Method Of Mitigating Sulfide Trees In Polyolefin Insulated Conductors

Abstract

In an electric cable provided with a plastic sheath and an insulating layer consisting of polyolefin series resin applied directly or with the aid of other insulating layer on the copper conductor, polyolefin-series resin insulated electric cable provided in a desired position with a sulfide-capture layer consisting of a polyolefin-series resin composition incorporated with such powder of metals, salts of the metals, or the mixture thereof as to form water-insoluble metal sulfides by reacting with water soluble sulfides.

Description

This invention relates to polyolefin-series resin insulated electric cables and, more particularly, to polyolefin-series resin insulated electric cables provided with a means of preventing the deterioration of the polyolefin-series resin insulation due to chemical trees (hereinafter referred to as sulfide trees).

Polyolefin-series resins (including polymers of an olefin, copolymers of two or more olefins and cross-linked polyolefins prepared by using an organic peroxide as a cross-linking agent) have inherently excellent electric insulating and anti-chemical properties. Accordingly, they have been extensively used as the electric insulating material for electric cables. However, it has recently been found that the polyolefin series resin insulated electric cables laid in chemical plants and in water often result in unexpected dielectric breakdown. Detailed examination of such troubled cables reveals that copper sulfide and copper oxide grow in tree-like form within the polyolefin-series resin insulation to constitute leakage paths (these tree-like leakage paths are hereinafter referred to as sulfide trees to distinguish them from the so-called trees, which are electrically created in the insulation in a high electric field). As result of extensive investigations conducted by the inventors about the formation of the sulfide trees, it has been verified that the sulfide trees will result if the polyolefin-series resin insulated electric cable is located at places where the water soluble sulfide will be produced. It frequently occurs in the pits of chemical plants that the cable is unintentionally exposed to the water-soluble sulfide either in gase or aqueous solution. Hydrogen sulfide is produced through the luxuriation of zostera on the sea-bed. It is also produced in the presence of sulfate-reducing bacteria. In these surroundings, hydrogen sulfide or sulfur ions will gradually penetrate by the aid of and together with water through the cable sheath and the insulator of the cable core to reach the periphery of the copper conductor, where the hydrogen sulfide or sulfur ions will react with copper conductor to produce the water insoluble copper sulfide. The copper sulfide thus produced will sometimes be oxidized to produce copper oxide. As copper sulfide is crystallizable, it grows as tree-like crystals within the polyolefin-series resin insulation at certain temperatures. The growth rate is usually slow, but will be accelerated according to the electric field and crystal structure of polymer. The copper sulfide trees thus grown eventually penetrate the entire thickness of the insulation and cause the dielectric breakdown of the cable.

In order to prevent its phenomenon of the sulfide trees, various measures have been proposed. In one such measure, the cable sheath may further be covered with a metal, which is strongly resistant against the water-soluble sulfide, for instance, lead, to prevent the penetration of said sulfide into the electric cable. In a second measure, a laminate tape consisting of a polyolefin film and an aluminum foil may be applied along or wound on the periphery of the insulated cable core, so that the penetration of the water-soluble sulfide such as hydrogen sulfide is stopped by the metal layer interposed between the insulated cable core and the cable sheath.

These measures are indeed very effective. However, the former measure is only justified if it is known that there is water-soluble sulfide in the place where the cable is laid. If the cable is to be laid in places where the presence of the water-soluble sulfide is not prediatable, this measure is not advantageous because the metal sheath increases the cost and weight of the cable. The latter measure is also not economical because of the provision of the laminate tape. Besides, it is possible that during long period of use the water-soluble sulfide will permeate through the seam between adjacent tape edges to give rise to the growth of the sulfide trees. Thus, the second measure is not so effective in spite of the increased steps and cost of manufacture of the cable.

The invention, accordingly, is intended from the foregoing aspects, and it has an object of providing a polyolefinseries resin insulated electric cable, which is simple in construction and capable of preventing the growth of sulfide trees over a long period of use.

