Cable Management Systems

Presentation by Dave Burton of Fifth Dimension on Cable Management Systems and the issues that are raised within a railway environment

Our second FDAL free seminar was given by jointly by our director Dave Burton and a representative from Legrand on the evening of 30 November in our offices at Farringdon to a small audience of clients and colleagues.

Dave’s presentation began by asking why cable ‘management’ was needed. The observation was made that there is a very large number of cables on railway networks and on London Underground the density was exceptionally high. Cables cannot simply go anywhere. They are heavy and need support. They are vulnerable and need protection. They are ugly and need hiding, disguising or other aesthetic attention. For reasons of controlling EMC they often need separation by type. Finally they can get stolen and need to be managed in a way where that risk is considered. In short, they need managing.

Cables need different treatment in different places. The four obvious types of location for cables were trackside (outdoor) where theft was a particularly difficult issue and weather had to be considered, but there was usually plenty of space. In tunnels, especially tube tunnels, the issue was usually one of working within the tight structure gauge with cable positions of limited capacity. At stations, the public areas introduced the aesthetic demands, sometimes with limited cable route options and with difficult access requirements for installation posed by heavy passenger usage. Plant areas were possibly the simplest environment but at older stations these areas might already be very congested. Each of these environments required a different treatment.

In the open air the conventional means of managing cables were either to lay in concrete lidded troughs, as favoured for many years by British Rail and its successors, or to install within a cable run usually comprising closely-spaced vertical posts to which cable hangers were mounted (on lines with an air main the latter could be used to provide support intermediately between posts). Where the railway ran between walls and at bridges then the hangers could be fixed directly to the structure. Both systems required a substantial management system (cables could be numerous and very heavy). Particular issues arose where cables had to cross tracks. To go beneath tracks required load bearing protection such as a dedicated concrete cableway, while to go above tracks required routing via a suitable structure, or, if there were none convenient, a dedicated cable bridge. Minor cables could sometimes cross tracks in ballast mounted plastic tubes but this was not suitable for anything more substantial. Theft from (in particular) troughs was giving rise to a tendency to bury cables in the ground in ducts to discourage thieves but even this was not wholly successful and of course makes legitimate cable access more difficult. In tunnels either troughing could be used where there was space, or brackets mounted to the walls.

Dave drew attention to some of the issues that arose where cable ducts and trunking were used, as some challenging issues arose in complying with BS7671 (the IET wiring regulations). Protection against moisture was a requirement, and it was necessary to select cables which could withstand exposure to moisture for duct installation. The greatest concern was that where grouped cables were touching, they had to be de-rated to take account of the cumulative heating effect. This often meant that in a typical station installation, only 50% of a cable’s theoretical current capacity was available as so many cables need to be run in close proximity. This is a factor sometimes overlooked in design, particularly where spare containment capacity is used for new circuits. There was a plea for anyone installing a duct to leave it roped up so that the intended (and future) cables could be pulled through.

Traditionally, tube stations had managed with a small number of iron conduits fixed on and around the station; increasingly, from the 1970s, the cabling requirements came vastly to exceed anything as basic as this and numerous high capacity runs were necessary. These posed huge demands on providing aesthetic solutions in very limited space. A common platform solution was to run cabling behind the name frieze, as that, too, had to run along the length of a platform. These name panels were raised, hiding cables behind; with good design, this could be made to look pleasing. The Victoria Line first used this approach, with cabling recessed into the false wall. Other solutions included mounting in (for example) trays above a false ceiling, and in some cases proprietary metal CMS systems could be used, now available with coloured powder coatings that made them acceptable in passenger areas. Modern lighting systems could also be obtained with attractive integral CMS (old fluorescent systems might have this, but it wasn’t necessarily attractive).

Plant areas did not usually pose too many issues and neatness was possibly the most worthy objective. Getting the cables in and out of plant areas was more of a challenge though. This often required using disused passageways, cable ducts, shafts, below platform areas and the like — often places with very difficult access and various other hazards. This posed many different issues but could often require the installation (at huge cost) of ladders, walkways, enhanced lighting (even if there were any before) and other measures to mitigate identified risks. Undesirable perhaps, but in many cases there was simply no choice about the route a cable had to take. Matters were further complicated by the regulations requiring access to any “connection” point, which led to a debate about what it meant. Avoiding joints or connections in difficult areas seems an obvious thing to do anyway, though.

Dave talked the audience through the various complicated issues around cable entry to CMS systems (where there were various not-obvious issues), connections to the system, bonding of the CMS, fire rating and segregation, and theft and measures to mitigate it.

The lecture was followed by a presentation by Legrand about their product range and how they sought to address the various issues that had been raised. After that, and some questions, the discussion was continued over some drinks and nibbles.