An essential sub-system of any railway signalling or traffic management system is the signalling power supply (SPS). This typically includes: source of supply (usually the distribution network operator – DNO), principal supply point (PSP), signalling power distribution system (SPDS) and signalling equipment.

The majority of distribution systems are autonomous cable networks that are not interconnected although, in new installations, an increasing number are double-end fed systems. Thousands of kilometres of power cable interconnect the power supply points and signalling equipment housings positioned along the railway via functional supply points.

As with all electrical supply systems, safety is paramount. Network Rail has a rolling programme to improve electrical safety in the Signalling Power Systems which drives significant performance improvements in our network. Signalling power cable failures can be very disruptive to railway traffic. Even short-term interruptions can have a wide impact on performance across the network.

The introduction of new technologies, in the form of Distribution Interface Transformers Assemblies (DITA’s) together with a new tool for modelling network electrical performance, Target Earth Calculation Model (TECM), will support asset managers, designers and contractors to deliver significant improvements in electrical safety and performance.

To support this, Network Rail has drawn up a new standard, NR/L2/SIGELP/27416 ‘Alteration to Signalling Power Systems (SPS)’. This introduces a new set of tools, methods, processes and technologies for driving electrical safety in the largest non-traction power supply system in operation, which covers the entire rail network.

Network Characteristics

The SPS network is not homogeneous but has a number of key characteristic that are similar for the entire network installation. Some of these are driven by the age and technologies used at the time of installation. Any network electrical safety and performance assessment requires asset managers and designers to characterise the network. This can be challenging where some installations have an asset age exceeding 50 years.

The new NR/L2/SIGELP/27416 standard now sets out a supporting appendix describing typical electrical configurations across the network and a guide for characterising the network electrical system and configuration.

  • Signalling Power Systems can be made up of electrical systems that are:
  • Single-phase, IT individual with 2-core unarmoured cable;
  • Single-phase, IT individual with IT grouped, with 2-core unarmoured cable mixed with 2-core armoured cable;
  • Single-phase, IT individual with IT grouped, with 2-core unarmoured cable mixed with 3-core armoured cable;
  • Single-phase, IT, Class II signalling power distribution systems with 2-core enhanced unarmoured cable.

IT – Individual Terra – is a form of earthing that has its earthed connection isolated from the power supply and components are individually earthed and double insulated with the supply fed through distribution interface transformers. This earthing system is tolerant of cable earth faults. Cable sizes can range from 6mm2 to 185mm2. The number of cores is dependent on electrical configurations. Cable networks can comprise multiple feeders, installed in a range of outdoor environments including cable troughing, under-track crossings (UTX) or direct buried. These can be subjected to humidity and water immersion with typical network lengths (multiple feeders) combining to give a total cable network length in the range of 30 to 70km. Individual feeders can range in length from 5 to 30km.

This can cause leakage capacitance to earth in the range of 1μF (microfarad) to 100μF depending on the age and size of the network, the cable type and cable installation method. The cable network connects into signalling locations comprising metallic parts with a local resistance to earth values less than 1 ohm or values up to 1000 ohms (or more). This can be a key driver for assessing the electrical safety compliance of the installation.

Cables can have multiple in-line and transition cable joints and networks comprise both single phase and/or three phase AC systems.

Reasonable opportunity Introducing alterations to an existing SPS is often a highly disruptive process, since it affects many sub-systems across a large part of the network. However, this disruption in itself creates an opportunity to carry out other minor works which, hitherto, would have been postponed because the upheaval required is large compared with benefit gained.

In the past, minor work that could have been carried out at the same time as alteration work may have been ignored and managed by derogation, largely because it is outside the scope of the alteration work defined. Although this means that only work defined within the work scope gets done, it also means that any additional work related to improving electrical safety, that would be sensible to carry out at the same time, is not included.

A key principal of the new standard now requires designers to undertake a reasonable opportunity assessment alongside the asset manager when developing a design to introduce alterations to an SPS. Reasonable opportunity assessments determine any additional work that it would be reasonable to carry out at the same time as the alteration work, to address or improve electrical safety and/or performance.

Improving electrical safety

A central strategy for managing the network electrical risk is feeder sub-division. This drives the network to be subdivided into smaller and more manageable sub-networks. It also has the benefit of reducing network capacitance and reducing the risk of electric shock, along with improving performance by allowing asset managers to identify emerging faults easier than in larger networks.

