1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 , TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU#/.3425#4)/. G0G0).34!,!4)/.G0G0!.$02/4%#4)/.G0G0/ modified at Geneva, 1976, Malaga-Torremolinos, 1984 and Melbourne, 1988)1 GeneralBecause of the technological progress made in the use of aluminium, aluminiu
2、m cable sheaths are being usedon an increasing scale and their favourable characteristics can now be fully exploited.These characteristics include: low density (almost a quarter that of lead); much higher mechanical strength than lead, so that the sheath is lighter not only because aluminium islight
3、er than lead, but because the thickness may be less than for lead; very high resistance to vibration; high conductivity, so that a better screening factor and more effective protection from overvoltages ofatmospheric origin can be obtained.It is now found that the stiffness of an aluminium sheath do
4、es not give rise to any additional serious problemsduring laying.However, because aluminium is more vulnerable than lead to electrochemical and electrolytic corrosive action,aluminium cable sheaths and the joints between individual factory lengths (jointing sleeves and adjacent sections ofcable) req
5、uire a Class II (see 1) outer protective covering of plastic material.As can be seen from the foregoing, an aluminium sheath has many advantages over a lead sheath. Thegeneralized use of aluminium for sheathing cables is therefore desirable, at least whenever cable costs would not beincreased compar
6、ed with the use of lead, and also whenever aluminium sheaths satisfy the technical requirements to agreater extent. The use of cables with aluminium sheaths is particularly interesting in the case of trunk cables.2 Types of aluminium sheath2.1 Extruded sheathsThis type of sheath is obtained by extru
7、ding the aluminium directly around the cable core. The press may be ofthe continuous type or not. If it is not continuous, care must be taken to ensure that no problems are caused in the zonesaffected by the intermittent nature of the process.2.2 Welded sheathsThis type of sheath is made by applying
8、 around the cable core an aluminium strip which is longitudinallywelded.2.3 Quality of sheath materialIn order to make the means of protection against corrosion effective, great care has to be taken concerning thequality of the sheath. In case pure aluminium is used, the purity of aluminium for the
9、sheath should not be lower than99.5% grade, for both the extruded sheath or the welded sheath.2.4 Choice of sheath shape and thicknessAfter the sheath has been extruded or welded it may either be shrunk on to the cable core (noncorrugatedsheath) or corrugated by a variety of methods (corrugated shea
10、th).The sheath may be corrugated or noncorrugated, depending on the diameter of the cable core, the minimumradius of curvature during laying and on the mechanical characteristics of the aluminium used (see 2). As a roughguide it can be stated that the sheath should be corrugated in the case of cable
11、s of more than 40-mm core diameter.As stated in 1 above the thickness of the metal used for aluminium sheaths is usually less than for leadsheaths.2 Volume IX - Rec. L4The thicknesses given in Table 1/L.4 are suggested although the values given in this table apply to bothextruded and welded sheaths;
12、 however, extruded sheaths may not be less than 0.9 mm and welded sheaths may not bemore than 1.4 mm, that being the maximum thickness which can be welded by existing methods.The use of lesser thicknesses than those indicated in Table 1/L.4 is not excluded and, conversely, in the case ofcoaxial cabl
13、es without armouring, the thickness of metal for all sheaths may have to be increased to improvemechanical protection. The increase in the thickness may be as much as approximately 0.3 mm.Values different from those given in Table 1/L.4 may, of course, be adopted in certain cases (for example, ifext
14、remely favourable screening factors are required).3 Protective coveringsAs stated above, since aluminium used in an underground environment is more liable to corrosion than lead, animpermeable (Class II) covering should be provided in accordance with reference 1 to ensure the protection of thecable
15、sheath and the jointing sections of individual factory lengths of cable (jointing sleeves and adjacent sections ofcable).Two types of plastic material can be used at present for protective coverings:a) polyvinylchloride (PVC);b) polyethylene.Polyethylene is preferable since its general characteristi
16、cs and its low permeability for water vapour give betterprotection to the aluminium.To ensure that moisture which may have penetrated the protective covering (for example, because of a defect inthe covering) does not spread along the surface of the sheath, extending the areas of corrosion, it is ess
