Electricity: electrical systems and devices – Safety and protection of systems and devices – Load shunting by fault responsive means
Patent
1982-08-10
1984-06-26
Moose, Jr., Harry E.
Electricity: electrical systems and devices
Safety and protection of systems and devices
Load shunting by fault responsive means
361119, H02H 904
Patent
active
044569408
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to an arrangement for overvoltage protection of circuits in a telephone exchange. Such overvoltages, for example, can be lightning transients on lines connected to the exchange or so called longitudinal disturbances on subscriber lines caused by short circuit to earth.
BACKGROUND ART
In the journal Electronics of Oct. 25, 1979, page 42 an arrangement for the protection of circuits connected to a telephone network is described. The arrangement is intended to protect against, for example, lightning transients on subscriber lines and consists mainly of four so called clamp diodes, which connect the two wires of the subscriber line to the two poles of the power supply source. By this arrangement the maximum voltage between the a-wires and the b-wires is limited to the pole voltage of the supply source. The disadvantage with this design of the protection circuit is that the heavy transient currents which can be conducted to the negative pole of the supply source give rise to a voltage-drop in the power supply among other things due to its inherent resistance. Such voltage-drops can destroy other equipment in its turn.
To handle this disadvantage it has been suggested, compare for example the Swedish patent specification No. 7714316-2, to conduct the negative transient currents to earth by means of Zener diodes instead of towards the negative pole of the battery. The disadvantage with this solution is that the transient protection is not active to voltages in the voltage gap between the varying battery voltage and the Zener voltage.
A further problem with known arrangements resides in the cooperation of the protection unit with the ringing relay of the subscriber line. Most often connection of the ringing signal to the line is made by means of a relay. The ringing signal generator generates an alternating voltage for example in the interval from -200 V t+150 V while the working interval for the line circuit connected to the line is between 0 V and the battery voltage, for example -56 V. The transient protection for the line circuit, thus intended to protect against voltages lower than -56 V and higher than 0 V, must be located between the ringing relay and the line circuit so that the protection circuit does not short circuit the ringing voltage. The disadvantage with such an arrangement is that the sometimes heavy transient currents during, for example, lightning disturbances, pass through the relay contact, which consequently must to be dimensioned for this load.
BRIEF SUMMARY OF THE INVENTION
The technical problem to be solved by and an object of the present invention resides in the simultaneous elimination with one arrangement of the above mentioned disadvantages according to prior art.
Briefly, the invention contemplates apparatus for the protection of circuits in a telecommunication equipment against overvoltages appearing on lines connected to the circuits having at least two non-linear electronic circuit elements, which selectively shunt said overvoltages to ground, wherein the conduction state for at least one of the non-linear electronic circuit elements is controllable.
The advantages with the solution according to the invention are obvious as the protection circuit is effective for all realistic values of the disturbance voltage at the same time as the ringing relay only need be dimensioned with reference to its actual function as connecting element for ringing voltage to the subscriber.
BRIEF DESCRIPTION OF DRAWING
The invention will be described below with reference to the accompanying drawing is which
FIG. 1 illustrates an arrangement according to prior art;
FIG. 2 illustrates an arrangement according to prior art;
FIG. 3 illustrates an arrangement according to prior art;
FIG. 4 illustrates an arrangement according to the invention; and
FIG. 5 illustrates more in detail an arrangement according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1, according to the above, illustrates an arrangement according
REFERENCES:
patent: 3126452 (1964-03-01), Pettersson
patent: 3558830 (1971-01-01), Bender
Ghisler Walter
Hammerberg Lars E.
Hjortendal Royne G.
Moose Jr. Harry E.
Telefonaktiebolaget L M Ericsson
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