Water alarm and fault-locating for air core plastic-insulated telephone cable

[57] ABSTRACT

A broad design for remote electrical detection of relative humidity level and liquid water in a telephone cable, while avoiding false alarms due to high relative humidity. The alarm circuit relies on a plastic barrier between one pulp insulated conductor and a second conductor. The barrier is interrupted at separated intervals along the cable, for which points only the pulp insulation separates the two conductors.

Claims, 8 Drawing Figures

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PATENTED MAY 9197? 3,662,367

SHEET 1 OF 3

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G.E DE YE AU, JR,

INVENTORS W.Q.NUTT

G.H.WEDSTER

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ATTORNEY

PATENtEDMAY 91972

3.662,367

SHEET 2 OF 3

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PATENTEDMAY 91972 3,662,367

SHEET 3 OF 3

FIG.

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3,662,367

WATER ALARM AND FAULT-LOCATING FOR AIR CORE PLASTIC-INSULATED TELEPHONE CABLE

FIELD OF THE INVENTION

This invention relates to sheathed electrical cable; and in particular to accurate detection and locating of water in an air core multipair plastic insulated telephone cable.

BACKGROUND OF THE INVENTION

The problem of water damage to air core plastic insulated conductor telephone cable (PIC cable) is sufficiently critical to have commanded great and increasing attention. Water entering a sheath break in this type cable moves insidiously away from the break without noticeably degrading, at least initially, service on the affected pairs. Ultimately, as more and more water enters, the transmission is degraded, either directly by electrical effects or indirectly by corrosion effects.

Locating the cable fault by this time is electrically complicated by the likely presence of water in a long stretch of the cable. Schemes relying on impedance or dielectric irregularities occasioned by the presence of water in PIC cable are not altogether effective. Furthermore the electrical fault, when it is located, may prove to be far removed from the sheath break. And still another result of the time delay is the need to remove possibly large quantities of water.

Simple methods are, however, successfully used for paper, or pulp, insulated conductor cable. Water incursions cause the paper insulation to swell locally and effectively block longitudinal flow, while also drastically changing the insulation resistance and the dielectric breakdown characteristic between pairs of a conductor. The change in breakdown characteristic is commonly exploited by applying a high voltage, high current source to the pair to achieve a weld at the fault. This will permit an accurate resistance measurement for fault locating, or what is even more convenient a tone may be placed on the shorted pair. The tone is then “traced” along the cable with an inductive pickup coil, the tone stopping at the short.

Many plans for avoiding or coping with water in air core PIC cables have been propounded. Recently, for example, the filling of the cable core with a petroleum jelly-polyethylene mixture has been commercially useful. This approach, however, is presently limited to small or medium sized cables; and even here has disadvantages including manufacturing costs and increased cable diameter. Incentives still exist, therefore, to achieve a more effective fault detection and location scheme for air core plastic insulated telephone cable, especially since such cable has superior transmission characteristics especially at frequencies above audio.

Accordingly, one object of the invention is to reduce the cost of maintaining air core PIC telephone cable.

A second inventive object is to locate the presence of water in such a cable at about the time of its entry, and before appreciable longitudinal flow occurs.

A third inventive object is to achieve the first two objects by a system that does not produce false alarms if relative humidity in the cable is high.

A fourth inventive object is to achieve all of the three above objects while not interfering with the electrical performance of the cable.

A fifth inventive objective is to provide a means for monitoring the relative humidity in a cable through the use of sensitive measurements of insulation resistance.