The present invention is based on the following finding: In polyolefin-series resin insulated electric cables provided with a plastic sheath outside the insulated cable core comprising an insulating layer consisting of polyolefin-series resin applied directly or with the aid of other insulating layer on the copper conductor or, if necessary, around a desired number of insulated cores, stranded together, it was found that hydrogen sulfide, ammonium sulfide, etc. penetrating into the cable from outside could be trapped in a sulfide-capture layer in the form of water-insoluble metal sulfides and that the sulfide tree could be prevented in the polyolefin-series insulating layer, by providing a sulfide-capture layer of polyolefin-series resin composition containing such powder of metals, salts of the metals, or the mixture thereof as to form water-insoluble metal sulfides by reaction with water-soluble sulfide, in a desired position that is on the inside, outside, or both sides of said polyolefin-series resin insulating layer or around a desired number of insulated cores stranded together.

The aforesaid sulfide-capture layer made of a polyolefin-series resin composition containing metals or metal salts capable of reacting with the water-soluble sulfide such as hydrogen sulfide to produce a water-insoluble metal sulfide may concurrently serve as a semiconducting layer usually provided on the outer or inner side or on both sides of the polyolefin-series resin insulating layer, or it may be provided separately from the semiconducting layer. When the sulfide-capture layer is provided on the outer side of the polyolefin-series resin insulating layer, it may concurrently serve as the protective sheath layer. In 3-core electric cables, for example, the jute filler beneath the protective sheath may be replaced with the polyolefin-series resin composition containing metals or metal salts according to the invention. Thus, the end of preventing the growth of the sulfide trees can be achieved by providing the sulfide-capture layer around the copper conductor of the polyolefin-series resin insulated electric cable.

To this end, in accordance with the invention it is preferable that a metals or metal salts to be incorporated into the sulfide-capture polyolefin-series resin layer are water-insoluble and that the metal sulfide that will be produced by the reaction of metals or metal salts with the water-soluble sulfide such as hydrogen sulfide are also water insoluble. The above metals may be zinc, cadmium, silver, cobalt, strontium, bismuth, gold, tin, iron, copper, lead, nickel, antimony, manganese, vanadium, tellurium, and the above metal salts may be oxides, hydroxides, sulfates, chlorides, nitrates, carbonates and aliphatic and aromatic organic acid salts of aforementioned metals. The grain size of these metals or metal salts should not be too large, and is preferably below 100 meshes. Among the aforementioned metal salts, such salts of lead as lead oxide [PbO], lead hydroxide [Pb(OH).sub.2 ], lead carbonate [PbCO.sub.2 ], lead nitrate [Pb(NO.sub.3).sub.2 ], lead chloride [PbCl.sub.2 ], lead acetate [Pb(CH.sub.3 CO.sub.2).sub.2 ], lead sulfate [PbSO.sub.4 ], lead chromate [PbCrO.sub.4 ], lead peroxide [PbO.sub.2 ], red lead [Pb.sub.3 O.sub.4 ], lead sesquioxide [Pb.sub.2 O.sub.3 ], white lead [2PbCO.sub.3 -- Pb(OH).sub.2 ], lead stearate [Pb(C.sub.18 H.sub.35 O.sub.2).sub.2 ], mono-basic lead acetate [Pb.sub.2 O(C.sub.2 H.sub.3 O.sub.2).sub.2 ], basic lead silicate [PbO.sup.. H.sub.2 O.sup.. 2PbSiO.sub.3 ], tribasic lead sulfate [3PbO.sup.. PbSO.sub.4.sup.. H.sub.2 O], dibasic lead phosphite [2PbO.sup.. PbHPO.sub.3.sup.. 1/2H.sub.2 O], dibasic lead phthalate [2PbO.sup.. Pb(C.sub.8 H.sub.4 O.sub.4)], tribasic lead maleate [3PbO.sup.. Pb(C.sub.4 H.sub.2 O.sub.4)H.sub.2 O], lead salicylate [Pb(C.sub.6 H.sub.4 (OH)CO.sub.2).sub.2 ] and dibasic lead strearate [2PbO.sup.. Pb(C.sub.17 H.sub.35 COO).sub.2 ], such salts of zinc as zinc oxide [ZnO], zinc hydroxide [Zn(OH).sub.2 ], zinc sulfate [ZnSO.sub.4 ], zinc chloride [ZnCl.sub.2 ], zinc carbonate [ZnCO.sub.3 ], zinc stearate [Zn(C.sub.17 H.sub.35 CO.sub.2).sub.2 ], zinc laurate [Zn(C.sub.11 H.sub.23 CO.sub.2).sub.2 ], and zinc ricinoleate [Zn(C.sub.17 H.sub.32 (OH)CO.sub.2).sub.2 ], and such salts of cadmium as cadmium oxide [CdO], cadmium hydroxide [Cd(OH).sub.2 ], cadmium sulfate [CdSO.sub.4 ], cadmium chloride [CdCl.sub.2 ], cadmium carbonate [CdCO.sub.3 ], cadmium stearate [Cd(C.sub.17 H.sub.35 CO.sub.2).sub.2 ], cadmium laurate [Cd(C.sub.11 H.sub.23 CO.sub.2).sub.2 ] and cadmium ricinoleate [Cd(C.sub.17 H.sub.32 (OH)CO.sub.2).sub.2 ] are particularly preferable as they are readily miscible with polyolefin-series resin, inexpensive and readily available.

