GB2082406A - Monitoring electrical cables and joints for the ingress of moisture - Google Patents

Abstract

Two detecting conductors such as moisture detecting tape 2 with two metallic conductors laminated to an insulating substrate are secured in or to the cable and terminated in a fixed resistance 6. The terminating resistor causes a small current to flow continuously through the detection conductors and should the line open resulting in a loss of protection, the absence of the monitoring current will signal an alarm indicating a system fault. The ingress of water into the cable system will result in an increased current in the monitoring line which is then detected by the remote electronics equipment and an alarm is activated. Coded signals are used to indicate the position of a faulty splice, and to effect isolation of a selected cable portion. <IMAGE>

Description

SPECIFICATION Apparatus to monitor electrical cables including splice joints and the like for the ingress of moisture The invention relates to the monitoring of electrical cables for the presence of moisture within the cable system.

A major and continuing concern of telephone companies is the need for constant vigilence and protection against the presence and effect of moisture in outside cable plant.

For the past fifteen to twenty years, the main weapon in the battle has been "Pressurization". Pressurization has proved to invaluable; however, it is costly in its initial setup and carries certain maintenance and nuisance burdens. For the most part, its' use has been confined to large feeder and important cables and its overall efficiency has been largely dependent on the efficiency of material and physical isolation blocking or damming. In instances where filled telecommunication cables are used in conjunction with non-filled cable types, pressurization can be extremely difficult if not physically impossible. The monitoring and protection system described within is an electronic system and therefore not subject to the constraints limiting a physical protection system such as pressurization.The invention offers much in simplicity and economy and may be incorporated in all types of cable plant, cables large and small, filled and unfilled.

The invention as designed, does not prevent or retard the ingress of moisture. It does, however, react instantly to its presence and provides for pin-pointing the location of the breach. Except in cases of catastrophic failure, the system provides warning and facilitates remedial action before outages occur.

The present invention is an electronic system which continuously monitors outside cable plant for the presence of moisture in the cable or at splices and other critical locations.

At the first trace of moisture an alarm is activated and the system indicates whether the cause is a cable sheath or other fault.

Ideally, the general monitoring system operates over a special moisture detection tape which is an integral part of the cable construction. The system may, however, be installed on conventional cable to monitor splice points and other locations. Where "detection tape" cable is used only one dedicated conductor pair, besides the detection tape, is required; the dedicated conductor pair being used for line interrogation. When conventional cable is involved, two dedicated conductor pairs are required, one being for splice alarm operation and the other for line interrogation.

Fundamentally the invention operates by detecting the increase in conductance that occurs when a water path exists between two separate and isolated conductors. A monitoring voltage is impressed between two detecting, which may take the form of a moisture detecting tape with two metallic conductors laminated to an insulating substrate. In order to insure the continuity and therefore secure operation of the system the far end of the detection conductors is terminated in a fixed resistance of a large value. The terminating resistor causes a small current to flow continuously through the detection conductors and this verifies the continuity of the monitoring line. Should the line open resulting in a loss of protection, the absence of the monitoring current will signal an alarm indicating a system fault.The ingress of water into the cable system will dampen or moisten the insulating dielectric between the detecting conductors and result in an increased current in the monitoring line. The increase in current is detected by the electronics at the equipment office and an alarm is activated signalling the presence of moisture. The moisture detection conductors are accessable at the equipment office and by employing well known bridge techniques the exact location of the breach may be determined. The sensitivity of the system to changes in current, as related to a break in the monitoring line or dampening or moistening of the monitoring line, is adjustable over a wide range.The nominal monitoring line current as determined by the termination resistance falls into a safe current "win dow". Should the line current exceed the preset maximum or minimum current level, an alarm is signalled which identifies the presence of moisture or loss of protection.

