SURGE PROTECTOR FOR TELECOMMUNICATIONS EQUIPMENT

Abstract: A protector for protecting equipment, especially telephone equipment, against high voltage and/or current surges such as are caused by lightning in the vicinity of the equipment or the cables to which it is connected, comprises one or more overvoltage protectors (42, 58; 234, 236) mounted upon one face of an insulating support (18; 218). A generally planar contact member (62; 290, 312) mounted upon the support member has a ground contact member (90; 296) to make contact with a ground electrode (114; 310) in the equipment to be protected and with each overvoltage protector. A spacer (60; 320, 322) of fusible plastics material coupled thermally to the protection device melts when a sustained fault occurs. As the spacer melts, it permits an electrical connection between the contact member and the line to which the overvoltage protector is connected, effectively short-circuiting the line to ground. Overcurrent protection may be provided by means of a resistor (116) disposed in series with the line and located close to the plastics spacer so as to heat the spacer when an overcurrent occurs. In one case, the spacer comprises a film (60) interposed directly between a pair of contacts (40, 74') which serve to short-circuit the protector. In another case, the spacer comprises a limb (324) with relatively thin lateral projections (328, 338) which melt and shear. In both cases, the spacer preferably comprises high density, high molecular weight polyethylene.

Description: TITLE: SURGE PROTECTOR FOR TELECOMMUNICATIONS QUIPMENT TECHNICAL FIELD: * ι • This invention relates to protectors for protecting equipment against high voltages and/or currents such as are • -•' caused by lightning in the vicinity of the equipment ©A fre » \» cables to which it is connected. Embodiments of the inv.erki n, are especially, but not exclusively, applicable to overvoltage protectors used for protecting telephone equipment. ' BACKGROUND ART: Conventional such overvoltage protectors usually t Hi comprise a pair of gas tubes mounted coaxial ly within a housing. Fusible elements, typically discs of soldejr, are associated one with each of the gas tubes. The arrangement is such that, when an overload condition persists, for example , when a power line contacts the telephone linjβ, the „h ,a»t generated in the gas tube will cause the fusible element to melt and short-circuit the gas tube, either directly or by releasing a spring-loaded plunger. In U.S. patent specification number 4,066* * 840, issued November 1977, P.S. Lundsgaard et a l discio'se^such a protector having coaxial gas tubes with a fusible dielectMc pellet mounted directly upon each gas tube. Melting f the dielectric pellet allows a resilient conductive member to short-circuit the gas tube. In U.S. patent specification humber '4,S51,957, issued July 1989, K.H. Chung discloses a lead oV plastics „ pel let mounted directly upon a gas tube. The pellet maintains ontact member away from the electrodes of the gas tube. When the pellet melts, the contact member short-circuits the gas tube. An alternative overvoltage protector is disclosed 5 in U.S. patent number 4,212,047 by Napiorkowski, issued July 8, 1980, to which the reader is directed for reference. Napiorkowski discloses a protector having a sleeve of fluoroplastics material around the gas tube and a clip of spring metal surrounding the sleeve. When a sustained fault 10 occurs, the heat generated causes the fluoroplastics material to melt, allowing the metal clip to contact the gas tube and effect the desired short circuit. In practice, these known devices are susceptible to problems concerning the suitability of plastics materials for 15 use in overvoltage protectors of the kind used in telecommunications, especially in central offices and at subscriber's premises. As discussed in U.S. patent specification number 4,056,840, such protectors are designed to be "self-restoring" i.e. return to its open-circuit 20 condition once the fault has been cleared. Typically, such a protector can be expected to operate many times during a useful life of as long as forty years without being subjected to a fault severe enough and sustained long enough, to fuse the plastics material and short-circuit the gas tube. 25 Although such repeated operations do not generate enough heat to melt heat the plastics material, the plastics material nevertheless is subjected repeatedly to relatively high temperatures because it is in direct contact with the gas <l - ' fι'(. tube. This can lead to creepage of the plastics material and to premature failure. * » » The plastics material should also preferably exhibit a clearly defined and abrupt transition between its s Ad a-n molten states. The range of plastics materials which exhibit these characteristics and are capable '6f wi hstanding" the relatively high temperatures associated with d * jtøθct contact with the gas tube or other protep,tion device is ,.1-jnvi ed. DISCLOSURE OF THE INVENTION: According to one aspect of the present 'invehflon, a protector for protecting telephone equipment against excessive voltages comprises: a support member; a protection device supported y said support electrode means for coupling said' protection device to the equipment to be protected; mutually proximal first and second con ac ® sRaceαAfrrom said protection device, the first and th rd " contacts being biased one towards the other; interconnecting means connecting one terminal of t ρaid ι protection device to the electrode means" * &nd * to thfe f%rst contact; a contact member connecting the second contact to a second terminal of the protection* device and coijiprising, a ground contact for connecti ng to a g'round el adjacent the protector when the protector i s i nstal l ed i n the equi pment ; a spacer of thermoplastics material preventing electrical connection between the first contact and the second contact, there being a heat transfer path between the spacer and the protection device; the arrangement being such that excessive heating of the protection device causes the spacer to melt and permit electrical connection between the first contact and the second contact. Preferably the spacer comprises a high density, high molecular weight polyolefin, for example polyethylene. The spacer may comprise a film. Alternatively, the spacer may comprise a limb and at least one lateral projection abutting a heat-conductive element for transmitting heat from the resistance element, one of the pair of contacts acting against the limb to urge it towards the heat-conductive element, the arrangement being such that heat generated by an excessive current melts the spacer at the junction between the lateral projection and the limb and the limb is displaced until the contacts make contact. The support member, with the contact member assembled to it, may be housed in a housing having at least one aperture in a side juxtaposed to the support member. The ground contact portion may then be resi1iently-biased to project through the aperture. The support member may comprise a block of insulating material having a deflection temperature greater then the melting point of the spacer. The protection device may be disposed in a recess in the block. 