In accordance with the invention, the polyolefin-series resin, to which the aforementioned powdery metals or metal salts are to be incorporated, may include high density polyethylene, medium density polyethylene, low density polyethylene, ethylene-vinylacetate copolymer, ethylene-ethylacrylate copolymer, ionomer, isotactic polypropylene and isotactic polybutene-1. These polyolefin-series resins are suitable for they are inherently excellent in water proofness, and less likely to undergo the degradation of physical properties even when the aforementioned powdery metals or metal salts are added thereto.

The invention will now be described by having reference to the accompanying drawings, in which:

FIG. 1 is a sectional view of a polyethylene insulated electric cable provided with a sulfide-capture layer, which contains a metal or a metal salt capable of reacting with the water-soluble sulfide to produce a water-insoluble metal sulfide between a copper conductor and a polyethylene insulating layer, embodying the invention;

FIG. 2 is a sectional view of a cross-linked polyethylene insulated electric cable provided with a sulfide-capture layer, which contains a metal or metal salt capable of reacting with the water-soluble sulfide to produce a water-insoluble metal sulfide, covering a cross-linked polyethylene insulating layer, embodying the invention;

FIG. 3 is a sectional view of a polyethylene insulated three-core electric cable provided with a sulfide-capture layer surrounding three polyethylene insulated cable cores, embodying the invention; and

FIGS. 4 and 5 illustrate the sulfide-capture effects obtainable in accordance with the invention.

In the embodiment of FIG. 1, a copper conductor 101 having a desired diameter is covered with a sulfide-capture coating 102 consisting of a polyolefin-series resin composition containing the aforementioned metals or metal salts. The sulfide-capture coating 102 is covered with a polyolefin-series resin insulating layer 103 having a desired thickness, which is in turn covered with an outermost plastic cable sheath 105.

In the embodiment of FIG. 2, the sulfide-capture coating 102 of a polyolefin-series resin composition containing the aforementioned metals or metal salts nature is provided on the outer side of the polyolefin-series resin insulating layer 103.

In the embodiment of FIG. 3, three insulated cable cores, each having a copper conductor 101 and a polyolefin-series resin insulating layer 103 thereon, are stranded together with a filler 104. These stranded cable cores are covered with the sulfide-capture layer 102, which is in turn covered with the outermost protective plastic cable sheath 105.

