pass through an elastomer plug (15), which exerts physical pressure on them to assist with opening during sustained overvoltage. Hot melt (18) around solder (17) of the thermal fuse limits heat conduction to back-f ill sand.

Description

Transient Voltage Surge Suppression INTRODUCTION Field of the Invention The invention relates to transient voltage surge suppression.

Prior Art Discussion At present, in industrial type applications, such protection is often provided by a power distribution panel having a suppression module included inside. This suppression module typically consists of metal oxide varistors (MOV) which provide the surge suppression function. However under certain fault conditions the coating on the MOVs can burn and/or the MOV may rupture causing fragments to be expulsed.

To safeguard against these events a typical suppression module will contain some form of thermal disconnect component and special fusing components to open prior to the MOV rupturing. Components are housed in an enclosure capable of withstanding some level of internal explosion and flames. Additional electronics are also included to indicate whether either the thermal disconnect or the fusing has operated.

At present it is known to assemble the discrete components either on a printed circuit board or by means of some mechanical joining method, (e.g. attached individually or to a busbar) and then to enclose the assembly with a suitable enclosure which would prevent expulsion of fragments of a component should a catastrophic failure occur under fault conditions. In addition, the enclosure must also contain a fire should a component combust under fault conditions. These requirements require relatively expensive enclosures which in some cases may be filled with a flame/arc damping material such as sand. It has been known for the enclosure to be a significant portion of the total cost of the total module. Since the main components such as the MOV, fuse and thermal disconnect are all individual components special attention needs to be taken to ensure that the combination of the components will operate as required.

The invention addresses this problem.

SUMMARY OF THE INVENTION According to the invention, there is provided an integrated fuse device comprising a varistor, a thermal fuse, and a current fuse within an enclosure having device terminals, wherein the varistor is connected to the thermal fuse by a first terminal and is connected to a device terminal by a second terminal, and wherein said first terminal has greater thermal conductivity than said second terminal.

In one embodiment, the first terminal is of copper, and the second terminal is of steel.

In another embodiment, the second terminal comprises at least two plates.

In one embodiment, the second terminal has a cross-sectional area of less than Zmmz.

In one embodiment, the first terminal has a total cross-sectional area of at least Ommz.

In another embodiment, the thermal fuse comprises at least one link having a melting point to melt with sustained overvoltage.

In one embodiment, the or each link has a diameter in the range of 2mm to 4mm.

In one embodiment, each thermal fuse link is of solder composition.

In one embodiment, the thermal fuse is configured to also act as an over-current fuse in specified conditions.

In one embodiment, the thermal fuse comprises a thermal insulator coating to limit heat flow to the surrounding environment within the device enclosure In a further embodiment, the thermal fiise passes through a body which exerts inward pressure around the thermal fiise.

In one embodiment, the body is of deformable material.

In one embodiment, the thermal fiise comprises at least one thermal link extending through the body.

In another embodiment, the thermal fuse comprises two stages, a first stage with an encapsulant around a link and a second stage with a link passing through a deformable body which exerts inward pressure on the thermal element.

In one embodiment, the thermal fuse comprises a shape memory metal having at least one bend along its length.

In one embodiment, the first and second varistor terminals are integral with varistor electrodes, providing electrical and mechanical connection.

In one embodiment, the varistor electrodes have recesses adjacent varistor element edges.

In one embodiment, the second varistor terminal includes holes arranged so that it also acts as a current fuse.

In one embodiment, the current fuse extends from the thennal fuse to a device terminal.

In another embodiment, the current fuse comprises at least one length of conductor having apertures.

In a further embodiment, the current fuse is bent between its ends whereby the lengths of conductor are longer than the distance between the thermal fuse and the device terminal.

DETAILED DESCRIPTION OF THE INVENTION Brief Description of the Drawings The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:- Fig. 1 is an outside perspective view of a protection device of the invention; Fig. 2 is a perspective view and two diagrammatic sections showing the internal components of the device; Fig. 3 is an exploded perspective view of a varistor stack of the device; Fig. 4 is a device schematic diagram; Fig. 5 is a set of three X—ray images showing operation of the device; Fig. 6 is a perspective photograph of a bank of three of the devices in a typical application arrangement; Fig. 7 is a set of temperature vs. time plots; Figs. 8 and 9 are diagrams illustrating alternative devices; and Fi g. 10 is a perspective view of an alternative varistor stack.

