Question 6/V - Coordinated protection schemes for telecommunication cables (Continuation of Question 6/V. 1985-1988; revised wording) (To be coordinated with Question 4/VI) In the cases where telecommunication cables need protection against the effects of the electromagnetic induction by a.c. power and traction lines, against lightning and against corrosion, it may be convenient to provide coordinated measures to give protection against all these factors. Studies under this Question should be directed to coordinated protection schemes for optical fibre cables, in particular coordinating the protection of cables and working staff against overvoltages due to electromagnetic induction by power and traction lines with that for lightning protection (see also Recommendation K.25 "Lightning protection of optical fibre cables"). Note - Studies on this Question should also include the revision of the reply annexed to the Question. ANNEX (to Question 6/V) Coordinated protection schemes for telecommunication cables 1.Introduction In the cases where telecommunication cables need protection against dangers and interference, from a.c. power and traction lines, against lightning and against corrosion, it is convenient to provide coordinated measures to give protection against all these factors. The environment, where the cable is laid, is the main factor to be considered to decide whether or not protection is necessary, in particular, the following criteria should be taken into consideration: a) for protection against lightning -keraunic level, -orographic conditions, -altitude, -soil resistivity; b) for protection against effects of electric lines -soil resistivity, -inducing current, -distance between the telecommunication cable and the electric line; c) for protection against corrosion -soil resistivity, -corrosive components (ions) in the soil, -stray currents in the soil (d.c. sources are d.c. traction rails, d.c. earthing electrodes, metallic structures with cathodic protection and the like). Protection of underground communication cables against corrosion is mainly achieved with protective plastic coverings on the metal cable sheath. As regards the environment, the installed cable behaviour is different if an insulating or conductive plastic material is used as protective covering. Therefore, coordinated protection schemes are also different and the two cases should be separately considered. 2.Combined protection with insulating plastic coverings1 Combined protection of telecommunication cables against induction, lightning and corrosion with insulating plastic coverings requires: -suitable metal sheath; -adequate covering with right value of dielectric strength; -ground connections of metal sheath; -use of shield wire. Periodic supervision of the insulating covering does not seem to be necessary because the administrations had a positive experience concerning corrosion. Such a control could be recommended only in the areas where stray current or interference current corrosion have been observed. The distance (d) between electrodes and their resistance (R) can be established using specific methods and considering thunderstorm activity, induction from electric lines and electric characteristics of telecommunication cables. Nevertheless, the following indications about the values of (d) and (R) can be taken into account: -local cables are usually earthed only at each end; -long-distance cables can be earthed only at the repeater stations with about 10-20 ohm of earth electrode resistance or more frequently (at each joint) but with no excellent value of (R). Multipoint sacrificial anodes may be considered as earth electrodes; in such cases, the earth resistance values have to be controlled during their lifetime. Shield wires are used for both local and long-distance cables when conditions warrant. Generally their use is considered: -for local cables when the soil resistivity is greater than 1,000 ohm m in rural areas only; -for long-distance cables when the soil resistivity exceeds 100 ohm m. The Handbook "The protection of telecommunication lines and equipment against lightning discharges" contains useful information for determining the need for shield wires. 3. Combined protection with conductive plastic coverings Combined protection of telecommunication cables against induction, lightning and corrosion with conductive plastic covering requires: -suitable covering characteristics, -use of shield wires when conditions warrant. Generally their use is considered, as well as for bare cables, when the soil resistivity is greater than 1000 ohm.m. The Handbook "The protection of telecommunication lines and equipment against lightning discharges" contains useful information for determining the need for shield wires. Ground connections of the metal sheath are not necessary. The covering thickness and compound type are the same as for insulating coverings. On the basis of experience of one Administration, the possible limit values of the main properties of the conductive plastic covering to be considered are shown in Tables 1 and 2. In particular the chemical characteristics in Table 1 are recommended to make negligible the effects of the galvanic corrosion between conductive plastic covering and metals used in buried telephone plants. TABLE 1 Limit values of conductive plastic covering characteristics + + Conductive covering Properties + Test method PE PVC + + - + + Carbon black contest (%) < 13 < 25 ASTM D 1603-76 modified* Polymer density (g/cm3) 0,920-0,935 1,3-1,4 IEC 811 p. 5.3 Melting temperature ( C) _ 115 - ASTM E 793-85 Cold bend test at -15 C no visible no visible ASTM D 2633-82 crack crack Melt flow index (g/10 min) 0,05-0,1 - IEC 811 App. B + + * Thermogravimetric analysis TABLE 2 Limit values of electrical, mechanical and physical characteristics of the conductive plastic covering + + Conductive covering Properties + Test method PE PVC + - + - + + - Volume resistivity (ohm.m) < 10 < 10 IEC 93 Transversal resistivity (ohm.m) < 20 < 20 - Tensile strength at break (MPa) > 9 > 9 IEC 811 Percent elongation at break (%) > 200 > 130 IEC 811 Environmental stress > 200 - COM VI n. 26 cracking resistance (h) 1968-72 Water absorption < 1 < 1 IEC 811 - 24h; 100 C - (%) + + The periodic supervision of the covering is not possible but this fact is not important because the low corrosion rate of the metal sheath make negligible the corrosion damages. The protection need against electromagnetic induction is unlikely and the personal safety is assured by the continuous earthing of the metal sheath. The conductive plastic covering can be also used above a metal moisture barrier of an optical fibre cable; up to now this coordinated protection scheme has not been experienced in optical cable plants.