Circuit protection arrangement

Abstract

 A circuit protection arrangement for protecting a circuit against damaging signals, such as surges and transients, comprises the combination of a series protection element which becomes open circuit in the event of a damaging signal and a shunt protection element which becomes a low resistance circuit in the event of a damaging signal. The series protection element is a normally-closed switch device 4 which is actuated in response to heat generated in the shunt protection element, e.g. in the form of a gas tube 1, so as to open-circuit the series path and provide a short circuit of the shunt path. The protection arrangements may be applied to both unbalanced lines and balanced lines, such as L1, L2, feeding an input E1, E2 of the equipment to be protected.

Description

[0001] The present invention relates to circuit protection arrangements for protecting a circuit against damaging signals, such as high voltage or current surges and transients, and which comprise both a series protection element and a shunt protection element arranged so that in the event of a damaging signal the series element tends to open circuit and the shunt element tends to short circuit.

[0002] In practice with existing arrangements there is generally some finite leakage current across an "open circuit" series element; whilst an appreciable resistance, and hence a voltage drop, occurs across a "short circuit" shunt element; thereby reducing the effectiveness of the protection arrangement.

[0003] It is an object of the present invention to provide a circuit protection arrangement in which the aforementioned disadvantages are substantially reduced or avoided.

[0004] From one aspect of the invention provides a circuit protection arrangement for protecting a circuit against damaging signals, such as surges and transients, comprising the combination of a series protection element which becomes open-circuit in the event of a damaging signal and a shunt protection element which becomes a low resistance circuit in the event of a damaging signal, wherein the series protection element is a normally-closed switch device which is actuated in response to heat generated in the shunt protection element to open-circuit the series path and provide a short circuit of the shunt path.

[0005] The invention also provides a circuit protection arrangement for protecting a circuit against damaging signals, such as surges and transients, comprising a changeover switch device which is noramlly restrained in one position in which it completes a series circuit and forms a series protection element, and a shunt protection element in thermal relationship with said changeover switch device such that heat generated in the shunt protection element in response to a damaging signal causes actuation of the changeover switch device to its other position in which it opens the series circuit and short circuits the shunt path.

[0006] The invention further provides a circuit protection arrangement comprising a changeover switch device which is normally retained in one position to provide a series protection element and which is thermally actuated to its other position by heat generated in a shunt protection element associated with said changeover switch device, and wherein when in said other position the switch device open circuits a series path and short circuits a shunt path.

[0007] The switching action is arranged to be irreversible, so that when the thermally actuated switch device has been switched to its other position, it will remain in this position even when the heat which caused its operation is removed.

[0008] In a preferred arrangement the switch device includes a contact which is biassed towards the short circuit shunt position but which is retained in the normally closed series circuit protection position by means of a thermally softenable material, e.g. soft solder or fusible alloy, which softens to release the contact upon the application of sufficient heat from the shunt protection element.

[0009] Thus, the switch contact may be formed of a resilient wire or strip, which may include one or more turns to impart an enhanced springiness to the contact.

[0010] Advantageously the shunt protection element comprises a gas discharge tube and the heat generated by the tube when it is struck is employed to cause the thermally responsive operation of the switch device.

[0011] The protection arrangements according to the invention may be applied to both balanced and unbalanced circuits and lines and advantageously the arrangements according to the invention may be in the form of a module which can be readily attached to and replaced in a circuit arrangement which is to be protected.

[0012] The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:-

Figures 1a and 1b are schematic diagrams of the circuitry of two balanced protection arrangements according to the invention,

Figure 2a is a diagrammatic plan view of one embodiment of circuit protection arrangement according to the invention,

Figure 2b is a side elevational view based on the arrangement of Figure 2a and showing the components mounted on a circuit board,

Figure 3 is a diagrammatic plan view of another embodiment of circuit protection arrangement, and

Figures 4a and 4b are respectively a perspective view and a side elevational view of a further embodiment of circuit protection arrangement.

[0013] Referring to the drawings, the arrangements to be desribed are a balanced arrangement and similar references are used for corresponding parts of the protection arrangement in each line.

[0014] As shown in the schematic diagramms of Figures 1a and 1b, a pair of lines L balanced with respect to earth and having input terminals L1 and L2 are connected to equipment to be protected via terminals E1 and E2. Associated with the lines is a shunt protection element comprising a double gas discharge tube 1 having its common electrode connected to earth via connecting pin 2 and its outer electrodes respectively connected to the two lines L via connecting pins 3 which are also connected to the normally-closed switch contact 4 disposed in series in each line. These contacts 4 are each part of a changeover switch and are activated in response to heat generated in the gas discharge tube 1 so as to open-circuit the series path between the input lines and the equipment and short-circuit the shunt path provided by the tube 1. In Figure 1a, actuation of the switch contacts 4 serves to short-circuit the equipment side across terminals E1 and E2 whilst in Figure 1b, actuation of the switch contacts 4 serves to short circuit the input line side across terminals L1 and L2. If desired, the changeover switch could be arranged to short-circuit both the input side and the equipment side.