Description of the Embodiments Referring to Figs. 1 to 4 a protection device 1 comprises a fibre-glass tube 2 and crimped Cu end caps 3. The device 1 is used in the TVSS (Transient Voltage Surge Suppression) field. A TVSS module is typically found in a power distribution panel within a facility such as a factory or office block. The purpose of the TVSS module is to suppress voltage transients which can occur on the power line due to events such as lightning, and so protect electronic equipment connected to the power line from damage.

Varistor terminals 10 are connected to an end cap 3. The terminals 10 are of 0.4mm steel, are 4mm wide, and are 20mm long. The terminals 10 extend from a stack 11 of three varistors in parallel, described below in more detail with reference to Fig. 3.

A thermal fuse comprises links 12 of solder material, solder 17 securing the links 12 to Cu varistor tenninals 20, and hot melt adhesive 18 over the adhesive 17. The thermal fuse links 12 are c. 12 mm long and have a round cross-section of 3mm diameter. The Cu terminals 20 have an exposed length of 5mm and are of 0.8mm Cu plate and are 20mm wide. The links 12 are reflowed to the Cu terminal 20 by the (lower melting temperature) solder paste 17, covered by the coating of hot melt adhesive 18, covering this connection. The links 12 may alternatively be directly soldered to the Cu terminals 20. The thermal fuse link 12 connection to the Cu terminal 20 is coated in the material 18 to give a level of thermal isolation from surrounding filler material. The purpose of the coating 18 is to minimise heat lost to the filler material. This material is deposited such that at a minimum the connection points of the links 12 and the solder 17 on the copper terminal 20 are covered. In this embodiment the coating material 18 is a hot melt adhesive of a polyamide composition and the filler material is sand.

The thermal fuse links 12 pass through an elastomer plug 15. This is of silicone rubber material. The diameters of through-holes 16 in the plug 15, when relaxed, are less than that of the links 12. They therefore exert pressure on the links 12, especially when they soften. In one embodiment the hole 16 dimensions are of 0.8mm diameter. It is also of benefit that, as illustrated, the holes in the plug do not extend all the way through. This increases the pressure on the thermal fuse links 12 at the point where they are forced through the remaining portion of the plug 15. In one embodiment, this remaining portion of the plug material is 0.4mm in depth. The plug 15 has an overall dimension of 16.3mm by 14mm (length by width) and 4.4mm thick. The corners have a radius of 4mm.

An indicator lead 2l extends from a Cu terminal 20 out through one end cap 3. While both fuse elements, current fuse element 13 and thermal fuse 12, are intact the supply voltage will appear on the indicator lead. In the event that either fuse element is opened then the voltage on the indicator lead will be removed. This on/off feature can be utilised for the purposes of alarm indication.

A current fuse 13 comprises a pair of perforated length of Cu. The metal may alternatively be Ag. The holes have a 2mm diameter. The length and hole dimension is chosen to suit the device ratings.

The tube 2 is back filled with sand, which surrounds all of the components shown in Fig.2.

Referring particularly to Fig. 3 the varistor stackll comprises three MOV elements 25 each having an electrode 26 and a ring of passivation 27. Each electrode 26 extends under the passivation 27 but not to the edge of the MOV elements 25. The Cu terminals 20 are identical. The end terminals 10 include a thin (0.4mm) steel plate sandwiched between MOV elements 25. The very large difference in thermal conduction paths will be clear from this diagram, the terminals 10 being thin and the Cu terminals 20 having a much greater cross-sectional area. Also, the thennal conductivity of steel is c. 16W/(M—K) and that of Cu is c. 400 W/(M—K). The differences in physical cross-sectional area (l0:l) and in thermal conductivity (2521) together give a thermal path to the thermal fuse 12 which is much greater than that to the end cap 3.

