The first street trams were powered by prime movers in the form of horses or steam tractors, with no need for electric wires or any other external equipment. Unfortunately, however, they also produced pollution directly into the city centre. Regular horse trams added their pollution to the already difficult conditions in urban streets of the period, heavily trafficked with horse drawn vehicles and with loose-dressed surfaces. For the Victorians the advent of electric traction not only gave an increase in mobility, but sent the pollution elsewhere, leaving the cities with a much better environment. Moving the pollution to a distant power station at least meant that it could be dealt with more efficiently on a larger scale.
There are alternatives to overhead wires, such as slot conduit systems, but these are expensive
to install and only applicable where overhead wires are considered totally unacceptable.
Overhead contact lines have been around now for well over 100 years. In that time, it might be thought that standard systems would have been developed to provide a good balance of electrical efficiency, minimum cost, and good aesthetic appeal. In the real world, each new system had its own design process, and a different end result. What standardisation there was grew out of a merging of separate systems, and was brought by market forces from the supplier companies.
Everyone likes to be able to see the sky, and any restriction of the view must be a disadvantage, In this context, therefore, there are no such things as 'good looking overhead wires', for any will hide the sky to some extent. If they spoil the view in a major town, however, the whole system may suffer adverse publicity before or after the opening. In extreme situations the whole scheme may fail. Our objective must be to minimise the intrusion.
'If you can't hide it, make it a feature'.
A Brecknell, Munro & Rogers centre pole
on the St. Petersburg tramway, installed 1903
Picture courtesy of David Hartland
The first point is to consider the actual equipment to be supported. This is predominantly an electrical design process, where substation positions are decided with regard to the service density and volt drop along the line. Care must he
taken with electrical design not to have too pessimistic a simulation. More substations means less copper. Substations are expensive, both in equipment and land usage, so there is an advantage in minimising the number. Fewer substations, however, mean more copper is needed along the line, increasing overhead costs and spoiling the appearance. There is a balance to give minimum cost, but this may not be the same balance as that for best appearance.
Having set the amount of copper needed to feed the system, the next decision is how much of this needs to be up on the overhead, and how much can be placed in ducts beside the track.
Clearly the more copper is placed underground, the less equipment is visible, but cables underground add to the system cost. (They may, however, reduce the on-going maintenance costs).
Contact wires and poles
Clearly there must be at least one contact wire above the line, but I would argue that one is normally sufficient. For a 750V system running up to 80km/hr with suitable pantographs, one contact wire only is required. Twin contact wires not only stand out against the sky, but are a maintenance difficulty in that they do not wear evenly and this implies premature replacement of both.
Poles must be reduced to an absolute minimum, choosing wall fixings or direct structure fixings instead wherever possible. Spacing is a tricky subject. They must not be placed too far apart or the wire will sag excessively, and the UK practice is to design so that even if one pole is knocked down in a road accident, the wire will not hang so low as to be touched by bystanders.
The most effective reduction is to place poles as centre bracket arms, located in the 'six foot' between lines. The result is usually a well-balanced system putting all overhead equipment in the track area and away from side walkways and third-party ground. Because poles are evenly loaded, they may be of modest size. The next best approach is probably to use double-track bracket arms, where one pole supports both lines from one side; the pole must be of larger diameter as it is loaded unevenly, but the overall effect remains acceptable. If poles must be placed on both sides of the track there are advantages in combining the function with street lighting, or station canopy supports, and reducing the total of upright supports visible.
Picture courtesy of David Hartland
Midlands Metro tram 7 negotiates Bilston Road at
Wolverhampton. Notice the single contact wire; the
small, fully insulated cross span rope; and the use
of combined traction and lighting poles.
Having minimised the poles, the next stage is to look at the detailed design. There are two approaches - to emphasise or camouflage. Victorians accepted that overhead poles brought them great advantages, and encouraged embellishment in scrollwork and spandrels, to emphasise their presence. Nowadays we would probably work hard to hide them into the background, and here shape and colour are the key features.
Shape of poles and colour
The most acceptable shape to the human eye is a taper with the diameter reducing from the base upwards. This, it could be argued, mimics the natural and relaxing image of a tree trunk profile, and we should do well to take note. It is also right in mechanical terms, for the bending moment on the pole reduces further from ground level. Plain profile poles, of whatever cross section, are taboo - the straight pieces of 'I' beam seen on main line railways are not acceptable for any public area. They also make it very tricky to incorporate cable feeds or balance weights.
Taper poles may be made of concrete but the material is only really at home in an environment of all-concrete buildings where the colour and texture matches the surroundings. To construct a steel pole with taper is difficult, and will need to be of thin section and large diameter for strength. The traditional waisted pole is formed by joining two or three progressively smaller sections together, and this remains the neatest way of giving a nice-looking taper.
