1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 + TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU02/4%#4)/.G0G0!).34G0G0).4%2 modified at Malaga-Torremolinos, 1984)A metal sheath provides a cable with electrostatic screening and a degree of magnetic screening. A plasticsheath has no intrinsic screening p
2、roperties. Some plastic-sheathed cables, for example those with paper-insulatedcores, incorporate a metal screen as a water barrier. Such a metal screen, which is usually in the form of alongitudinally applied aluminium tape, provides the same screening properties as a nonferrous metal sheath of the
3、 samelongitudinal conductivity. The tape must, however, be connected to the telephone exchange earth electrode systems atits ends and/or to conveniently located earthing points, such as metal cable sheaths, along its length. It is also importantthat at jointing points the tape be extended through by
4、 connections of very low resistance. Although the degree ofscreening provided by the tape may be small at 50 Hz, it can be considerable at frequencies which give rise to noiseinterference. The presence of a screen on a cable also reduces the induction arising from the high-frequencycomponents of tra
5、nsients caused by power-line switching and also induced transients from lightning strokes; suchtransient induced voltages are of increasing importance with the increasing use of miniaturized telecommunicationequipment with very small thermal capacity.On the basis of the above considerations and expe
6、rience with the use of plastic-sheathed cables,the CCITT recommends that the following provisions be observed:1 Since plastic-sheathed subscriber distribution cables without a screen give satisfaction for distribution from theexchange to subscribers, they may be used in localities where there are no
7、 alternating current electrified railways.However, account must always be taken of the risk of noise interference that may arise in the vicinity of electricrailways, especially those with thyristor controlled equipment in the locomotives. Consideration should also be given topossible interference by
8、 radio transmitters which operate in the same frequency range as the circuits in the plastic-sheathed cable.2 Trunk and junction cables should contain a screen which can have the form of an aluminium-tape water barrier.Cables provided with a screen having a conductance of the order of half that of a
9、 cable having the same core diameter,but with a lead sheath, have given complete satisfaction where there are no risks of severe magnetic induction.3 If a plastic-sheathed cable is provided with a screen of a conductance equivalent to that of a conventional lead-sheathed cable, then in the presence
10、of induction the plastic-sheathed cable can be used in entirely the samecircumstances as the lead-sheathed cable.4 If the effect of the screen according to 2 and 3 above is not sufficient to limit the magnetic induction atmains frequencies, or to these harmonics arising from neighbouring power lines
11、 or electric railways, to permissiblevalues the screening factor can be improved by increasing:4.1 the inductance of the metal sheath, if necessary, by a lapping of steel tapes;4.2 the conductance of the existing screen by additional metal tapes or wires which are arranged below the screen.An improv
12、ed screening effect may also become necessary if there is the risk of noise interference in the vicinityof electric railways equipped with thyristor controlled devices.5 The screen must be connected to the earth electrode systems of the telecommunication centres. In the case ofsubscribers cables the
13、 remote end should be connected to a suitable earth. It is also important for the screen of thecable to be extended through at cable joints by means of connections of very low resistance.6 In view of the increase in the number of electrical installations and the level of harmonics resulting from new
14、techniques, it is to be expected that the effects of interference will become worse. This being so, it may be extremelyuseful to improve the screening effect of plastic-covered cables as indicated above.7 If cables have to be laid in areas where there is a danger of atmospheric discharges, attention
15、 is drawn to theimportance of the metallic screen and of its construction in the protection of cables against lightning and also to theimportance of the interconnections between the screen and other structures. (See the manual cited in 1.)2 Volume IX - Rec. K.148 Screening factorThe following consid
16、erations enable the screening factor at the mains frequency to be determined fairlyaccurately for all types of cable regardless of the outer plastic covering used. In particular, they show how thescreening factor to be used in practice may vary depending on the conditions in which the cable is used.
