US3679810A

US3679810A - System for maintaining low relative humidity in telephone cables and other enclosures

Info

Publication number: US3679810A
Application number: US99820A
Authority: US
Inventors: Wendell Glenn Nutt
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1970-12-21
Filing date: 1970-12-21
Publication date: 1972-07-25
Anticipated expiration: 1989-07-25

Abstract

This disclosure describes a system for maintaining a reduced level of relative humidity in telephone cables. A moisturepermeable tube of, for example, polyvinyl chloride with a pulpinsulated pair within it is included in the cable core. The pulp insulation, being a natural desiccant, absorbs moisture that passes through the tube wall from the cable, thus holding down the relative humidity inside the cable. When the pulp is saturated, a longitudinal flow of air coupled with applied electrical current sufficient to heat the pair, purges the moisture from the tube. The pulp insulation is ready thereafter to absorb moisture anew.

Description

0 United States Patent [151 3,679,810 Nutt [451 July 25, 1972 [s41 SYSTEM FOR MAINTAINING LOW 3,234,045 2/1966 Larsen ....174/11 R x RELATIVE HUMIDITY IN TELEPHONE 2,081,803 5/1937 Ell ..174/11 R x CABLES AND OTHER ENCLOS L s FOREIGN PATENTS OR APPLICATIONS [721 Invent Wendell Glen" 501,593 2/1939 Great Britain ..174/14 [73] Assignee: Bell Telephone Laboratories, Incorporated,

Murray Hill, NJ. Primary Examiner Lewis I-l. Myers Assistant ExaminerA, T. Grimley [22] Flled' 1970 Attorney-R. J. Guenther and Edwin B. Cave [21] Appl. No.: 99,820

[57] ABSTRACT [52] 0.8. CI. ..174/l6 R, 174/10, 174/27, This disclosure describes a System f maintaining a reduced 174/115, 219/276 level of relative humidity in telephone cables. A moisture- [51] Int. Cl. .J'lOlb 7/28 permeable tube of for example, polyvinyl chloride with a [58] Field of Search 174/10, 11 R, 14 R, 27, 70 R, pulpdnsulated pair within it is included in the cable core The 174/122 R, 124 R, 115, 113 R, 16 R; 219/209, 276; pulp insulation, being a natural desiccant, absorbs moisture 34/151 218 that passes through the tube wall from the cable, thus holding down the relative humidity inside the cable. When the pulp is [56] References Cited saturated, a longitudinal flow of air coupled with applied elec- UNITED STATES PATENTS trical current sufficient to heat the pair, purges the moisture from the tube. The pulp insulation 1s ready thereafter to ab- Patterson R o -b moisture anew 233,492 10/1880 Field ...175/15 C X 199,506 l/1878 Brooks 1 74/14 R 5 Claims, 4 Drawing Figures PATENTEDJULZB m2 8.679.810

//v VENTOR W G. NU 7'7 wim A TTOR/VE V 1. Field of the Invention This invention relates to multipair telephone cables and in particular to a system for controlling the relative humidity within such a cable.

2. Background of the Invention Although great care is taken in the design of telephone cable sheath to assure watertight integrity, during use over periods ranging from weeks to years, humidity inevitably builds up in the cable interior through moisture permeation of the sheath. In the case of some sheath designs, the relative humidity approaches 100 percent if the outside conditions are sufficiently conducive, as in the case of buried plastic insulated conductor cable.

This level of humidity makes it difiicult to design a liquid water-detecting alarm system that will respond to the presence of liquid water but not to mere high humidity conditions. Also, many insulating materials such as foamed plastic degrade in the presence of high relative humidities. Moreover, with the advent of plastic insulated aluminum conductor cable, it is necessary to avoid the corrosive effects upon the conductor of such high relative humidity levels.

It is well known to periodically purge multipair air core cable with a pressurized gas such as nitrogen to rid the cable of moisture. This does not assure, however, that in the interim the cable atmosphere will not exceed a critical relative humidity level. Furthermore, this wholesale purging method is time-consuming, costly and inefficient since the gas tends to channel" through relative open portions of the cable core while bypassing portions that need to be dried.

Accordingly, one object of the invention is to maintain a desired level of relative humidity within a multipair telephone cable core, simply and at low cost.

Another objective is to monitor the relative humidity of the atmosphere within the cable.

An added inventive object is to extend the useful service life of such cable, particularly aluminum conductor cable.

