1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 , TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU#/.3425#4)/. G0G0).34!,!4)/.G0G0!.$02/4%#4)/.G0G0/ modified at Melbourne, 1988)1 Type of armouring1.1 The most common forms of armouring are:a) Tape armouring This consists of overlapping steel tape or tapes,
2、 applied in helical form with a short lay,over the cable sheath.b) Wire armouring This is formed from round, flat or trapezoidal steel wires applied helically around thecable sheath with a relatively long lay.1.2 These two types of armouring are used in combination with other protective layers (jute
3、, plastic) forconstructional or mechanical reasons, or for protection against corrosion.2 Choice of armouringIn deciding whether or not to use armouring and in choosing between the various types of construction, verycareful consideration should be given to the local conditions of installation, such
4、as:a) whether the cables are laid in duct or direct in the soil;b) whether the cables are laid in a trench alongside a road or on private land;c) what material is used for the cable sheath;d) whether other cables are or may be laid along the same run;e) the nature of the soil: rocky, sandy, corrosiv
5、e or not; presence of micro-organisms;f) the depth of the trench, which in any case should not be less than 50 cm, and for large cables 80 cm;g) the risk of induction;h) the risk of attack by rodents or insects;i) the degree of exposure to lightning;j) whether the size and importance of the link jus
6、tifies special precautions, in which case steel-wirearmouring provides additional protection, particularly in manholes;k) whether a long draw-in is required, e.g. crossings under rivers (as cases of this are infrequent, no need isenvisaged for a new design of land cable incorporating a central strai
7、n wire).3 Protection providedWith cables laid directly in the soil, armouring contributes to safe installation and reliability of operation byensuring protection of the cables against:a) mechanical damage caused by stones and excavation equipment or tools;b) rodents and insects;c) chemical or electr
8、olytic corrosion;d) effects of atmospheric discharges;e) indication phenomena due to the proximity of power lines.2 Volume IX - Rec. L.34 Tape armouringTape armouring is to be preferred for protection against damage by pointed digging tools, sharp stones, etc. It isalso useful for providing magnetic
9、 screening for circuits within the cable, for which wire armouring is much lesseffective, because the air gaps between the individual steel wires, which are arranged circumferentially around thecable, greatly reduce the magnetic coupling between the armoured sheath and the conductors within the cabl
10、e.5 Wire armouringWire armouring gives considerable additional tensile strength to a cable and is useful where pulling-in stressesare high (long draw-in) or where high stresses arise from conditions of use, for example where there is groundsubsidence in mining districts and where cables are run in w
11、ater and bogs or in shafts leading to deep level locations.6 General type of armouringFor cables with a metallic sheath of lead or aluminium, the type of armouring in most common use consists oftwo helical windings of steel tape between layers of impregnated paper and jute with an external protectio
12、n of jute yarnor other fibre. This type of armouring ensures good protection in all five cases listed in 3 above.For plastic-sheathed cables, a light armouring may be used, formed of metallic tapes (steel, aluminium orcopper) between two coverings of plastic material (polyethylene or PVC). Cables of
13、 this design are protected chieflyagainst the hazards mentioned in 3b) and 3c) above and to a certain extent against hazards 3a) and 3d) above.7 Armouring for main cablesThe major cables in a long-distance network are certainly best protected by a watertight metallic sheath and theconventional armou
14、ring described above but the price of such protection is relatively high.The cost of cables can be reduced by using a thin welded-steel sheath protected against corrosion by abituminous compound and a plastic covering. This protects the cable, though to a lesser degree, against hazards 3a), b),c), d
15、) above; some protection against induction may be obtained by inserting conductor elements or copper oraluminium bonds under the steel sheath.8 Through-connection of armouringIn case long-distance cables or similar cables are provided with metal armouring, this should be through-connected electrical
16、ly at the splicing points. This should be done to obtain maximum protection against the effects ofatmospheric discharges and protection against induction.Metal armouring on cables forming part of the distribution network should also be through-connected in casesuch protection is needed.In case metal
17、-armoured cables are also provided with a metal sheath, it may be desirable to through-connect thesheath and the armouring electrically at the splicing and/or repeater points. This should be done to neutralize anydifferences in potential between the armouring and metal sheath, and to obtain maximum
18、protection against magneticinterference. Through-connection may create corrosion problems, which will usually reduce the lifetime of the metalarmouring.9 Omission of armouringOn directly buried cables, metal armouring can be dispensed with in case the cable is provided with a strongplastic sheath, f
19、or example of polyethylene. A further prerequisite is that the soil and laying conditions should befavourable.Additional protection, for example of optical fibre cables, may be obtained by providing the cable sheath withan external layer of polyamide (thickness 0.4 - 0.5mm). This has a favourable ef
20、fect as a wearing surface when drawingthe cable over long distances. Moreover, the layer gives a certain degree of protection against light mechanical attacks.Volume IX - Rec. L.3 310 Corrosion considerations cables with metal sheathsBoth tape and wire armouring are useful in mitigating corrosion at
21、tack; largely because they tend to keep theimpregnated coverings lying beneath them in good order and so safeguard the metal sheath from the effects ofdifferential aeration, etc.11 Rodents and insectsDamage from rodents and insects to direct buried cables may be high in some areas. In those location
22、s, it maybe advisable to consider the application of some type of armouring. For detailed information regarding armourprotection against rodent and/or insect attack, the reader is directed to Part IV-B, Chapter II of the CCITT manualOutside plant technologies for public networks, mentioned in Recomm
23、endation L.1.12 Tropical countriesIn tropical countries special attention must be paid to 6 and 7 above and to the danger from micro-organisms.In general, it is safe to dispense with armouring only when: cable is laid in duct; no magnetic screening is required, or where this is provided by some othe
24、r metallic layer included for thepurpose; when there is no risk of corrosion or where corrosion protection is provided by some other layer includedfor this purpose; in the case of directly buried cables, where the soil is homogeneous and contains no flints or rocks likely todamage the cable, and where there is no danger of damage by rodents and insects.However, special local conditions may still make armouring necessary, even in the above cases.
