Earthing and Bonding

Earthing and Bonding is a fundamental part of railway electrical safety and electromagnetic compatibility. It establishes intentional electrical connections between conductive railway assets and appropriate earthing or return-current systems so that dangerous potential differences are controlled during both normal operation and electrical fault conditions.

In electrified railways, particularly AC traction systems such as 25 kV AC, earthing and bonding must be coordinated with the traction power return circuit. Rails may form part of the traction return path, while additional return conductors, earth wires, bonding conductors and connections may be provided depending on the electrification arrangement.

Typical assets requiring consideration include:

Running rails and traction return conductors
Overhead Line Equipment (OLE/OCS) structures and supports
Metallic cable trays, trunking and containment
Trackside equipment and equipment enclosures
Signalling and telecommunications equipment
Station and wayside structures
Platforms, fences, barriers and other accessible metallic components
Bridges, tunnels and other conductive structural elements
Electrical substations and technical buildings

Earthing generally refers to connecting an electrical system or conductive part to earth through an appropriate earthing arrangement.

Bonding refers to electrically connecting conductive components together, or to the traction return/earthing system as appropriate, to control potential differences and provide electrical continuity.

Railway earthing and bonding design must also consider touch voltage, accessible voltage, step voltage, fault-current paths, traction return current, stray current, electromagnetic compatibility (EMC), signalling compatibility and lightning protection.

The exact earthing and bonding arrangement depends on the traction system, signalling technology, infrastructure configuration and applicable railway standards. For AC railway applications, EN 50122-1 is a key reference covering protective provisions relating to electrical safety and earthing associated with fixed installations.