SUMMARY OF THE INVENTION

In accordance with the inventive principle, a plastic barrier is placed between a pulp insulated conductor, and a second conductor which may or may not be pulp insulated. The plastic barrier assumes different forms in several illustrative inventive embodiments; but in all instances is utilized to interrupt, over a large percentage of the length, the de leakage path between conductors and thereby prevent false alarms due to high humidity. At the same time, the desirable water sensitivity and electrical breakdown characteristics of paper cable are In a second inventive embodiment, a pulp insulated conducpreserved. 75 tor 20 and a plastic insulated conductor 21 are twisted

In a first embodiment of the inventive monitoring pair, one of two pulp insulated conductors is given a longitudinal serving of thin plastic film such as 0.006 inch thick polyethylene film. One-half inch wide gaps or windows are cut in the film 5 every 50 inches or so. The film-covered pulp insulated wire is then twisted together with the regular pulp-insulated wire. Thus, only 1 percent Or less of the pulp insulation is exposed. The film covering along the remainder 99 percent of the length supplies the excellent electrical property of polyethylene for high insulation resistance, even in the presence of liquid water. Yet the windows allow pulp-to-pulp contact to occur for applying standard high voltage breakdown procedures to locate liquid water.

In a second inventive embodiment, a pulp insulated conduc¹³ tor and a plastic insulated conductor are twisted together. Gaps are created in the plastic wire insulation, again at intervals of 50 inches or so. As in the previous design, the only insulation separating the wires at the gaps is wood insulation, in 2θ this case, that of the one pulp insulated conductor. Thus, the same electrical breakdown tests can be applied here; and as in the previous design, polyethylene protection is achieved over some 99 percent of the length of the monitoring pair.

In a third inventive embodiment, two pulp insulated con25 ductors are laid into separate interstices of the cable core. Every 50 inches or so, the two wires are brought together and twisted for approximately one-half inch. In this case, the polyethylene barrier between the wires for the separation intervals is the polyethylene insulation of the cable conductors.

In a fourth inventive embodiment, a sensing element is applied longitudinally as in the third embodiment, but in addition the wires are undulated to allow longitudinal compliance so that the cable may be flexed.

The invention and its further objects, features and ad35 vantages will be more readily appreciated from a reading of the descriptions to follow of illustrative embodiments.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1 and 2 are side perspective sketches of a first inven40 five embodiment;

FIG. 3 is a side perspective view of the second embodiment;

FIG. 4 is a side perspective sketch of a third embodiment ;

FIG. 5 is a side perspective schematic showing the third embodiment applied in a cable;

FIG. 6 is a schematic diagram of an entire monitoring system illustrating two inventive embodiments;

FIG. 7 is a side perspective sketch of a fourth inventive embodiment;

FIG. 8 is a schematic diagram further illustrating the fourth inventive embodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT

The embodiment disclosed in FIGS. 1 and 2 involves a detecting element 10 consisting of a longitudinal serving of plastic film 11 which, for example is polyethylene, shaped around a paper insulated conductor 12. The serving 11 is made with abutting edges which are adhered together with an 60 adhesive; but other ways of placing a polyethylene layer loosely around conductor 12 are readily envisioned.

At intervals denoted S in FIG. 2 along the length of each element 10, there are placed one or more window cutouts 13. An element 10 and another like element except with no plastic 65 film 11 then are twisted together in a monitoring pair 14, as shown in FIG. 2, with the window areas 13 in contact or closely adjacent to the paper insulation of the other conductor. If S is, for example, approximately 50 inches, the window widths are approximately one-half inch. Only 1 percent of the pulp 70 insulation of pair 14 is exposed, the other 99 percent being protected by the polyethylene film. The monitoring pair 14 in FIG. 2 may be incorporated into a cable in the manner shown in FIG. 6.

3,662,367 together. The conductor 20 is, for example, a standard 22gauge pulp insulated wire; and the conductor 21 is standard 22-gauge polyethylene insulated conductor. Pursuant to this aspect of the invention, gaps such as 13α of about one-half inch in length are placed through the plastic insulation of conductor 21, at intervals of approximately 50 inches. As in the first embodiment, the plastic insulation intervenes at all points between the two wires of conductors 20, 21, except at gaps 13α, where the only intervening layer is the wood pulp insulation of conductor 20. The resulting monitoring pair 14α is likewise placed in a cable in the manner shown in FIG. 6.

