A DEVICE FOR INDICATING A DESTROYED ARRESTER

Abstract: With the intention of providing reliable indication as to whether an overvoltage protective device has been destroyed or not, there is provided a protective device and a system which are so constructed as to indicate exclusively that the protective device has been destroyed and nothing else. The most important feature of the system resides in a novel type of failsafe device having an alarm function - an alarm failsafe device (12) - which behaves in precisely the same manner as a typical failsafe device that short-circuits the terminals of the protective device (8) when the protective device has been heated to a critical temperature (10) and, at the same time, short-circuits a signalling circuit which signals the position of the device. To this end, the protective device includes an additional electrical contact spring - an alarm spring (2) - which is held retracted by the same piece (3) of meltable material as that which holds a main failsafe spring (1) retracted. When the material (3) melts as the protective device is heated, the main failsafe spring (1) and the alarm spring (2) are released. Because the failsafe device is connected to earth through a centre pin on the protective device, the protective device and the signalling circuit will also be earthed when the springs are released. Protective devices can be monitored and malfunctioning devices easily and reliably registered, by providing all incoming telecommunications lines with alarm failsafe devices in groups in cassettes connected to a monitoring system.

Description: A DEVICE FOR INDICATING A DESTROYED ARRESTER. TECHNICAL FIELD The present invention relates to a device for registering the malfunction of a fuse or protective device included, for instance, in an overvoltage protection system in telecommuni¬ cations equipment. In the event of an overvoltage protective device being overheated as a result of an overvoltage with risk of damage to the system, the event is registered by the inventive device so as to enable the protective device to be replaced. DESCRIPTION OF THE BACKGROUND ART A three-pole noble gas discharge tube (hereinafter called a protective device) is normally used to protect telecommunica¬ tions equipment and telecommunication lines from overvoltages caused by lightening, or induced alternating voltages, or as a result of direct contact with power cables. The two wires of an incoming telecommunications line are normally protected with a protective device at the input to the main distribution frame of the station. The function of the protective device is to short-circuit the wires to earth should an overvoltage occur on the line, thereby protecting line interface boards and other equipment connected to the incoming telecommunica¬ tions line from the damaging overvoltage. The protective device normally returns automatically to its standard state of very high resistance, when the overvoltage ceases to appear. When a protective device is subjected to an a.c. overvoltage for a long period of time, the power loss in the protective devicewill cause the protective devicetoheat-up. This is liable to damage the cassette in which the protective device is mounted, since the cassettes are normally made of a plastic material. Heating of the protective device may also result in its own destruction. If the protective device is destroyed, it will act as a very large resistance between line and earth and be unable to stop any further overvoltages on the telecommunications line, these overvoltages instead reaching the connected telecommunications equipment and causing damage thereto. Anotherproblem is that the functional state of a protective device cannotbe determined and measured without disconnecting the protective device from the telecom¬ munications line. It is necessary to remove the protective device from the cassette, in order to be able to measure its electrical properties and therewith establish whether or not the protective device has been destroyed. There has been developed a failsafe device with the intention of solving part of this problem, this device having been in use for several years. A failsafe device functions to short- circuit the protective device immediately prior to its potential destruction. The failsafe device is triggeredbythe heat emitted by the protective device and is typically comprised of a spring-loaded electric contact which is held retracted in a piece of material which melts when heated, so as to release the electrical contact when the protective device reaches a given temperature. As the electrical contact is released, it moves towards the poles or terminals of the protective device and short-circuits the terminals to earth. When a protective device fitted with a failsafe device is de- stroyed, the station is able to carry out measurements on the connected telecommunications line to ascertain whether or not the line has been short-circuited to earth. However, a telecommunications line can be short-circuited to earth by a number of different causes, and not only because a protective device provided with a failsafe device has been destroyed. Thus, it is not possible for station personnel to be sure that a specific telecommunications line has been earthed due to a destroyed protective device or as a result of an external short-circuit. Earthing of a telecommunications line is therefore an unreliable indication that a protective device has been destroyed. SUMMARY OF THE INVENTION With the intention of giving a reliable indication as to whether or not a protective device has been destroyed, there has been constructed a protective device and a system which are designed to indicate reliably that a protective device has been destroyed and nothing else. The most important part of this system is a novel type of failsafe device that includes an alarm function, hereinafter called an alarm failsafe device, which behaves in precisely the same manner as a typical failsafe device in short-circuitingpins/terminals on a protective device when said device is heated to a critical temperature and which, at the same time, short-circuits a signal circuit to indicate the position of the failsafe device. This is achieved by providing the protective device with an additional electric contact spring - an alarm spring - which is held retracted by the same piece of meltable material as that in which a main failsafe spring is held retracted. Whereas the main failsafe spring is designed and biassed to move towards the protective device, the alarm spring is designed and biassed to move away from the protec¬ tive device and towards a contact rail or bar. If the protective device is heated to a temperature at which the material melts, the main failsafe spring will be released and move towards the protective device, therewith making contact between terminal pins and a centre pin on said device. The alarm spring is released at the same time and moves away from the protective device and into contact with a metal rail or bar mounted in the cover of a cassette in which the protec¬ tive device is fitted. Because the failsafe device is now earthed through the centre pin on the protective device, both of the terminal pins on the device will be earthed and the metal rail or bar in the cassette cover will also be earthed when the springs are released. The cassette cover is also provided with a light-emitting diode which is connected so as to begin to light-up when one of the protective devices in the cassette has been destroyed. Simple and reliable monitoring and registration of malfunctioning protective devices can be achieved by providing all incoming telecommunications lines with alarm failsafe devices arranged in groups in cassettes connected to a monitoring system. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates a spring element and a pin for an inventive alarm failsafe device, said element and pin being shown in a flat state. Fig.2 shows the spring element of Fig. 1 bent or curved to fit a three-terminal protective device of the noble gas discharge tube type. Fig. 3 illustrates an inventive alarm failsafe device. Fig.4 illustrates inventive alarm failsafe devices connected to the incoming telecommunications lines, where the right protective device has been destroyed. Fig. 5 illustrates an electric circuit corresponding to Fig. 4. Fig. 6 illustrates a cassette which includes a cover member andwhich accommodates ten alarm failsafe devices, said Figure also illustrating alarm failsafe devices and a cassette. Fig. 7 illustrates a system which includes alarm units and buzzer units connected to cassettes that house alarm failsafe devices. BEST MODE OF CARRYING OUT THE INVENTION Figs. 1-3 are detailed views of the spring part of an alarm failsafe device 12. For the sake of illustration, the spring part of the alarm failsafe device is shown in Fig. 1 in its state prior to being curved and fastened around the cylindri¬ cal body of a protective device 8. The spring part of the alarm failsafe device is comprised of a resilient or springy T-shaped plate 5 made of beryllium copper, for instance. The main failsafe spring 1 of the alarm failsafe device is designed so that when released it will contact the end terminals 9 on the protective device 8. The alarm spring 2 of the alarm failsafe device is constructed so that when in its tensioned position, it will lie generally inwardly of the main failsafe spring 1 and is held fixed in this position by means of a meltable rod or pin 3. One end of the meltable pin may be fitted and fixed in a hole 4 on the main failsafe spring 5 of the alarm failsafe device. The main failsafe spring 5 also includes a hole 6 which receives a centre pin 11 provided on the protective device. Reference numeral 7 identifies a connecting point at which the failsafe device is in contact with a centre electrode annulus 13 on the protective body. Fig. 2 illustrates the alarm failsafe device with the spring part 5 bent or curved for fitting the device to the protective body. Fig. 3 shows the alarm failsafe device and its main failsafe spring and alarm spring mounted on the protective body 8. Fig.4 shows howthe protective devices/alarm failsafe devices 12 are coupled to incoming telecommunications lines 14 connected to line interface boards 15, wherein the right protective device has been destroyed in the illustration and its terminal connector pin 10 has been short-circuited via a pin connector 11 in the centre of the main failsafe spring. The alarm spring 2 has moved away from the protective device and is in contact with a metal rail 16 fitted to a cover member 17 on a cassette 18 in which the protective device is mounted. As a result, there is formed an electric circuit which passes from earth 19 to an electrical connection through the alarm failsafe device 12, the metal rail 16, a light-emitting diode 20, a resistor 21 and an alarm unit 22. Fig. 5 illustrates an electric circuit which is an equivalence to the circuit shown in Fig. 4. Fig. 6 is a simplified illustration which shows how the alarm failsafe devices 12 are adapted for insertion into the cassette 18 and covered with a cover member 17 which incorpo¬ rates a rail 16 that can be connected to a signal circuit. Also fitted to the cover member 17 is a light-emitting diode 24 which lights-up when the alarm spring 2 of the alarm fail¬ safe device contacts the rail 16 subsequent to melting of the rod 3. Fig. 6A shows the cover member 17 and the light- emitting diode 24 fitted thereto. Fig. 6B shows ten alarm failsafe devices 12 for insertion into respective cassettes 18, as shown in Fig. 6C. Fig. 6D shows the cover member 17 fitted to a cassette 18. Fig. 6E shows the cassette cover member 17 from above, with the light-emitting diode 24 well visible. Fig.7 illustrates the connection of a respective buzzer unit 25 to the alarm units 26, each of said units being connected to a commonvoltage source and including a functional electric switch. Each alarm unit is connected to a holder 27 which accommodates a plurality of cassettes. If an overcurrent should cause abnormal heating of an alarm failsafe device, the rod 3 will melt and the alarm circuit will be connected to earth, wherewith the light-emitting diode 24 begins to light- up and the alarm unit 26 serving the group of alarm failsafe devices will register this earth connection and deliver a signal to the buzzer unit 25. When a protective device malfunctions, the malfunction is noticed immediately and the buzzer heard by station personnel, wherewith the protective device is exchanged for a functioning device.