[0015] The convention adapted regarding input and output relates to an incoming fault current and any output in fault current let-through into the equipment from the protection arrangement. Obviously, wanted signal currents may pass in both directions and the protection arrangement may be regarded as transparent to such signal currents.

[0016] Figures 2a and 2b show an embodiment of the invention in which the components are mounted on a circuit board. Each switch contact 4 is in the form of a shaped resilient wire or strip which may be coiled about a post 5 to increase its flexural range and which has one end soldered to a point 6 connected to the terminal E1 or E2. The spring contacts 4 are tensioned so that they are biassed towards the centre earthed pin 2 of the gas discharged tube, but they are constrained into contact with the pins 3 and also into contact with posts 7 connected to the line terminals L1 and L2 by fusible joints at points P1 and/or P2. It will be understood that both points P1 and P2 may be fusible joints or that the desired effect can be achieved if only one point is a fusible joint and the other point is merely a pressure contact between the parts. The fusible joints may be formed by a soft solder or a fusible alloy.

[0017] The fusible joints are intended to be softened by heat generated in the gas discharge tube 1 and whilst there is direct heat transfer to a fusible joint at points P1 via the pins 3, it is desirable to provide some form of heat transfer member, such as a metal strip between the insulating envelope of the gas discharge tube 1 and the posts 7 when fusible joints are located at the points P2.

[0018] The released position of a switch contact 4 showing it in contact with the centre earthed pin 2 and out of contact with the pin 3 and post 7 is shown in dashed lines for the upper contact in Figure 2a. This is the position which the contact assumes under its natural spring bias when the fusible alloy at points P1 and/or P2 has softened sufficiently to release it from its normally constrained position.

[0019] Figure 2b shows the arrangement mounted on a circuit board B.

[0020] Referring now to Figure 3, in the embodiment each switch contact 4 is again a resilient wire or strip which in this case forms a generally vee-spring coiled at an intermediate point of its length about a post 5 and has one arm 4a bearing against a pin forming the terminal E1 or E2. The contacts 4 are tensioned so that the other arms 4b are biassed towards the centre earthed pin 2 of the gas discharge tube, but they are constrained into contact with the pins 3 and also into contact with pins forming the input terminals L1 and L2 by fusible joints at point P1 and/or P2. It is again possible for both points P1 and P2 to be fusible joints or the desired effect can be achieved if only one point is a fusible joint and the other point is merely a pressure contact between the parts. The fusible joints may be formed by a soft solder or a fusible alloy.

[0021] As in the previous embodiment, the fusible joints are intended to be softened by heat generated in the gas discharge tube 1 and whilst there is direct heat transfer to a fusible joint at points P1 via the pins 3, it is desirable to provide some form of heat transfer member, such as a metal strip between the insulating envelope of the gas discharge tube 1 and the posts P2 when fusible joints are located at the points P2.

[0022] The released position of a switch contact 4 showing it in contact with the centre earthed pin 2 and out of contact with the pin 3 and post L1 is shown in dashed lines for the upper contact in Figure 2. This is the position which the contact assumes under its natural spring bias when the fusible alloy at points P1 and/or P2 has softened sufficiently to release it from its normally constrained position. In the released position, the tension in the coiled portion of the spring ensures its continued pressure against the output pin E1, E2 as well as against the earth pin 2. In practice, both springs operate at about the same time, so that the inputs L1,L2 are isolated and the outputs E1 and E2 are earthed.

[0023] In operation of the above devices, brief, infrequent, and low-energy transients are suppressed or "clipped" by the gas discharge tube 1 without raising its temperature sufficiently to release the spring contacts 4. The tolerance of the arrangement to these transients may be increased by providing a heat-sink for the gas discharge tube and/or by increasing the thermal resistance between the gas discharge tube and the blobs of fusible alloy at P1 and/or P2.

[0024] When, however, the temperature of the gas discharge tube rises sufficiently to melt the alloy and release the spring contacts 4, the equpment terminals E1.E2 are fully disconnected from the lines L1,L2 and shorted directly to earth at pin 2. Of course only one of the contacts 4 may so operate instead of both.

[0025] This temperature rise could occur as a result of a heavy surge caused by lightning or by induction from a power line under fault conditions, or a steady AC or DC surrent of a few amps resulting from accidental contact between the line and a power cable. It is the object of the present invention to provide effective protection from these and similar hazards.

[0026] For balanced lines comprising both L1 and L2 and a common earth, the module requires two spring contacts as indicated. The resulting configuration is known as a 5-point arrangement. For unbalanced or "line-and-earth" circuits, a 3-point arrangement with a single spring contact only is required. Either arrangement may readily be constructed as a module.

