Earthing and Bonding for AC Electrified Railways

The complexity of integrating the earthing of a 25kV electrified railway with various electrical distribution systems and exposed conductive parts, means that it is impossible to prescribe one earthing and bonding design solution that addresses the needs of every railway. Therefore, this creates different earthing designs for individual railways.

This guide aims to assist infrastructure owners, railway designers and installation contractors in adopting a harmonised approach towards E&B design philosophy for 25kV railway infrastructure.

The guide implements a common earth system formed by bonding lineside assets and structures to the traction return system, effectively forming a low impedance meshed earth network.

Electrical safety is required to minimise the potential difference between all exposed conductive parts and systems sharing the system earth.

This new guide explains the principles of a ‘common earth’ system (traction and non-traction) which is able to provide a robust and low impedance path to earth.

Section 1: Introduction

Section 2: Description of the AC traction electrification distribution system

The AC electrification systems have been developed specifically for railway traction purposes and to utilise the availability of universal 50/60Hz and 60Hz HV public electricity supplies.

The main feature of this section is to describe the principle operation of the railway AC traction system as a single-phase overhead electrification system with one pole intentionally earthed.

Section 3: Mass of earth as part of the traction return system.

The mass of earth is an integral part of the traction return circuit with the traction running rails and structure foundations inextricably linked to mother earth.

Therefore, this section looks at the electrical characteristics between earthing elements, including the rails, overhead line masts, foundations and earth mats to remote earth.

The 25kV traction return circuit has a combined neutral/earth return path and the rail voltages fluctuate due to the rail impedances and the flow of traction load current and fault current.

The current flow through the earth paths is responsible for conductive and inductive effects on auxiliary systems.

Computational analysis has been provided in this section to provide a deeper understanding of the characteristic behaviour of both ‘classic’ and ‘autotransformer’ distribution and return circuits.

Section 4: Traction return requirements and circuit configuration

The traction return circuit are the running rails and the foundations of the overhead line masts and this is the negative return of the 25 kV distribution system.

The circuit is required to provide the return path for both the traction current and act as a protective conductor during a 25kV short circuit.

This Section identifies the elements of the traction return circuit, its configuration and functional requirements.

The traction return circuit is also required to maintain the rail potentials with the limits prescribed in national regulations and international standards as defined in BS 7671, EN 50522, IEEE80, EN 50122-1 and EN 50122-3.

Section 5: Protective provisions for humans

The human body responds in several ways to electrical current flowing through it. The sensation of electric shock is only one such effect and this can be extremely painful and fatal.

When a shock is received, the electric current may take multiple paths through the body and its intensity at any one point is difficult or impossible to predict.

The passage of electric current may cause muscular contractions, respiratory failure, seizures, fibrillation of the heart, cardiac arrest or injury from internal burns. Any of these effects can be fatal. 

The need to protect people who work with electricity and the public has been a fundamental objective throughout history.  The objective of this section is to explain the requirements enshrined in the national and international standards and codes of practice, and national statutory legislation.

Section 6: Electrification system assets excluding the traction return

This section discusses assets within the electrification system that are not considered part of the traction return system but form part of the electric traction system and require bonding to the traction return circuit. 

These assets include the Overhead Catenary System; 25kV cables; Switching Stations; Feeder Switching Stations and Substations.

Section 7: Assets not forming part of the traction return

There are assets along the railway that can be endangered by an electric traction system.  These are not part of the electric system and include civil and structural assets such as stations, bridges and tunnels and electrical assets such as low and high voltage distribution systems.

The threats from the electric traction arise under both normal and fault conditions.  Traction fault currents can flow in assets that have not been designed to carry such large currents following insulation breakdown or breakage of live overhead conductors. 

Under normal conditions, inductive coupling from the overhead catenary system and galvanic coupling means that traction current can flow through structural reinforcement and protective conductors. 

This section describes the protective provisions that are typically applied to protect these assets and systems.

Section 8: System measurements

This section considers the testing required to validate the operation of the earthing and bonding system in compliance with standards and specifications.

During the commissioning of the 25kV electrification system a variety of additional test activities are undertaken that provide confidences of integration of the railway earthing system with the 25kV electrification system and other railway and non-railway assets.  

Section 9: Functional requirement of lightning protection for civil railway structures

Electrified railways are exposed to lightning and this includes HV power line and HV grid substation, 25kV electrification infrastructure, viaducts and bridges, return rails and LV systems installed within the railway.

The design of the railway is required to protect the electrification infrastructure and humans from the effect of lightning surges induced into the overhead equipment and maintain a reliable operation of the train service. 

This section has addressed the behaviour of lightning on the 25 kV infrastructure and the approach to lightning protection that is required to ensure the reliable and safe operation of the railway.

Section 10: Principles of earthing at AC DC Interface

Traction electrification systems operate with AC or DC distribution and use either HV overhead catenary (AC or DC) or a third rail (DC).

Railway companies are looking to integrate the operation of their railway systems', making it easier to provide interchanges for passengers.

It is not surprising, therefore, that railway companies seek to operate trains over different electrification systems.

Where DC and AC railways operate in close proximity, there is the possibility of electrical disturbances due to physical separation and electro-magnetic coupling.

Physical interfaces include track separation, air clearances, arrangement of the overhead lines.

Electro-magnetic coupling includes galvanic coupling, electric & magnetic fields and radiated fields.

The electric traction return circuit is provided by the running rails and these rails are earthed for AC electrification and floating with DC Electrification with respect to the earth's mass.

When the systems operate independently each system is required to comply with earthing and bonding requirements as detailed in EN 50122-1 and section 4 of this book.

However, when they co-exist and are interconnected or are adjacent to each other, the technical challenges of combining the different requirements become significantly more complicated, this aspect is detailed in EN 50122-3.

The consequence of parallel operation also includes 50Hz electro-magnetic induction into lineside conductors.

Section 11: Safe working during maintenance, renewal and decommissioning

This section describes the requirements for working safely on an overhead contact system during maintenance and renewal of the 25 kV electrification system.

Additionally, the essential operational & safety requirements for 25kV electrification systems in accordance with National Safety Regulations are addressed.

Humans require protection from direct contact from live 25kV conductors.

This section addresses the safety requirements for maintenance and renewals of the overhead lines and the requirements to prevent death or injury to those working on or close to live conductors.

During maintenance, the designed protective measures are essentially bypassed; therefore, alternative protective measures are required to achieve the safe working practice including disconnection and isolation at all supply points. 

Staff must also be made aware of the limits of working and understand the earthing of the isolated equipment at the point of disconnection and the necessary procedural arrangement.

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