A specific inventive object is to purge a cable of moisture by substantially the same mechanism that, in between purges, keeps the relative humidity low; thereby to perform two tasks with one structure.

SUMMARY OF THE INVENTION Pursuant to the invention, conductor means insulated with a desiccant, such as one or two pairs of pulp-insulated conductors, are disposed in a tube whose walls have a selected permeability to moisture. The tube for example a plastic such as polyvinyl chloride, is placed in the interior of the cable. In use, the moisture permeates slowly through the cable sheath, but difiuses relatively rapidly through the tube wall. The pulp insulation on the conductors in the tube absorbs most of the moisture that enters the cable. At 50 percent relative humidity, for example, pulp absorbs 7 /2 percent of its own weight in water from the surrounding atmosphere. The insulation on a pulp pair of wires may thus hold 100 times what is held in the atmosphere in the cable at that relative humidity.

Then, when it is desired to reduce the relative humidity within the cable, a current is passed through the conductors thereby to heat the moisture-laden desiccative insulation. Concurrently, gas pressure is applied to one end of the tube. Since the latter presents an extremely low longitudinal pneumatic resistance, a substantial high-velocity flow of dry air can be placed down the tube, thus purging the moisture at the far end.

In an alternate embodiment, the tube wall is rendered more permeable to moisture by the addition of a system of small perforations along its length. The prime mode of moisture passage is in this instance convection, rather than diffusion.

The invention and its further features, objects and advantages will be readily apprehended from a reading of the description to follow of an illustrative embodiment thereof.

DESCRIPTION OF THE DRAWING FIG. 1 is a front perspective view of a buried cable embodying the invention;

FIGS. 2 and 2A are two forms of purging tube housing the desiccative-insulated conductors; and

FIG. 3 is a side schematic diagram of a system embodying the invention.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT As seen in FIG. 1, a multipair telephone cable designated 10 and consisting of an outer sheath 11 with a plurality of twisted conductor pairs 12 therein, is shown cross-sectionally buried in damp earth. The conductor pairs 12 are, for example, the aforementioned plastic insulated conductors. The sheath 11 may, for example, consist of so-called ALPETH which is an aluminum layer 13 and an outer jacket 14 of polyethylene. This sheath is permeable to moisture.

Pursuant to the invention, a tube 15 as shown in all figures is placed in the cable and traverses the entire length thereof. As seen in FIGS. 2 and 2A tube 15 advantageously is a polyvinyl chloride tube which itself is highly permeable to moisture, even more so than the sheath 11. Within the tube 15 there are provided one or more pulp insulated conductor pairs 16. As shown in FIG. 3 the pair or pairs 16 are terminated at one end of the cable to a battery 17, for example, or other suitable power source; and at the other end of the cable, are grounded or connected to one another.

In one embodiment of the invention, the walls of tube 15 are provided with a plurality of tiny perforations 18 along their entire length, to increase their ability to pass moisture present in the cable interior to the tubes interior for absorption by the pulp-insulated pair. As shown in FIG. 3, the tube 15 is connected to a blower 18 for occasional purging; and at its other end exhausts to the atmosphere.

As an example of the operation of the invention, suppose that it is desired to maintain the relative humidity inside cable 10 to some value less than percent; such as, for example, 50 percent. At 50 percent relative humidity, wood pulp insulation absorbs about 7% percent of its weight in water. Thus, one or two 22-gauge pulp-insulated pairs such as pairs 16, each pairhaving approximately 0.4 gram of insulation per foot, will absorb as much moisture as the sheath 11 would pass per unit length in days. Furthermore, this is 100 times the amount of moisture contained in the air within the cable 10.

At the 50 percent relative humidity level, a standard pulp insulation withstands 1,000 volts of dc current without breaking down. A thin undercoat of plastic, varnish, or lacquer applied to the wire beneath the pulp substantially increases the breakdown voltage. If a 2,000-ohm loop (amounting to 11 miles of cable) of pulp-insulated conductor is at its far end, it will pass one-half ampere of current. Heat will be dissipated at a level of approximately 45 watts per mile. Given a sufficient time, this dissipation will warm the pulp insulation and evaporate whatever moisture has been absorbed. However if more wattage is desired, the two conductors may be connected in parallel using some other conductor to complete the circuit. Using the conductors in parallel and a low-resistance return, the same 1,000 volts would increase the wattage by almost a factor of four. The mentioned other conductor could, for example, be the metallic shield of the cable sheath. By the same expedient, it is possible to complete a circuit by using a single pulp-insulated conductor within the tube and the cable sheath.

