A railway must be supported with a safe and reliable power system for signals, points, interlockings and communications. This power system must be tolerant against failure and provide a constant, stable supply in order for the control and communications equipment to operate correctly.
Often either taken for granted or overlooked, power systems for railways can be difficult to provide, given that the infrastructure is long and narrow and not always close to an external power source. In addition, the earthing arrangements differ from domestic, industrial and commercial electrical systems, a situation that has caused compliance issues with safety requirements.
The signalling power supply must always be available, with a continuity of supply similar to other safety-critical applications, for example airport ground lighting or hospital operating theatres. So, it is not acceptable to turn everything off when a fault arises, which has been a traditional electrical engineering solution for such hazards.
The power system must be safe, both for maintenance staff and for members of the public, when the equipment is located on platforms or at level crossings, even should faults occur that result in accessible metal becoming live.
Railways have invested a lot of time, effort and money in providing reliable power supplies with multiple sources of energy, but, in some cases, they have neglected the power distribution network that moves the power to the current-using equipment. However, over the last few years, new maintenance processes and the introduction of Class II equipment have improved matters, after a lot of effort by both signalling and power engineers in Network Rail and the supply industry.
Principal Supply Point
A signalling power supply comes onto the rail infrastructure from a Distribution Network Operator (DNO) network via a Principal Supply Point (PSP). DNOs are companies licensed to distribute electricity in the United Kingdom. At the PSP, the incoming supply is converted to the distribution voltage of, typically, 650V. The incoming supply is usually 400V, but can be other voltages.
To maintain continuity of supply, PSPs use additional sources in addition to the 400V DNO supply, including the railway’s own three-phase high voltage traction-power system and standby generators.
A PSP for an important route may also have an uninterruptible power supply (UPS), so that the supply is maintained during the changeover between the two sources. In simple terms, a UPS is a big battery fitted with an inverter that converts the battery’s DC output to AC. It may also provide insulation monitoring equipment.
Functional Supply Point
The major elements of a railway signalling power system are the power sources, the distribution network and electrical loads (sometimes referred to as current-using equipment). The power supply voltage is often transformed down to a lower voltage to minimise voltage drop when distributed over longer distances. Historically, a two-wire 650V AC distribution system has been used for railway signalling power in the UK.
Within a trackside case, equipment known as the Functional Supply Point (FSP) normally converts the 650V AC to 110V AC using transformer(s). The FSP contains transformers and rectifiers to convert the power distribution voltage to the AC and DC voltages used by the track-side train control system equipment. It may also accommodate lockable switchgear, to allow for safe working practices on the power distribution and train control equipment.
Location Cases (Locs) accommodate railway signalling equipment to detect the location of trains, control the trackside signals and switch the points. Locs and FSPs can be the same physical asset, containing both power equipment and train control equipment. The equipment may typically use both AC and DC voltages less than 110V, so transformers and rectifiers are required to obtain the correct voltage from the incoming supply.
Class I and Class II protection
Protection arrangements in power distribution networks are generally Class I or Class II. In Class I, exposed metal parts must be connected together and connected to electrical earth by a separate earth conductor (coloured green/yellow in the UK). The basic requirement is that no single fault can result in dangerous voltage becoming accessible so that it might cause an electric shock and that, if a fault occurs, the supply will be automatically disconnected. Traditionally, railway signalling power supply distribution systems have been based on Class I individual earth designs, which require an effective earth arrangement.
Class II, or double-insulated, electrical equipment is designed in such a way that it does not require a safety connection to electrical earth. The basic requirement is that no single fault can result in dangerous voltage becoming accessible so that it might cause an electric shock, and is achieved without relying on an earthed metal casing. This is usually met by having at least two layers of insulating material between live parts and the user, or by using reinforced insulation. Not only is there a safety benefit with Class II, but the availability is far greater as the supply is not tripped due to a cable fault.
In domestic situations, Class II power supplies (such as laptop chargers) will have a two-wire power cord as opposed to a three-wire power cord with a safety earth connection. Products designed with Class II insulation are often labelled as “Class II” or “double insulated” or will have a concentric square safety label symbol.
BS 7671 (the IET Wiring Regulations – informally called “the regs” by some) covers the primary types of power system earthing arrangement called TN, TT or IT. These use the French terms for Terre (earth), Neute (neutral) and Isolé (isolated).
The first letter indicates the connection between earth and the power-supply equipment, so ‘T’ indicates a direct connection of a point with earth (Terre) while ‘I’ means that no point is connected with earth (Isolé).