5 In preferred embodiments of the invention, the support member has a pair of mutually-spaced protection devices, third and fourth mutually proximal contacts and a thermoplastics spacer maintaining electrical separation between the third and fourth contacts. Second interconnecting means may then connect the third contact to a terminal of the second protection d,evj,ce and to a second electrode means for connection to the, equipment. A heat transfer path between the second protection device and the second spacer provided* for melting of the second spacer by heat generated by excessive current i^t e second protection device. The contact member may have ' second g ! τ-όund f contact portion and the housing corresponding second aperture. contact of the contact member. ' ' * ' The or each interconnecting means, conveniently a flat metal strip, may extend along one side ,jcf .Jibe, r rø en j itp ., |» I. 4 J- ^ !l <(; ' the corresponding electrode means. Wϊere two protection devices are provided, their respective interconnecting means may extend along opposite sides of the support member. Preferably the or each protection device is a solid- state device. Aspects of the invention also concern protection . against excessive or so-ca -l ,led„ sneH«ki *v f iUls<irrϊfe.lh"t,, * - * I" , , t ι; ϋIn telecommunications equipment there is a need for protection against relatively low level , current purges which eu jq. accompanied by a voltage surge sufficient to operate the usual , » H* overvoltage protector but nevertheless ύ 1 n m sti,tlLl dama • *g**e' ^th 1 e telecommunications equipment. To satisffy thife " need, 'sneak ■n , \w < current protectors are used which incorporate a current sensitive element. Known sneak current protectors employ a heat coil which fits into a three dimensional space frame. The heat coil comprises a resistive wire, in series with a telephone line or the like, and wrapped around a spool. A spring loaded plunger is soldered into the spool. When excessive current causes the heat coil to heat up, heat is transferred to the solder causing it to melt and the plunger to short the telephone line to ground, thereby shunting the excessive current away from the equipment. Usually, in an overvoltage protector of the gas tube kind described above, the heat coils are wound around the plunger and melt the same solder discs as the gas tubes. Such known sneak current protectors are not entirely satisfactory because they involve a relatively complex set of thermal masses and conduction paths. Their current protection levels are not easy to change and the protectors usually have to be custom made for each application which increases production costs. Accordingly, protectors embodying the first aspect of the invention may further comprise a substantially planar resistance element, for example a ceramic chip resistor, connected to the electrodes so as to be in series with a line to be protected. The resistance element may be disposed in thermal proximity to the spacer of thermoplastics material. According to a second aspect of the invention, a protector for protecting equipment against excessive overvoltages and excessive currents comprises:- an overvoltage protection device; a pair of electrodes for connecting the protector to equipment to be protected; interconnecting means connecting respective ones of said pair of electrodes to one terminal of said protection device; a contact member for connecting a second terminal of said overvoltage protection device to a ground electrode of the equipment; first and second contacts connected to - respective terminals of said protection device and biased one towards the other; a thermoplastics spacer maintaining separation of said first and second contacts; a substantially planar resistance element connected in series between one of said electrodes an-d said one terminal of said overvoltage protection device; the interconnecting means serving to conduct heat from both said resistance element and said protection device to said thermoplastics spacer; and the arrangement being such that excessive heatin'g of either one of the resistance element and the protection device causes the spacer to melt and permit contact between said first and second contacts. The spacer may comprise a central limb and at least one lateral projection or arm, and comprise a fusible material. The projection may then abut a heat^conductive element connected to the resistance element. One of the pair of contacts acts against the central l mb to urge it towards the heat-conductive element. When an excessive current in the 8 resistance element heats the spacer sufficiently, the plastics material at the junction between the lateral projection and the central limb melts and the central limb is displaced until the contacts meet. 5 In preferred embodiments of the second and third aspects of the invention, the resistance element is mounted upon a substrate, for example a printed circuit board, and the interconnection means comprises printed circuit elements connecting the resistance element in series with the equipment 10 to be protected. The plastics spacer is also mounted upon the printed circuit board or substrate. The length and cross-sectioned area of the printed circuit elements may be controlled to compensate for different amounts of heat generated in the protection device and the 15 resistance element respectively. According to another aspect of the invention, a protector for protecting against excessive currents in telecommunications equipment comprises a resistance element and electrodes for connecting the resistance element in series 20 with a line to the equipment which is to be protected, such pair of contacts being mutually proximal and biased one towards the other but maintained apart by an insulating spacer. The spacer comprises a central limb and at least one lateral projection or arm, and comprises a fusible material. 25 The projection abuts a heat-conductive element connected to the resistance element. One of' the pair of contacts acts against the central limb to urge it towards the heat- conductive element. When an excessive current in the resistance element heats the spacer sufficiently, the plastics 9 materi al at the j uncti on between the l ά'tferάl " pr^όjec'tfdV) * and the central limb melts and the central limb is displaced until the contacts meet. According to yet another aspect of the , a protector for protecting telephone equipment against excessive voltages comprises: a support member supporting an overvol.tage protection device; electrode means for coupling said protection device to the equipment to be protected; interconnecting means connecting one terminal of said protection device to said electrode means and having a first contact; a contact member comprising a ground contact • for contacting a ground electrode extending adjacent said protector when the protector is installed in said equipment and a second contact juxtaposed to s-aid first contact, said first contact and said second contact being biased one toward the other; a spacer of thermoplastics material separating said first contact and said second contact, said spacer comprising a limb and at least one lateral projection, the projection abutting a heat-conductive element for transmitting heat from the protection device, one of said first and second contacts acting against the limb to urge it towards the heat-conductive element, the arrangement being such that said excessive heating melts the spacer at the junction between the lateral projection and the limb permitting the limb to displace until the first and second contacts make contact. 10 BRIEF DESCRIPTION OF DRAWINGS: An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which:- 5 Figure 1 is an exploded isometric view of a protector for use in equipment in a telephone central office and having a thermal shunt; Figure 2 is a plan view of a planar spring metal contact member of the protector showing current flow in the 10 contact member after operation of the thermal shunt; Figure 3 is a cross-sectional view on the line 3-3 of Figure 2; Figure 4 is a fragmentary cross-sectional view on the longitudinal centre line of the protector; and 15 Figure 5 is a detail view illustrating an embodiment of a second aspect of the invention; Figure 6 is a view of a protector according to a third aspect of the invention: Figure 7 is a plan view of the protector of Figure 20 6; Figure 8 is a view corresponding to Figure 6 after the protector has operated: Figure 9 is an end view of a detail before operation of the protector; 25 Figure 10 is a view corresponding to Figure 9 but after operation of the protector! Figure 11 is a sectional partial view of the protector after operation; and 11 Figure 12 is a plan view of a 'printed circuit * Board assembly of the protector of Figure 6. ' >, MODE(S) FOR CARRYING OUT THE INVENTION':' "" ' , ' Referring to Figure 1, an overvoltage protector for