GB2186763A - Telephone interface protection circuit - Google Patents

Abstract

A protection circuit for a digital telephone exchange which is adapted to be connected in the line feed path of a subscriber's unit and having protective equipment in the form of a shunt for protection against overvoltages, and including transistor means (21, 33) for detecting the level of current in either of the rails (15, 16) of the line feed path, and FET transistors (37, 38) for blocking the passage of current should the current detected exceed a specified value, the transistors (37, 38) being switched by a monostable circuit (30). <IMAGE>

Description

SPECIFICATION Telephone interface protection circuit The present invention concerns protective arrange mentsfor use in telephone exchanges, and in particularfor use in digital telephone exchanges.

Each subscriber's line terminates at an exchange at an interface which includes a line card. It is already known to provide protection at the interface against environmental orfaultconditions in the line that produce overvoltagesthat might damage the line card or equipment connected to the line card.

Examples ofthese conditions are lightening strikes, contactwith power lines, and induction from fault currents in power lines. Such fault conditions are comparatively rare and it may be acceptable that the subscriber connected via a line subjected to such a fault loses service while a fuse or other protection component is replaced.

However there are other types of fault condition that cause excessive current to be drawn from the line feed power supply without causing an overvoltage. These faults can often occur during rewiring or routine maintenance and can be transitional. In particular a short circuit between wires ofthe subscriber's line, or a shortcircuitfrom one wire to earth orto another power supply may occur comparatively often and may be short-lived.

Accordingly the present invention has for an object to provide a protective arrangement which in the event of an overcurrent limitsthe currentwhich can be drawn whilst minimising power dissipation, and does not activate any overvoltage protective equipment.

Thus according to one aspect of the invention there is provided a protection circuit for a digital telephone exchange and adapted to be connected in the line feed path of a subscriber's line unit, the circuit including means for detecting the level of current in either of the rails of the line feed path, and means for blocking the passage of current in both line feed rails should the current detected in either rail exceed a specified value.

In orderthatthe present invention may be more readily understood an embodimentthereofwill now be described by way of example and with reference to the accompanying drawings in which: Figure lisa diagram showing a subscriber line and interface at an exchange, and Figure 2 is a circuit diagram of an overcurrent protection arrangement according to the invention.

Referring now to Figure 1 of the drawings, this shows a digital telephone 1 connected by a subscriber pair2to a local exchange generally indicated at3.

Only those elements required for an adequate understanding of the invention will be described. The line unit atwhich the subscriber pairterminates is also only generally indicated at 4 as the bulk of its components aretotally conventional.

As shown inthe drawing the line unit4 includes a shunt which is intended to protectthe exchange equipment against overvoltages of the kind already referred to in this specification. In the presence of an overvoltage the device conducts to earth, and if the overvoltage is sustained the fuse will blow. Such protection equipment would conventionally inciude a pairofelectrodes in a gas-filled enclosure and responding to a break-down voltage to strike an arc so astodiverttheovervoltagefromthe load. Also included are a pair of fuses 6 and 7 connected between the line and the line feed power supply via windings 8, 9 of a transformer 10.Powerforthe subscriber line is provided at 11 via an overcurrent protection unit 12 which will be described more fully with reference to Figure 2 of the drawings.

The protection circuit arrangement shown in Figure 2 is a thick-fil m hybrid circuit and is designed to give protection to -75 V line fuses such as fuses 6,7. It operates by interrupting the line current to the fuses 6, 7 when the value of the line current rises to a threshold value of 25 mA +30%.

In the event of an overcurrent being detected currentflow in both legs of the line is interrupted for a period of 300 ms after which interval line feed is reapplied. If the fault which caused the overcurrent is still present the circuit will interruptthe current flow again so that in the case of a persistentfaultthe sequence ofsensing and interruption will be repeated continuously.

The circuit basically comprises two rails 15, 16 extending between the line feed supply side and 19,20 on the load side. The sensing elements of the circuit are two transistors 21 and 33 which are turned on by voltage drops across resistors 23,24 respectively.

Associated timing components in the form of resistors 25,26 and capacitors 27,28 provide a delay. This delay is very important for the successful operation of the circuit. The output load on the ciruit on start-up includes shunt capacitances due to the line and to the input stages ofthe remote Line Transmission Ter mination. In addition the Line Transmission Termination includes a DC-DC converter which requires high transient currents on start-up. Thus when line feed is initiated, or reapplied after a fault condition, the total load on the circuit may resemble a short circuit. These normal conditions would result in spurious operation of the protection circuit, that is line feed would be interrupted immediately after it was reapplied even in the absence of a fault condition.Thus the timing components 25-28 are arranged to provide a delay of 10-20ms.

On detection of a fault condition a monostable 30 with associated timing components in the form of a capacitor 31 and resistor 32 provide a 300 ms period.

Overcurrent in either of rails 15 or 16 will cause transistors 21 or 33 or both to switch. Transistor 33 acts directly as an input to pin 5 of monostable 30, whilst if transistor 21 switches it acts indirectly as a monostable input, causing transistor 22 to conduct via the diode 34 and resistors 35,36.

The output of monostable 30 is followed by monostable 70 and is taken to control the action of a pair of series-pass FET's 37,38. In the quiescent state pin 7 is high so that FET38 is turned on via resistor39.

