Overview OfOHLE Systems

1 Introduction

The aim of this page is to provide an overview of the different types of ohle systems that could be relevant to modelling in Rail3D, to try to define the terms used reasonably clearly and identify where and why different systems would be found in the prototype. Those who know something about this area should feel free to correct and add to the outline here.

Overhead line equipment (ohle) is essentially a system for supporting an electrical conductor above the track in a position where traction current can be drawn from it via some kind of sliding contact device mounted on the train. In most cases, the wheels of the train and the rails form the other side of the electric circuit. Thus ohle consists of two basic components: the electrical conductor and the system for holding it up.

2 Wires

In real life the contact wire is quite a complicated structure, since it needs to be mechanically strong, a good conductor, and resistant to corrosion. Usually, it is also given a special profile to allow it to be fixed to support hardware while leaving the conductive lower surface free of obstructions. However, for Rail3D purposes we don’t need to worry about any of these things. The contact wire is just a line at a certain height above the track.

The illustration above shows a piece of “low-profile” catenary under a bridge, where the support wire is only a short distance from the contact wire. You can just about make out the grooves in the sides of the contact wire for the fixing clamps.

Another slightly unusual feature in the picture is the duplication of the contact wire: this is Dutch 1500V dc equipment, and the second wire is needed at this point, where trains are starting out of a station on an up-grade, to ensure that enough current is available.

2.1 Tram-type

In tram-type overhead, the contact wire is supported at intervals of 20m or so by bracket arms or span-wires. In between the supports it hangs freely in space. The advantage of this system is that it’s cheap, simple, and not very intrusive visually — an important factor when trams run through city streets. The disadvantage is that you need lots of supports, and that the wire sags in between supports. At higher speeds the alternation of supported and unsupported lengths sets up oscillations in the current collector, which can lead to arcing and damage.

You find tram-type overhead on many older urban tramway systems and on some other lines where speeds are low, for instance mountain railways and goods yards.

In old-style tram overhead the contact wire is directly connected to the span wire or bracket arm by an ear; in more modern systems there is usually a flexible or elastic element in between (see also the notes on trolleybus overhead below).

2.2 Catenary

Catenary overhead works like a suspension bridge: the strength is provided by a steel support cable that hangs between two supports, while the contact wire is supported from it like a bridge deck by short vertical droppers, secured to the contact wire by clamps known as ears. (Catenary is the mathematical term for the curve a chain makes when hanging freely between two supports.)

The advantage of this system is that the contact wire is supported uniformly along its length with a relatively small number of supports (typically one every 50m or so on straight track). Obviously it is more expensive and complex than tram overhead, and looks rather ugly when used in city centres (cf. Manchester, The Hague, etc.).

Catenary overhead is the most common arrangement on main line railways, and is used on many modern tramways.

2.3 Three-phase

Most railway electrification schemes use either direct current or single-phase alternating current. Either way, they need two conductors to connect the train to the power station: the contact wire and the earth return through the rails.

Three-phase alternating current can be transmitted through wires very efficiently. Induction motors powered by multiphase current are also very simple to build and efficient in use, thus it was inevitable that some engineers came up with the idea of running trains off three-phase in the early days of electrification. The trouble is that you need three conductors, i.e. the track and two overhead wires. This makes everything very complicated, especially at junctions. Three-phase electrification was used extensively in Italy, and sporadically in other parts of the world, but is now obsolete except for a few special cases like mountain railways (Gornergrat, Jungfrau). The survivng examples all seem to use tram-type suspension.

2.4 Trolleybus

Trolleybuses also need two overhead wires, because they run on rubber tyres and don’t have any rails for current return.

Trolleybuses invariably use tram-style suspension, usually with spanwires stretched across the road.