A practical cable system may be constructed with several branch cables extending from a main cable run. At the point of a branch cable run, the detection line on the branch cable is bridged across the detection line on the main cable run, through a remote disconnect unit. The remote disconnect unit provides the facility for sectionalizing and isolating branch cable runs or at locations wherever system isolation is required. The remote disconnect unit forms part of the total monitoring system and is powered and signalled from the equipment office on one dedicated conductor pair. A number of disconnect devices may be tied to the same dedicated conductor pair as they are uniquely addressable by means of modulation coding. It is therefore possible to parallel several monitoring lines on one main cable monitoring line and remotely disconnect those lines for purposes of fault locating.In the event of a break in the monitoring line or the entry of moisture, the faulting cable section is determined by cyclically selecting and transmitting the signalling codes to each remote disconnect unit. Activating a particular remote disconnect unit causes the monitoring line associated with the unit, to be replaced by a non-faulted line simulation. When substitution results in cessation of the alarm it is obvious that the faulty cable has been isolated.

It is known that splice points and other sensitive locations frequently fail allowing the ingress of moisture with resulting deterioration of the cable system and without an alarm and detection system similar to the present invention, damage may occur to the cable for a considerable distance on either side of the fault before an outage indicates a fault has occurred and even then, location of the fault is often time consuming and difficult.

In addressing this problem the invention provides a special sensing device which is placed at these locations. At all critical points, such as cable splices, the sensor device, hereinafter referred to as a Splice Sentry Unit, is connected in parallel on the moisture detection line. The Splice Sentry Unit provides a sensing input which is tied to the moisture detecting conductors. The moisture detecting conductors are then placed appropriately inside of the splice closure or other sensitive cable system components and/or other locations. Should water breach the protection at the location, the moisture detecting conductors are dampened and activate the electronics in the Splice Sentry Unit. When activated, the Splice Sentry Unit modulates the line with a coded signal.The coded signal is transmitted back along the detection line to the equipment office where it is intercepted by tuned line amplifiers and a moisture fault alarm results. The electronics in the equipment office decodes the incoming fault signal and luminates the corresponding indicator. The exact location of the fault is thus uniquely identified. The remote Splice Sentry Unit is powered by the detection line voltage. The device, unless triggered by the presence of moisture, appears as an open circuit on the line. Therefore, no standby current is drawn which would result in a false moisture alarm.

Figure 1 is a schematic diagram showing apparatus and circuitry for monitoring the detection line current in a cable.

Figure 2 is a schematic diagram of the signal decoder.

Figure 3 is a schematic diagram of the line interrogation device.

Figure 4 is a schematic diagram of the portion of the device which may be installed at a remote location along a cable (Splice Sentry Unit).

Figure 5 is a schematic diagram of a remote disconnect unit used in conjunction with Fig.

5.

In the drawings like characters of reference indicate corresponding parts in the different figures.

The apparatus for monitoring the detection line currents at the equipment office (Fig. 1) senses both the DC monitoring and AC coded signals.

An electrical cable 1 with a moisture detecting tape 2 helically wrapped around the outer layer of the cable core between the cable core and protective outer jacket is shown. The moisture detection tape 2 with parallel and separate conductors is connected to the monitoring circuit through resistors 3 and 4. The input resistors 3 and 4 limit the direct line current and provide a suitable termination for any signalling currents. A regulated and protected DC power supply 5 provides the detection line voltage and powering for any remote splice sentry units.

The termination resistance 6 at the end of the detection line establishes the nominal line current thus verifying line continuity. The line current develops a proportional voltage at the negative input of a summing amplifier 9. the voltage at the output of the amplifier 9 is reduced to zero for the nominal line current via the offset adjustment control 7 connected to the positive summing input 9 and the DC power supply 5. A filter capacitor 8, is connected from the negative summing input of 9 to ground to bypass alternating current components of the signal. The output of the summing amplifier 9 is connected to the inverting input of a following summing amplifier 10 which exhibits unit gain. A decrease in line current such as caused by a line open will cause the output voltage of the summing amplifier 9 to go positive.The positive voltage is connected via a control 1 2 and the output of the control 1 2 is connected via a lead to an SCR 14. The control 1 2 determines the threshold voltage from the output of amplifier 9 at which SCR 14 will trigger on. The loss of the detection line 2 current will result then in the triggering of SCR 14 which causes a current to flow through a relay coil 1 5 and system fault indicator LED 1 7. The energizing of the relay coil 1 5 causes contacts 22 to open, introducing a resistance 20 at Scan terminals 24, 25 for purposes of remote alarming. A system line fault is thus detected and alarmed.