[0027] The spring contacts could conceivably be thin enough to be heated by an excess of current and so either cause or aid melting of the fusible alloy. However, a study of working conditions on practical lines shows that such a level of current can only arise as a result of an intrusive voltage high enough to "strike" the gas discharge tube and hence the tube conducts and so generates enough heat unaided.

[0028] Consequently, a further advantage of this invention is that the spring contacts may be heavy enough to (i) avoid annealing and loss of tension due to the heating effects of fault-currents, (ii) add negligible series loss resistance to the signalling circuits and (iii) provide adequate contact pressure at any contact points which are not soldered.

[0029] Referring now to Figures 4a and 4b, there is shown an embodiment of protection arrangement formed as a module which is mechanically interchangeable with existing types of protection modules, as employed for example in telephone exchange equipment. By virtue of the use of metal strip contacts rather than wire, it provides better pulse current carrying capacity, due both to the larger cross-section of the conductors and the larger pressure contact areas.

[0030] A gas discharge tube 1 is supported from a platform 10 of insulating material by two arms 11 formed of metal strip, which are respectively attached to annular terminals 3 connected to the outer electrodes of the gas discharge tube 1. The lower end of the arms 11 are connected to tag strips 12 depending below the platform 10 and forming the input connections L1 and L2. Also connected to or formed integrally with the lower ends of the arms 11 are U-shaped springs 4 which are biassed such that they tend to contact the ends of a bridge contact 13 having one central limb 13a in contact with the annular terminal 2 connected to the common electrode of the gas discharge tube 1. A second central limb 13b is connected to an earth pin 15 extending through the platform 10 and also serves to locate and support the bridge contact 12 above the ends of the U-shaped springs 4. Also attached to the platform 10 and having one end disposed below the ends of the springs 4 are tags strips 14 forming the output connections El and E2.

[0031] The springs 4 are normally held in contact with the ends of the strips 14 by means of a soft solder or fusible alloy, as shown in the drawings, thus providing a series connection between the input terminals L1,L2 and output terminals E1,E2. However, upon overheating of the gas discharge tube 1, heat is conducted via the metal strips 11 and 13 to melt the fusible alloy, thereby releasing the U-shaped springs 4 so that they break contact with the strips 14 and make contact with the bridge 13, hence effectively open-circuiting the series paths between Ll,El and L2,E2 and short circuiting the gas discharge tube.

[0032] Whilst particular embodiments have been described it will be understood that various modifications may be made without departing from the scope of this invention. Thus, the spring contacts performing the switching function may be of various other forms besides that specifically shown. Also the shunt protection device which generates the heat could be other than a gas discharge tube.

Claims

1. A circuit protection arrangement for protecting a circuit against damaging signals, such as surges and transients, comprising the combination of a series protection element which becomes open circuit in the event of a damaging signal and a shunt protection element which becomes a low resistance circuit in the event of a damaging signal, wherein the series protection element is a normally-closed switch device which is actuated in response to heat generated in the shunt protection element to open-circuit the series path and provide a short circuit of the shunt path.

2. A circuit protection arrangement as claimed in claim 1, comprising a changeover switch device which is normally restrained in one position in which it completes the series circuit and forms the series protection element, and the shunt protection element is in thermal relationship with said changeover switch device such that heat generated in the shunt protection element in response to a damaging signal causes actuation of the changeover switch device to its other position in which it opens the series circuit and short circuits the shunt path.

3. A circuit protection arrangement comprising a changeover switch device which is normally retained in one position to provide a series protection element and which is thermally actuated to its other position by heat generated in a shunt protection element associated with said changeover switch device, and wherein when in said other position, the switch device open circuits a series path and short circuits a shunt path.

4. A circuit protection arrangement as claimed in claim 1, 2 or 3, in which the switch device includes a contact which is biassed towards the short circuit shunt position but which is retained in the normally closed series circuit protection position by means of a thermally softenable material, which softens to release the contact upon the application of sufficient heat from the shunt protection element.

5. A circuit protection arrangement as claimed in claim 4 in which the thermally softenable material is a soft solder or a fusible alloy.

6. A circuit protection arrangement as claimed in any preceding claim, including one or more switch contacts formed of a resilient wire or strip.

7. A circuit protection arrangement as claimed in any preceding claim, in which the shunt protection element comprises a gas discharge tube and the heat generated by the tube when it is struck is employed to cause the thermally responsive operation of the switch device.

8. A circuit protection arrangement as claimed in any preceding claim, in the form of a module which can be readily attached to and replaced in a circuit arrangement which is to be protected.

9. A circuit protection arrangement as claimed in any preceding claim in combination with a pair of balanced lines, in which a switch device is connected in series with each line and the shunt protection element is connected across the lines.

10. A circuit protection arrangement as claimed in claim 9, in which the shunt protection element is a double gas discharge tube, having its common electrode connected to a reference potential, such as earth.

Drawing