When a pole is viewed from ground level, the upper two thirds appears against the sky as background. To hide the pole, it should he painted pale blue or grey. The lower one-third appears against the scenery, and there is more freedom to integrate it with the background. In rural areas this lower part can be green; in urban areas painted to match other street furniture. Blues and reds are best avoided because of a tendency to fade.
Picture courtesy of David Hartland
Note the ball and spike finial, the balanced design and the colour scheme.
Painting the top two-thirds a pale colour has helped to camouflage the pole.
It could be argued that pale blue would have been a better choice.
Finials and mountings
Finials are a means of preventing water ingress to a hollow pole, but they also provide an opportunity for endless debate amongst the planners. In fact, from the contractor's point of view, it is worth encouraging such debate long and hard, because the more time is expended on discussing the shape and the colour of finials, the less is the opportunity for general artistic debate which may delay the main design process. There is a great range of possible shapes, which range from the traditional spike and ball arrangement, to the unusual and the obscure.
Wherever possible, wires can be supported from span wires fixed directly to a convenient wall or facade of a nearby building. This fixing is very neat, and very cost-effective, but the major difficulty is legal formality. To obtain permission from the building owner will involve a complex legal process to obtain the wayleave, and the whole process rapidly becomes cumbersome and time-consuming, Typically it will take two years to agree to a building fixing. This timescale is unacceptable in the normal plans for a project of this type, and certainly impossible in the timing of the usual overhead line sub-contract. If wall fixings are to be used on a new construction they must be agreed in principle by the client or main contractor in the very early stages of the project. The alternative is extra poles which obstruct pavements, look unsightly, and cost more.
Contact wires must be separated electrically from the supports, and include double or triple insulation. Where the wires are supported by span wires, there are two possibilities for this. The span may be formed in a single insulated polymeric rope, or it may be formed in steel wire using separate discrete insulators. The former gives a smooth appearance to the eye, but is black and of larger diameter than the steel, which is lighter in colour but requires separate discrete lumps as insulators. It is a matter of some debate which has the better visual appearance.
The overhead system must be separated along its length by section insulators and isolator switches for maintenance and emergencies. Switches may be mounted at the pole top, with linkage down to a handle at shoulder level, but the resulting assembly is ugly and handles are a collision danger. It is preferable to mount switches in a lineside cubicle with cables running in ducts to the poles, and inside poles up to the contact wire.
Section insulators are the most visual of all overhead components, and create more maintenance effort and wear and tear than any other component. There are great practical advantages to reducing mass and size, let alone visual benefits, and the challenge is to develop new designs capable of higher speeds in a smaller size.
Tensioning and junctions
The contact wire must be tensioned to limit the sag. For street-running applications, this is done by using a fixed-termination system, so that the wire is tensioned when installed and the value of tension, and sag, vary with the temperature. This is acceptable for speeds up to 50km/hr, but above this figure the wire must be tensioned automatically which implies breaking it into sections individually tensioned and overlapped with the adjacent lengths. It is here that there are major visual disturbances to cope with.
To apply the tension, either stacks of weights may be used with a pulley mechanism, or a gas tensioner. The former is a real eyesore, involves regular maintenance, and leaves the system open to vandalism. The latter has the disadvantage of higher initial cost. At overlaps, there are two sets of contact wires and this inevitably creates a visual disturbance, especially at the point of passing current over the gap between wires a flexible loop is required which takes up space and may be awkward to camouflage. The best philosophy is to locate overlaps where the local geography hides them from general view.
At complex junction layouts, all the skill of the overhead designer is needed to position poles or wall fixings to be of use in supporting the complex wire arrangement. Modern road junctions have the advantage of many traffic islands which give extra possible locations for poles, but on the other hand buried services give restrictions. The neatness of junction layouts is perhaps the most challenging task facing the overhead designer, and the most open to subjective comment.
Modern building is conducted by a joint venture of a civil engineer and E & M partners, with the overhead generally by a sub-contract. The question is, therefore, at what stage the subcontract should be issued, and by whom. Should the overhead system be a subcontract to the E&M or to the Civils? The E&M side has a major input in substation and power design, but the overhead system is predominantly a task of locating various structures at points along the line, and overall system layout design is normally located within the Civil Engineer's remit.
Whichever choice is made, the overhead line contractor must have an input early in design or it may be too late to influence decisions which are crucial to the overall visual success of the scheme. Likewise, aspects of overhead appearance may be affected by pantograph performance. It is logical that the pantograph and overhead should be within the one contract to ensure the best combined solution is adopted. The full discussion of these questions could fill a whole magazine!
I would like to thank my colleagues for their ideas and comments, and register the help of an unknown but loudmouthed British tourist in Neuchatel High Street last summer - it was his comments on the overhead wires there that gave me inspiration for writing this paper.
(This article has been adapted from a presentation to the IMechE conference Inner City Urban Railways on 27 April 1999.) AUGUST 1999 Tramways and Urban Transit