17、8.1 GeneralThe screening effect produced by the metal screen of a cable mainly depends on: the frequency of the induced e.m.f. The limitation of this e.m.f. mains frequency (16 2/3 Hz, 50 Hz, 60 Hz)is therefore a determining factor in the choice of a cable from the standpoint of safety of staff andi
18、nstallations. On the other hand, the screening factor at higher frequencies should also be taken into accountin seeking to protect equipment against interference. A substantial reduction of the induced e.m.f. at themains frequency may suffice for complete protection; the level of induced e.m.f. per
19、unit length in the case of screens made by ferromagnetic material. Thescreening effect of such a cable is optimum for a given value of induced e.m.f. per unit length, so that acable designed for the reduction of high induced e.m.f. per unit length may be of no practical use forprotection against low
20、 induced e.m.f. per unit length. The composition of the screen must be adapted to thelevel of the induced e.m.f. per unit length; the quality of its earthing. The screening effect is determined by the value of the current circulating in themetal screen. The resistance of the parts ensuring current f
21、low between screen and earth is thereforedecisive. For cables with an insulating plastic outer covering, if earth connections are provided only at theends, they must be of very low resistance: the sheath should preferably be earthed at intervals along theline. When the plastic outer covering is cond
22、uctive, the sheath is in practice continuously earthed; the length of the induced section of the link to be protected. It is easier to improve the screening effectwhen this section is long. The concept of length in this case relates to the quality of earthing required.8.1.1 The screening factor (for
23、 explanation of symbols, see Appendix I)The following most frequently used screening factors are defined in the Directives: Nominal screening factor, kn(see Figure 1/K.14). This factor can easily be measured in a laboratory and isused to qualify the efficiency of the screening effect. Screening fact
24、or related to distant earth, k(see Figure 2/K.14). This factor must be taken into account inensuring protection against danger and interference, the conductors of the subscriber pairs being connectedat their terminals to a neutral earth through certain parts of the equipments, without transformers.V
25、olume IX - Rec. K.14 3 Screening factor related to the sheath km(see Figure 3/K.14). This factor must be taken into considerationin cases where the only accessible earths are those used for earthing the screen. This relates to cablesconnecting telecommunication centres to one another, their screens
26、being connected to the earths of thecentres.The Directives contain very detailed explanations and formulas for the accurate calculation of these factors in awide variety of situations. On the other hand, these screening factors can be evaluated on the basis of simpleexpressions which often provide a
27、n adequate degree of accuracy. These expressions differ according to whether theouter cable covering is insulative or conductive and use the constants and variables listed in Appendix I.8.2 Cables with insulating outer coveringThe outer covering of the metallic cable sheath is made of an insulating
28、plastic material. To obtain a screeningeffect, this sheath must be earthed at both ends and possibly at points in between.4 Volume IX - Rec. K.148.2.1 Calculation of the screening factorThe screening factor can then be calculated by means of the expressions (see also the Directives, Vol. II):k= ZiEL
29、WAWBZeELZsLWAWB+(8-1)km= ZiELZeELZsLWAWB+(8-2)Strictly speaking, the use of these expressions presupposes that the sheath is earthed only at the ends. It may beassumed, however, that in fairly comparable situations only the earths near the ends have any influence on thescreening effect. The expressi
30、on thus gives a good approximation of the screening effect in the case of intermediateearths.As a general consequence, earthing connections at intermediate points tend to improve k, but, on the otherhand, make kmworse.8.2.2 Influence of lengthWhen the earths of a sheath required to obtain a screenin
31、g factor kclose to nominal value knhave aresistance value which makes earthing very difficult, the link may be considered to be “short”. In the contrary case, it isregarded as “long”.Note “Link” is held to mean the cable length actually subjected to induction.8.2.2.1 “Long” linksScrutiny of Equation
32、s (8-1) and (8-2) shows that for very long links, screening factors kand kmare close tokn. This is true of lengths in excess of about10 WWZABiE+In this case, a non-armoured cable ( ZeEclose to ZiE) may be used. Moreover, the longer the link, the higherthe resistance value of the sheath earthing may
