WO2005074241A1

WO2005074241A1 - Overvoltage protection in telephone/data lines, suitable for installations with low-quality earth connections

Info

Publication number: WO2005074241A1
Application number: PCT/GB2005/000252
Authority: WO
Inventors: Walter Gustavo Gutnisky
Original assignee: Tyco Electronics Argentina Sa; Tyco Electronics Uk Ltd
Priority date: 2004-01-29
Filing date: 2005-01-25
Publication date: 2005-08-11
Also published as: AR042973A1; BRPI0507310A; EP1709786A1; MXPA06008534A; PE20060181A1

Abstract

A protection device (10) uses a single-coil (15) latch relay (11) to decouple a telephone and/or data installation from DC transmission lines (12) in the event of a line AC/overvoltage disturbance. A trip circuit (19) and a reset circuit (21) are respectively connected to normally-closed (OUTA1, OUTB1) and normally-open (OUTA2, OUTB2) terminals of the relay to monitor the line voltage using an earth connection as a reference voltage, supplying respectively trip (23) or reset (25) currents to the coil for switching the relay. Hence, the protection device may operatively tolerate an inexpensive high-resistance earth connection. The reset current is supplied to the relay coil by a transistor circuit (Q1, Q2) only after the disturbance has ceased, the nominal transmission voltage has been restored and the line has stabilized for some seconds.

Description

OVERVOLTAGE PROTECTION IN TELEPHONE/DATA LINES, SUITABLE FOR INSTALLATIONS WITH LOW-QUALITY EARTH CONNECTIONS

FIELD OF THE INVENTION: The present invention is generally related to telecommunications and particularly refers to protecting electrical installations and persons from disturbances and accidents caused by overvoltage and other undesirable electrical conditions in voice and/or data transmission lines. More specifically, the invention concerns a method and a circuit for protecting telephone or data equipment and installations against electrical line disturbances introduced either by fault or induction, through the service wires extending from the power mains supply post to the premises where the installations or equipment is located, as well as from direct current (DC) overvoltage. Voltage on a telephone line is nominally direct current 48 volts negative against earth (-48VDC). This is a safe value in terms of human safety. However, many telephone equipment is powered from the AC mains supply (220 VAC) or is frequently close to such high-voltage power lines thereby exposing the telephone lines to undue voltages which may be dangerous to people or affect in- and outdoor installations or miscellaneous equipment installed by data and telephone service providers both at subscriber homes as well as company premises.

SUMMARY OF THE PRIOR ART: Examples of such protector systems may be found in U.S. patents 4,661,878, 5,623,388 and 6,104,591. U.S. patent 4,661,878 discloses an overvoltage protection circuit for directing a surge current on a telephone line to a ground terminal when the surge current exceeds a predetermined level. U.S. patent 5,623,388 discloses an overvoltage protection circuit using thyristor devices to provide respective current paths from tip and ring terminals of a conventional telephone communication line to ground. U.S. patent 6,104,591 discloses a failsafe module that mechanically connects either the tip or ring telephone line conductors to ground in response to an overcurrent, thereby shunting the overcurrent power to ground. The common denominator in the above patents as well as in conventional overvoltage and overcurrent protector devices in general is that, for proper operation, they require a low- resistance earth connection so as to be able to shunt high-power electrical disturbances to earth. Otherwise, such conventional protector devices may not operate in a proper and predictable manner or not at all. A proper earth connection should have a resistance of not more than about 6 Ω. However, an earth connection of such quality is frequently not available or is costly to install. A typical earth installation may require a 3 mm² cable connected to a javelin between 1-5 and 3 meters long depending on electrical conductivity of the soil. As a result, in practice many installations have inadequate or no protection at all, resulting in damage to the installations or equipment and, in some cases, death by electrocution. Another drawback in some types of protector devices is that they may not automatically reset once the reason they tripped has ceased. SUMMARY OF THE INVENTION: An object of the invention is to protect installations, equipment and persons from undesirable electric signals such as, e.g., AC voltages in excess of predetermined magnitudes, on electrical telecommunications lines even when the resistance of the earth connection is high or non-existent. Another object of the invention is to provide a ground connection at a lower cost compared to other, conventional protection systems. To achieve the foregoing and other objects, the protection device of the invention uses the ground connection as a reference to determine the presence of undesirable electrical disturbances and simply opens the circuit to isolate such disturbances, in contrast to conventional systems that drain the excess power to earth. As soon as the undesirable condition is detected, the electrical circuit between the disturbed line and the installation under protection is opened. Another feature in a preferred embodiment is the use of a latch relay having a single coil which is tripped during the presence of an undesirable condition and is not reset until the return of preset conditions to automatically reconnect the installation to the line.