Claims: CLAIMS 1. A device for registering the malfunction of safety devices, for instance the malfunction of an overvoltage protective device for telecommunications equipment, wherein abnormal heating of a protective device with risk of damage releases parts of a spring element, wherewith the protective device and a signal circuit are connected to earth, charac¬ terized in that the spring element includes a main failsafe spring (1) and an alarm spring (2) which are mutually separa- ted by meltable material when in a tensioned state and in electrical contact with an earthable centre pin 12 on the protective device and when in a released state are also in electrical contact with a device mounted terminal pin (10) and a rail or bar (16) , said device-mounted terminal pin and said rail or bar being connected to earth. 2. A device according to Claim 1, characterized in that the meltable material has the form of a rod (3) . 3. A device according to any one of the preceding Claims, characterized in that a signal circuit (20-23) is connectedto the rail or bar (16) , wherewith faults are indicated with the aid of light and/or sound signals.

Inventors: ERIKSSON TOMAS, ERIKSSON, TOMAS
Applicants: ERICSSON TELEFON AB L M, ERIKSSON TOMAS, ERIKSSON, TOMAS, TELEFONAKTIEBOLAGET LM ERICSSON
Application year: 1995
Main classification: H02H9
IPC codes: H01T1, H01T4, H01T15, H02H3, H02H7