33、be.This need not be taken into account in the choice of a cable, which can be based on the curve of values ofnominal screening factor knfor different values of induced e.m.f., since the efficiency obtained will be very similar.8.2.2.2 “Short” linksIn this case, the value of ZiEL is approximately the
34、 same order of magnitude as the sum of the extreme terminalearth values WWAB+ . Screening factors kand kmmay be calculated by means of Equations (8-1) and (8-2).Armoured cables must be used to protect such links, and the screening effect is then provided through theincrease in the value of impedance
35、 ZeEobtained by using material with high magnetic permeability for the outer part ofthe sheath.Volume IX - Rec. K.14 5To evaluate kand kmby means of Equations (8-1) and (8-2), it is necessary to know the curve of variationsof ZeEas a function of the current flowing through the sheath (Figure 4/K.14)
36、.The calculation then calls for some simple successive approximations for evaluating ZeEafter choosing a valueof WWABand corresponding to earths which may be expected to be feasible in view of the ground resistivity at theends of the link.8.3 Cables with conductive outer coveringThe outer covering o
37、f the metallic cable sheath is made of a conductive plastic material providing electricalcontact between the sheath and the earth surrounding the cable.Intermediate connections of the sheath to the earth other than at the ends will be unnecessary if the resistivity ofthe conductive material is close
38、 to or better than that of the surrounding earth (values of about 50 m are easilyobtained).The current flowing through the sheath varies along the link, particularly near the terminals, and in the middlepart remains at a value very close to IM= e / ( ZeE+ Zs), corresponding to the current which woul
39、d circulate in thesheath if it were completely earthed (earths with zero resistance value).To calculate screening factor k, we can thus use an equivalence consisting in replacing this cable by one witha sheath connected to the earth at each end by zero resistance earths and of a length equal to that
40、 of the link L,shortened at each end by a length l such that P l = 1.This means that the cable has a nominal screening factor on a shorter length equal to L 2l.kcan then be evaluated approximately by means of the following expression:kklLlLff n=+122(8 - 3)6 Volume IX - Rec. K.14In the same way, kmca
41、n be expressed by:kklLfm n=12Equation (8-3) is not applicable in cases where the earthing of the metallic sheath is really excellent. The link isthen considered to be “long” and k= km= kn.The parameters required for the calculation are those of the cable ()ZZeEiE, , the induced e.m.f. per unit lengt
42、hand the admittance per unit length Y of the sheath in relation to the earth, which may be chosen according to groundresistivities between 1 S and 10 S (1 S should be chosen if nothing is known about earthing quality).8.3.1 Influence of lengthThe remarks relating to cables with insulating covering a
43、re also applicable in this case.8.3.2 “Long” linksThe screening factor is close to kn. The cable may or may not be armoured, according to the results required.8.3.3 Short” linksScreening factor kmay be estimated by means of Equation (8-3). The cable should be armoured in mostcases.8.4 Determination
44、of cable parametersIf the nominal screening factor and impedance per unit length ZiEcan be measured by means of thearrangement described in the Directives (Vol. IX), determination of impedance per unit length ZeEcan be based: either on a calculation based on the phaser diagram, plotted from the meas
45、ured parameters I, Uoiand Uoe; or on the measurement of the voltage Uoeappearing between the end of a conducting wire laid on theoutside of the sheath and reference point 3, the other end of the wire being connected to the sheath(Figure 5/K.14).For certain cables with screens consisting of several n
46、on-ferromagnetic, highly-conductive layers, theseparameters can be measured more approximately by a coaxial-type measuring device.Volume IX - Rec. K.14 7ZiE: Internal impedance per unit length with external return. For power frequencies, this value is close to resistance per unit length for direct c
47、urrent.ZeE: External impedance with external return per unit length.Zs: Ground return impedance per unit length.Y: Admittance per unit length of the sheath-earth circuit.P: Propagation constant of the sheath-earth circuit.K: Characteristic impedance of the sheath-earth circuit.WWAB, : Impedance valu
48、e of earths at the ends of the sheath.L: Length of link subject to induction.e: Induced e.m.f. per unit length.E: Total induced e.m.f.I: Current flowing through the sheath.Reference1 CCITT manual The protection of telecommunicaton lines and equipment against lightning discharges,Chapter 4, 2.1, ITU, Geneva, 1974, 1978.