BRIEF DESCRIPTION OF THE DRAWINGS: The invention and the manner in which it may be implemented will be hereinafter described with reference to the accompanying drawing wherein: Figure 1 is a circuit schematic of the protection device according to a preferred embodiment of the present invention. Figure 2 is a graph illustrating the response of the protection device as a function of the input voltage. Figure 3 is a graph illustrating the switching of the protection device as a function of time.

DETAILED DESCRIPTION OF A PREFERED EMBODIMENT: A protector circuit 10 according to the present invention is illustrated in figure 1. The circuit 10 comprises a latch relay 11 having two contacts defining a pair of fixed terminals INI and IN2 which are connected to the incoming telephone line 12, usually the service connection pair coming from a telephone exchange (not illustrated). Line 12 can be a telephone cable containing tip and ring conductors transmitting signals at a voltage of -48 VDC (i.e. negative polarity relative to earth). The output of the relay 11 includes a pair of normally-closed terminals OUTAl and OUTA2 which are connected to the telephone or data- processor installation or equipment (not illustrated) to be protected. The installation may have an earth connection (not illustrated) having a high resistance, sometimes as much as several hundred ohms as will be described hereinafter, which is connected to the ground 13 of the circuit protector 10. The relay terminals are labeled with "IN" and "OUT" prefixes merely for convenience sake; it is understood that signals may be transmitted through the relay in either direction, i.e. from the line 12 to the equipment of the load installation and vice versa. "Ground" is defined as reference voltage for the protector circuit, usually a chassis potential or the like; "earth" is an external 0-volt reference for the line 12. A single coil 15 switches the contacts of the relay 11 between the pair of normally- closed terminals OUTAl and OUTA2 and a pair of normally-open terminals OUTBl and OUTB2. Protection is triggered by tripping the relay 11 from the normally-closed state to the normally-open state when the coil is driven by a predetermined positive voltage at its terminal 17 not connected to ground whereas a predetermined negative voltage at said coil terminal 17 causes switching in the opposite direction, that is from the normally-open state to the normally-closed state. A capacitor C4 connected in parallel to the coil 15 of the relay 11 prevents oscillations. The protector circuit 10 further comprises: a tripper or decoupler subcircuit 19 for detecting the presence of disturbances having a polarity (sign) opposite to the nominal (negative) line polarity, such as undesirable AC semicycles, and a magnitude substantially higher than the nominal line value, such as an overvoltage surge; and a reset subcircuit 21 for detecting both when such disturbances come to an end and that the line voltage on the telephone cable 12 is back to nominal DC parameters. In normal operating conditions, that is, when no undesirable signals are present, and having detected normal voltage conditions (-48 VDC) on the telephone line, the signals at terminals INA and INB are present through the relay 11 at the outputs OUTAl and OUTBl thereof connected to the installation under protection. Referring to the tripper subcircuit 19, the signals OUTAl and OUTBl are sensed through capacitors Cl and C2 which decouple DC components. The diodes D3 and D4 select the positive semicycle relative to ground 13. The resistors Rl, R2 and R3 limit the current through the relay 11. As soon as an AC signal (e.g. 220 VAC) is detected over a certain trip threshold, the relay 11 switches over, opening the circuit at the outputs OUTAl and OUTA2. This switching event takes place in response to a positive voltage relative to ground 13 of a suitable value (e.g. 170 volts) in the coil 15 of the relay 11. The arrow 23 indicates the path of the trip current for the relay 11. The diodes Dl and D2 discharge the capacitors Cl and C2 during the negative semicycle relative to ground 13. After the relay 11 has switched over, the signals at the line terminals INI and IN2 are passed through to the output terminals OUTBl and OUTB2 which are thereafter sensed by the reset circuit 21 for determining when the preset conditions for reconnecting the output terminals OUTAl and OUTA2 are re-established. In this embodiment, such preset conditions are the absence of an AC signal above the threshold and the presence again of normal telephone-line and data-line DC parameters 12 on the line. In other words, switchback or resetting the connection of the installation takes place in this embodiment when there is a suitable voltage, negative relative to ground 13 at the relay-coil terminal 17. The reset circuit 21 which detects this condition comprises transistors Ql and Q2. The diodes D6 and D7 are for operating with the negative AC semicycles and with negative DC values, as measured relative to ground. The transistor Q2 is arranged to respond to the restoration of nominal DC parameters on the line 12 so as to drive the coil 15 negatively for thereby resetting the normal state of the relay 11, unless the transistor Ql biasing its base is saturated because of the persistence of AC on the line, thereby clamping the driver transistor Q2 in the cut-off state. In order to prevent transient states, the capacitor C5 forces the clamping transistor Ql to conduct during switching transients and the appearance of stray disturbances. The array formed by the resistors R5, R6 and R7 and the diode D5 