There are a number of different systems in use: in the one above, the contact wires are straight and have to be kept the correct distance apart (typically about 0.6m) by insulated spacer bars. A pair of spacer bars is attached to the span wire by a V-shaped bridle. On curves, the bridles would be arranged to pull the wires outwards, so the spacer bars wouldn’t be needed. An alternative to using spacer bars is to introduce a slight zig-zag into the wires by offsetting the suspension points of the bridles alternately to the left and right. Another variant is to use elastic droppers that look rather like thick rubber bands instead of wire bridles.

In old-fashioned systems, as used in Britain until the 50s and 60s for example, flexible suspension was not used, and the contact wires were attached directly to the span wires using similar ears to those used for tram wires (see the Carlton Colville photo below).

A special feature of trolleybus overhead is that the trolleypoles have to be guided along the correct route at junctions. Trolleybus wires thus have “points” in the overhead wire. Systems to allow drivers to select the correct route are similar to those used on trams: typically an insulated section of wire detects whether or not the vehicle is drawing current when it passes, and operates a relay to set the points accordingly; in more modern systems a short-range radio signal is used. Rarely-used turnouts might be operated by a keyswitch.

More interesting for Rail3D is that, where routes split, the facing points are often placed some distance before the junction, with an extra pair of wires running up to the junction itself, so that a trolleybus stuck in a traffic queue on one route will not hold up the other route as well.

In the image above, the moving switch elements are on the right, and the crossing “frog” is in the centre. Metal “rails” are used to define smooth passage through the turnout, while jumper cables connect the different bits together electrically. The whole arrangement is suspended from two sets of span wires.

The metal rails used in the turnout are also often used on sharp curves, e.g. in turning loops and at road junctions.

3 Supports

ohle support systems have to define the position of the contact wire both vertically (heigth above the rail) and laterally (distance from the track centreline).

In tram-type overhead these functions are usually combined; in catenary the vertical support is by droppers from the support wire, while the lateral postion is controlled by a radius arm that is free to swing up and down.

3.1 Simple masts

The most straightforward kind of ohle has masts placed to one side of the track. Each mast has an upper bracket arm (usually a triangular structure) extending over the track to hold the support wire, and a radius arm to guide the contact wire. Because the radius arm is designed to work under tension, on straight track it is necessary to mount adjacent radius arms so that they pull in opposite directions. This is done by mounting alternate arms “backhand” fashion on the end of a lower bracket arm, so that they pull in the direction away from the mast.

For simplicity, masts are usually all placed on the same side of the track, unless there’s a good reason not to…

3.2 Centre-poles

Centre-poles are quite often used on tramways, especially where they run on reserved track or in the median strip of a dual carriageway. They cut the number of masts needed by 50%, and have the added advantage of placing the masts well out of the way of road traffic. In The Hague, tram centre-poles often double as street-light supports.

It isn’t very common to use centre-poles on railways, although you do see occasional examples. The objection is presumably that you would need to increase the track spacing to make room for them.

3.3 Gantries

Gantries are often used in places where there are more than two parallel tracks. They avoid the need to place masts between the tracks.

3.4 Span-wires

A span-wire works like a gantry, but uses an assembly of tension wires stretched across the tracks to hold the contact wire and support wire up.

Span-wires are most common with tram-type overhead — if you attach the span-wires to existing buildings, you can oftne avoid the need to place masts in city centres altogether. In some places, e.g. Germany, it is common to use a kind of “transverse catenary” assembly instead of rigid gantries on multi-track railway alignments. You often see this at large stations.

3.5 Pull-offs

Normally, ohle goes in a straight line from one support to the next. If the track is curved, you have to add extra supports to prevent it from deviating outside the width swept by the pantograph head. On very sharp curves, as found on many trmaways and narrow-gauge railways, you would need an awful lot of masts to keep the wires in the right place. The solution is to run extra wires from the existing support masts to pull the contact wire sideways at intermediate points, rather like the guy-ropes of a tent. These are generically known as “pull-offs”.

One rather spectacular form of this is the “spider’s web” sometimes used to hold up tram overhead

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