In addition to the above embodiments, the aforementioned metals or metal salts may be incorporated into the outermost protective plastic cable sheath 105 or the semiconductive layer usually provided on the outer or inner side of the usual insulating layer 103 so that the sulfide-capture substance-incorporated layer may also serve as the sulfide-capture coating 102. It is particularly advantageous in manufacture and from the standpoint of economy to incorporate metals or metal salts capable of reaction with such water-soluble sulfide as hydrogen sulfide to produce metal sulfides in the semiconductive layer on the inner or outer side of the usual insulating layer of the electric cable. In any case, an excellent sulfide trapping effect can be expected from the addition of the aforementioned metals or a metal salts to any other layer surrounding the copper conductor than the polyolefin-series resin insulating layer 103.

In this invention, the lower limit of the amount of metals or metal salts to be added to a polyolefin-series resin composition constituting the sulfide-capture coating depends upon the coefficient of water permeability of the polyolefin-series resin composition. With a polyolefin-series resin composition whose water permeability coefficient at a temperature of 40.degree. C. is below 20.times.10.sup.12 (gcm/cm..sup.2 sec. cm. Hg), for instance, high, medium and low-density polyethylene and polypropylene, at least 1 part by weight of the above metals or metal salts should be added to 100 parts by weight of the resin. With a resin having a permeability coefficient of above this value, the addition of at least 10 parts by metal weight of the metals or metal salts to 100 parts by weight of the resin is necessary. The dependence of the range of the amount of the metals or metal salts to be added to the polyolefin-series resin composition constituting the sulfide-capture coating upon the water permeability of the resin used stems from the fact that the substantial proportion of the accidents due to the penetration of the water-soluble sulfide have actually taken place where the cable is laid in water, with water acting as the carrier of a sulfide. There is no upper limit of the amount of the above metals or metal salts to be added so long as the aforementioned end alone is taken into consideration. However, if more than 100 parts by metal weight of the above metals or metal salts are added to 100 weight parts of the polyolefin-series resin used, the degradation of physical and chemical properties of the resin becomes outstanding depending upon the kind of the resin. For this reason, the preferable range of the amount of the above metals or metal salts is practically less than 150 weight parts, and more practically 5 to 60 parts by metal weight, with respect to 100 parts by weight of the resin.

The effects according to the invention will become more apparent from the results of experiments given below.

EXPERIMENT 1

Sample rods 10 mm. in diameter and 100 mm. in length were made from compositions listed in Table 1 below. These sample rods were immersed either in the aqueous solution of ammonium sulfide or in the saturated aqueous solution of hydrogen sulfide for a certain time. After each of the successive time intervals they were drawn out of the solutions and radially severed to determine the sulfide penetration speed by measuring the thickness of the portion blackened due to formation of lead sulfide. The smaller the thickness of the blackened portion, the greater, it was judged, the sulfide trapping effect. The results of experiments using the aqueous solution of ammonium sulfide are shown in FIG. 4, and those using the saturated aqueous solution of hydrogen sulfide are in FIG. 5. --------------------------------------------------------------------------- TABLE

1 Sulfide- Sample rod Base Resin Capture Additive content additive (in weight parts) 1 Polyethylene White Lead 10 2 Polyethylene White Lead 20 3 Polyethylene White Lead 40 __________________________________________________________________________

As is apparent from FIGS. 4 and 5, so far as the effect of trapping the water-soluble sulfide is concerned, it is increased by higher content of the metal or metal salt additive capable of reacting with the water-soluble sulfide to produce a metal sulfide.

EXPERIMENT 2

The results of experiments conducted about the effect of adding the metals or metal salts capable of trapping the sulfide to the base resin on the processibility and mechnical properties of the resultant composition are given in Table 2 below. ##SPC1##

EXPERIMENT 3

Sample sheets were made of polyolefin-series resin compositions shown in Table 3 below. ##SPC2##

Fifty-two sample pieces were prepared, each measuring 1.0-mm.-thick, 40-mm.-wide, and 200-mm.-long and composed of a copper plate, 0.5-mm.-thick, 20-mm.-wide, and 160-mm.-long, completely covered by hot-pressing with different kinds of polyolefin-series resin composition. These sample pieces were then immersed in the saturated aqueous solution of hydrogen sulfide. After 35 days of immersion, each copper plate was stripped of its resin covering and checked for blackening due to formation of copper sulfide. In Samples Nos. 24, 33, 41, 44, 48 and 52, whose covering did not contain any powdery metal or metal salt, remarkable blackening of the copper plate was observed.