The metal oxide varistor stack 11 suppresses transient (very short term) overvoltages of the order of micro-seconds. In that time—frarne the varistor stack 11 absorbs and dissipates substantial electrical energy. However, the varistors are not designed to suppress a sustained overvoltage, ie. a situation where the voltage, for example l20\/ac, rises to 240Vac for a significant period of time. For a MOV a significant period of time may be of the order of seconds. Depending on the extent and time of the sustained overvoltage and the short-circuit current available the MOV 11 may overheat and become a fire hazard. A sustained overvoltage condition can occur during the installation of any electrical equipment, i.e. connection to the wrong supply voltage. However sustained overvoltages can occur even with correctly installed equipment. In industrial installations the supply voltage is typically supplied by 1, 2 or 3 phase systems. A most common type of incident which can lead to a sustained overvoltage is the impact of a “loss of neutral conductor” in a 2 or 3 phase system. If the electrical loads on the different phases are unbalanced and the neutral connection is lost then equipment normally operating at 120V can suddenly be supplied a voltage between 120V and 240V. Such a condition may not operate a circuit breaker and so the condition could last for some time. Other conditions can also lead to sustained overvoltages. For this reason Surge Suppression Devices (SPD) are subjected to sustained overvoltage conditions with varying short-circuit conditions to simulate conditions which can occur in the field.

Fig. 4 shows the three aspects of protection namely: Varistor stackl 1, Thermal fuse 12, transient surges; sustained overvoltage and short circuit (high current) conditions, to protect the vaiistor stackl l ; and Current fuse 13, very high currents of the order of kAmps.

Referring to Figs. 5(a) to 5(c), X-ray images of three fault conditions are illustrated as follows: Fig. 5(a): l0kAmp short circuit and abnormal overvoltage test thermal fuse links 12 intact, current fuse 13 open.

Fig. 5(b): 1kAmp short circuit and abnormal voltage test. Current fuse 13 intact, thermal fuse links 12 open.

Fig. 5(c): 500Amp short circuit and abnormal overvoltage test. Current fuse 13 intact. Thermal fuse links 12 open.

The tube enclosure is able to withstand the MOVs and the fuse fragmenting under fault conditions.

Fig. 6 shows how a bank of three devices 1 may be installed.

The protection device 1 integrates the basic functions of a TVSS module into a single, industry—standard package. The suppression component, thermal disconnect, and suppression fuse are contained within an industrial fuse body.

Thermal disconnect is effected by the thennal fuse 12, 17, 18. Under the defined fault conditions the MOV stack 11 generates heat. This heat melts the solder 12 and 17 of the fuse 12. However the back-filled sand acts as a heat sink and one end of the MOV stack 11 is connected to the metal end cap 3 of the device body, which also acts as a heat sink. The hot melt adhesive 18 minimizes the heat loss at the thermal fuse 12 due to the sand. Also, because of the high heat conductivity of the Cu tenninals 20 heat transfers much more quickly to the thennal fuse 12, 17, l8.The current fuse 13 is designed to open when subjected to currents of typically >1,000 Amps under the specified fault conditions. However a technical conflict arises due to the need for the complete device 1 to open at test points of 1OOAmps and 500Amps and for the current fuse 13 to be able to sustain up to 40,000Amp surge test (8/20usec). Reducing the dimensions of the current fiase 13 would enable it to open at the 100/500A current levels but it would not be sufficient to handle the 40kA surge test without opening.

The thermal fuse 12 acts for the current range of typically 100—1000A. Under the 100A — 1000A test the MOV 11 stack fails rapidly and will not generate enough heat to melt the thennal fuse and so the thermal fuse needs to generate its own heat to cause it to open under these test conditions. There are conflicting requirements on the thermal fuse: (a) it must not fail under the 40kA surge test, (b) it must open under the 0.5A-SA limited current test in a time of less than 7 hours, and (c) it must self-open under the 100A-1000A test condition. These test conditions are specified by industry standards.

