(i) Computer Based Interlocking (CBI):- The entire line including turn backtrack, transfer track, and sidings will be equipped with CBI system for the operation of points and crossings and setting of routes. The setting of the route and clearing of the signals will be done by workstation, which can be either locally (at station) operated or operated remotely from the OCC. This sub-system is used for controlling vehicle movements into or out of stations automatically from a workstation.

All stations having points and crossings will be provided with workstations for local control. Track occupancy, point position, etc. will be clearly indicated on the workstation. It will be possible to operate the workstation locally if the central control hands over the operation to the local station. The interlocking system design will be on the basis of the fail-safe principle.

The equipment will withstand tough environmental conditions encountered in a Mass Transit System. Suitable IS, IRS, BS standards or equivalent international standards will be followed in case wiring, installation, earthing, cabling, power supply and for material used in track circuits, axle counters, relays, point operating machines, power supply etc.

(ii) Track Vacancy Detection:- Primary mode for track vacancy detection system on main line may be through radio and
for secondary detection it can be through Track circuit / Axle Counter.

(iii) Signals:- Line side Signals: Multi Aspect Colour Light (LED) type Line side signals shall be installed on the Main Line and depot entry/ exit.

(a) At stations with point and crossing for point protection catering for bidirectional working

(B) At departure location at stations for normal direction of working

(iv) Point Machines:- Non-Trailable Electrical Point Machine capable of operating with either 110V DC or 3- phase 380V AC will be used on main line. The depot point machine will preferably be trailable type.

What is a Power Supply? 

1. Electrical supply used for Signaling purposes. Different types of equipment need different voltages.

2. Supply is extended by the Electrical department based on the requirement of signaling.

3. Signalling has to provide the details of locations and maximum load at each location.

4. Based on these requirements the main transformers and the feeder cables are designed.

5. At each location the distribution is the responsibility of signaling.

Various supplies are used in signaling.

1. 50 VDC – standardized for new signaling circuits with BR relays
2. 24VDC – used in old installations with 24V relays and also for standard AWS circuits.
3. 12VDC for old shelf type relays, lever locks, SPTs and old AWS circuits.
4. 120VDC for point operation
5. 110VAC for all general purposes of signaling.
6. 140V AC used in SSI

Supplies above 110V AC are considered to be high voltages which require proper protection.  Signal staff are authorized to handle supplies above110V AC.

Electricity at Work Regulations (1989) made under the Health and Safety at Work Act 1974.

• Employers and Employees are required to take suitable precautions when working with or near to electrical equipments
• All electrical and electronic equipments are required to comply to these regulations.

The main requirements are:

1. Protection from Direct Contact
2. Protection from indirect contact
3. Use of appropriate equipment
4. Over current protection
5. Ability to disconnect the supply
6. Sufficient working space
7. Competent staff
8. Accurate documentation

Distribution of mains power

1. Ring system is provided to feed the mains power to any Relay Room / loc from a feeder station.
2. Each location has facility for isolating itself from the supply.
3. Facility to be provided for isolating sections for maintenance / fault finding activities.
4. Load on the feeder is calculated before introducing a new location on the supply.
5. Load distribution and feeder cables are designed to ensure that a minimum of 90% supply is available at any point in case of ring breakage

Fuses and Circuit Breakers

1. Fuses are used to protect end equipment from high currents which can damage the equipment. Similarly circuit breakers are utilized. At the same time they isolate a fualty equipment from other equipement/circuit.
2. Fuses have to be replaced if they blow off. Circuit breakers can be reset.
3. UK railways have Porcelain type fuse holders and also Enterlec type. Fuses may be of varying values depending on requirements. Typical values are 1A, 2A, 3A, 4A , 6A , 10A , 15A etc.
4. CM type fuse holders are used for outgoing feeds / supplies to track for track circuits etc. They are also of various values – 32A, 63A ,100A etc.

