Railway Power Supplies and Earthing

It's not easy to provide the power system for a railway network, with its long and narrow infrastructure. The railway is not always able to access power externally, but its electrical systems must have a reliable, safe and constant supply. This power source must be able to tolerate failures and provide a stable, uninterrupted supply, so that communications and control equipment such as points and signals operate correctly. Furthermore, the earthing arrangements for railway systems are different to those used in commercial, industrial or domestic electrical systems, which has led to past complications with safety compliance.

The power supply for railway signalling is a safety-critical application, requiring the same constant availability as that used in hospital operating theatres, for example, or airport ground lighting. It can't just be turned off in the event of a fault, as would happen with typical electrical engineering hazards. It must also be entirely safe, both for the public and for railway maintenance staff, as equipment is frequently located at level crossings or on platforms.

Principal Supply Point

New maintenance processes, and the recent introduction of Class II protection, have greatly improved the distribution of power to those applications using the current. Licensed distributors of electricity supply power for signalling to the rail infrastructure, via a Principal Supply Point (PSP). This is where the incoming supply (usually, but not always, 400V) gets converted upwards to the typical distribution voltage of 650V.

As a back-up, PSPs also use supplementary sources to the DNO's 400V so as to ensure the continuity of supply. These sources include the railways own standby generators, plus a high voltage, three-phase traction-power system. On important routes, a PSP may also have a further backup provided by an Uninterruptible Power Supply, to ensure continuity of supply during any switching of power between sources.

Improvement Notice

In November 2013, Network Rail was issued with an Improvement Notice by the Office of Rail and Road (ORR). It addressed the non-compliance of legacy signalling equipment designs with the above regulations, and especially the safety hazards of exposed conductive parts that were publicly accessible. 650V power distribution networks were targeted by the ORR as being inadequately earthed. They were deemed to be in contravention of certain statutory provisions of the Health and Safety at Work Act 1974, and the Electricity at Work Regulations 1989.

Network Rail was therefore required to overhaul its publicly accessible 650V power distribution networks, and undertake remedial works on any power distribution installation that was susceptible to dangerous electrical fault situations. Such applications had to be suitably earthed and bonded, and inspection and maintenance provisions put in place to ensure that all its 650V Locs were in compliance.

After investigation, Network Rail designed and installed Class II protection solutions to the identified problems, such that the ORR accepted their strategies and closed down the Improvement Notice in August 2017.