Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1996-09-18
1998-09-08
Hofsass, Jeffery
Communications: electrical
Condition responsive indicating system
Specific condition
340638, 340664, 361119, 361124, 337 32, 337140, G08B 2100
Patent
active
058050701
DESCRIPTION:
BRIEF SUMMARY
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 telecommunications 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 telecommunications 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 telecommunications 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 device will cause the protective device to heat-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 the telecommunications line and earth and will be unable to stop any further overvoltages on the telecommunications line. These overvoltages will instead reach the connected telecommunications equipment and cause damage thereto. Another problem is that the functional state of a protective device cannot be determined and measured without disconnecting the protective device from the telecommunications 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 triggered by the 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 destroyed, 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
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patent: 4642723 (1987-02-01), Achtnig et al.
patent: 4876626 (1989-10-01), Kaczmarek
patent: 5172295 (1992-12-01), Hegnet et al.
patent: 5248953 (1993-09-01), Honl
patent: 5299088 (1994-03-01), Honl et al.
Hofsass Jeffery
Telefonaktiebolaget LM Ericsson
Wu Daniel J.
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