GB2119187A - Telecommunication exchange protection - Google Patents

Abstract

Telephone line circuit protection is made somewhat difficult with electronics in the line circuit since the electronic devices may blow more quickly than the fuses. To avoid this each line wire is connected to a threshold detection device (X1, X2), which responds when its line wire voltage is out of specification. Such a response is used to fire thyristors or triacs (TR1, TR2) to ground both the line wires and thus to blow one or both of the fuses (F1 or F2). To ensure that a permitted out of specification voltage, such as ringing does not erroneously cause fuse blowing further detection devices (X3-X6) are connected across the line wires and in series with one line wire. These detect whether the voltage comes from the exchange side, in which case it is OK, or from the line side, in which case it is wrong. If it is OK a logic unit LU, via which the thyristors or triacs are fired, is blocked. Thus the fuses are only blown in response to a genuine fault. <IMAGE>

Description

SPECIFICATION Telecommunication exchange protection This invention relates to the protection of telephone exchanges against fault voltages on subscriber or junction wires.

Certin fuses that have been used as fault protection devices at the line circuits or junction circuits may in certain cases pass a current which peaks at as much as 375 mA, or a continuous current of more than 350 mA. While this is relatively harmless with the conventional electro-mechanical exchanges it could be harmful with more modern exchanges where electronic devices are connected to the line wires. A further difficulty which may arise is that the fuses installed to protect electronic equipment may take longer to blow in a fault condition than that equipment. This could clearly have distressing consequences, and there could well be a fire hazard. Thus a system of detecting excess voltages is needed in which the fuses can be relied on to blow, or cause some other safety device to trip.In producing such safety devices it is desirable to ensure that the ringing voltage, which is a relatively large alternating voltage, does not blow the fuse or trip the safety device. Hence an object of the invention is to provide such a detection and protection arrangement.

According to the invention, there is provided a circuit arrangement for protecting a.

telephone line circuit or junction circuit against damage due to improper voltage conditions on the line wires, which includes a plurality of threshold detection devices each connected to one of the line or junction circuit wires and arranged to give an output when the voltage condition of that wire moves out of specification, a logic circuit means to which the output of said threshold detection devices are connected and which gives an output in response to a said output from at least one of said threshold detection devices, and normally non-conductive triacs or thryristors each of which is connected between one of the wires and a reference potential such as ground, wherein when a said threshold detection device detects an output of specification voltage it causes said logic circuit means to give an output to switch the triacs or thyristors on, thus connecting the wires to the said reference potential.

According to the invention, there is further provided a circuit arrangement for protecting a telephone line circuit against damage due to improper voltage conditions on the line wires, which includes a fuse connected in each said line wire, two threshold detection devices each connected to one of the line wires and arranged to give an output when the voltage conditions of that wire moves out of specification, further threshold detection devices connected between the line wires and in series with one of the line wires and responsive to the current and voltage conditions present on those wires to indicate whether a voltage condition came from the exchange side or the line side, logic circuit means to which the outputs of said threshold detection devices are connected and which gives an output in response to the detection of an improper voltage condition by one or both of said first mentioned threshold devices, which output is inhibited by the logic circuit means if the response of said further threshold detection devices indicate that the voltage condition, although out of specification, is not improper, e.g. it is a ringing signal, and normally nonconductive triacs or thyristors each of which is connected between one of the line wires and a reference potential such as ground, wherein when one of said first-mentioned threshold detection devices responds to an out of specification voltage and that response is not inhibited by the response of any one of the further threshold detection devices said logic circuit means gives an output which switches the triacs or thyristors on so as to connect the line wires to said reference potential, so as to blow one or more of said fuses.

The accompanying drawing shows how this invention is applied to a telephone subscriber's line circuit, only the line wires and the protection circuitry being shown.

The arrangement shown includes a number of logic circuit devices X1 to X6, each of which has two inputs a and b and two outputs C and d. Each such logic device will only pass current in one direction, as indicated by the arrow between the a and b terminals.

These terminals a and b are totally common mode isolated from the output connections c and d. The inter-relation between the terminals of the device is such that if a is positive to b, then the output is true, i.e. an effective output signal is developed at its terminal c.

Such a device, which is used as, and hence herein referred to as, a threshold detection device, is highly sensitive and has a low resistance between its a and b terminals. In addition it is fast in operation. Such a device can take several forms, including an optocoupler based device, and certain read only memory elements. The latter arrangement especially has the advantage that it is readily implementable in integrated circuit form.

The circuit shown has the usual fuses F1 and F2 connected respectively in the line wires A and B. The threshold detection devices X1 to X6 are connected as shown to respond to various excessive voltage conditions. Thus X1 is connected to the wires A and B via the diodes D1 and D2, and responds if the B wire voltage moves negative to - 50 volts. In a similar way, X2 is connected to the wires A and B via diodes D1 and D2 so as to respond when the A wire move positive of the sys tem's zero voltage level. As will be seen later, the response of X1 to X2 may in certain cases be inhibited by the responses of X3-X6.

X3 and X4 are connected in the B wire, X5 and X6 are connected across the line to respond by polarity comparisons to a voltage on the B wire with respect to the A wire, as will be explained later.

Depending on the original fault condition, either F1, F2, F1 and F2, or neither F1 nor F2 may blow. Following such a fault, either or both of the triacs (or thyristors) TR1 or TR2 may latch on, holding the A or/and B wires to earth, dependent on current from the exchange. With certain fault conditions, e.g.