use in protecting equipment in a telephone ' central office * comprises a housing 10 in the form of an elongate box, of square cross-section, closed at one end and open at the other. A handle 12 projects from the closed end. Adjacent the. same end, a lever 14 extends generally parallel to the, housing -iO . and terminates in a lip 16. The lever 14 has a depending detent (not shown) which serves to limit withdrawal of the protector from the equipment at a predetermined position until the lever 14 is flexed to disengage the .de'te t. In its surface shown uppermost in Figure ϊ v a support member 18 of generally parallelepiped, shape has two recesses 20 and 22, respectively, spaced a art alo ø, its length, with a shoulder or land 24 between them. The housing _ 10 and support member 18 are made of insulating material, for example a synthetic plastics material such as Valox 42QSE0 (Trade Mark), a polyester by General Electric Corporation. An interconnecting member, in the form of a flat cop e β^trip 26, extends along one side of the support member 18 between the shoulder or land 24 and a U-shaped groove 28 in the side of the support member 18. The groove 28 houses a U-*shaped« electrode 30 such that its limbs 32 and 34 protrude frond the end of the support member 18 that will be adjacent the op n» end of housing 10 when installed. TheΑimbs 32 and 3-4 se>γe as electrodes or contact pins to mate with complementary contacts in the equipment cabinet when the protector is inεtal led. The interconnecting strip 26 overlaps the U-shaped electrode 30 and is connected to it by crimping or soldering as at 36. The interconnecting strip 26 has two tabs 38 and 40, respectively, formed by bending the edge portions of the interconnecting strip 26 at right angles. Tab 38 extends along the bottom of recess 20 and tab 40 projects inwardly a short distance across the upper surface of shoulder or land 24. Tab 40 serves as a contact, as will become apparent in the ensuing description. A solid state overvoltage protection device 42 is accommodated in recess 20 and rests upon contact tab 38. The solid state overvoltage protection device 42 comprises two metal discs 42A and 42B, the latter larger than the former, which serve as terminals. The actual solid state device, typically a voltage-triggered semiconductor device providing bi-directional overvoltage protection, is sandwiched between the metal discs 42A and 42B. A second interconnecting strip 44 extends along the opposite side of the support member 18 and is similar to interconnecting strip 26 in that it has a contact tab 46 projecting inwardly a short distance across shoulder or land 24 and is connected to a U-shaped electrode 48 housed in a groove 50. The limbs 52 and 54 of the U-shaped electrode 48 ι project from the end of the support member 18 in a similar manner to limbs 32 and 34. The second interconnecting strip 44 differs from the first interconnecting strip 26 in that its second contact tab 56 extends across the bottom of the second recess 22. A second solid state overvoltage protection device 58, similar to device 42, rests upon tab 56 in recess 22. The dimensioning of the recesses 20 and 22 and the solid state overvoltage protection devices 42 and 58 is such ' that he upper faces of the devices 42 and 58 are flush with the uppermost surface of the support member 18. A film 60 of synthetic plastics insulating material , specifical ly high density, high molecular weight polyethylene, for example SCLAIR WCI 46C (Trade Mark) resin by Du Pont Tnc, which has low creepage, extends across the upper surface of shoulder or land 24 of support member 18 and is clamped between it and the underside of a spring contact member in the form of a plate 62. This spring contact plate &2 is formed, conveniently by stamping and/or chemical milling, from a piece of flat stock beryllium-copper, or other suitable resilient contact material, as illustrated in Figure 2. The contact plate 62 comprises a medial portion 64 deformed out-of-φlane to form two semi-cylindrical thermal shunt springs 66 and 68, respectively, one each side of a central hole 70. The side margins 72 and 74 of medial portion 64 of the plate 62 serve as load distributors, extending longitudinally to overlap, and bear against, the underlying portions 72' and ,74', respectively, of the plate 62. Each distal portion of the contact plate fe2 comprises three limbs 76, 78 and 80, respectively. Each middle limb 78 is generally T-shaped and arched, a lateral piece 82 at its extremity forming the cross bar of the "T" bearing against the adjacent end portions of limbs 76 and 78. 14 The spring contact plate 62 and plastics film 60 are clamped to the support member 18 by a fastener in the form of a screw 84 which extends through the hole 70 in the spring contact plate 62 and a corresponding hole 86 in the plastics film 60 5 and into a screwthreaded hole 88 in the middle of shoulder or land 24 of support member 18. The edge portions 72' and 74' beneath the load distributors, marginal portions 72' and 74', respectively, overlie contact tabs 46 and 40 and serve as contacts also. 10 Contact tab 40 forms with contact portions 74' a first pair of contacts. Contact tab 46 forms with contact portions 72' a second pair of contacts. The edge portions 72' and 74' are biased towards the support member (downwards in the drawing) by the associated thermal shunt springs 66 and 68, 15 respectively. Thus, when screw 84 is tightened, the thermal shunt springs 66 and 68 are deformed both elastically and plastically into their final position to obtain maximum and consistent spring travel and force. The plastics film 60 is clamped resiliently between contact plate 62, specifically the 20 marginal contact portions 72' and 74', and contact tabs 40 and 46, and the solid state overvoltage protection devices 42 and 58 make good electrical contact with the underside of the contact plate 62 as shown in Figure 4. With the spring contact plate 62 and plastics film 25 60 assembled onto the support member 18, the assembly can be slid into housing 10. The arched 'portions 90 and 92 of middle limbs 78 of spring contact plate 62 will protrude through elongate longitudinal slots 94 and 96, respectively, in the juxtaposed wall of housing 10, as illustrated in Figure 4. 15 An end cap 98 has four holes 100, 102, 104 and 106 to receive the contact pins 32, 34, 52 and 54j respectively, when the end cap 98 is fitted onto the end of support member 18 in housing 10. Lugs 108 (only one is shown protrude laterally from the edges of end cap 98 to engage in holes 110 in extensions 112 protruding from the side walls gf housing 10 and secure the assembly in the housing 10. In use, contact pins 32 and 34 will be corinected to the "tip" conductor and contact pins 52' and 54 will be connected to the "ring" conductor of the telephone line. The protruding arched limbs 90 and 92 of spring- contact plate, 62 •v will make contact with a ground plane 114 (see Figure 4) in the equipment cabinet. When an overvoltage occurs on the "tip" conductor, solid state overvoltage protection device 42 will operate to short-circuit the "tip" conductor, by way of interconnecting strip 26, to ground plane 114. If the overvoltage condition is sustained, the heat generated in the solid state overvoltage protection device 42 will be transmitted via tab 38 to tab 40 and to contact plate 62, eventually causing the plastics film 60 to melt. In view of the pressure applied by screw 84, the plastics material between the tab 40 and the spring contact- plate 62, w,i : l 1 be exuded, allowing electrical connection between the tab 40 and the spring contact plate 62 to ground the "tip", effecting mechanical short-circuiting of the overvoltage protection device 42. Figure 2 shows the directions in which current flows in the spring contact member 62 following a fault affecting the "tip" conductor. If the overvoltage occurs on the "ring" conductor, overvoltage protection device 58 will operate, sustained fault conditions leading to melting of the plastics film 60 trapped between tab 46 and contact plate 62 to effect