An increase in detection line 2 current as caused by moisture on the detection line 2 will result in an increased voltage at the output of the unity gain amplifier 10. The output from amplifier 10 is connected via a lead to a control 11 and the output of the control 11 is connected via a lead to an SCR 1 3. The control 11 determines the threshold voltage from the output of amplifier 10 at which the SCR 1 3 will trigger on. An increase in detection line 2 current as a result of dampening of the detecting line 2 will cause the triggering of the SCR 1 3 which results in a current through a relay coil 1 6 and illumination of the moisture alarm LED 18. The energizing of relay coil 1 6 causes contacts 23 to open introducing a resistance 21 at Scan terminals 24, 24 for the purposes of remote alarming. A moisture penetration fault is thus detected and alarmed.

A local audible alarm may be installed at the equipment office and connected in series with the alarm circuitry via terminal 1 9.

In the event of moisture penetration at a splice or other critical location, the electronic device (Fig. 4), installed at the remote location, activates. The current drawn by the remote electronic device (Splice Sentry Unit) exceeds the preset maximum value and an alarm is signalled illuminating an LED 18. The Splice Sentry Unit generates a coded signal which transmits down the detection line 2 to the equipment office. An instrument amplifier 26 is connected across the conductors of the detecting tape 2 and intercepts the coded signal (see Fig. 1).

The instrument amplifier 26 increases the strength of the coded signal and rejects any unwanted common mode noise voltages on the moisture detection line 2. The output of the instrument amplifier 26 is applied to the input of an active band pass filter 27 which rejects all noise voltages outside of the frequency band of the code signals. A frequency range of 1 50 Hz to 1050 Hz is selected as the preferable in-band range for the code signals. This frequency range is compatible with typical transmission characteristics of communication cable conductor pairs and the moisture detecting tape 2. The amplified signal from amplifier 26 and the band limited output from the active Band Pass Filter 27 is connected to the input of an Automatic Gain Control amplifier 28.

The AGC amplifier 28 automatically adjusts its gain to the level of the incoming signal from the Band Pass Filter 27 such that a constant output voltage is realized from the AGC amplifier 28. The dynamic range of the AGC amplifier 28 ensures a constant output voltage of 2 V RMS for a voltage level of 3 MV RMS to 3 V RMS at the input of the instrument amplifier 26 (these values as exemplary only).

The output of the AGC amplifier 28 drives an indicator LED 29 and is connected via a terminal 30 to a Signal Decoder 31 (Fig. 2).

The Signal Decoder 31 accepts the Modulated Code signal from the output of the AGC amplifier 28 and demodulates the signal. The output of the Signal Decoder drives the code display 33 illuminating the LED which corresponds to the code frequency of the signal generated by the Splice Sentry Unit (Fig. 4).

The Splice Sentry Unit (Fig. 4) is comprised of four main components, namely, the code generator 47, the current sink 49, an SCR 48 and a moisture detecting conductors 51. The device is tied parallel across the moisture detecting line 43 of the communication specifically identified as 42, 45. The SCR 48 is in series with the code generator 47 and current sink 49 and, unless triggered by moisture on the detect conductors 51, prevents current from passing through the device.

Moisture dampening the conductor's dielectric 51 results in a current through the conductors, current limiting resistor 50, and the input gate of the SCR 48. The SCR 48 turns on and the voltage on the cable detection line 43 is applied across the code generator 47 and current sink 49.

