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Definition: Overhead Wire/Catenary
Both trolleys (original as well as heritage) and light rail
systems are powered by electricity, normally drawn from wire suspended above the
track. However, the choice of overhead wire construction style can have a major
impact on the appearance of the system. Two principal types of overhead wire construction exist: Direct
suspension trolley wire and catenary construction:
Direct suspension trolley wire is typical of urban streetcar
systems and consists of a single wire running parallel to the center of the
track, suspended at a height of about 18 feet. The wire is suspended either by
span wires which connect to poles or buildings at the side of the street or by
bracket arms that are attached to poles next to the track. Both bracket arms and
poles can be decorated with creative designs to enhance the appearance of the
streetscape. Carefully designed direct suspension overhead can be quite simple
and thus be minimally intrusive to the appearance of the urban area served by
the line.
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Simple direct suspension trolley wire supported by span wires from line poles on San Francisco's Embarcadero. |
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Direct suspension trolley wire supported by attractive bracket arms at Fisherman's Wharf in San Francisco. |
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Catenary overhead on the Hudson-Bergen light rail line in New Jersey. Such construction is more complex than needed for heritage trolleys. |
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Catenary construction is more typical of interurban
trolleys running between cities, modern light rail lines, or electrified main line railroads. Catenary
construction consists of a messenger wire suspended loosely between suspension
points (thus naturally forming what mathematicians call a "catenary" curve).
From the messenger wire short vertical drop wires connect to the contact wire
which is at a constant height. Catenary suspension, especially modern designs,
is considerably more complex than direct suspension and thus is more visually
intrusive and more expensive. The advantage of this design is that it holds wire
at a more constant height and tension allowing higher speed operation, and
suspension points can be further apart than for direct suspension. Catenary
construction is typically not used in streetcar or heritage trolley construction
and should be avoided if visual appearance and historical accuracy are valued.
Today many people new to the electric transit field
inappropriately use the term "catenary" to refer to any type of overhead wire
construction. However, planners developing heritage trolley systems should
specify carefully the type of wire construction to be used as the system
chosen can have a significant effect on both the cost of construction and the
attractiveness of the finished product.
Overview Presentation Follow this link for a presentation given by John Aurelius at the committee meeting in Seattle in January 2010 outlining the different types of overhead contact systems used by heritage and modern streetcars.
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