Were the pair 16 to be contained within the cable I0 without the surrounding tube such as 15, water thus evaporated would condense on cooler parts of the cable. However, pursuant to the invention, the tube 15 provides an extremely low longitudinal pneumatic resistance; and consequently a purging of the moistened air therein by action of blower I8 quickly replaces the moist air within the tube with dry air.

For example, if the tube has an inside diameter of 0.6 inch, a flow of 30 cubic feet per hour results in a pressure drop of typically 0.1 psi per kilofoot. Thus if psi is applied in blower 18, 30 cubic feet per hour flows through tube 15. The exit pressure 11 miles away still would be approximately 5 psi. If 5 the flow is straight through tube 15, the velocity can be expected to be approximately 200 feet per minute. If the holes 18 are tiny, such as 0.002 inch in diameter, or if the moisture gets inside this tube by permeation only, then this 200 feet per minute velocity is readily realized and the moist air swept cleanly away.

An alternative to positive gas purging with above-atmospheric pressure, is to draw a vacuum on the tube from one end. One advantage of the latter expedient is an enhanced evaporation rate under the reduced pressure conditions.

Viewed more broadly, the invention is seen to be useful additionally in maintaining desired humidity levels in many telephone repeater housings, splice closures, power cables and many other applications. One implementation of its use in, for example, repeater housings is to form the tube as a coil within the housing, to expose added desiccative insulation.

An added advantage of the invention is its relative humidity monitoring capability. Conventional measurements of the dielectric properties of the pulp-insulated pair made from a remote location are translatable into a reading of relative humidity. These tests include measurement of insulation resistance, capacitance increase, dissipation factor and others. The tests can be performed manually, or can readily be applied automatically on either a continuous or intermittent basis. lf automatic, electrical measurement corresponding to a predetermined relative humidity level can be adapted to trigger a cycle of heating and gas purging.

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.

What is claimed is:

1. Apparatus for maintaining a desired relative humidity within the core of a sheathed plastic insulated conductor, communications cable, comprising in combination, a sheathed plastic insulated conductor communications cable with a core;

conductor means insulated with a desiccative material;

tube means disposed within said core, surrounding said conductor means and having a greater degree of moisture permeability than said cable sheath;

means for electrically energizing said conductor means to heat said material; and

means for longitudinal gas-purging of said tube.

2. Apparatus in accordance with claim 1, wherein said material comprises wood pulp insulation.

3. Apparatus in accordance with claim 2, wherein said tube further comprises plural perforations longitudinally disposed along said tube for enhancing moisture passage from within said cable sheath to within said tube.

4. A system for maintaining the relative humidity within the interior of a sheathed multipair plastic insulated conductor telephone cable to a desired level comprising in combination, a sheathed multipair plastic insulated conductor telephone cable;

a compliant moisture-permeable tube disposed along the length of said cable interior and including therein one or more pairs of wood pulp insulated conductor;

means connected to one end of said tube for applying a flow of dry gas thereinto;

means connected to said one or more conductor pairs for applying electrical current thereto sufficient to warm said pulp insulation; and

actuation of said named means combining to evaporate and purge from said tube moisture taken up by said desiccative insulation.

5. The system of claim 4, wherein the moisture-permeability of said tube is relatively high compared to that of the sheath of said cable.

Claims

1. Apparatus for maintaining a desired relative humidity within the core of a sheathed plastic insulated conductor, communications cable, comprising in combination, a sheathed plastic insulated conductor communications cable with a core; conductor means insulated with a desiccative material; tube means disposed within said core, surrounding said conductor means and haviNg a greater degree of moisture permeability than said cable sheath; means for electrically energizing said conductor means to heat said material; and means for longitudinal gas-purging of said tube.

4. A system for maintaining the relative humidity within the interior of a sheathed multipair plastic insulated conductor telephone cable to a desired level comprising in combination, a sheathed multipair plastic insulated conductor telephone cable; a compliant moisture-permeable tube disposed along the length of said cable interior and including therein one or more pairs of wood pulp insulated conductor; means connected to one end of said tube for applying a flow of dry gas thereinto; means connected to said one or more conductor pairs for applying electrical current thereto sufficient to warm said pulp insulation; and actuation of said named means combining to evaporate and purge from said tube moisture taken up by said desiccative insulation.