AU654814B2

AU654814B2 - A circuit protection device

Info

Publication number: AU654814B2
Application number: AU77440/91A
Authority: AU
Inventors: John Morton; Benjamin Wright
Original assignee: Telstra Corp Ltd
Current assignee: Telstra Corp Ltd
Priority date: 1990-04-26
Filing date: 1991-04-23
Publication date: 1994-11-24
Anticipated expiration: 2011-04-23
Also published as: AU7744091A

Description

A CIRCUIT PROTECTION DEVICE

FIELD OF THE INVENTION

The present invention relates to two devices to protect electrical equipment from lightning strikes and more particularly but not exclusively to protect telephone equipment such as facsimile machines.

BACKGROUND ART

Gas arrestors in the past have been commonly used to protect

telephone circuits by discharging the lightning to ground. However these arrestors are relatively slow and are therefore not totally successful.

One attempt to overcome the above problem is described in Australian Patent 590491. This particular patent teaches the use of Zener diodes, in series with a parallel arrangement of capacitor and resistor. However this particular arrangement has the disadvantage that it is frequency- dependent. More particularly, it is only effective during rapid changes in voltage. In this regard it should be appreciated that the problem is high voltages, not high rates of change in voltage.

OBJECT OF THE INVENTION

It is the object of the present invention to overcome or substantially ameliorate the above disadvantages.

SUMMARY OF THE INVENTION

There 1s disclosed herein a protective circuit for a

telecommunications system, the circuit comprising;

a pair of signal lines to be attached to the telecommunications system;

a gas filled protector operatively connected to each line and to an earth line means;

a first and second varistor, each varistor being connected to a respective signal line and to a further earth line means;

a first resistor in each signal line between said protector and the associated varistor; and

a second resistor in each signal line to connect each first resistor and its associated protector to the telecommunications system.

Preferably, there would be a plurality of said protective circuits, arranged in parallel. It is still further preferred that the earth line means are earth lines having a total cross-sectional area greater than the cross-sectional area of the signal lines, so that the signal lines will fail prior to the earth lines. It is still further preferred that the protective circuit is of a three or more layered construction, with the first layer consisting of the first resistors and the protectors, a further layer consisting of the earth line means, and the third layer consisting of the second resistors, with the third layer being adapted to be attached to the telecommunications system.

BRIEF DESCRIPTION OF THE DRAWING

A preferred form of the present invention will now describe by way of example with reference to the accompanying drawing, which schematically depicts a protective circuit for a telecommunications system.

BEST MODE AND OTHER EMBODIMENTS OF THE INVENTION

In the accompanying drawing, there is schematically depicted a protective circuit 10 for a telecommunications system. The circuit 10 includes a plurality of subcircuits 11, with the subcircuits 11 being arranged in parallel.

The circuit 10 includes signal lines 12 and 13 between which extends a gas filled protector 14. The gas filled protector has the object of clamping the voltage at 230 volts. Once the voltage rises to a certain level, the protector 14 starts to ionize and operatively connects the signal lines 12 and 13 to an earth line 15.

There is further provided in each line 12 and 13, resistors 15 and 16, with each resistor 15 being approximately 4.7 ohms, and each resistor 16 being 2.2 ohms.

Also extending between the lines 12 and 13 is a pair of varistors 17, which are preferably metal oxide varistors. The varistors attached to an earth line 18.

Preferably, the total sum of the cross-sectional areas of the earth lines 15 and 18 is greater than the cross-sectional area of each of the lines 12 and 13 so that the lines 12 and 13 will destruct prior to the earth lines 15 and 18.

In the abovedescribed circuit 10, the first stage of protection is provided by the varistors 17. They are effective for voltage surges above 140 volts. If the varistors 17 fail to clamp the voltage at 140 volts, the second stage of protection comes into operation. The second stage of protection 1s provided by the gas filled protectors 14 which act to clamp the voltage to 230 volts. If this second stage of protection fails, the "sacrificial" design in respect of the relative cross-sectional areas of the earth lines 15 and 18 relative to the lines 12 and 13 comes into operation. That is the lines 12 and 13 will fuse. Accordingly, any overwhelming strike on any one of the ten subcircuits 11, will "take out" the lines 12 and 13, leaving the untouched lines 12 and 13 still guarded. If all the lines 12 and 13 were struck at the same time, again the lines 12 and 13 would fail before the earth link.

A desirable construction for the circuit 10, is a multi-layered construction, including three or more layers. For example the first layer could include the protector 14, and resistors 17 and 19. The second layer would include the earth lines 15 and 18. The third layer would include the resistor 16.

Claims

1. A protective circuit for a telecommunications system, the circuit comprising;

a pair of signal lines to be attached to the telecommunications system;

2. The circuit of claim 1, wherein said circuit consists of a three more layered construction, with a first layer consisting of the first resistors and the protectors, and a further layer consisting of the earth lines, and a third layer consisting of the second resistors, with the third layer being adapted to be attached to the telecommunications system.

3. The circuit of claim 1 or 2, wherein the first and second varistors are metal oxide varistors.

4. A circuit protection device including a plurality of protective circuits according to claim 1, 2 or 3, wherein said circuits are arranged in parallel, and wherein the earth line means are earth lines, with the total cross sectional area of the earth lines being greater than the cross sectional area of the signal lines, so that the signal lines will fail prior to the earth lines.

5. A protective circuit for a telecommunications system,

substantially as hereinbefore described with reference to the accompanying drawing.