As is apparent from the above three kinds of experiments, for formation of the sulfide-capture layer, it is desirable to determine the content of sulfide-capture additives, i.e., metals or metl salts, according to their kinds as well as the kind of the base resin in which they are used.

Some examples of the fabrication of the polyolefin-series resin insulated electric cable according to the invention will now be described.

EXAMPLE 1

On a copper conductor 22 sq. mm in cross section was formed a 5-mm.-thick insulating layer of polyethylene (with density of 0.92 and melt index of 2.0), which was then extrusion-coated to a thickness of 2.0 mm. with a semiconductive polyethylene composition consisting of 100 weight parts of ethylene-vinylacetate copolymer (with a vinylacetate content of 3 percent), 20 weight parts of carbon black and 20 weight parts of white lead, which was in turn covered with a polyethylene sheath, 1.5-mm.-thick, to complete a polyethylene insulated electric cable. As a comparison sample, an electric cable similar to the above, but free from white lead was also produced.

These electric cables were then immersed for 245 days in an aqueous solution of ammonium sulfide held at room temperature. Then, they were taken out of the solution, and their coverings were removed off the conductor. In the cable according to the invention, the copper conductor was found blackened only in a minor degree, and the formation of lead sulfide was observed in the conductive polyethylene layer, but no sulfide trees were observed in the polyethylene insulating layer. On the other hand, in the comparison cable the copper conductor was observed to be blackened, and tree-like crystals of copper sulfide about 0.6 mm. long were seen growing in the polyethylene insulating layer.

EXAMPLE 2

A copper conductor 22 sq. mm. in cross section was extrusion-coated to a thickness of 1.2 mm. with a semiconductive polyethylene composition consisting of 100 weight parts of ethylene-vinylacetate copolymer (with a vinylacetate content of 3 percent), 20 weight parts of carbon black and 20 weight parts of litharge, on which was then formed a cross-linked polyethylene insulating layer, 3 mm. in thickness, which was in turn extrusion coated to a thickness of 1.2 mm. with the semiconductive polyethylene composition consisting of 100 weight parts of ethylene-vinylacetate copolymer (with a vinylacetate content of 3 percent), 20 weight parts of carbon black and 20 weight parts of litharge, and the resultant cable core was finally covered with a polyethylene sheath, 1.5 mm. in thickness to complete a polyethylene insulated electric cable. A comparison cable similar to the above, but free from litharge was also produced.

These electric cables were immersed for 490 days in the saturated aqueous solution of hydrogen sulfide held at a temperature of 50.degree. C. Then they were taken out of the solution, and their copper conductor was stripped of the coverings. In the cable according to the invention, no copper sulfide crystals were observed in the cross-linked polyethylene insulating layer. On the other hand, in the comparison cable extreme blacking of the copper conductor was observed and tree-like copper sulfide crystals, about 1.0 mm. long, were observed in the cross-linked polyethylene insulating layer.

EXAMPLE 3

Three insulated cable cores, each comprising a conductor consisting of seven copper wires (0.8 mm. in diameter) stranded together and polyethylene (with density of 0.92 and melt index of 2.0) insulating layer, 0.8-mm.-thick, thereon were stranded together with jute. On the resultant strand was wound a cotton tape, which was then extrusion coated to a thickness of 2.0 mm. with a polyethylene composition consisting of 100 weight parts of polyethylene, 2.5 weight parts of carbon black and 40 weight parts of white lead to complete a 3-core polyethylene insulated control cable. A comparison cable similar to the above free from white lead in the cable sheath was also produced.

These control cables were left immersed for 147 days in an ammonium sulfide solution while being charged with 200 volts at room temperature. Thereafter, they were broken for examination. In the cable according to the invention, no formation of sulfide copper crystals was recognized in the polyethylene insulating layer. On the other hand, in the comparison cable tree-like copper sulfide crystals about 0.4 mm. long, were seen growing in the polyethylene insulation.