Sub-dividing the network into smaller parts using Distribution Interface Transformer Assemblies (DITA) allows a reduction in system capacitance termed First Fault Current Reduction (FFCR). FFCR may be used in combination with earthing to reduce the maximum permissible earth resistance value to control touch voltages under first fault conditions in IT Electrical Systems. This can be quantified by using a new tool in accordance with NR/L3/SIGELP/27420 – ‘Target Earth Calculation Methodology for Signalling Power Supplies’. The tool, which is available as a free download, forms a key part in allowing designers to establish lengths and target earth values and it also helps determine feeder sub-division lengths. First Fault Current Reduction (FFCR) may be used in isolation or in combination with local earthing, traction bonding and Class II/hybrid equipment. This may be useful in installations where: The magnitude of electric shock risk is to be reduced for all installations on the feeder network without the need to address each Signalling Apparatus Housing; Individual feeder Insulation Monitoring indications allow a response time for faults to be achieved within the maximum permitted time; Individual soil conditions and access arrangements make it difficult for the construction of local earth farms and the installation of Class II / hybrid equipment.

Feeder sub-division

Where existing installations have multiple feeders derived from a single transformer, opportunities can now be taken to consider either sub-dividing the Principal Supply Point (PSP) transformers such that each outgoing feeder has an individual isolation transformer, and or sub-dividing the feeder into smaller sections by using DITAs in accordance with network rail standard NR/L2/SIGELP/27419.

Distribution Interface Transformer Assembly (DITA) concept

New DITA technologies have now been deployed in several installations including Blackburn King Street; Stirling, Dunblane and Alloa; Halton Curve, Northern Programmes and Bristol Area Resignalling. All now have achieved trial product certification. They are now ready for wider implementation to drive electrical safety and performance improvements. DITAs in accordance with NR/L2/SIGELP/27419 can now also be used in signalling power distribution systems for a number of purposes.

DITAs can be used for segregation between a Class I and a Class II installation to preserve the integrity of a Class II-based feeder when interfaced with a Class I feeder. This is critical where the Class I system has been installed for a number of years and may not be compliant with the requirements of BS 7671.

This would equally apply if a Class I installation was the source and a Class II installation was installed downstream.

A DITA can also provide segregation between feeders and spurs or branches, to divide a signalling power system feeder into sections, particularly long feeders or those having multiple spurs or branches. Similarly, DITAs can segregate distribution feeders supplied by the PSP, from a single isolating transformer serving a network to individual isolating transformers for each feeder, and segregate distribution feeders by dividing an existing feeder into electrically smaller sections.

In addition, DITAs can step-up the voltage (voltage boost). This can be beneficial in installations where smaller cable sizes have been deployed to achieve value engineering or where aluminium cables are used to reduce the likelihood of theft or drive efficiencies.

To minimise the flow of stray traction current between different designs of a railway traction system with different traction earthing arrangements, DITAs can be installed at the interface between an auto transformer traction system and classical AC traction system, between AC and DC traction systems, or between a traction power system and a non-electrified area. If correct isolation is not provided, stray DC traction currents could flow through a signalling system earthing arrangement at apparatus housings, potentially causing electrolytic corrosion.


The DITAs have been designed to consist of the switchgear, isolating transformer, protection and control unit, insulating monitoring equipment, distribution feeder soft-start equipment and alarm facilities.

The transformer section of the DITA may contain the incoming isolating transformer(s) and the main isolating device for the DITA system. This is a key component that allows the network to be segregated.

The protection control unit (PCU) contains the Definite Minimum Time electronic circuit protection for the DITA system, insulation monitoring, distribution feeder soft start (DFSS), bypass arrangements, alarms and auxiliary power supplies.

The DMT electronic circuit protection within the PCU, programmable with a definite minimum time characteristic for circuit disconnection in the event of a fault, allows the protection to be tuned to prevailing network characteristics.

The key innovation is the DFSS – distribution feeder soft start – an electronic device within the PCU that allows transformer inrush conditions during start-up to be damped, allowing protection sizes to be reduced and removing the risk of fuses being ruptured under start-up conditions. This technology has the potential to be used to manage high start-up currents in point operations.

National Rollout

John Fry, technical head of E&P for Network Rail Infrastructure Projects, said: “We are keen to support the deployment of new technology to drive electrical safety and performance improvements in our networks when undertaking alterations on the network driven by renewal or enhancement projects.” With the technology now available from three potential suppliers, and with a number of design houses with the experience of network modelling to introduce DITAs, now is the time to review the potential and consider their deployment.

This article was written by Tahir Ayub, a programme engineering manager (enhancements) at Network Rail Infrastructure Projects (Central) who has been the technical lead for the development of the new standard.

Read more: Rail Engineer April 2018 – Rolling stock and Infrarail preview