1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 + TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU02/4%#4)/.G0G0!).34G0G0).4%2 available space for underground services is used more efficiently; there is a reduced amount of roadway surfacing work and consequently less delay to traffic; the separation of po
2、wer and telecommunication cables is more precisely assured.2 Electrical safetyIf power and telecommunication cables are not easily distinguished from each other they should be clearlymarked.Power cables should generally be buried deeper than telecommunication cables.Power and telecommunication cable
3、s should be separated by a suitable distance according to:a) the voltage of the power cable;b) the type of the power cable;c) the type of the telecommunication cable;d) the nature of the separating material.The minimum distance is often stipulated in national standards.Under the following circumstan
4、ces national standards may allow reduced distances: the power cable having a concentric neutral operates at low voltage and the telecommunication cable has anearthed armouring, or the cables are separated by concrete fillings or similar material.If there is danger to staff doing manual excavation, h
5、igh voltage power cables should be protected by covers ofsuitable material (brick, concrete, etc.).3 Electromagnetic inductionIn order to avoid inadmissibly high danger and interference to telecommunication cables from power cables theDirectives must be observed. Such effects are especially to be ex
6、pected when:a) the power cable belongs to a network with a directly earthed neutral;b) the individual phase conductors of the power line are run in separate cables (e.g. three-phase single-corecables); orc) the currents in the power lines have a high harmonic content.Danger and interference are not
7、to be expected when: the power cable works under normal operational conditions, and in case of three-phase single-core cablethe individual phase cables are properly arranged and transposed; or the length of the parallel running is relatively small (e.g. some hundred metres).Proper arrangement and tr
8、ansposition of phase conductors of the power cable system are effective for reducingelectromagnetic induction.2 Volume IX - Rec. K.19Other metallic conductors in the tunnel (e.g. pipe-lines, concrete reinforcements) have normally a reducingeffect on the induced longitudinal voltages. The magnitude o
9、f this screening factor depends to a great extent on thearrangement of the various installations in the tunnel and on the construction of the tunnel and can, therefore, only bedetermined for each individual case.4 Other dangersThe joint use of trenches and tunnels may increase the exposure of teleco
10、mmunications staff to other dangerssuch as: striking power cables during excavation; access difficulties and isolation problems while working inside tunnels; explosions due to leakage from gas pipes if these are also present in jointly-used tunnels; foul air accumulations in tunnels.Suitable safe wo
11、rking methods to overcome such dangers should be incorporated in the joint workingagreement.5 Practical limitationsThe successful use of joint trenches and tunnels requires a disciplined cooperation by all parties concerned. Theduties and responsibilities of each party should be precisely defined. Special measures may be necessary to overcomelimitations of space underground and to facilitate subsequent maintenance of the cables, and such special measuresneed to be agreed before the joint construction work commences.