the short- circuit. The way in which the spring contact plate 62 is formed is particularly advantageous. The T-shaped ends 82 of limbs 78 can slide along the surface of outer limbs 76 and 80 as the curved portions 90/92 are flexed. This helps to ensure even distribution of the spring pressure and consistent contact with the ground plane. Moreover, this bifurcate contact between limbs 78 and limbs 76 and 80, shares current flow, increasing current carrying capacity as compared with a single contact. Likewise, the marginal portions 72 and 74 of the spring contact plate 62 can slide relative to the underlying edge portions 72' and 74' of the limbs 80 upon which they bear. This ensures that, as screw 84 is tightened, the force is applied evenly and directly above the corresponding tabs 40 and 46 so that there is little risk of an edge penetrating the plastics film 60 as a result of uneven force distribution. Two protrusions may be provided on the underside of the spring contact plate 62, each positioned to bear against one of the overvoltage protection devices 42 and 58. The protrusions will improve contact and provide a slight clearance, typically about 0.020 inches, between the top of the overvoltage protection device and the spring contact plate 62. 17 Referring now to Figure 5, which shows an overvoltage protector with an alte ' rnative form oA contact member and means for protecting against 1 'Of "tehee " * current, the overvoltage protector is in construction to that described with' ςpfe,r.enpe, to Figur9 ιl υ *to 4, but with "sneak" current protection røeans for' rotejcskting against excessive currents. Components corresponding to'those shown in earlier Figures are identified "by * i^he' same refe^ehce numerals but with the suffix "A". One significant difference comprises the* sp ing contact member 62A, which is formed by two strips of spring ^ contact material 63 and 65, the medial portions of^ ' K ό are riveted or welded together as at 67 and extend 'the fu l width of the support member 18. Spring contact strip 65 thus extends between spring contact strip 63 and the plastics material 60 on shoulder 24 of the support member 18. S r ng contact strip 65 projects both sides of the 1 shourTder 2Φ- to overlie and make electrical contact with the Solid tate overvoltage protection devices 42A and 58A, respectively. Spring contact strip 63 arches away from spring contact strip 65 to form two arched contact portions 90A and 92A to contact the ground plane 114 (Figure 4). Another significant difference is €hat a pair of planar resistance elements in the ' form of ceramic chip resistors 116 (only one is shown) are positio ed adjacent'the medial shoulder or land 24 of support member 18, one such resistor each side of the central fastening^ screw '84. Each ceramic chip resistor 116 is housed in a recess 118 in the ' * side of the support member 18. Instead of a single conn-ector strip 26, the interconnection for the ."tip" conductor is made by conductors 120 and 122, respectively. Conductor 120 is connected at one end to an electrode in the form of contact pin 34A and conductor 122 is connected to an electrode in the form of contact pin 32A. The contact pins 34A and 32A are separate and replace the single U-shaped electrode 28 shown in Figure 1. Conductor 120 has a first tab 38A extending beneath solid state overvoltage protector 42A and a second tab 40A, serving as a contact, extending across shoulder or land 24 beneath the thermoplastics film 60. An extension 124 extends from contact portion or tab 40A to overlie and contact one terminal 126 of the ceramic chip resistor 116. The other conductor 122 extends longitudinally of the support member 18 to connect to the other terminal 128 of chip resistor 116. Both conductors 120 and 122 may be soldered to the respective underlying terminals 126 and 128 of the chip resistor 116. The chip resistor 116 is thus connected between the contact pins 34A and 32A and hence in series with the "tip" conductor of the telephone line. Should a fault occur which is characterized by a sustained abnormally high current in the line, a so-called "sneak current", but not necessarily accompanied by a voltage high enough to operate the overvoltage protectors 42 and 58, the heat generated in the chip resistor 116 will be conducted via extension 124 at the conductor 120, to tab contact 4θA, to melt the plastics film 60, and connect the contact tab 40A to the spring contact plate 62, short-circuiting the protector. 19 I ! t* H • It should be appreciated that the connec io s ^ the opposite, "ring", side of the support bifurcated in a similar manner and the second chip rβθistor inserted in series with the "ring" conductor. An qverc-yrrent or "sneak current" in the "ring" line will resiϊl*t "in overheating of the second chip re stor to me,lt, „the pl^εrt^cs film and short-circuit the "ring" conductor to grόϋriα* "iW a similar manner to that described for the "tip" conductor. * » A potential limitation of protectors in which a thin plastics film is interposed directly between the cd tacts which are to short circuit, is that the thermal impedance of the film is relatively low, and becomes even less as the ilm* begins to melt and the contacts begin to close. nHence the ground contact can act as a heat sink and inhibit the heating of the film to its melting point. This could be a problem when the source of the heat is a sneak current. The embodiment disclosed i Figures 6 to 12 addresses this is'sue: Referring now to Figure 6, a combination overvoltage and overcurrent protector is shown with a housing 210 which is similar to the housing 10 shown in Figure 1. The housing 210 takes the form of an elongate box of square cross-section, closed at one end and open at the other. A handle 212 projects from the closed end. Adjacent the same end, a lever 214 extends generally parallel to the housing 210 and terminates in a lip 216. The lever 214 has a depending detent (not shown) which serves to limit withdrawal of the protector from the equipment at a predetermined position until he lever 214 is flexed to disengage the detent. The protector comprises a generally parallelepiped support block 218 which has one surface (that shown uppermost in Figure 6) stepped to provide in descending order, four steps 220, 222, 224 and 226 (see also Figure 7), respectively, with an inclined face 228 between step 224 and step 226. Two recesses 230 and 232 are provided in steps 220 and 224, respectively. The recesses 230 and 232 house overvoltage protectors 234 and 236, respectively. Interconnecting members 238 and 240, respectively, extend along the side of the support block 218 which is recessed to accommodate them. The interconnecting members 238 and 240 comprise wire rod. At one end, the interconnecting members 238 and 240 extend beyond the end face of the support block 218 and protrude through an end cap 242 as contact pins 224 and 246, respectively. When the protector is installed in the equipment cabinet, contact pin 244 will connect to outside plant and contact pin 246 will connect to a line to the central office. Interconnecting member 240 has a tab 248 (see Figure 7) which projects into recess 232, beneath overvoltage protector 236, and makes contact with one terminal of the over-voltage protector 236. A corresponding tab 250 (see Figure 7) of a corresponding interconnecting member 252 on the RING side of the support block 218 extends into recess 230 to make contact with overvoltage connector 234 in like manner. Adjacent its end remote from the contact pin 246, interconnecting member 240 has 1 a section which extends perpendicularly and terminates in a connector pin 254 which has a reduced diameter. The connector pin 254 extends into a terminal post 256 soldered into a printed circuit board 258 which is mounted upon the lowermost step 226 adjacent the end of support block 218. A ceramic chip resistor 0 is surface- mounted upon the printed circuit board ;-2'5-8 and has one terminal connected by printed circuit conductor 262 to terminal post 256. The other terminal of chip resistor 260 is connected by