preset the thresholds during which the transistor Ql conducts (as further explained hereinafter) and limits the base current thereof. The resistor R4 and the capacitor C3 set a time-delay for the operation of the transistor Q2 in order to prevent short-term switching, such as due e.g. to brief relaxing of disturbances. If this prevention is not provided, the resulting intermittent oscillations or tripping-resetting could damage protected equipment components and cause personal injury. The clamping transistor Ql stops conducting when the disturbance conditions disappear. Nevertheless, if no negative voltage is restored on the line 12, the transistor Q2 will not reset the relay 11. If the DC voltage is present at least during a time-interval set by the resistor R4, the capacitor C3 and the diodes D8, D9 and D10, the transistor Q2 conducts and drives the relay 11 with a suitable voltage to reset the circuit to its normal condition. The arrow 25 shows the path of the current resetting the relay 11. A resistor R8 limits the current at the base of the transistor Q2 and contributes to set the length of the time delay. The graphs in figures 2 and 3 represent the switching state of the relay 11 as respective functions of the input voltage variations and of time (in seconds). The graphs show that protector tripping action is triggered by a positive excursion of about 170 volt and is much faster, in the order of 20 milliseconds, than the action resetting the circuit (or reconnecting the installation) when the line voltage stabilizes around -48 volt after several seconds, e.g. 25 seconds. In short, the invention operates according to a simple and safe method essentially consisting in sensing the rectified voltage in a nominally-DC line in order to switch the relay 11 to its load-tripping position in response to the detection of overvoltage relative to a predetermined threshold. The relay 11 is thus switched by means of a sufficiently positive voltage at the input terminal 17 of the coil 15. With the relay 11 in its tripped position, the method of the invention proceeds by detecting the voltages of opposite polarity, since the coil 15 reverses the movement of the relay 11 in response to a DC current of a polarity opposite (in this case negative polarity) to that tripping the relay 11, which is drained by the input terminal 17 of the coil 15 by means of the power transistor Q2. The method first waits for the undesirable overvoltage to cease. This occurs when the transistor Ql becomes saturated by the negative semicycles of the AC overvoltage to cut off the base-emitter junction of the transistor Q2 to keep it in the cut-off state. After detecting that the undesirable overvoltage has ceased during a predetermined time- interval of several seconds set by the capacitor C3, the transistor Q2 becomes free to follow the negative voltage of the line until it reaches the nominal voltage of -48 volt to reset the relay and reconnect the load installation at the normally-closed output OUTAl, OUTBl back again to the beginning of the cycle. Hence, no earth connection is required for shunting high fault current intensities, but merely for providing a ground reference voltage, thereby draining relatively very low current intensities. For this reason, the protector device of the invention may operatively tolerate earth connections having a relatively high resistance, as much as 600 Ω. Protection is thus provided by simply opening the relay switch 11 to stop the electrical disturbance advancing towards the installation loading the line 12 at the normally-closed relay terminals OUTAl and OUTBl. As a result, if no earth connection is readily available, an inexpensive earth connection may be set up with a standard small-section cable, e.g. 5 mm , connected to a metal pipe or a short, say 20-cm long, electrode or javelin. A prototype of the preferred embodiment was designed with the following components: Part Value Part Value Cl : 0-63μF/400 Volts. C2: 0-63μF/400 Volts C3: 470μF/10 Volts. C4: O-lμF/50 Volts. C5: Adjusted during calibration. D1-D7: 1N4007 D8-D10: 1N4148 Ql : BC549 / 2N2222 / MPSA42 Q2: MPSA42 Rl: 2k2 R2: 56K R3: 56K R4: 100K R5: 330K R7: 2K2 R6 2K R8: 22K Relay 11: R-FP2 The tripper and reset thresholds are set by a suitable selection of component combinations. In the present embodiment, the trigger threshold is predetermined at 170 VCA with the values for Cl, C2, Rl, R2 y R3 indicated hereinabove. For the reset circuit 21, the combination of values of the resistors R5, R6 y R7 set the threshold for the transistor Ql to conduct to inhibit reconnection. In this embodiment, the threshold for transistor Ql to conduct is 120 V. The transistor Ql stops conducting as soon as the disturbance ceases, enabling conduction of the transistor Q2 to reset relay 11. The values given hereinabove for the resistors R4 and R9, in combination with the three series diodes D8, D9 y D10, set a threshold of -30 V for transistor Q2 to start conducting and reset the relay 11. It should be noted that the invention has been described, by way of example specifically for -48-volt telephone lines; however, those skilled in the art may envision other applications without departing from the general principles of the invention. The above description and accompanying drawings are intended to explain in a non- limiting manner the nature, objects and advantages of the invention. Those skilled in the art will realize that changes and/or variations may be made in the embodiment of the invention shown and described, insofar as such changes and/or variations are within the scope and spirit of the appended claims.