The second letter indicates the connection between earth or network and the electrical device being supplied, so ‘T’ corresponds to an earth connection by a local direct connection to earth (Terre) or ‘N’ shows that the neutral connection is supplied by the electricity supply network (Neutre).
The traditional way of distributing the power trackside along the railway for signalling has been using the IT earthing system. The output of the source transformer is isolated from earth, whilst all exposed conductive parts of the installation are connected to earth.
The source transformer output is isolated from earth, although the system will be still earthed by the stray capacitances of the cables. Should there be a fault with the cable insulation or FSP equipment, creating a direct short circuit to earth, there is no return path for the earth fault current with the equipment remaining operational.
The trackside cables are usually, but not always, two-core, and each FSP is individually earthed using an electrode formed of one or more buried earth rods. An appreciable earth fault current may flow, but the length and resistance of the feeder circuit conductor to the fault locations, as well as the resistance of the Loc connection to earth, can result in a fault current that is insufficient to cause automatic operation of the protective device in an acceptable time.
The general mass of earth can be variable, which makes the magnitude of earth fault current difficult to predict.
The problem is that this arrangement is not compliant with the Electricity at Work Regulations 1989, nor BS 7671 (Wiring Regulations). The highest permitted accessible voltage is 50V (BS7671) or 60V (EN 50122-1). This may be exceeded on some large legacy power distribution 650V networks.
An even bigger problem on a system with an IT Earthing arrangement is with a second earth fault situation, as an accessible harmful voltage is very likely to be present for an unacceptable duration. The ratio of the two earth fault resistances will determine how much of the 650V system voltage appears on each faulty equipment case.
It is believed that the non-compliance with BS 7671 may have arisen because there is an exemption for ‘railway signalling equipment’ in the standard, which railways relied upon for many years. However, the Office of Rail and Road (ORR) took the view that the exemption to the regulations is only for the ‘signalling equipment’ itself and not the electrical distribution networks feeding signalling equipment. In any case, the Electricity at Work Regulations 1989 are applicable, so non-compliance is not an option.
The legacy design, and, in particular, the hazard from exposed conductive parts of signalling equipment in public areas, resulted in the ORR issuing Network Rail with an Improvement Notice on 19 November 2013. In the notice, the ORR stated: “650V power distribution networks used to supply power to track-side signalling equipment at various locations on Network Rail managed infrastructure are not constructed to prevent, so far as is reasonably practicable, danger. Signal location cases, to which members of the public have access, are not adequately earthed and bonded to prevent danger should exposed conductive parts become charged at dangerous voltages”.
The scope of the notice applied to all of the 650V power distribution networks across Network Rail managed infrastructure, and the ORR considered that the situation contravened the statutory provisions contained in Section 3(1) of the Health and Safety at Work Act 1974 and regulations 4(1), 4(2) and 8 of the Electricity at Work Regulations 1989.
Network Rail was required to undertake a review of 650V power distribution networks to which members of the public have access, to:
- Identify assets with exposed conductive parts that are not adequately earthed and bonded to prevent danger, should they become charged at dangerous voltages;
- Subject to the findings of 1) above, undertake suitable remedial works, prioritised on the basis of risk, to ensure that 650V signal location cases which may reasonably foreseeably become charged as a result of an electrical fault are earthed and bonded;
- Devise and implement inspection and maintenance arrangements for ensuring that 650V signal location cases to which members of the public have access are maintained to prevent, so far as is reasonably practicable, danger;
- Implement any other equally effective means of achieving compliance with the notice.
A number of solutions were identified by Network Rail to comply with the Improvement Notice. These are linked and need to be combined in addressing the risks to the signalling power distribution problem.
A suite of Class II-based power system equipment and designs were developed and approved. Signal power network insulation monitoring and fault location equipment have been introduced, based on risk. The design and installation of signal power network earthing arrangements have been revised, together with a much-improved signalling power system inspection and maintenance regime.
Competences and training have been reviewed and greatly improved, both for signalling power system inspection, testing and maintenance, together with improved safe working practices for work on or near signalling power equipment.
The inspection and maintenance regimes have also been improved. Responsibility and accountabilities are clearly defined and include a requirement to inspect and take action based on risk. A consistent national method of classifying defects by codes and the required response, and by whom, has been implemented.
Additional resources, where required for the inspection and implementation of identified control measures, have been authorised, which includes the required competencies. Where a hazard is identified, a feasibility design, based on risk and the ground resistance, may require a Class II system to be installed retrospectively, should an improved earth electrode not be sufficient.