EXAMPLE 4

A conductor consisting of seven copper wires (0.8 mm. in diameter) stranded together was covered with an insulating layer, 1 mm. in thickness, of polypropylene (with density of 0.90 and melt index of 1.0), which was then extrusion coated to a thickness of 1.5 mm. with a polyethylene composition consisting of 100 weight parts of polyethylene (with density of 0.92 and melt index of 0.3) and 35 weight parts of zinc oxide (ZnO) to form a sulfide-capture layer, which was in turn extrusion coated to a thickness of 1.5 mm. with a polyethylene composition (with carbon black content of 2.5 percent, density of 0.93 and melt index of 0.3) forming the cable sheath, thus producing a model cable. A comparison cable similar to the above, but free from zinc oxide in the polyethylene composition polypropylene insulating layer was produced.

These cables were then immersed for 6 months in the saturated aqueous solution of hydrogen sulfide held at a temperature of 50.degree. C. while AC voltage of 400 volts was applied to them, thus causing forced deterioration of them. Before the forced deterioration, their insulation resistance was 7.17.times.10.sup.5 (m.OMEGA.-km). After the forced deterioration for 6 months, the insulation resistance of the comparison cable decreased to 3.90.times.10.sup.3 (M.OMEGA.-km), whereas the insulation resistance of the cable according to the invention decreased merely to 2.45.times.10.sup.5 (M.OMEGA.-km). Detailed examination of these deteriorated cables by breaking them revealed that in the cable according to the invention the copper conductor had undergone only slight blackening, and the polypropylene insulating layer contiguous to the conductor had undergone no change. On the other hand, in the comparison cable a great deal of black copper sulfide crystals about 10.mu. long were seen growing in the polypropylene insulating layer.

EXAMPLE 5

Three insulated cable cores, each comprising a conductor consisting of seven copper wires (0.8 mm. in diameter) stranded together and a 0.8-mm.-thick cross-linked polyethylene (with gel fraction of 78 percent and density of 0.92) insulating layer thereon, were stranded together with jute. On the resultant strand was wound a cotton tape, which was then extrusion coated to a thickness of 1.5 mm. with a polyethylene composition consisting of 100 weight parts of high density polyethylene (with desity of 0.96 and melt index of 0.2) and 45 weight parts of cadmium sulfate (CdSO.sub.4), which was in turn extrusion coated to a thickness of 1.5 mm. with a polyethylene composition (with carbon black content of 2.5 percent, density of 0.93 and melt index of 0.3) forming the cable sheath, thus producing a model cable. A comparison cable similar to the above, but free from cadmium sulfate in the high density polyethylene composition was produced.

These cables were then immersed for 6 months in the saturated aqueous solution of hydrogen sulfide held at a temperature of 50.degree. C. while AC voltage of 400 volts was applied to them, thus causing forced deterioration of them. Detailed examination of these deteriorated cables by breaking them revealed that in the cable according to the invention the copper conductor had suffered no corrosion. On the other hand, in the comparison cable a great deal of black copper sulfide crystals about 70.mu. long were seen growing uniformly in the cross-linked polyethylene insulating layer.

As is apparent from the above examples, the sulfide-capture layer may be provided between the copper conductor and the insulating layer thereon, or alternatively the sulfide-capture additives, which are powdery metals or metal salts capable of reaction with hydrogen sulfide, etc. to produce metal sulfides, may be added to a covering layer such as an outer semiconductive layer or protective plastic sheath layer.

It will be appreciated that according to the invention the end of preventing the dielectric breakdown of the polyolefin-series resin insulated electric cable (laid at places where they are affected by chemicals or on the sea-bottom) due to growth of sulfide trees in the insulation resulting from the reaction of a water-soluble sulfide with the copper conductor can be achieved by preventing the growth of the water-insoluble copper sulfide crystals in the insulation through addition of metals or metal salts, which can actively react with the water soluble sulfide entering the cable from outside such as hydrogen sulfide, to produce a water-insoluble metal sulfide, to at least one of the covering layers surrounding the copper conductor other than the polyolefin-series resin insulating layer. Thus, the above examples are by no means limitative, but various changes and modifications may be made without departing from the scope of the invention.

