1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 + TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU02/4%#4)/.G0G0!).34G0G0).4%2 electrical line equipment (track circuits) which, though necessary for railway safety installations, mayprevent the systematic connection to rail of metal structures near the rail
2、way; the characteristics of the protective devices required which, with a.c. electric traction systems, may be tosome extent affected by the presence (or absence) of booster-transformers; the degree of insulation of the contact system, which may also affect the nature of the protective devices,parti
3、cularly in the case of relatively low-voltage electric systems such as 1500 V d.c. lines; the means to be recommended for linking a metal structure to the rail in case of overvoltage withoutmaking a permanent connection (one method is to make the connection via a spark gap).2 A.c. electric traction
4、linesIt is recommended that neighbouring metal structures, for example all those within a certain distance from theline, be connected to rail, provided that there are no safety installations which make this impossible.If the structures cannot be connected to rail, it is recommended that they be eart
5、hed to an earth electrode havinga sufficiently low resistance.3 D.c. electric traction linesProtective measures should also take account of the need to avoid any risk of electrolytic corrosion. Suchmeasures may amount to connecting to rail only such metal structures as are sufficiently insulated fro
6、m the ground orlinking them via a spark gap or, in the case of metal structures carrying an adequately insulated contact system or lineswith a sufficiently low service voltage, connecting neither to rail nor to earth.4 Telecommunication cablesIn new installations, it is recommended that cables near
7、rails, at the entry to substations or over metal bridgesshould have an outer plastic covering, possibly of high dielectric strength, where it is necessary to prevent contactbetween the cables and such structures.If, on the other hand, cables with metal sheaths already exist, a good solution, at leas
8、t in the case of largerailway stations, may be to connect the sheaths to rail.5 Conditions to be fulfilled by PTT installations in the neighbourhood of electric traction linesThe following are the main precautions taken to protect such installations: placing them outside the danger zone; screening;
9、substituting insulating components for metal components, in particular the sheaths or covering of cables orin the construction of repeater cabinets or boxes.Note The above recommendations are inspired solely by technical considerations which are to be carefullyweighed up in each case. It goes without saying that every Administration must comply with the laws and regulationsin force in its country.