230 volts from - 660 ohms or more, there is a risk that neither fuse blows. If the fault voltage in an alternating voltage, and/or conditions are such that the triac or thryristor is operating below its holding current it may be necessary to provide a latch for the triac or thyristor firing circuit, and to ensure that under fault conditions the device TR 1 or TR2 is continuously fired means (not shown) would be provided to rest such a latch.

The c and d outputs from the threshold detection devices X1 to X6 are connected to a logic unit LU, which produces an output if the devices X1 to X6 detects a fault condition. As will be seen later, the main fault detection function is done by X1 and X2, but the responses of X3-X6 may be such as to inhibit that of X1 or X2. Such inhibit operation can be done readily in the unit LU. The output of LU is applied to a firing circuit FC, which fires the two triacs or thyristors TR1 and TR2, which devices when fired each grounds its one of the line wires. Hence any electronic devices connected to the line wires are protected, whether or not the fuses blow.

It will be appreciated that identification of which threshold detection device responds can be used to give some indication of the probable nature of the fault.

Devices X1 and X2 detect if the voltage on the A and/or B wires goes outside the defined limits. Thus the outputs of X1 and X2 are "false" if the A and B wire voltages are in the range 0 to - 50 volts. At some A or B wire voltage positive with respect to 0 volts, the output of X2 goes "true", while at some A or B wire voltage negative to - 50 volts the output of X1 goes "true". Either of these conditions indicates that there may be a fault voltage on the A or B wire, and it it is in fact a fault causes LU to fire TR1 and TR2 via FC to ground the line wires. Resistors R1 and R2 are included to ensure that the ringing signal source will not be excessively loaded. Such grounding of the line wires blows at least one of the fuses.

Devices X3, X4, X5 and X6 detect, by polarity comparison, if a voltage appearing on the B wire with respect to the A wire comes from the subscriber side in which case it is most likely to be a fault voltage, or from the exchange side, in which case it is a permutted voltage, e.g. ringing. Thus assume that the B wire goes positive to the A wire and that the voltage is applied from the exchange side, e.g. ringing. Current then passes through X5 and X3 in parallel, so that they each give a "true" output. However, if the same voltage were applied from the subscriber side, it would "see" X3 and X5 in series, with the result that the voltage would not pass through X3 but would pass through X4 and X6. Thus the fact that in one case X3 responds but in the other case it does not, would define the origin of that voltage.X4 and X6 perform an equivalent function for a voltage of the other voltage conditions.

There are only two combinations of X3-X6 outputs which are allowed to inhibit an overvoltage condition detected by X1 or X2, and this inhibit must operate when a subscriber is being rung. Thus if X3 output is false, X4 is true, X5 is false and X6 is true, with the B wire negative to the A wire, then it is assumed that the exchange is ringing. Similarly if X3 is true, X4 is false, X5 is true and X6 is false, with the B wire positive to the A wire, it is assumed that the exchange is ringing.

Thus if either X1 or X2 (or both) produce an output which is not inhibited by the logic unit LU as a result of the outputs of any of X3-X6, then both TR1 and TR2 are fired via FC, which grounds both the A and the B wires and thus short-circuits them both to ground.

This so-called "crowbar" action causes F1 and/or F2 to blow. Hence the overall effect of a fault, if it is not adequate to blow the fuses, is to augment its effects on the fuses such that at least one fuse blows.

Claims (3)

1. A circuit arrangement for protecting a telephone line circuit or junction circuit against damage due to improper voltage conditions on the line wires, which includes a pluraity of threshold detection devices each connected to one of the line or junction circuit wires and arranged to give an output when the voltage condition of that wire moves out of specification, a logic circuit means to which the output of said threshold detection devices are connected and which gives an output in response to a said output from at least one of said threshold detection devices, and normally non-conductive triacs or thyristors each of which is connected between one of the wires and a reference potential such as ground, wherein when a said threshold detection device detects an output of specification voltage it cases said logic circuit means to give an output to swich the triacs or thyristors on, thus connecting the wires to the said reference potential.

2. A circuit arrangement for protecting a telephone line circuit against damage due to improper volage conditions on the line wires, which includes a fuse connected in each said line wire, two threshold detection devices each connected to one of the line wires and arranged to give an output when the voltage conditions of that wire moves out of specification, further threshold detection devices connected between the line wires and in series with one of the line wires and responsive to the current and voltage conditions present on those wires to indicate whether a voltage condition came from the exchange side or the line side, logic circuit means to which the outputs of said threshold detection devices are connected and which gives an output in response to the detection of an improper voltage condition by one or both of said first mentioned threshold devices, which output is inhibited by the logic circuit means if the response of said further threshold detection devices indicate that the voltage condition, although out of specification, is not improper, e.g. it is a ringing signal, and normally nonconductive triacs or thyristors each of which is connected between one of the line wires and a reference potential such as ground, wherein when one of said first-mentioned threshold detection devices responds to an out of specification voltage and that response is not inhibited by the response of any one of the further threshold detection devices said logic circuit means gives an output which switches the triacs or thyristors on so as to connect the line wires to said reference potential, so as to blow one or more of said fuses.

3. A circuit arrangement for protecting a telephone line circuit, substantially as described with reference to the accompanying drawings.