The monitoring pairs 14 and 14α act alike during fault location because of the similarity of their twist length, which is in each case short enough to create a snug copper-pulp-copper junction at the window location to make if possible at times of fault-locating to achieve dielectric breakdown with the standard 600 to 700 volts breakdown potential. At the same time, the twist length must be long enough to allow application of a test signal large enough to be detected in conventional fashion outside the cable sheath.

In a third inventive embodiment, shown in FIG. 4, two pulp insulated conductors 30, 31 are made up with a wide longitudinal separation W except for intervals occurring every S inches, where the conductors 30,31 are brought together in a series of twists 32. This monitoring pair 14b is placed with the separated portions disposed in circumferential interstices on opposite sides of a core 33 of twisted telephone pairs within a sheathed cable 34. The two conductors 30, 31 cross and twist for approximately one-half inch every 50 inches or so, separated for the great majority of their length by the polyethylene insulation of the conductors in core 33. This scheme is especially suitable for fault detection because the extremely long apparent twist length caused by the lay of the core 33, and the wide separation of the two wires, create very favorable conditions for using tone testing signals. The inventive embodiment of FIG. 4 is rendered flexible to withstand bending, by the common helix angle of all elements in the cable core, as shown in FIG. 5.

In this embodiment, the sensing element comprises a monitoring pair 14c. Pair 14c comprises two standard paper insulated wires 51,52; or alternatively, one of the other combinations previously mentioned. Wires 51,52 are normally spaced and parallel, but are brought into juxtaposition for about onehalf inch every 49½ inches, for example. Wires 51,52 may be transposed at each twist section by having an odd number of half twists. This expedient reduces outside interference on the monitor pair and also the interference that may be caused on cable pairs when fault-locating signals are applied. Also the direction of successive twists, for example, the sections 54 and 55 are reversed. Thus, it is convenient to form two twists at once without involving the rotation of high inertial components.

The wires 51, 52 are given longitudinal compliance, so that they may be applied longitudinally. The compliance is necessary for the cable to flex without unduly straining the wires. The compliance may be provided by undulating the wires as seen in FIG. 8 or, if somewhat less compliance is needed, it may be provided by forming the wires into elongated rectangles.

It may be advantageous in manufacture to reinforce the monitor pair by affixing it to a plastic tape 53. The whole monitor pair assembly may be fed from a desiccated storage box (not shown), applied to the cable, and immediately sheathed, obviating any requirement for drying of the core or reducing the relative humidity of the factory space.

Importantly, in each of the above-described inventive embodiments, the amount of exposed pulp insulation is sufficient to absorb most degrees of moisture diffused within the cable core. The embodiments of FIGS. 2 and 4 supply the most pulp for absorbing diffused moisture, their capacity being approximately 2 grams per 100 feet. The embodiment of FIG. 5 is approximately twice this capacity. The actual amount of moisture that need be absorbed is a function of, among other things, the sheath permeability. Sheaths sufficiently impervious to be compatible with these amounts of moisture absorption are readily designed.

Breakdown of pulp pairs in the field for fault location is done in the manner depicted in FIG. 6, which shows the monitoring pairs 14, 14α or 145 connected to a test set 40 which consists of a bank of batteries supplying 630 volts and a shortcircuit current of up to 5 amperes. Water across the monitoring pairs at the fault causes current to flow at that point upon the application of test voltage. This current flow causes vaporization of the water and carbonization of the pulp. The more conductive the pulp becomes, the stronger the current flow until the copper at the junction melts and itself flows, causing a weld between the conductors.

Since the number of pulp pairs available for breakdown in a plastic insulated conductor cable is far less than normally is available in a pulp insulated conductor cable, it is necessary to assure that the monitoring pairs of the present invention short out in the above-described manner. To enhance the probability of a good short, one controlling factor is that the pulp insulation between the conductors must be thick enough to prevent inadvertent shorts, but as thin as possible to aid in the breakdown. In the case of the embodiment of FIG. 3, the plastic insulated conductor and pulp insulated conductor pair, the insulation at the gaps 13α is not as great as in the other embodiments described. Twist tightness is also a controllable factor, the tighter twists being preferred for successful breakdown welding. An added advantage of the windows made in plastic is that when breakdown battery is applied, the leakage current is concentrated, greatly improving the chances of forming an arc that results in a weld between conductors. Paper helps achieve a weld by starting an arc without an explosion.