However a combination of thermal fuse 12 link cross-sectional area, alloy composition, metal composition of the MOV ll terminals, and elastomer plug 15 accommodates all of the test requirements. The elastomer plug 15 aids the separation of the thermal fuse links 12. Each hole 16 in the plug 15 has a diameter less than that of the thermal fuse 12 link and so when the thermal fuse 12 heats and softens the plug applies pressure to help the thermal fi.lS€ links separate. In one embodiment the thermal fuse link alloy composition is Bismuth/Lead/Cadmium in the ratio .5%/37.7%/8.5°/o which is a standard low melt solder alloy.

Referring to Fig. 7, the temperature rise impact of different metal combinations used in the MOV stack 11 is shown. The purpose is to attain the maximum temperature rise on the Cu terminals 20, connected to the thermal fuse 12. The MOV stack 11 is the heat source under this specific fault condition. Fig 7 demonstrates that the use of steel tenninals 10 on one end of the stack 11 helps to increase the rate of temperature rise on the Cu terminals 20.

The following table demonstrates the ability of the selected components to sustain kA (8/20usec) transient pulse condition without issue, The following table sets out test results which demonstrate that the selected components meet all the current (design critical) specific fault test conditions. est 320V Quantity 150V Qua Limited Current e 183 n 182 O.5A 2.5A Tested Passed Failed % Pass % 100% A 1 00% 10A 1 00% A A A A Pulse Test 10kA ted shot) % ‘I 00% 100% 100% % 100% .0 This illustrates that the device 1 operates under the specified test conditions covering the range O.5A up to 2kA, and in addition the peak pulse condition of 40kA. In addition, further testing has been carried out to demonstrate that the unit operates as designed under short-circuit test conditions including 5kA, l0kA and 200kA.

It will be appreciated that the invention provides a major improvement over the prior art by incorporating all components into a single body. Since industrial fuses are required to be constructed so as to provide containment from rupture and fire under fuse fault conditions it is advantageous to include the additional components for surge suppression and thermal disconnect within a fuse body. This will eliminate the need for a further enclosure by the end user. Although some enclosure will be utilised to suit the end application, its specification will be greatly simplified.

While in the current embodiment the current fuse element is attached to the thermal fuse and then to the MOV ll stack, an alternative connection/arrangement can be utilised. Since the MOV stack 11 has an electrode which is a fired silver material it has been found that a silver current fuse element can be formed as part of the MOV terminal and co-fired between 500-800’C such that the MOV electrode is bonded to the MOV ceramic material and in addition is bonded to the silver current fuse/terminal. This eliminates the need for a soldering operation, which can cause a leakage current issue arising from the flux required during the soldering process. In addition, suitable holes may be incorporated into the terminal 10 to act as the only or as an additional current fuse 13. This is shown in Fig. 3, indicated by the numerals l0(a). The configuration of the links and holes are chosen according to the required specification and whether the links are replacing the current fuse 13 or are complementary.

For very low limited current fault conditions e. g. typically <0.5Amp, where the heat generated in the stack 11 does not greatly exceed the melt temperature of the thermal fuse links 12 the siliconee rubber 15 can act as a heat sink and therefore not allow the solder links 12 to melt. However given that the silicone rubber is an important feature in the 100A — 1000A fault region an alternative is required to address the low current fault conditions.

An alternative protection device, 40, is shown in Fig. 8. This comprises end caps 41 and 42, terminals 43 connected to a stack 44 of varistors, a first thermal fiise link 45, a bridge 46, a second thermal fuse link 47, and a current fuse 48. The first thermal fuse link 45 has a hot melt coating/encapsulation 49 and the second thermal fuse link 47 has the elastomer device 15. To ensure minimum heat sinking the first solder link may be covered with a low thermal conductivity material and therefore is able to melt under the low current fault conditions.

Referring to Fig 9, in a protection device 60, a first thermal fuse link is a shape memory metal alloy 66. A coating material 67 allows the shape memory metal to contract. There are standard solder connects at both ends. Shape memory alloy, such as Nickel Titanium, has the ability to be deformed at room temperature and when heated will return to its original shape. For this application the alloy has an original form in one embodiment of a coil, it will then be deformed or stretched between the bridge 46 and the stack of varistors 44. The connection to the varistor stack terminal and the bridge 46 is with solder or conductive epoxy.