Claims

1. A device for protecting a DC installation from undesirable overvoltage and AC signals transmitted on a transmission line to which said installation is connected as a load for transmission of DC signals of a nominal voltage and polarity, said protecting device including a ground for electrical connection to earth and characterized by further comprising: (a) electrical-disturbance detector means for detecting the signal voltage on said line relative to said ground and for producing a fault output signal in response to the detected voltage being in excess of a predetermined trip threshold or of a polarity opposite to said nominal polarity; and

(b) tripping means for disconnecting said installation from said line in response to said fault signal.

2. A device according to claim 1, characterized in that said line is a telephone and/or data-transmission line and said DC installation comprises telecommunications equipment.

3. A device according to claim 1, characterized in that said electrical earth connection has a finite high resistance.

4. A device for protecting a telecommunications installation from undesirable overvoltage and AC signals transmitted on a telephone and/or data-transmission line to which said installation is connected as a load for the transmission of DC signals of a nominal voltage and polarity, said protecting device including a ground for electrical connection to earth and characterized in that the electrical earth has a finite high-resistance and is connectable to said line to provide a reference for determining the line voltage and in that said protecting device further includes means for decoupling said installation from said line in response to said undesirable overvoltage and AC signals on said line.

5. A device according to claim 3 or 4, characterized in that said electrical earth connection resistance is less than 600 ohms.

6. A device according to claim 3 or 4, characterized in that said electrical earth connection resistance is over 6 ohms.

7. A device according to claim 1, characterized by further comprising (c) resetting means for reconnecting said installation to said line in response to said fault signal being absent during a predetermined time interval.

8. A device according to claim 7, characterized in that said resetting means further includes normal-condition detector means for detecting the signal voltage on said line to thereby produce a normal output signal in response to the line signal having a voltage, relative to said ground, of said nominal polarity and not exceeding a predetermined reset

5 threshold, said predetermined reset threshold not exceeding said predetermined trip threshold, whereby said resetting means reconnects said installation to said line only in response to said normal output signal being present again after said fault signal has been absent during a predetermined time interval.

9. A device according to claim 2, characterized in that said tripping means includes relay 0 means for connecting said installation to said line, said relay means including normally- closed terminals connected to said installation, normally-open terminals and coil means for switching said relay means between said terminals in response to said fault signal.

10. A device according to claim 9, said telephone and/or data-transmission line comprising two line conductors for transmitting said electric telephone and/or data signals at said5 predetermined nominal direct-current voltage, characterized in that said device further includes first rectifiers connected with a like polarity to each of said relay normally-closed terminals; a tripper circuit arranged in series with said first rectifiers and with said relay coil for0 energizing said coil with trip current in response to overvoltage in said line to switch said relay to the normally-open terminals thereof; second rectifiers connected with a like polarity to each of said relay normally-open terminals, said first and second rectifiers arranged with a opposite polarities; and a reset circuit arranged in series with said second rectifiers and with said relay coil for 5 energizing said coil with reset current to switch said relay to the normally-closed terminals thereof in response to said overvoltage ceasing in said line and the voltage on said line substantially reverting to the predetermined nominal DC voltage.