printed circuit conductor pad 264 to a second terminal post 266 which is mounted upon the printed circuit board 258. A connector pin 268, formed as a reduced-diameter section of interconnecting member 238, protrydes into teripinal post 266. Hence, the chip resistor 260 is connected in series between the contact pins 244 and 246, respectively, and hence in series with the line between the central office and the outside plant once the protector is installed. A second chip resistor 270 is mounted upon printed circuit board 258 and is connected in series with the other wire of the line. Thus one terminal of chip resistor 270 is connected by printed circuit conductor 272 to a terminal post 274 which receives a connecting pin 276 of interconnecting member 252. The other terminal of chip resistor 270 is connected by printed circuit conductor pad 278 to a terminal post 280 which receives a connecting pin 282 formed at one end of an interconnecting member 284. The ends of i terconnecting members 252 and 284 remote from the printed circuit board 258 protrude through end plate 242 as contact pins 286 and 288, respectively. Hence, chip resistor 270 is connected in series between contact pins 286 and 288 and hence in series with the line between the central office and the outside plant. A spring contact member 290 overlies the support block 218 and is fastened to it by a central screw 292. The spring contact plate 290 has three finger portions 294, 296 and 298 at one end and three finger portions 300, 302, 304 at the other end. Middle fingers 296 and 302, respectively, curve away from the support block 218 to protrude through respective slots 306 and 308 in the housing 210. In use, the curved fingers 296 and 302 will make contact with a ground plate 310 of the equipment into which the protector is instal led. A bifurcate contact plate 312 has a bight portion 314 at one end which is clamped between spring contact plate 290 and the step surface 222 of support block 218. The limbs 316 and 318 of the bifurcate contact plate 312 extend longitudinally of the protector to a position above the printed circuit board 258. Their distal ends are supported by plastic spacers 320 and 322, respectively, so that limbs 316 and 318 extend adjacent, but not touching, connecting pins 276 and 254, respectively. The contact plate 312 is resilient, for example spring metal, and so shaped that the limbs 312 and 318 are urged towards the adjacent ends of connecting pins 276 and 254 but prevented from contacting them by the plastic spacers 320 and 322, respectively. Figure 9 is a detail diagrammatic end view of the printed circuit board 258 showing the limbs 316 and 318 of contact plate 312 supported by plastic spacers 320 and 322, respectively, in the normal or "open-circuit" position. The plastics spacers 320 and 322 are of cruciform shape, having central limbs or "web" portions 324 and 326, respectively, and lateral arms 328/338 and 330/331, respectively. The web portions 324 and 326 support, at one end, the contact plate 23 limbs 316 and 318, respectively. Their opposite ends extend a small distance into corresponding holes 332 and 334, respectively, in the printed circuit board 258. The lateral arms 328 and 329 of spacer 320 rest upon the " subjacent printed circuit conductor sections 336 and 338, respectively, and support the spacer 320 against the spring force^exerted by the contact plate limb 316. In like manner, lateral arms 330 and 331 of spacer 322 rest upon subjacent printed circuit conductor sections 333 and 335, respectively. Figure 12 shows the conductor pattern of the printed circuit board 258 in more detail. Conductor 262 has at one end a solder pad area 340, to which the terminal post 274 will be soldered, and at its other end a conductor pad 342 to which one terminal of resistor 260 is soldered and which extends to join pad 335 beneath the lateral arm 331 of spacer 322. Conductor pad 264, the bonding pad for terminal post 266, is connected to a second bonding pad 346 for the other terminal of resistor 260. Portion 333 which extends between the resistor bonding pad 346 and terminal pad area 264 and beneath the arm 330 of spacer 322, is relatively wide a compared with conductor 262. Conductor 262 which interconnects the resistor bonding pad 342 and the terminal bonding pad 340, is relatively long and narrow. The width of the conductor strip 262 and its length, are selected so that the thermal resistance between the terminal pad 340 and the resistor 260 is much greater than that between the spacer 322 ' and the resistor 260. This determines the sensitivity of the sneak current protection by controlling heat sink effects due to the thermal mass of ground electrode 114 (Figure 4) (318 in FiguYe 7). The interconnecting member 240, connects pad 340 to SSOVP 236 which in turn is connected by contact member 290 to ground electrode 310, which is, in effect, a virtually infinite heat sink. On the other hand, interconnecting member 238 connects pad 344 to electrode 244 which, in use, is connected to wiring having a relatively low heat sink effect. Conductors 272 and 278 associated with resistor 270 and spacer 320 are formed in a similar way to conductors 262 and 264 and so will not be described in detail. A through slot 348 extends down the middle of the printed circuit board 258 to provide thermal isolation between the resistors 260 and 270. When an overvoltage condition operates SSOVP device 234 heat is conducted via printed circuit conductor 272 to conductor section 338 to heat arm 329 of spacer 320. Heat is also transmitted by way of resistor 270 to conductor section 336 to heat the other arm 328 of the spacer 320. When SSOVP device 236 operates, heat is transmitted in like manner via printed circuit conductor 262 to heat the lateral 330 and 331 of spacer 332. When an excessive (sneak) current heats one of the resistors 270 and 260, the heat is conducted by printed circuit conductors 333 and 335, or 336 and 338, directly to the lateral arms 328 and 329, or 330 and 331 of the associated spacer 320 or 322. When sufficient heating has occurred, the lateral arms 328 or 330 melt or shear with a snap action from the associated web portion 324 or 326. The associated limb 316 or 318 of the spring contact plate 312 displaces the web portion 324 or 326 into the corresponding hole 332 or 334 until the limb 316 or 318 makes contact with the corresponding connector pin 254 or 276 to short circuit the associated RING or TIP to ground. Figures 10 and 11 illustrate such displacement of spacer 320 permitting contact between limb 318 and connecting pin 254. As mentioned previously, the ground electrode 114/318 in the equipment is, in effect, a virtually infinite heat sink and the contact plate 312 is connected to it. The height of the central limb of the spacer provides thermal insulation between the contact plate 312 and the resistance element. This ensures that the heat generated by the resistance element is available to melt the lateral projections. It should be appreciated that an overvoltage protector will usually produce more heat, when conducting, than a resistance element conducting a sneak current. In the specific embodiment of Figure 6 - 12, the spacers 322 and 320 straddle conductor sections.333/335 and 336/338, respectively, and hence are in parallel with the corresponding resistance element 268 or 270. Consequently, heat from the protector is transmitted to one of the lateral arms via the associated resistance element and directly to the other lateral arm. A modification is envisaged in which both arms of each spacer rest on the same conductor section and hence are both heated to the same extent. This entails enlarging conductor sections 338 and 335 to encompass the holes 332 and 334, respectively, and provide an area of conductor around the hole to contact both arms of the spacer. A separate conductor would connect the resistance element to its associated terminal pad. The chip resistor could be stocked in various values and readily substituted during manufacture to allow economical manufacturing of protection devices with