Traditionally, feeder insulation monitoring equipment can identify that a fault has occurred, but not where the fault is located. The fault could be anywhere on the power network, which can total more than 30km in a large signal box area, requiring time-consuming inspection and testing to locate the fault. Network Rail is currently trialling ‘smarter’ insulation monitoring equipment, which is able to trace the location of any fault better. The equipment is based on designs used in the offshore oil and gas industries, and the results are encouraging. Full approval is expected soon.
The Class I collective earthing system (which complies with the regulations) was not specifically identified as part of the improvement notice response, but such systems had been used selectively around the network for a number of years, such as on the Mickle Trafford resignalling in 2006 and Crewe-Winsford remodelling in 2008. With the Class l collective earthed system, a three-core armoured cable is used rather than a two-core 650V supply cable. The third core and armour are used together in parallel as a circuit protective conductor (CPC) to bond all the equipment together equipotentially. The bond ensures that, in a second fault situation, there is a low impedance path present.
The problem with this arrangement is a financial and environmental one, as it requires 50 per cent more conductor than the Class 1 individually earthed arrangement, increasing not only cost but also the risk of theft.
Class II power supply design
Class II was selected, not just as a means of eliminating safety risk arising from second earth faults, but also as a means of eliminating harmful voltage on accessible conductive parts with first earth faults. An effective local electrical safety earth electrode is no longer required at an FSP on a Class II power distribution network.
Using Class II for railway signalling power distribution satisfies the requirements of BS 7671 regulation 410.3.3 by utilising double or reinforced insulation instead of the traditional automatic disconnection of the supply as protection against electrical shock. This also has the benefit of lower capital cost. However, effective continuous monitoring of the distribution system insulation remains an essential safety feature of any Class II power distribution network.
A Class II-based design solution has been identified for new-build signalling power distribution systems and the renewal of legacy signalling power distribution systems, using two-core unarmoured cables. A number of resignalling schemes have already been installed using Class II and, over the next two years, a significant amount of signalling power system renewals will replace Class I with Class II. This includes the West Coast power signal boxes north of Crewe.
Location case and equipment buildings will be provided with Class II switchgear housings and Class II signalling transformers, with approved conduit and fittings, will be used to provide supplementary insulation and protection to wiring between the items of Class II equipment. Distribution equipment can be connected by either two-core enhanced unarmoured cable or two-core armoured solid-bonded cable, provided that the armour is not allowed to traverse the Class II-fitted functional supply points. However, extensive testing identified that an enhanced unarmoured cable option presents the highest overall level of safety.
The overall Class II strategy will result in improvements to personnel safety, compliance with standards, greater tolerability to DC corrosion, simplified earthing and bonding in AC electrified areas, along with reduced capital costs, better reliability, less maintenance and the ability to integrate into legacy systems.
To support the strategy, Network Rail has issued a range of new standards. The key standards are issued as both signalling and electrification/plant (SIGELP) standards, are designed to be user friendly and to inform the right person what they need to do to control the risk.
The strategy was accepted by the ORR and the Improvement Notice was closed down on 22 August 2017, with the ORR monitoring the situation.
Thanks to Graeme Christmas, Martin O’Connor and especially Graeme Beale, of Network Rail, for their help with this article.
The challenge of implementation…
In history, there are often moments or circumstances that turn necessary actions into unforeseen opportunities. As a result of the improvement notice, the introduction and roll out of retrofittable Class II as a compliance strategy now provides Network Rail with the tools and the methodology to deliver widespread enhancements to all signalling power supplies, with minimal disruption.
As with most new systems and technology, one key challenge is its implementation within the existing infrastructure. This challenge is one of understanding and joined-up process driven by survey, integrated design, option selection and efficient deployment. It has been discovered that, over the past four to five years, this package of works is ideally suited to support Network Rail’s existing route delivery teams. An example of this efficient process can be seen above.
New technology born out of change!
Maintenance of the existing signalling power network places a high demand on existing human resources. Routine testing and the assessment of the existing 650V signalling power cable presents existing maintenance teams with both safety and logistical challenges.
Cable and system monitoring technology offers a significant improvement in the management of these assets. Insulation monitoring and fault location devices enable rapid response and proactive maintenance regimes to be implemented. The key success of the roll out of this technology is the ability to deploy alongside the Class II retrofitting process, thereby leaving the asset in both a fully compliant and digital-ready state.
There have already been several examples of significant infrastructure reliability improvements as a result of Class II deployment. One is the elimination of low insulation monitoring values as a result of prescribed Class II retrofits, a second is confirmation of potential cost savings in unnecessary cable replacement across a large section of the Thameslink Route.
Lewis Westbury – Ilecsys Rail