1、INTERNATIONAL TELECOMMUNICATION UNION#)44 + THE INTERNATIONALTELEGRAPH AND TELEPHONECONSULTATIVE COMMITTEE02/4%#4)/.G0G0!).34G0G0).4%2b) protection against effects of electric power and traction lines: soil resistivity (in deep layers), inducing current, distance between the telecommunication cable
2、and the electric power and traction line;c) protection against corrosion: soil resistivity (in the top layer), corrosive components (ions) in the soil, stray currents in the soil (sources of d.c. stray currents are d.c. traction rails, earthing electrodesassociated with d.c. sources, metallic struct
3、ures with cathodic protection and the like).Protection of underground communication cables against corrosion is achieved mainly with protective plasticcoverings on the metal cable sheath.Regarding the environment, the installed cable behaviour is different if an insulating or conductive plasticmater
4、ial is used as protective covering. Therefore, coordinated protection schemes are also different and the two casesshould be considered separately.2 Coordinated protection with insulating plastic coverings2.1 Metallic cablesCoordinated protection of telecommunication metallic cables, i.e. cables with
5、 metal conductors (symmetricalor coaxial pairs), against induction, lightning and corrosion with insulating plastic coverings requires the considerationof: suitable metal sheath; adequate covering with adequate dielectric strength;2 Recommendation K.29 earth connections of the metal sheath; use of a
6、 shield wire. The manual “The protection of telecommunication lines and equipment againstlightning discharges” contains useful information for determining the need for shield wires.If an Administration has positive experience with respect to corrosion, then periodic inspection of theinsulating cover
7、ing is not necessary. In areas where stray currents exist or corrosion due to interference currents has beenobserved, periodic inspection may be recommended.The distance, d between earthing electrodes and their resistance, R can be established on the basis of: the specific calculation methods, takin
8、g into account the limit values for the voltage between any metalcomponents and earth due to induction from power and traction lines (ref. “Directives concerning theprotection of telecommunication lines against harmful effects from electric power and electrified railwaylines”); the transfer impedanc
9、e of telecommunication cables; the thunderstorm activity, i.e. number of thunderstorm days per year or ground flash density.Nevertheless, the following indications about the values of d and R can be taken into account: local cables are usually earthed only at the ends; for extended cable installatio
10、ns it is recommended toearth the cables additionally at intermediate points; long-distance cables are earthed at the repeater stations or additionally at intermediate locations.Multipoint sacrificial anodes may be considered as earth electrodes; in such cases, the earth resistance valueshave to be c
11、ontrolled during their lifetime. Such a control can be recommended for normal earth electrodes.2.2 Optical fibre cables2.2.1 Metal-free cablesMetal-free cables do not require protection against power induction, lightning damage and metal corrosion,although lightning damage to such cables is conceiva
12、ble when they are installed below ground in metal ducts.2.2.2 Metal components in the cable core and sheathFrom the protection point of view, this type of cable is the same as a metal cable; therefore the coordinatedprotection, described in 2.1, applies.Nevertheless, the poor shielding effect of the
13、 moisture barrier may not allow a sufficient reduction of theovervoltages inside the cable and may require, as a consequence, the installation of surge arresters between the metalpairs and the moisture barrier, and the bonding of the strength member to the moisture barrier.The use of cables which ha
14、ve metal components, but adequate resistibility to lightning surge currents (inagreement with Recommendation K.25 “Lightning protection of optical fibre cables”), can offer an adequate shieldingeffect against induction from power or traction lines if the metal sheath is earthed following the criteri
15、a indicatedin 2.1.Recommendation K.29 32.2.3 Metal-free cable coreIn this case the cable has only one metal component, the sheath(s).The protection against lightning can be achieved selecting a cable with a higher capability to withstandlightning surge currents or with the installation of shield wir
16、es following the criteria suggested in 2.1.The protection against induction can be obtained keeping the shield(s) continuous at splices, providingappropriate earthing at repeaters and providing earth electrodes at splices only where required to limit the shield-earthvoltage to values below the limit
17、s.Another coordinated protection scheme can be the use of both of the following protection means: the installation of shield wires; the interruption of the metal shield, i.e. the moisture barrier, at each splice or additionally at intermediatelocations as required to keep the induced shield-earth vo
18、ltage values below the limits.3 Coordinated protection with conductive plastic coverings3.1 Metallic cablesCoordinated protection of telecommunication metallic cables, i.e. cables with metal conductors (symmetricalor coaxial pairs), against induction, lightning and corrosion with a conductive plasti
19、c covering, requires theconsideration of: suitable covering characteristics; use of shield wires when conditions warrant. Generally their use is considered as well for bare cables.The manual “The protection of telecommunication lines and equipment against lightning discharges” containsuseful informa
20、tion for determining the need for shield wires.Along the line, ground connections of the metal sheath are not necessary.The compound type below the conductive coverings may be the same as for non-conductive coverings.On the basis of some experimental results, the possible limit values of the main pr
21、operties of the conductiveplastic covering to be considered are shown in Tables 1/K.29 and 2/K.29.In particular the chemical characteristics in Table 1/K.29 are recommended to render negligible the effects ofthe galvanic corrosion between conductive plastic coverings and metals used in buried teleph
22、one plants.Though periodic inspection of the covering is not possible, it is also not important since the low corrosion rateof the metal sheath renders the corrosion damages negligible.Safety for personnel is assured by the continuous earthing of the metal sheath; the need for protection againstelec
23、tromagnetic induction is unlikely, although, in some cases it may be necessary.4 Recommendation K.29TABLE 1/K.29Limit values of conductive plastic covering characteristicsTABLE 2/K.29Limit values of electrical, mechanical and physicalcharacteristics of the conductive plastic covering3.2 Optical fibr
24、e cablesThe conductive plastic covering can also be used around a metal moisture barrier of an optical fibre cable; inthis case the coordinated protection described in 3.1 applies.Up to now, conductive plastic covering has not been experienced in optical cable plants.PropertiesConductive coveringTest methodPE PVCCarbon blackcontent (%)9 9IEC 811-1-1Clause 9Percent elongation atbreak (%) 200 130IEC 811-1-1Clause 9Environmentalstress crackingresistance Procedure B IEC 811-4-1Clause 8Water absorptionat 24 h at 100 C (%) 1 1IEC 811-1-3Clause 9