11. A device according to claim 10, characterized in that said tripper circuit includes capacitors for decoupling the nominal DC voltage on said line conductors, each o capacitor connected in series with the respective first rectifier.

12. A device according to claim 10, characterized in that said tripper circuit further includes means for limiting current through said coil, said current limiting means adjusted for tripping said relay at said predetermined trip threshold.

13. A device according to claim 10, characterized in that said reset circuit comprises: a first transistor having an input connected to said second rectifiers and biased in order for said first transistor to saturate and thereby conduct said reset current through said coil in response to predetermined nominal DC voltage on said line conductors, and a second transistor having an input also connected to said second rectifiers and biased in order for said second transistor to saturate in response to overvoltage on said line conductors, said second transistor having an output connected through a time-delay circuit to said first transistor, whereby the saturation of the second transistor clamps said first transistor in the cut-off state, thereby inhibiting reset of said relay.

14. A device according to claim 10, characterized in that said coil is connected between ground and a circuit node common to both said tripper and reset circuits and said trip and reset currents pass through said coil in opposite directions.

15. A device according to claim 9, characterized in that said relay means comprises a latch relay having a single coil for switching said relay between said outputs in both directions by means of coil currents of opposite polarity.

16. A device according to claim 1, characterized in that said predetermined DC voltage is adjusted to a nominal transmission voltage of a telephone exchange to which said line is connected.

17. A device according to claim 16, characterized in that said predetermined nominal DC voltage on the line is about -48 volt and said overvoltage trip threshold is about 170 volt.

18. A method for protecting a telecommunications installation from undesirable overvoltage and AC signals on a telephone/data line to which said installation is connected, comprising the steps of: (a) providing an electrical connection to earth as a reference voltage; (b) monitoring voltage on said line relative to said reference voltage; (c) detecting a disturbance condition in the monitored voltage consisting of at least one of (i) the magnitude of said monitored voltage exceeding a predetermined trip threshold and (ϋ) the polarity of said monitored voltage being opposite to a predetermined nominal polarity of said line;

(d) preventing the detected fault condition from reaching said installation, characterized by electrically disconnecting said installation from said line; and (e) reconnecting said installation to said line only in response to said monitored line voltage becoming restored to a magnitude below a predetermined reset hreshold having said nominal polarity.

19. A method according to claim 18 wherein said line has two conductors, characterized in that both said conductors are individually monitored relative to said earth connection and said installation is disconnected from both said lines in response to detecting one of said fault conditions in at least one of said lines.

20. A method according to the claim 18, characterized in that said earth connection has a relatively high resistance.

21. A method according to the claim 18, characterized in that the magnitude of said predetermined trip threshold is around 170 volts, said predetermined reset threshold is around 120 volts and the nominal line voltage is -48 volts.

22. A method according to the claim 18, characterized by comprising the steps of: (a) inserting a relay in said line, said relay having a normally-closed output for connecting said installation thereto, a normally-open output and coil means for switching said relay between said outputs; (b) rectifying a line signal present at said normally-closed output and applying the rectified signal to said coil to switch said relay, thereby disconnecting said normally-closed output and connecting said normally-open output to said line in response to the voltage at said normally-closed output exceeding the predetermined trip threshold; (c) thereafter rectifying the line signal present at said normally-open output to detect when the overvoltage thereof in excess of said threshold ceases; (d) thereafter waiting for a time-interval of a few seconds to elapse during which no such line overvoltage signals are detected; (e) after said time-interval has elapsed, detecting the restoration of nominal DC line voltage to thereafter switchback said relay to reconnect said installation through the normally-closed output to said line; and (f) recycle to step (b).

23. A method according to claim 22 wherein said predetermined nominal voltage on said line is DC negative relative to earth, characterized in that in the step (b) positive overvoltage signals are rectified and detected on the line and in the step (e) negative voltage signals are rectified and detected on the line.

24. A method according to claim 18, characterized in that a low-intensity current is passed through said earth connection even during a fault condition.