Basic of Third rails for Railways (DC Traction System)

Contact lines for Railways

1. Overhead Contact Line

a. Single Wire Systems

b. Catenary Systems

2. Overhead Conductor Rail

a. Overhead Conductor Rail

3. Conductor Rail

a. Third rail

b. Fourth rail

Conductor Rails.

the oldest form of electrical traction current supply lines for electric railways.

Why 750 V DC Third Rail System for Metro Rail Systems, and what are its limitations?

750V dc third rail system has been extensively used in metros and more than 60% of existing metro systems in the world utilize 600-750V dc third rail system.

The system does not affect the aesthetics of the city as it is laid alongside the track and also require smaller tunnel diameter.

Third Rails current collection type

1.?????? bottom of? the rail

2.?????? side of? the rail

3.?????? top of? the rail

Types of Conductor rails

1.?????? Steel conductor rails

2.?????? Aluminium-steel composite rail

Technical Aspect of Using Aluminium/Steel Composite Third Rail

The method for reducing current losses (and thus increasing the spacing of feeder/substations, a major cost in third rail electrification) is to use a composite conductor rail of a hybrid aluminium/steel design. Aluminium is a better conductor of electricity, and the running face of stainless steel gives better wear.

750 V dc Third Rail Bottom Collection with Composite Aluminium Steel Third Rail.

For Bangalore Metro/Bhopal Metro, 750V DC third rail bottom current collection with the use of composite Aluminum steel third rail on main lines is envisaged from reliability and safety considerations.

The life of composite third rail is expected to be 25-30 years.

Stinger system

In the main workshop areas, for the purpose of safety of working personnel, for power supply stinger system is installed.

In this system, the conductor rails are installed on the gantry of the buildings and vehicles are powered through flexible cables.

Components of Third Rail System

The Third Rail system consists basically of the following main components:

????? a)??? Conductor Rail

????? b)??? Splice Joint assembly for third rail

????? a)??? Expansion Joint

????? b)??? Anchor Assembly

????? c)??? Insulated Joint assembly

????? d)??? Power Feed Assembly

????? e)??? Ramp

????? f)? Support Assembly Protective Cover system

Conductor Rail

The Conductor rail, ie third rail is made of an aluminium body designed for a nominal current of 4500A and a stainless steel contact path for current transfer to the collector shoes. The third rails is manufactured in standard lengths such as 15m length, which are joined together by special splice joints.The third rail is kept in defined positions on the concrete support/rail sleeper and is insulated from ground by an insulated support system. The normal support spacing is of the order of 5m. An insulated joint assembly is used in order to separate the different power sections along the track.

Splice Joints

For joining the conductor rails (including ramps, insulated joints and expansion joints).It is an end to end joining through a bolted connection incorporating aluminum splice plates. The splice plates are of sufficient length and designed to provide sufficient cross section area to maintain the electrical and mechanical properties of the conductor rail across the joints. The? Splice plates are manufactured from a similar aluminium alloy to that of the conductor rail and designed to clamp tightly into the web area of the rails using a taper on upper and lower edges.

Expansion Joint Assembly

In order to compensate for temperature-dependant rail expansion, expansion joints assemblies are installed per calculated distance which depends on the ambient temperature and heat rise. At the mid-span between two expansion joints, an anchor assembly is mounted to avoid unidirectional creeping of the rail caused by thermal expansion. These are sliding expansion joints? provided at intervals.

The Anchor Assembly

The anchor assembly is designed to fix, with respect to the guide way, the midpoint between expansion sections. The assemblies is necessary for each third rail section between expansion joints. The anchor design is? based on the same profile as that of the aluminum splice plates to ensure the high adhesion in the web of the conductor rail with no corrosion.Normally these anchors shall be located at the mid point of rail lengths, between expansion joints, but there may be occasions when anchors are needed at other places, for example near cable terminals (typically 22.5m from cable terminal).

Insulated Joint Assembly

These are provided at various locations for the purpose of sectioning of the conductor rail being powered from different traction substations. This is an insulated joint used to separate different sections provided by two different substations. ?Every insulation joint is factory made from a length of conductor rail (of about 4m long) in 3 (three) pieces with oblique cut at 30° angle (or other suitable angle). The middle part of conductor rail is about 400mm long and there is about 100 mm air gap between conductor rail pieces.?

Power Feed Assembly

The Power Feed Assembly consists of cable terminals that are arranged on the conductor rail located in accordance with the plans of the traction power supply. At intervals along the conductor rails, Feeding points consist of two aluminium blocks and associated Huckbolts, firmly connected to the third rail. One aluminium block is the same as used for splice joints and the other one has a specific extrusion plate that is dedicated for cable lug fastening. It is possible to arrange the plate in a variety of angles and shapes to suit the incoming cable lugs.

Ramps

Different kind of ramps are used on turnouts or where needed in order to bring effect a smooth transfer of collector shoe to conductor rail. The conductor rail is? installed on trackside at the specified distance to the track centerline / top of the rails. In the double-track line, the conductor rail is? installed between both tracks in order to leave sufficient space for walkways.

The conductor rail ramps are of two types:

a)??? Typically 5.0 to 5.5 m for high speed ramp (over 100 kmph) used on the main lime (slope 1/50)

b)??? Typically 3.0 to 3.5 m for low speed ramp (upto 25 kmph) used in depot area (slope 1/30)

Protective Covers and Fasteners

The entire conductor rail, including all ramps, expansion joints and cable terminals are covered by an insulated protective cover to protect against the weather and to avoid unintentional contact by staff working/walking on the line. Each cover system is designed for high standards of electrical performance, fire and smoke properties, and long life. The covers are coloured grey or yellow.

INTERRUPTION

Bridgeable Gap

Installed at any location where the third rail needs to be interrupted while still providing traction power continuity

Non Bridgeable Gap

Installed? to separate two feeding sections in a way that shunting of the both sections by the inter connected collector? shoes of a single motor car is not possible