This guide aims to assist train operators, railway designers and installation contractors in adopting a harmonised approach towards E&B design philosophy for 25kV railway infrastructure. This will be in line with implementing a common earth system formed by bonding lineside assets and structures to the traction return system, effectively forming a low impedance meshed earth network.
The guide will therefore undertake to explain the principles of a common earth system (traction and non-traction) which is able to provide a robust and low impedance path to earth. Electrical safety is required to minimise the potential difference between all exposed conductive parts and systems sharing the system earth. The accessible voltages are required to be controlled with the limits defined in BS 7671 BS EN 50122-1 and BS EN 50122-3.
The traction return system is formed by inter-bonding at regular intervals the rails, aerial earth wires, buried earth wires and the 25kV HV traction substation.
The common earthed system is formed by bonding together the traction return system with multiple non-traction conductive structures to a single common reference point. Examples of non-traction railway infrastructure that may typically be common bonded include civil conductive structures, lightning protection system, walkways, lineside plant and structures as well as signalling and telecommunications equipment.
Although non-traction electrical systems maybe connected to a separate earth, they will be connected to a common earth system via a single point connection to avoid the hazard of touch potential between systems with separate earths.
Definitions of Earth
Definition of earth - conductive mass of the earth, whose electric potential at any point is conventionally taken as equal to zero [IEC 60050-826-04-01].
Definition of Earthing - connection of conductive parts to an appropriate earth electrode
Earth Electrode - conductor or a group of conductors in intimate contact with and providing an electrical connection to earth [IEC 60050-461-06-18]
Definition of Bonding
Equipotential bonding - provision of electric connections between conductive parts, intended to achieve equipotentiality [IEC 60050-826-13-19]
Traction Cross bond- electrical connection intended to connect in parallel the conductors of the return circuit
Traction Rail-to-rail cross bond- electrical bond that interconnects the running rails of the same track
Track-to-track cross bond- electrical bond that interconnects tracks
Proposed Structure of the Guide
It is proposed to split the work into Eleven Chapters:
- Chapter 1 – Introduction;
- Chapter 2 - Common principles of earthing and bonding;
- Chapter 3 - The traction return current system;
- Chapter 4 - E&B of civil assets;
- Chapter 5 - E&B of non-traction lineside LV (requirements of BS 7671) installations (e.g stations, equipment rooms, lineside systems and assets;
- Chapter 6 - E&B of non-traction HV grid connections and lineside distribution;
- Chapter 7 - E&B of train control and detection systems and operational-lineside; telecommunications systems, radio, telephony;
- Chapter 8 - E&B Interfaces with other railway and tram systems including AC DC interfaces;
- Chapter 9 - E&B Bonding of the traction depot
- Chapter 10 - TNO grid connections for the 25kV traction railway
- Chapter 11 - Operation and maintenance requirements
Please send any comment you may have to Andrew Cooney, Portfolio Development Manager at acooney@theiet.org
Dr R D White and Allen McDonald