Enterlec type fuses : representation

Porcelain fuse: Representation

CABLES AND POWER FEEDERS

1. Cables are used to run 110V AC Power supply between the locations/cases.
2. 2 core 2.5 sq.mm (f) type B2 are used.
3. Internal power wiring is done by single core cables
4. We generally use 1 core 2.5 sqmm (RD or BK) for 110V
5. Power feeder locations are provided at regular intervals generally nearer to a RR.
6. It is important that the voltage variation is accounted for and as per Standards a variation of +/- 10% over the supply voltage is permitted.

 INDICATIONS

• Generally the failure of power is indicated to the signaller by means of an audible and/or visual alarm.

Earthing

Requirements for Earthing

1. All electronic equipment needs to be earthed to protect against Electrical Surges. So to protect the equipment from surge earthing is provided is known as a fast transient earth (FTE).
2. There has to be only one earth at any given location and earth connections have to be bonded together.
3. Any other earthed signaling equipment, loc box is present within touching distance of other metal works, then it is required to be bonded together to avoid the possibility of a potential difference between the items.
4. FTEs should have low inductances.

1. Typical values may be 10 ohms
2. As a direct requirement all location cases within 2.0m of each other, one of which is earthed, are required to be bonded together to avoid the possibility of a potential difference between 2 metal works causing a flow of current when any person accidentally comes into contact with both cases simultaneously.

Lightning Protection      

What is a lightning strike? – A stroke of a lightning flash attaching to equipment.

Lightning protection system – Whole system of conductors is used to protect equipment from the effects of lightning.

What to do for protection? – Provide a direct and low resistance path to the earth for the lightning currents.  Concept of FTE with R=10 ohms or less.

Earthing

RT/E/C/11210 App 2J10 says that all metal work in a relay room must be earthed unless it is double insulated. Other metal works also need to be earthed if under fault conditions they come in contact with high voltages. Equipment required to be earthed are:

In addition to this all electronic equipment needs to be earthed to protect against interference. This earth is known as a fast transient earth (FTE). There has to be only one earth at any given location so all such earth points have to be bonded together.

If any earthed signaling equipment , loc box , or other equipment is present within touching distance of other metal works then it may be required to be bonded together to avoid the possibility of a potential difference between the items.

EARTH ELECTRODES

Requirements

Typical Bonding Arrangements

Typical earth arrangement in a loc

Equipotentail Bonding

Purpose is to protect against indirect contact with exposed conductive parts and extraneous conductive parts made live by an electric fault.

Earthed equipotential bonding within apparatus housings is associated with signalling power supply system.

Equipotentail bonding of extraneous conductive parts that are not within apparatus housings is associated with external electrical systems such as traction supply.

As a direct requirement all location cases within 2.0m of each other, one of which is earthed, are required to be bonded together to avoid the possibility of a potential difference between 2 metal works causing a flow of current when any person accidentally comes into contact with both cases simultaneously.

RT/E/C/11600 Part J

Equipotential Bonding

Where earthing systems are provided, a single main earth terminal or a busbar should be provided at each location to which the equipotential bonding should be connected.

Earthing and equipotential bonding conductors should be shown on the circuit diagrams and should be so designed that they can be tested for effectiveness (like by provision of disconnection links).

In AC electrified areas equipotential bonding to the traction return system should be used to protect against traction faults. All line side metal works should therefore be bonded to the traction return system instead of the true earth.

In DC electrified areas , metal works should not be bonded to the traction return system nor should the traction return be bonded to any earth system as it may provide a alternate return path. Damage to structures may also occur due to these stray currents.

Lightning Protection

What is a lightning strike? – A stroke of a lightning flash attaching to equipment.

Lightning protection system – Whole system of conductors used to protect an equipment from the effects of lightning.

What to do for protection ? – Provide a direct and low resistance path to the earth for the lightning currents. Concept of FTE with R=10 ohms or less.

Surge arrestors – discharges the surge voltages due to lightning strike. Should not be connected to earth unless the equipment has been specifically designed to resist multiple earth faults.