In this state transistor 40 is also turned on so that FET 37 is turned on viathe potential divider 41,42.

Transistor 43 and resistor 50 act to limitthe current drawn. Transistor 53 and resistor 41 provide the same function in the other leg. Thus line current can pass through the circuitto pins 19,20.

lfeithertransistors21, 33 isturned on, due to overcurrent in the earth or -75V legs respectively, monostable30 istriggered so that pin 7 goes low for 300 ms. This action turns off FET's 37,38. When the outputtransistorswitches 37,38 are cut-off the excessive line current is interrupted for the period of monostable30.Attheend ofthistime monostable30 resets and line current is restored. If it is still excessive thewhole process repeats until thefaultis cleared.

The period of retriggerable monostable 70 is longer than that of monostable 30. Its output Qtherefore remains high forthe whole period of the fault condition. This high output is supplied via the junction between series resistors 80,81 connected to the base of a transistor 82 providing an inputto a Schmidttrigger gate 83. Gate 83 is a NAND-gate acting as an inverter and as a result a logic high appears at overcurentalarm output 84.

When the monostable30 returnsto its previous state and pin 7 goes hightheturn-on rate of the FET's 37 and 38 is limited by the high gate impedance. This minimises interference to external circuitry.

Additional features of the overcu rrent protection ciruict are as follows. Source resistors 23,44 and 24, 43 provide a constant current limiting action to 150 mA in each leg. During the sensing delay period of 10-20 ms this current limit preventscircuit damage. The output diodes 45,46 block reverse voltages which might be applied two the line duringfault conditions.

The monostable30, in this embodiment, operates from 1 5\( supplies situated between -60V and -75V.

The - 60V rail is provided by a diode 47, resistor 48 and a transistor 51 decoupled by capacitors 49,52.

The circuit also includes means for detecting the presence ofan undercurrent in either of rails 15,16.

This is an important indication to an engineer as to the nature of a fault. The undercurrent detection means comprise a resistor 60. If there is insufficient voltage drop across this resistorto supply base drive to a transistor 61 dueto an undercurrent situation, then the collector of transition 61 goes open circuit. In such a situation base current is supplied via a resistor 63 to transistor 62so that the latter is saturated. This results in a logic low at a Schmidt-trigger gate 64. Gate is a NAND-gate being used as an inverter. As a result a logic high appears at an undercurrent alarm output 80.

This arrangement is similar to that used for overcurrent alarm.

Other elements of importance in Figure 2 are a pair of diodes 90,91 associated with the alarm logic gates which prevent the inputs to the logic gates 64,83. Also in the undercurrent detection circuit diodes 92,93 limit the voltage drop across resistor 60 to reduce potential power loss.

It will be seen thatthe circuitjust described is simple yetwill provide protection to the fuses 6 and 7 in overcurrent situations. Thus minor fau Its of the type which can occur during routine maintenance can mean that a subscriber is subjected to only a minor inconvenience rather than having the line cut offfor a considerable length of time whilst blown fuses are found and replaced.

The device just described has a number of important advantages. Thus curent drawn from the line feed power supply is at all times limited to be less than a specified value. This ensures that a fault on any single line circuit cannot trip outthe powersupplyfor a group of line circuits. Thus protection is provided against a single line fault taking a group of subscribers out of service. Provision of extra power capacity from the line feed power supply to caterforfault conditions is therefore not now necessary, saving cost. Additionallythe device is not a current limit circuit, and therefore dissipation is minimised in the event of a fault condition. (Service cannot be maintained and therefore it is acceptable to cut the line feed supply.) Finally, the device is located on the line circuit, thereby providing distributed current control. This has the advantage of simplifying the line feed power supply design and minimising wiring between the supply and the line cards.

Claims (9)

1. A protection circuit for a digital telephone exchangewhich is adapted to be connected in the line feed path of a subscriber's unit and having protective equipment in the form of a shuntfor protection againstovervoltages, and including meansfordetect- ing the level ofcurrent in either ofthe rails of the line feed path, and means for blocking the passage of current should the current detected exceed a specified value.

2. A protection circuit as claimed in Claim 1, wherein the detecting means comprise a pair of transistors each associated with a respective one of the rails and also associated with timing components which provide a delay so as to prevent spurious operation ofthe protection circuit.

3. A protection circuit as claimed in Claim 2 and further including a first monostable which is switched from its stable state when one orthe other ofthe transistors detects an overvoltage in its associated rail.

4. A protection circuit as claimed in Claim 3, and wherein the monostable is associated with the timing components.

5. A protection circuit as claimed in Claim 3 or Claim 4, wherein the output ofthe monostable is connected to a pair of FETtransistors in rails respectively so that on triggering of the monostable both transistors are turned off for a period determined by said timing components.

6. A protection circuit as claimed in Claim 3 or either of Claims 4 or 5 when dependent on Claim 3, and including alarm means forgiving an indication that an overcurrent condition has been detected.

7. A protection circuit as claimed in Claim 6, wherein said alarm means comprise a second monostable having a longer period than said first monostable and connected to said first monostable so as to be switched when the latter is switched, an output of said second monostable being connected to indicating means.

8. A protection circuit as claimed in Claim 6, and including means for detecting the occurrence of an undercurrent

9. A protection circuit substantially as hereinbefore described with reference to the accompanying drawings.