different current ratings. These advantages are not realised by existing protectors which employ heat coils in the form of wire-wound resistors around plunger pins of gas tube protection devices. Of course, being planar itself, the ceramic chip resistor is particularly advantageously employed in a protector which employs generally "planar" or "epitaxial" components as disclosed hereinbefore. In the afore-mentioned US patent number 4,056,840, the plastics film is required to protect the gas tube against failing open circuit. Embodiments of the present invention protect the plastics support member 18 and plastics housing 10 against overheating. The characteristics of the plastics material used for the spacer (film 60, cruciform spacers 320, 322) are determined according to the operating requirements of the device. Thus, since Valox 420SEO, th^ material of the support member (18, 218), has a heat deflection temperature of about 200°C, and the typical maximum environmental temperature is about 70°C, the plastics spacer must melt at a temperature in the range 70-200°C. A melting temperature closer to the upper end of the range is preferred since the device would then be less susceptible to early demise due to AC faults. Especially where a thin film, preferably about 0.002 inches, is employed so as to minimize travel of the thermal spring shunt, good 27 dielectric strength, greater than 500 V/mil , tfe Required as also is good creep resistance since the usual design life of such a device is about 40 years. Similar considerations apply to the lateral arms 328, 329, 330, 331 since they also are relatively thin as compared to the length of the central limb 324, 326 which is intended to provide good thermal insulation between the conductors 333, 335, 336 and 338 and the contact limbs 316 and 318. The preferred material, high molecular weight, high density polyethylene, meets these requirements. The specific material described herein (SCLAIR WCI 46C) has a melting point of about 125°C, dielectric strength of about 5000 V/mil, low creep rate, is available in thin films, and the products of its combustion are not corrosive or particularly toxic. SCLAIR WCI 46C is a polyolefini polymer of the polyethylene class. It has a clearly defined melting point at the required temperature. It also keeps its conformation requirements up to the melting temperatures. Other polyethelynes, indeed other polyolefines such as-polypropylene or polybuteric, can be used providing that the degree of crystal 1inity, stereospecificity and molecular weight combine to give the required thermal and mechanical properties. Other polyolefins might also be suitable. A high density, of at least about 0.941, has been found to give a suitably clear and quick transition between the solid and liquid states. For those embod'iments of the invention iri which the plastics spacer is in the form * of " film, manufacturing limitations may limit the specific gravity to about 0.965. A high molecular weight, at least 250,000, has been found to provide the necessary strength to meet the long term creepage requirements. It is envisaged that other crystalline polymers, such as nylon, could be used since they exhibit a clearly defined melting point. However, because they melt at a higher temperature than SCLAIR WCI 46C, the other components used in the protector, particularly the synthetic plastics support member 18, 218, would need to tolerate the higher temperature. It will be appreciated that, where a synthetic plastics support member is used, its heat deflection temperature should be greater than the melting point of the thermoplastic spacer or film. Further si plification and economy might be achieved by laminating the plastics film to the spring contact plate before assembly. Moreover, the end cap could be an integral part of the support member. In the case of the embodiment of Figure 5, the two parts of the spring contact member 62A might be clamped together by means of the screw 84A rather than welded or riveted. It should be appreciated that the spring contact members 62 (Figures 1-4) and 62A (Figure 5) are interchangeable, regardless of whether or not the overcurrent resistors are incorporated. It should also be appreciated that the sneak current protector could be used alone. INDUSTRIAL APPLICABILITY: An advantage of spacing the fusible thermoplastics spacer away from the actual overvoltage protection device is that the thermoplastics material may have" a lower melting point than, for example, fluoroplastics. Even though the contacts juxtaposed to the spacer or film are connected to the terminal of the protection device, the interconnection will limit heat conduction. This enables materials to be used which have a lower melting point and which do not produce corrosive byproducts of combustion. Polyethylene is such a material . , . Fluoroplastics, such as are disclosed in the cited patent specifications, have a poorer resistance to creep., a broader range of deflection temperatures, and the r * cςmbus i n <.> • can produce hydrofluoric acid (in the presence of water), which is undesirable in a central office where hundreds of protectors might operate simultaneously, and in subscribers premises. ,. An advantage of the "planar" or "laminar" form of protector disclosed herein, i.e. wherein the protection devices and contacts are assembled onto top and 1 side faces of the support member, is that it facilitates automatic manufacture. It is anticipated that this Will lea * d' o "Coέt savings compared to known protectors which employ coaxial gas tubes, heat coils, solder discs, and soon. Placing the shunt away from the protection devices (SSOVPl enhances the life expectancy of the device since it avoids early tripping of the protector on low level AC faults that normally can be handled by the SSOVP device. This is in contrast to certain g s ϋb- devices which have a solder pellet mounted directly to " the'tøas tube, and the gas tube device disclosed in the afore-mentioned U.S. patent specification number 4,212,047 which has a fusible plastics sleeve mounted di ectly upon the gas tube. The use of a plastics film which ruptures to allow short-circuiting of the protection devices advantageously reduces the size of the protector and permits an epitaxial form of construction with a generally planar contact plate assembly on one face of a support member. A disadvantage of protectors employing gas tubes is that the gas tubes are relatively bulky and assembly of the various components is relatively complicated and hence relatively costly. The size of the protector may be reduced by using solid-state devices instead of gas tubes. Such a solid state protector is disclosed in U.S. patent number 4,796,150 by Dickey et al , issued January 3, 1989, to which the reader is directed for reference. Dickey et al disclose a protector having a plurality of disc-shaped solid state protection devices located in recesses in opposite sides of a support member and contacting a central ground plane. These devices are secured to the support member by means of U-shaped spring clips which also provide a path to ground for the surges. Although the individual surge-prctection devices are relatively small, the arrangement is still relatively bulky and complicated to assemble. In preferred embodiments of each aspect of the invention the overvoltage protection device is a solid-state device. It will be appreciated, however, that an alternative protection device, such as a gas tube, could be used. Mounting the resistance element and circuit elements on a substrate provides a more robust, thermally efficient and reliable protector since it avoids having to solder the resistance element directly to the interconnection means. Since the "sneak" currents are relatively small, it is preferable to maintain a relatively large separation of the first and second contacts so as to reduce heat absorption. At the same time, however, it is desirable to melt only a relatively small amount of material to cause the contacts to close. The cruciform spacer effects fi satisfactory solution to this paradox.