17、ential to apply aleakproof layer consisting of an adhesive tape or a suitable mixture.The leakproof layer must adhere well to the aluminium, especially when PVC is used for the covering, sincethis material, unlike polyethylene, does not cling tightly to the sheath after extrusion.The protective cove
18、ring on the aluminium sheath should be sound. One form of test with the cable on the drumis to measure the insulation resistance of the covering.TABLE 1/L.4Suggested thicknessCore diameter (mm) Metal thickness (mm)Minimum Maximum Noncorrugated sheaths Corrugated sheaths a)101520253035404550607010152
19、02530354045506070800.7 to 1.00.7 to 1.00.9 to 1.01.11.1 to 1.21.1 to 1.31.1 to 1.41.51.60.5 to 0.90.6 to 0.90.7 to 0.90.8 to 0.90.90.9 to 1.01.11.1 to 1.21.1 to 1.21.1 to 1.31.1 to 1.41.3 to 1.5Volume IX - Rec. L4 3a)If it is intended to obtain approximately the same screening factor with a corrugat
20、ed sheath as with a noncorrugated one, the thickness should be the same as with a noncorrugated sheath.In the case of corrugated sheaths, the bituminous mixture must fill the corrugations sufficiently to allowcomplete contact with the outer covering.Special tests should be made of the efficiency of
21、the leakproof layer. A common test consists in removing a partof the protective covering from a sample of the aluminium sheath and submitting it to electrolytic attack using anoutside source of e.m.f. After some time, a check must be made to see whether the corrosion is confined to the placefrom whi
22、ch the protective covering was removed. The effectiveness of the protective covering can be assessed bymeans of a test to check the adhesion of the bituminous compound to both the aluminium sheath and the plasticcovering.To ensure the permanent effectiveness of the protective covering when cables ar
23、e laid in areas exposed tolightning discharges (in particular as concerns avoiding perforations due to lightning discharges) the indications givenin the manual cited in 3 should be taken into account.If a test of the protective covering is necessary in the manufacturing process, an electric spark de
24、tect method ora voltage resistance test method with the cable submerged in water is effective. In the process of installation andoperation, if the factors that might cause damage to the protective covering or decrease the protective coveringsinsulation resistance are to be found, the test should be
25、carried out and the faults should be eliminated.4 Jointing of aluminium sheathsJointing is undoubtedly a more difficult operation for aluminium than for lead sheaths, although thesedifficulties have been minimized by improved techniques.There are several methods of jointing aluminium sheaths: jointi
26、ng by means of lead sleeves; jointing by means of lead rings or cones which are plumbed using a normal method or fixed with specialglue to the aluminium sheath to permit subsequent soldering to lead sleeves; jointing by means of aluminium sleeves joined to the aluminium sheath by pressure welding (e
27、xplosion,pressure or cold welding); other methods including the use of adhesive tapes and epoxy pastes.The methods used for the jointing of aluminium sheaths must meet the conditions recommended in the bookletcited in 4.For an aluminium-sheathed cable subjected to significant temperature variations,
28、 tensions due to cablecontraction should not be borne by the joints as this can lead to joint failure, particularly with noncorrugated sheaths.5 Cathodic protectionThe corrosion protection of aluminium sheaths depends mainly on a high quality anti-corrosion protectivecover. However, if there is seri
29、ous risk of damage to the protective cover, and particularly if it is not possible to re-establish the protective cover to its original specifications after repair, the cover should be protected with specialmeasures such as sacrifice anode electrical chemical protection. Aluminium alloy sacrifice an
30、ode, which has theadvantage of a higher current capacity per unit weight, an appropriate protective potential, an abundant raw materialresource base, and ease of manufacture, is an effective measure to protect aluminium sheathed cables. Tests show thatgood results can be obtained if the protected al
31、uminium sheath potential value with respect to ground is limited withinthe range of 0.85 to 1.20 V (relative to a Cu/CuSO4electrode).6 If there are no special requirements in using aluminium sheaths for optical fibre cables, the same sheathmaterial and manufacturing process may be used as for metall
32、ic conductor cables.References1 CCITT manual Outside plant technologies for public networks, Part IV-A, Chapter III, 1.2.2, ITU, Geneva1988.2 Ibid., Part I, Chapter III, 6.2.2.3 CCITT manual The protection of telecommunication lines and equipment against lightning discharges, ITU,Geneva, 1974, 1978.4 CCITT manual Jointing of plastic-sheathed cables, ITU, Geneva, 1978.
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