Plant Maintenance
Maintenance Planning
Track Design
Vehicle Systems Design
Rail Manufacturing
Track Renewals
Electrification Testing
Electrification Design and Construction
Overhead Line and Track Renewals
 

From my experiences in dealing with overhead line, it appears that there is little concern for the electricity flowing through it. By this, I mean that it is more mechanically orientated rather than electrically orientated. The role of the OHL engineer is to ensure that the wires stay up, not electrified.

OHL has several key parts to it, these being:

• The contact wire
• The catenary wire
• The return conductor
• Earthing points
• Droppers
• Structures

The contact wire is the wire immediately in contact with the pantograph of the train. The current is transferred from the wire to the pantograph. On top of each pantograph is a set of carbons. These eventually wear due to the continuous contact with the OHL and this is the reasoning behind stagger.

Stagger

All OHL is staged in relation to the track to promote even wear of the carbon strips on train pantographs. The stagger is represented in the following simple diagram.

All OHL has to be kept at a height of 4.7m from the ground. This is the standard height. Under special circumstances, this height will change. It is lowered when fed under a bridge, and highered when passed over a level crossing.

To be kept at this height structures have to be used. There are several kinds of structures for holding up the OHL.

If there is a lot of lines, for example, just outside of a station, then portals may be used. These span across all tracks underneath and are the best option to use. This is because if a pantograph were to take down a wire, then it would only be that route rather than the whole span

However, due to expense and amount of steel used in each one, it cannot be justified that these should be standardised across the zone. The alternative is to use normal masts, which have wire spans across the tracks.

Portal structure.

Finally, all OHL is in tension of 11 kN. That is, balance weights can be found along a route.

Electrical Control

The electrical control room is where the Electrical Control Operator (ECO) can be found, and it is this person who can authorise power isolations for testing purposes. Within the control room is the relevant equipment for controlling feeder and sub-feeder stations as well as TSL's. This is all known as electric traction equipment.

When work has to be carried out on the OHL equipment, whether it is due to repairs or renewals, a full power isolation has to be granted. The process for gaining power isolation can be a long process, especially if the work is non-urgent.

The OHL feed system is operated by the ECO via a computer. The sections are divided in to feeder and sub feeder stations, TSL and OHL switches.

Various other sub divisions exist for the electrical control at place such as crossovers, junctions and neutral sections. All information regarding power isolation and supply can be found on power isolation diagrams. These indicate what can be shut off and what will be effected.

During an emergency, when the electricity has to be cut off, some rules should be adhered to:

• Staff should comply with general instruction No 6 of BR.29987 'Working Instructions for AC electrified lines'.
• No person or object in contact with the OHL should be approached until the ECO confirms that the section has been isolated.

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