Claims: CLAIMS 1. A protector for protecting telephone equipment against excessive voltages, comprising: a support member (18) a protection device (42) supported by said support member (18); electrode means (32, 34) for coupling said protection device (42) to the equipment to be protected; mutually proximal first and second contacts (40,74') spaced from said protection device (42), said first and second contacts being biased one toward the other; interconnecting means (26) connecting one terminal (42B) of said protection device (42) to said electrode means (32, 34) and to said first contact (40); a contact member (62) connecting said second contact to a second terminal (42A) of said protection device (42) and comprising a ground contact (90) for connecting to a ground electrode (114) extending adjacent said protector when the protector is installed in said equipment; spacer means (60) of thermoplastics material preventing electrical connection between said first contact and said second contact, there being a heat transfer path between said spacer and said protection device; the arrangement being such that excessive heating of the protection device causes the spacer to melt and permit electrical connection between said first and second contacts. 33 2. A protector as claimed in claim 1, characterized in that the support member (18) supports a second protection device (58), said protector further comprising second electrode means (52, 54) for coupling said protection device (58) to the equipment to be protected, mutually proximal third and fourth contacts (46,72'), the four contacts being spaced from both protection devices (42,58), said third and fourth contacts being biased one toward the other, second interconnecting means (44) connecting one terminal of said second protection device (58) to said second electrode means (52, 54) and to said third contact (46), said contact member (62) connecting said fourth contact to a second terminal of said second protection device (58), said spacer means (60) preventing electrical connection between said third contact and said fourth contact, there being a heat transfer path between said spacer means and said second protection device; the arrangement being such that excessive heating of the second protection device causes the spacer means to melt and permit electrical connection between said third and fourth contacts. 3. A protector as claimed in claim 1, characterized in that the contact member (62), protection device (42,58) and interconnecting means (26,44), are housed in a housing (10) having at least one aperture (94) in a side juxtaposed to said support member (18), said ground contact (90) being resi 1 iently-biased to project through said aperture. 34 4. A protector as claimed in claim 1, characterized in that the interconnecting means (26) extends along one side of the support member (18) to connect to said electrode means (30). 5 5. A protector as claimed in claim 2, characterized in that the first and second interconnecting means (26) extend along opposite sides of the support member. 10 6. A protector as claimed in claim 1, characterized in that said protection device (42) is located in a recess (20) in said support member (18) and said contact member (62) extends across the mouth of said recess to contact said protection device (42). 15 7. A protector as claimed in claim 2, characterized in that said second overvoltage protector device (58) is located in a second recess (22) in said support member (18) and said contact member (62) extends across the mouth of said second 20 recess to contact said second overvoltage protector. 8. A protector as claimed in claim 1, characterized in that said contact member (62) has slits defining a resilient longitudinally extending limb that is biased away from said 25 support member to serve as said ground contact (90). 9. A protector as claimed in claim 1, characterized in that said contact member (62) has a medial portion (64) formed as a spring (66, 68), the second contact comprising a marginal portion of said contact member urged towards said face by said spring. 10. A protector as claimed in claim 2, characterized in that said contact member (62) has a medial portion (64) formed as a spring (66, 68), the second contact comprising a marginal portion of said contact member urged towards said face by said spring and said fourth contact comprising an opposite marginal portion of said contact member. 11. A protector as claimed in claim 9, characterized in that said medial portion (64) is substantially semi- cylindrical with its cylindrical axis extending longitudinally of said contact member (62). 12. A protector as claimed in claim 10, characterized in that said contact member (62) is made of resilient material, and the protector further comprises fastening means (84) for bearing against said medial portion to urge said marginal portion of said contact member towards said support member. 13. A protector as claimed in claim 1, characterized in that said spacer means (60) comprises high density, high molecular weight polyolefin. 14. A protector as claimed in claim 1, characteri ed in that said spacer means (60) comprises polyethylene having a molecular weight of at least 250,000. 15. A protector as claimed in claim 1, characterized in that said spacer means (60) comprises polyethylene having a molecular weight in the range 250,000 to 500,000. 16. A protector as claimed in claim 1, characterized in that said spacer means (60) comprises polyethylene having a specific gravity of at least 0.941. 17. A protector as claimed in claim 1, characterized in that said spacer means (60) comprises polyethylene having a specific gravity in the range 0.941 to 0.965. 18. A protector as claimed in claim 13, characterized in that said spacer means (60) comprises polyethylene. 19. A protector as claimed in claim 1, characterized in that said spacer means (60) comprises a film. 20. A protector as claimed in claim 1, characterized in that said spacer means (320,322) comprises a limb (324,326) and at least one lateral projection (328,329,330,331), the projection abutting a heat-conductive element (333,335,336,338) for transmitting heat from the protection device (234,236), one of said first and second contacts acting against the limb to urge it towards the heat-conductive element, the arrangement being' such that said excessive heating melts the spacer at the junction between the lateral projection and the limb permitting the limb to displace until the first and second contacts make contact. 21. A protector as claimed in claim 1, further comprising overcurrent protection means comprising a resistance element (116) connected to said electrode " means (30) so as to be in series with a line to be protected, such that heating of said resistance element by excessive current therethrough will cause said spacer means (60) tσ melt. 22. A protector as claimed in claim 21, characterized in that said resistance element (116) is substantially planar and disposed in a recess (118) in said' support member (18). 23. A protector as claimed in claim 22, characterized in that said resistance element (116) is a ceramic chip resistor. 24. A protector, for protecting equipment against excessive voltages and excessive currents, co prising:- at least one overvoltage protection device (42;234,236) ; a pair of electrodes (32A,34A;244,246,286,288) for connecting the overvoltage protection device to equipment to be protected; interconnecting means (120, 122; 238 ,240 τ 250, 252 ) connecting said pair of electrodes to one terminal of said protection device; a contact member for connecting a second terminal of said overvoltage protection device (42) to a ground electrode of the equipment; first and second contacts (40,74' ;316,254,276,318) connected to respective ones of terminals of the protection device; said first and second contacts being biased one toward the other; thermoplastics spacer means (60,320,322) maintaining separation of said first and second contacts; and a resistance element (116;260,270) connected to said interconnecting means (26,44;238,240,252,284) and in series between one (32A;238,284) of said electrodes and said one terminal (42A) of said overvoltage protection device (42); means (24;262,272,333,335,336,338) serving to conduct heat from both said resistance element and said overvoltage protection device to said spacer means; the arrangement being such that melting of said spacer means by heat from either one of said protection device and said resistance element permits electrical connection between said first and second contacts. 