1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 + TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU02/4%#4)/.G0G0!).34G0G0).4%2 this voltage is, of course, related to the breakdown voltage.At very little extra expense, sleeves and joints can be made to have the same dielectric strength as the insulationbet
2、ween the conductors and the metallic sheath, although transformers and terminal equipments must be suitablyprotected when their dielectric strength is not up to the conditions concerned.If the source of the induced longitudinal e.m.f. is a high- reliability power line, as defined in the Directives,t
3、here is only a very small probability that staff will be in contact with a line at the precise moment when such a voltageof short duration occurs in the telecommunication cable. Any danger to staff is very slight given due observation of thesafety precautions for maintenance staff working on telepho
4、ne lines in which high voltages may be induced byneighbouring electricity lines.For a cable not having its circuits terminated by transformers the above conditions also apply provided thatsurge voltages are prevented from reaching the telecommunication equipment by the striking of the lightning prot
5、ectorsinstalled at the ends of the circuits.For these reasons, the CCITT is unanimously of the opinion that:1 It is possible to make telecommunication cables with conductors that are insulated from each other and from themetallic sheath by high breakdown strength plastics. For such cables, when ther
6、e is a fault on a neighbouring electricityline, the value of induced longitudinal e.m.f. that can be allowed is that which does not exceed 60% of the test voltageapplied between the conductors and the metallic sheath for checking the dielectric strength (this test voltage, which isgiven in the indiv
7、idual cable specifications, is related to the breakdown voltage) provided the following conditions areobserved:a) circuits in such cables are terminated at their ends and at branching points on transformers or are providedwith lightning protectors;b) equipment, joints and cableheads associated with
8、such cables must have a dielectric strength at least equalto that of the insulation between the conductors and the metallic cable sheath of the cable, given that thetransformers mentioned in a) above must be provided with lightning protectors when their dielectricstrength does not meet the required
9、conditions;c) the power line causing the induction must meet the conditions for high-reliability power lines given in 1;d) staff working on telecommunication cables must take the safety precautions specified in 2.2 When the circuits of such a cable are connected direct to the telecommunication equip
10、ment, that is, when notransformers or lightning protectors are inserted, and when the condition laid down in 1c) above is fulfilled, themaximum permissible induced longitudinal e.m.f. should be 650 V.2 Volume IX - Rec. K.13References1 CCITT manual Directives concerning the protection of telecommunication lines against harmful effects fromelectric power and electrified railway lines, Vol. VI, ITU, Geneva, 1988.2 Ibid., Vol. VII.