The code generator 47 develops a predetermined square wave code signal which is fed to and drives the current sink 49. The driven current sink 49 draws approximately 0 to 7 ma current between the cable detection tape conductors 43. As the impedance of the detection tape is in the order of 600 ohms, a resultant modulation wave form of about 4.2 volts peak to peak is impressed between the detection conductors 43. The wave form contains the coded information unique to that particular splice sentry unit and propogates back to the equipment office where it is detected. The location of the fault is thus uniquely identified.

Where branch cable moisture detection tape is spliced parallel to the main cable moisture detection tape, a remote disconnect unit (Fig.

5) is placed to enable isolation and fault locating. At the equipment office a line interrogation device (Fig. 3) generates a modulated signal to power and activate the remote disconnect unit. The modulated signal is transmitted to the remote disconnect devices via a dedicated conductor pair 35 within the cable 34. A sine wave generator 36 of selected code frequency is connected to a modulation control 37. The adjustable output from the modulation control 37 is connected to the negative input of a summing amplifier 39. A line current bias control 38 is connected to the positive input of the summing amplifier 39 and controls the direct line current through the interrogation pair conductors 35. The power supply 40 provides the DC potential enabling the nominal line current.

The interrogation line current is set at 1 6 ma (for example) which series powers the remote disconnect devices. Depressing the interrogation switch 41 completes the circuit and composite powering and modulation currents excite the conductors of the interrogation conductor pair 35. The selected frequency signal output from the summing amplifier 39 has a peak to peak amplitude of approximately 5 volts.

The composite signal from the line interrogation unit (Fig. 3) propogates down the interrogation pair 35 to the location of the remote disconnect device (Fig. 5). The interrogation conductor pair 44 series powers and parallel signals the remote disconnect device (Fig. 5).

The direct powering current of approximately 1 7 ma develops an approximate 5 volt potential across the zener diode 56 and filter capacitor 57. The 5 volt potential powers the signal decoder device 55. The coded alternating signal is coupled to the input of the signal decoder 55 through the DC blocking capacitor 58. If the signalling frequency at the input corresponds to the preset code frequency of the signal decoder a voltage potential of about 5 volts is developed across the relay coil 54.

The energized relay coil causes the relay contacts 52 to pull in. The moisture detection conductor connections from 43 to 45 are disconnected and the detection conductors 43 are terminated by a precision resistance 53.

The moisture detection conductors 43 from the incoming cable 42 are thereby isolated from the moisture detection conductors 45 on the outgoing cable 46.

Finally it should be noted that although the device and system herein described, teaches the use of frequency coded signals, it will be appreciated that the system can readily be modified to send and interpret digitally coded signals if desired, the modification being apparent to those skilled in the art to which the invention relates.

Claims (10)

1. Apparatus for monitoring electrical cables and specific locations therealong, such as splice joints and the like, against the ingress of moisture and including a source of electrical power; comprising in combination a pair of main separate and dielectrically isolated monitoring conductors operatively installed along the cable being monitored and being operatively connected to the source of electrical power, a terminating resistor across the distal end of said monitoring conductors to ensure continuity of current through said monitoring conductors, thereby verifying the continuity thereof, and electronic detection means operative connected to said monitoring conductors, said electronic detection means including indicating means to signal a lack of current continuity through said monitoring conductors and further including means to signal an increase of current flowing in said monitoring conductors, beyond a predetermined value due to a lessening of the resistance of the insulating dielectric between the two monitoring conductors, as by the presence of moisture.