25. A protector as claimed in claim 24, characterized in that said spacer (320,322) comprises a limb (324,326) and at least one lateral projection (328,329,330,331), the projection abutting a heat-conductive element for transmitting heat from the resistance element, one of the pair of contacts acting against the limb to urge it towards the heat-conductive element, the arrangement being such that heat generated by an excessive current melts the spacer at the junction between the lateral projection and the limb and the limb is displaced until the contacts make contact. 39 26. A protector as claimed in claim 24, characterized in that said resistance element (116) is substantially planar and disposed in a recess (118) in said support member X18). 27. A protector as claimed in claim 24, characterized in that said resistance element (116) is a ceramic chip resistor. 28. A protector as claimed in claim 2, further characterized by overcurrent protection means comprising resistance .elements (116) each connected to a respective one of said first and second electrode means so as to be in series with a line to be protected, there being a heat trahs iSsiVe path between each resistance element and a respective one of the spacer means, such that heating of a said resistance element by excessive current therethrough will cause said melting of said spacer means. 29. A protector as claimed in claim 1, characterized in that said overvoltage protection device (42) is a solid-state protector. 30. A protector as claimed in claim 28, characteri ed in that each said overvoltage protection device (42) is a solid-state protector. 31. A protector for protecting against excessive currents in telecommunications equipment, comprising a resistance element (260,270), electrodes (244,246,286,288) for connecting the resistance element in series with a line to the equipment which is to be protected, a contact member (290) for connecting to a ground electrode when the protector is in use, a pair of mutually proximal contacts (316,276;318,254) connected one to the contact member and the other to one of the electrodes, said contacts being biased one towards the other, a spacer (320,322) of insulating material preventing electrical contact between said contacts, said spacer comprising a limb (324,326) and at least one lateral projection (328,329,330,331), the projection abutting a heat- conductive element (333,335,336,338) connected to the resistance element, one of the pair of contacts acting against the limb to urge it towards the heat-conductive element, the arrangement being such that heat generated by an excessive current in the resistance element melts the spacer at the junction between the lateral projection and the limb and the limb is displaced until the contacts make contact. 32. A protector as claimed in claim 31, characterized in that the resistance element (260,270) is mounted upon a printed circuit board (258). 33. A protector as claimed in claim 32, characterized in that said resistance element is connected to said electrodes by circuit elements (262,272) printed on said printed circuit board. 34. A protector as claimed in claim 33, characterized in that at least one of said circuit elements is circuitous 41 and dimensioned to provide a desired thermal inφfedahσe bet eten . t '< > said resistance element and said spacer. « * » , 35. A protector as claimed in claim 31, charatttøri'zeri in that said spacer (320,322) is mounted upqn a, suRpo ^ aving a hole (332,334), said heat-conductive element being disppsed adjacent said hole and said limb being aligned with said hole, said projection being supported at the pePiphe^y cMffe hόh ,,j such that melting of at least part of the spacer in the vicinity of the junction between the arms and the limb allows the limb to enter the hole. ' ' * " ** ' << ' 'R ,! * 36. A protector as claimed in claim 35, characterized in that the resistance element (26b,27θV is tet ted upon a printed circuit board. 37. A protector as claimed in claim 5δ, character zed in that said interconnecting means comprise* circui ejl τne>hts (262,272) printed on said printed circuit b 38. A protector as claimed in claim 37, characterized in that said circuit elements are circuitous and dimensioned to provide a predetermined thermal impedance between said resistance element and said interconnecting member. 39. A protector as claimed in claim 31, comprising a i <'ι ! 'M t * A" second resistance element, connected to electrodes for connection in series with a second line tp - * » 42 protected, and a thermal barrier (348) between the first resistance element and the second resistance element. 40. A protector as claimed in claim 39, characterized 5 in that said second resistance element is connected to a second pair of contacts, such contacts being maintained apart by a second insulating spacer having a limb and a second projection projecting laterally from the limb, said projection being supported by a second heat-conductive element, the 10 second pair of contacts being urged one towards the other by one of the second pair of contacts, the arrangement being such that excessive current in said second resistance element effects melting of at least part of the second spacer in the vicinity of the junction between the projection and the limb 15 allowing the limb to displace until the contacts meet. 41. A protector as claimed in claim 31, characterized in that said resistance element (116) is a ceramic chip resistor. 20 42. A protector for protecting telephone equipment against excessive voltages comprising: a support member (218); an overvoltage protection device (236) supported by said 25 support member; electrode means (244,246) fbr coupling said protection device to the equipment to be protected; interconnecting means (24 of said protection device to sa d electrode means and hav ng a first contact (254); a contact member (290,312) comprising a ground contact (296) for contacting a wg adjacent said protector when the protector is installed in said equipment and a second contact (318) juxtaposed to said first contact, said first contact and said second contact being biased one toward the other; a spacer of thermoplastics material (322) separating said first contact and said second contact, said spacer comprising a limb (326) and at least one lateral projection (330,331 ), the projection abutting a heat-conductive element (335) for transmitting heat from the protection device, one of said first and second contacts acting against* the llmtoJlqMUjfaaeii it, towards the heat-conductive element, the arrangement being such that said excessive .heating .j>, ( junction between the lateral p i permitting the limb to displace unti 1 contacts make contact. 43. A protector, for protecting equipment against excessive voltages and excessive currents, compπsing:- at least one overvoltage protection device, ,, 236.) ; a pair of electrodes (244,246) 'for the overvoltage protection device to equipment to be protected; interconnecting means (238,240) connecting said^pai r< of electrodes to one terminal of said protection device; a contact member (290,312) for connecting a second terminal of said overvoltage protection device (236) to a ground electrode (310) of the equipment; first and second contacts (254,318) connected to respective ones of terminals of the protection device, said first and second contacts being biased one toward the other; thermoplastics spacer means (322) maintaining separation of said first and second contacts; and a resistance element (260) connected to said interconnecting means and in series between one (244) of said electrodes and said one terminal of said overvoltage protection device (236); the interconnecting means serving to conduct heat from said resistance element to said spacer means; the arrangement being such that melting of said spacer means permits electrical connection between said first and second contacts, the resistance element and the protector being both coupled thermally to the spacer, the coupling between the protector and the spacer having a greater thermal impedance than the coupling between the resistance element and the spacer. 44. A protector as claimed in claim 43, characterized in that the interconnecting means (262) between the protector and the spacer is longer than that between the spacer and the resistance element. 45. A protector as claimed in claim 43, characterized i that the interconnecting means (262) between the protector and the spacer has a lesser cross-section than that between the spacer and the resistance element.

Inventors: ANDERSON JAMES EDWARD, HUNG FRANCIS YUN-TAI, TAYLOR PAUL ANDREW, TRUMBLE WILLIAM PATRICK, ANDERSON, JAMES, EDWARD, HUNG, FRANCIS, YUN-TAI, TAYLOR, PAUL, ANDREW, TRUMBLE, WILLIAM, PATRICK
Applicants: NORTHERN TELECOM LTD, NORTHERN TELECOM LIMITED
Application year: 1990
Main classification: H01C7
IPC codes: H01C7, H01T1