When heat is generated under fault conditions by the varistor stack the connection will melt or soften and the shape memory alloy will return to its original shape, i.e. in this case a coil, which will be shorter than the gap between the varistor stack 44 and the bridge 46. The coating material 67 is such that when heated it softens and therefore allows room for the shape memory alloy to move.

Referring to Fig. 10 an alternative terminal design 100, is shown. A portion of the terminal 104 has a reduced thickness portion 105 at a place which coincides with the edge of a MOV element 101. The purpose is to avoid the terminal lying on the MOV element at the edge which may promote an electrical arc across the edge of the MOV element 101 under high voltage surge conditions. In other embodiments the number of MOV elements in the stack may be different, such as two or only one instead of three.

The specification of the MOV stack depends on the overall device specification.

The invention is not limited to the embodiments described but may be varied in construction and detail.

Claims (1)

1. Claims An integrated fuse device (1) comprising a varistor (1 1), a thermal fuse (12, 17, 18), and a current fuse (13) within an enclosure (2) having device terminals (3), wherein the varistor is connected to the thermal fuse by a first terminal (20) and is connected to a device terminal (10) by a second terminal; and wherein said first terminal (20) has greater thermal conductivity than said second terminal (10). An integrated fuse device as claimed in claim 1, wherein the first terminal (20) is of copper, and the second terminal is of steel (10). An integrated fuse device as claimed in claims 1 or 2, wherein the second terminal (10) comprises at least two plates. An integrated fuse device as claimed in any preceding claim, wherein the second terminal (10) has a cross-sectional area of less than 2mm2. An integrated fuse device as claimed in any preceding claim, wherein the first terminal (20) has a total cross-sectional area of at least 10mm2. An integrated fuse device as claimed in any preceding claim, wherein the thermal fuse comprises at least one link (12) having a melting point to melt with sustained overvoltage. An integrated fuse device as claimed in claim 6, wherein the or each link (12) has a diameter in the range of 2mm to 4mm. An integrated fuse device as claimed in claims 6 or 7, wherein each thermal fuse link (12) is of solder composition. An integrated fuse device as claimed in any preceding claim, wherein the thermal fuse (l2l7,18) is configured to also act as an over-current fuse in specified conditions. An integrated fuse device as claimed in any preceding claim, wherein the thermal fuse comprises a thermal insulator coating (18) to limit heat flow to the surrounding environment within the device enclosure An integrated fuse device as claimed in any preceding claim, wherein the thermal fuse passes through a body (15) which exerts inward pressure around the thermal fuse. An integrated fuse device as claimed in claim 11, wherein the body (15) is of deformable material. An integrated fuse device as claimed in claim 12, wherein the thermal fuse comprises at least one thermal link (12) extending through the body (15). An integrated fuse device as claimed in any of claims 11 to 13, wherein the thennal fiise comprises two stages, a first stage with an encapsulant (18) around a link (12) and a second stage with a link (12) passing through a defomiable body (15) which exerts inward pressure on the thermal element. An integrated fuse device as claimed in any preceding claim, wherein the thermal fuse comprises a shape memory metal (66) having at least one bend along its length. An integrated fuse device as claimed in any preceding claim, wherein the first and second varistor terminals (10, 20) are integral with varistor electrodes, providing electrical and mechanical connection. An integrated fuse device as claimed in claim 16, wherein the varistor electrodes have recesses adjacent varistor element edges. An integrated fuse device as claimed in any preceding claim, wherein the second varistor terminal (10) includes holes (10(a)) arranged so that it also acts as a current fuse. An integrated fuse device as claimed in any preceding claim, wherein the current fuse (13) extends from the thermal fuse to a device terminal (3). An integrated fuse device as claimed in any preceding claim, wherein the current fuse (13) comprises at least one length of conductor having apertures. An integrated fuse device as claimed in claim 20, wherein the current fuse (13) is bent between its ends whereby the lengths of conductor are longer than the distance between the thennal fuse and the device terminal (3).