2. The apparatus according to claim 1 in which said electronic detection means is remotely situated from said electrical cable being monitored, said indicating means to signal a lack of current continuity including a first summing amplifier operatively connected to the distal ends of said monitoring conductors, a following summing amplifier, the output of the first summing amplifier being connected to the inverting input of the following summing amplifier, relay coil means nd systems fault indicator means operatively connected to the output of said first summing amplifier said relay coil means being connectable to remote alarm means, the loss of said monitoring current operatively connecting said first amplifier to said fault indicating means and to said relay coil means, further relay coil means and further system fault indicator means operatively connected to the output of said following summing amplifier, said further relay coil means being connectable to further remote alarm means, the increase of said monitoring current beyond said predetermined amount, operative connecting said following summing amplifier to said further fault indicator means and to said further relay coil means.

3. The apparatus according to claim 1 which includes means for monitoring specific locations along the length of the electrical cable, such as splice joints and the like, said last mentioned means including an electronic, specific-location-monitoring unit, said unit being operatively connected to the source of electrical power and including code generating means, a localized moisture detecting means operatively connected to said code generating means and means in series with said code generator means and said specificlocation-monitoring unit, for preventing current passing through said monitoring unit unless said localized moisture detecting means is activated and means operatively connecting said specific-location-monitoring unit with said further indicating means.

4. The apparatus according to claim 2 which includes means for monitoring specific locations along the length of the electrical cable, such as splice joints and the like, said last mentioned means including an electronic, specific-location-monitoring unit, said unit being operatively connected to the source of electrical power and including code generating means, a localized moisture detecting means operatively connected to said code generating means and means in series with said code generator means and said specificlocation-monitoring unit, for preventing current passing through said monitoring unit unless said localized moisture detecting means is activated and means operatively connecting said specific-location-monitoring unit with said further indicating means.

5. The apparatus according to claim 3 or 4 in which said localized moisture detection means includes a pair of separate and dielectrically isolated moisture conductors.

6. The apparatus according to claims 3, 4 or 5 which includes branch cable monitoring means operative connected in parallel to said pair of main separate and dielectrically isolated monitoring conductors, a remote discon- - nect device operatively connected between said branch cable monitoring means and said main monitoring conductors, dedicated cable means operatively connecting said remote disconnect device to said electronic detection means, said electronic detection means including a line interrogation means for generating a modular coded signal along said dedicated cable means to said remote disconnect device for actuating same for connecting and disconnecting said branch cable monitoring means from said main monitoring conductors, the disconnecting of said branch cable monitoring means from said main monitoring conductors including means to connect the ends of said main monitoring conductors together at the location of said remote disconnect device thereby simulating a non-faulted monitoring conductor situation.

7. The apparatus according to claim 6 which includes a plurality of branch cable monitoring means, a remote disconnect device for each of said branch cable monitoring means, each remote disconnect device having a different signal in code for identifying and activating said individual remote disconnect device selectively.

8. A method of monitoring electrical cables for the ingress of moisture therein consisting of the steps of wrapping around said cable, in helical fashion, a pair of main separate and dielectrically isolated monitoring conductors mounted on the dielectric substrate, connecting a resistor between the distal ends of said conductors, connecting a source of current to the other ends of said monitoring conductors thereby causing a current to flow therethrough of a predetermined value, continuously sampling the current flowing through the monitoring conductors and operating indicating means if said monitoring conductor circuit is broken and operating further indicating means if the resistance between the separate isolated monitoring conductors lessens beyond a predetermined amount thereby indicating the presence of moisture therebetween.

9. The method according to claim 8 which includes the additional steps of connecting an electronic specific-location-monitoring unit to said main monitoring conductors along the length thereof connecting the source of electrical power to said monitoring unit, connecting a local monitoring conductor means to said monitoring unit, the activation of said local monitoring conductor means actuating said monitoring unit and transmitting a predetermined coded signal along said monitoring conductors to said indicating further indicating means.

10. The method according to claim 8 or 9 which includes the additional steps of connecting a remote disconnect device to said specific-location-monitoring unit, and operatively connecting said remote disconnect unit to the source of electrical power, operating said remote disconnect device, whereby said monitoring conductors are disconnected at the location of said monitoring unit and connecting said disconnected ends together thereby simulating a non-faulted monitoring conductor situation.