As has been described in the foregoing, according to the invention the sulfide-capture layer surrounding the copper conductor of the polyolefin-series resin insulated electric cable and containing metals or metal salts capable of reacting with the water-soluble sulfide to produce a water-insoluble metal sulfide, completely captures the water-soluble sulfide entering the cable from the outside and renders it into a metal sulfide insoluble in water, so that the complete prevention of the growth of copper sulfide crystals, the so-called sulfide trees, constituting the leakage paths in the polyolefin-series resin insulating layer may be ensured to provide stable insulating characteristic of the cable insulation over a long period of use.

Claims

What we claim is:

1. The method of mitigating formation of sulfide trees in an insulating layer of an insulated electric cable having at least one insulated cable core comprising a copper conductor encased within an insulating layer of polyolefin-series resin and a separate plastic sheath surrounding said cable core which comprises providing said electric cable with a sulfide capture layer separate from said insulating layer and said plastic sheath, said sulfide capture layer consisting essentially of a layer of polyolefin-series resin composition containing about 5 to 150 parts by weight per 100 parts of resin of a substance or mixture of substances capable of reacting with water-soluble sulfides to form water-insoluble sulfides selected from the group consisting of powdered zinc, cadmium, silver, cobalt, strontium, bismuth, gold, tin, iron, copper, lead, nickel, antimony, manganese, vanadium and tellurium, and the oxides, hydroxides and salts thereof.

2. A method according to claim 1, wherein the material of said polyolefin-series resin insulating layer is selected from the group consisting of high-density polyethylene, medium-density polyethylene, low-density polyethylene and cross-linked polyethylene.

3. A method according to claim 1, wherein the number of said insulated cable cores are at least three.

4. A method according to claim 1, wherein said sulfide-capture layer surrounds said insulated cable core.

5. A method according to claim 1, wherein said sulfide-capture layer is contiguous to at least one side of said polyolefin-series resin insulating layer of said insulated cable core.

6. A method according to claim 1, wherein the material of said sulfide-capture layer is of a composition consisting of 100 parts by weight of at least one base resin selected from the group consisting of high-density polyethylene, medium-density polyethylene, low-density polyethylene, ethylene-vinylacetate copolymer, ethylene-ethylacrylate copolymer, ionomer, isotactic polypropylene and isotactic polybutene-1 and 5 to 60 parts by metal weight of at least one substance selected from the group consisting of powdery metals salts of said metals, and the mixture thereof capable of reaction with the water-soluble sulfide to produce a water-insoluble sulfide.

7. A method according to claim 1, wherein the material of said sulfide-capture layer contains at least one substance selected from the group consisting of oxides, hydroxides, sulfates, chlorides, nitrates, carbonates and aliphatic and aromatic organic acid salts of lead, zinc, bismuth, cadmium, copper, iron and tin.

8. A method according to claim 1, wherein the material of said sulfide-capture layer contains a lead salt selected from the group consisting of lead oxide, lead hydroxide, lead carbonate, lead nitrate, lead chloride, lead acetate, lead sulfate, lead chromate, lead perioxide, red lead, lead sequioxide, white lead, lead stearate, monobasic lead acetate, basic lead silicate, tribasic lead sulfate, dibasic lead phosphite, dibasic lead phthalate, tribasic lead maleate, lead salicylate and dibasic lead stearate.

9. A method according to claim 1, wherein the material of said sulfide-capture layer contains a zinc salt selected from the group consisting of zinc oxide, zinc hydroxide, zinc sulfate, zinc chloride, zinc carbonate, zinc stearate, zinc laurate and zinc ricinoleate.

10. A method according to claim 1, wherein the material of said sulfide-capture layer contains a cadmium salt selected from the group consisting of cadmium sulfate, cadmium chloride, cadmium carbonate, cadmium stearate, cadmium laurate and cadmium ricinoleate.