Water conductivity is obviously not controllable, but depends to some extent on the mineral content of the soil around the cable. Even with distilled water, however, a reasonably low resistance occurs at the water saturation point because of impurities in wood pulp.

Pursuant to another aspect of the invention, any of the monitoring pairs 14,14α, 145,14c may further be utilized for helping to dry a cable core. Normally, dry nitrogen gas is not an optimum method of removing cable water by evaporation, because of the low evaporation rate of water into nitrogen. Even when nitrogen is passed through a plastic insulated conductor cable at a fast flow rate, the slow rate only saturates the nitrogen slightly.

Pursuant to this aspect of the invention, the inventive monitoring pairs, and particularly the wood pulp insulated pair of FIG. 4, liberate moisture absorbed by the pulp, due to the heating of the wet pulp at the time the high current from test set 40 is applied. The rapid water liberation also aids in hastening saturation of the nitrogen. When the pulp pair has lost its moisture, the heating current is turned off, and the pulp again commences to absorb moisture. The cycle is repeated until the desired cable humidity is reached.

Pursuant to a further inventive feature, any of the monitoring pulp pairs 14,14α, 145,14c may be employed to measure the relative humidity of the atmosphere within the cable. For all of the monitoring pairs 14,14α, 145 and 14c that the interposition of a plastic layer for 99 percent of the length increases by a factor of 100 the tolerance to moisture which the respective pair would have without the plastic layer. For relative humidity levels of, for example, 60 or 70 percent, the test voltage produces a very high rate of leakage across the conductors at the window or proximity points. However, since these points occur for only one one-hundredth of the total length, the leakage current per unit length is very low. On the other hand, if liquid water should envelop one of these points on the monitoring pair, a very sharply defined leakage point is obtained that can readily be detected and later welded by the dielectric breakdown technique described above.

A correlation between relative humidity and, for example, insulation resistance of the monitoring pair can be con3,662,367 structed. Thus, by a spot measurement of insulation resistance, the cable core relative humidity is determined. Correlations between cable core relative humidity and other parameters, such as wire-to-mate capacitance or wire-to-mate conductance may also be determined for the same purpose. Still other measures of dielectric properties may also be employed. For the inventive embodiments depicted in FIGS. 4 and 5, a 10-foot length of each was tested at 65 percent relative humidity; 1,000 volts breakdown potential was withstood and the insulation resistance was 100 megohms. At this relative humidity level, a conventional twisted pulp pair would have an insulation resistance of 1 megohm, which for a 10mile cable would translate to about 200 ohms, a certain false alarm.

In an implementation of the invention depicted in FIG. 6, a continuous monitoring of the cable water condition may be achieved automatically, by attaching the monitoring pair 14, 14α or 14* to, say, a standard telephone line relay 41. Line relays are designed to operate with adequate sensitivity and margins. An impedance in the range of 0 to 2 kilohms presented to the line relay 41, normally signifies closure of a switchhook signaling service request. On the other hand, to provide margin against undesired relay operations a leakage resistance greater than 10 kilohms is desired.

A 75 percent relative humidity level along the entire length of a 10-mile pulp insulated pair would cause the insulation resistance to drop well below 1,000 ohms as seen by the line relay 41. If, however, the members of the monitoring pair come together only 1 percent of the time as in all embodiments of the invention, the effective resistance is 10 kilohms. This figure approaches the margins within which a line relay 41 operates.

Accordingly, a monitoring pair constructed pursuant to the present invention may be attached to a conventional line relay to use the relay as signaling device for fault detection when the monitoring pair becomes water-saturated. The relay 41 operated, trips an alarm 42.

It is to be understood that the embodiments described herein are merely illustrative of the principles of the invention. Various modifications may be made thereto by persons skilled in the art without departing from the spirit and scope of the invention.