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 railway’s 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.
Functional Supply Point
The Functional Supply Point (FSP) is a track-side application containing transformers and rectifiers, used to convert the power distribution voltage to those required by the train's control system equipment. Although the power supply is usually converted up from 400V to 650V for distribution purposes, when it passes down to the current-using equipment, this voltage often has to be changed down again (usually to 110V). This is to avoid any voltage drops that may occur in distribution over longer distances.
Class I & Class II Protection
Traditional railway signalling systems used Class I individual earthing designs, whereby any exposed metal parts must have a separate earth conductor (in the UK, usually green/yellow) so that no single fault can produce sufficient voltage to cause an electric shock. In the event of such a fault occurring, the supply is automatically disconnected. Class II electrical equipment is double-insulated, and doesn't require this kind of safety connection direct to electrical earth. Instead, it uses reinforced insulation, or has two or more layers of insulating material situated between the user and any live parts. Class II is therefore intrinsically much safer, and provides a far greater continuity of supply, as it's not tripped by cable faults.
BS 7671 (IET Wiring Regulations) covers the main types of earthing arrangement for power systems. These are known as TN, TT or IT, based on the French for Earth (Terre), Neutral (Neute) and Isolated (Isole), where the first letter of the combination indicates a connection between the power-supply equipment and Earth. 'T' therefore denotes a point that has a direct connection with Earth, while 'I' denotes a point that doesn't.
The second letter in this combination indicates the connection between the relevant electrical device and the Earth or a Neutral distribution network. 'T' denotes that the device is connected to Earth by a local direct connection, while ‘N’ shows that an electricity supply network is providing the neutral connection.
The railways have traditionally used the IT earthing system at track-side to distribute the power for signalling, so that the source transformer's output is isolated from Earth, but all the installation's exposed conductive parts are directly connected to Earth. However, this system has proved insufficiently effective over an extended railway network, with distance negating the power of the earthing current to trigger protective devices within an acceptably safe time.
Nor is the typical track-side earthing arrangement compliant with BS 7671 or the Electricity at Work Regulations 1989, as typical 650V legacy power distribution networks greatly exceed the limits of permitted accessible voltages. More importantly, in the event of a second simultaneous earth fault occurring, a dangerous accessible voltage is highly likely to endure over an unacceptable period.
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.
Class II Power Supply Design
Class II protection eliminates the safety risks associated with second earth faults, as well as eliminating dangerous voltages where first earth faults occur on accessible conductive parts. It also satisfies regulation 410.3.3 of BS 7671 by using double or reinforced insulation to protect against electrical shock, instead of automatically disconnecting the supply. Two-core enhanced unarmoured cable provides the highest overall safety levels for connecting Class II distribution equipment.
New cable and system monitoring technology can be deployed while Class II systems are retrofitted, so that the asset is both digital-ready and fully compliant. These include fault location devices, and insulation monitoring, which enable the implementation of proactive maintenance regimes and rapid responses, reducing costs and the demand on human resources.
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