The cost of railway signalling systems has long been a concern to infrastructure management and several initiatives to introduce cheaper, more efficient practices have emerged in recent times. One such is ‘plug and play’ cabling for lineside signalling circuits. Network Rail recently staged a couple of one-day seminars for signalling suppliers and engineers to examine the technology, look at the benefits and be honest about the downsides.

The term ‘plug and play’ is copied from the computer industry but it is a misnomer in the rail context. Plug is correct, but the system as designed is very far from Play since the free mixing of cable terminations is certainly not intended.

The concept

The installation and termination of lineside cabling is time consuming, expensive, manpower intensive and carries safety risks. If the majority of this work, including subsequent testing, could be transferred to a factory environment, then significant cost and efficiency benefits will result.

Signalling systems are made up of a set of basic components:

  • An intelligence unit, normally known as an interlocking;
  • A control desk from which train movements are directed;
  • Lineside apparatus cases where the ‘intelligence’ is locally terminated;
  • End devices such as signals, points, level crossings, track circuits and axle counters that determine the passage of trains.

Whilst the intelligence and circuitry has advanced from vast banks of relays to computer based systems, the cabling to connect these components together has remained substantially similar. Multicore copper cables of various sizes are still the norm, designed originally for DC circuits but, with the advent of solid state interlockings, now also carrying low-bit-rate digital signals.

Run out in standard lengths, sometimes from a cable train but now generally hand pulled, the cable cores are terminated on 2BA sliding links mounted vertically
in equipment rooms and lineside cases. Since the latter occur at regular intervals, the cable ends are fully terminated every time and instances of direct in-line jointing of cables are rare. After the termination is completed, surplus cable invariably becomes scrap.

What, therefore, if all this cabling could be made to measure and plug coupled into the various termination points and end devices? Network Rail has been studying and implementing the concept and started by setting up a partnership with cabling equipment suppliers and the signalling contractors. From the beginning, it was realised that, for this to work, significant standardisation would be required. The robustness and reliability of the connectors would be important and the MIL-5015 standard, as developed over many years for the military and aircraft industries, is the chosen design.

Specified to IP 67 rating, the couplers have been tested for dust and water ingress, fire protection and resistance to carbon arcing. They passed with flying colours and a 30 year life is predicted. From this has evolved a series of standards and guidance notes on the use of plug couplers, the principal one being NS/L2/ SIG/300027 supported by a number of T-series drawings. The ‘Play’ element within the Network Rail vision relates to the combinations of joints and disconnection boxes that can be used for the optimum configuration within a project.Plug & Play Rack - high res Credit Unipart [online]

Lessons from elsewhere

Plug-coupled cables were very much part of the design for the A380 Airbus, each aircraft having 40,000 connectors and 530km of cabling. Graeme Boyd, now with Network Rail but who previously worked with Airbus Industries, told of the supposed advantages that this would bring but noted that it all went horribly wrong resulting in a £6.1 billion loss. So why did this happen and what should the rail industry learn from it?

As is often the case, it was lack of attention to detail that caused the major difficulties:

  • Mixed CAD packages and different versions of CAD software;
  • Wiring harnesses made to the incorrect specification and to incorrect lengths;
  • Resistance to change by the aircraft assembly workers;
  • Failure to produce a single project team;
  • Lack of conformity within the supply chain;
  • Too much customisation on individual aircraft resulting in an excessive product range. Airbus has learned from all of this and has simplified the design, routing and production of the cabling, involved the design teams in the cabling process and reduced the amount of customisation that is allowed with aircraft orders.

Trialling plug-coupled cables

Proving the workability of the concept was carried out at Leicester where plug-ended cables were laid in a demonstration yard including various types of end equipment and ducting (issue 99, January 2013). An initial trial at Kingswinford involving 114 plug-coupled cables yielded some valuable lessons, both good and bad. The time to undertake the installation was reduced by 75% and the ordering of the cables was easily standardised. For cables to fit, however, the measuring of lengths must be super accurate and there were issues with wrong contacts within the couplers, wrong delivery to the required sites and inadequate quality of installation. Also emerging was the unsuitability of the current lineside location design to accommodate plug couplers.

A more extensive deployment has been on the Cardiff Area Re- signalling project. Conor Linnell from Atkins, the main contractor, gave his assessment on suitability. Atkins had picked up that measuring accurate cable lengths would be important and attempted to do this by aerial survey to avoid manual surveying. However, the accuracy was not good enough and reverting to a high-quality traditional measuring wheel became necessary. Even then, the desired accuracy of within 3cm was not possible for any one cable and a figure of ± 1 metre is about as good as one can get.

Mistakes were made with the running in of cables ending up with adjacent male connectors on cables to be joined. The use of in-line joints within the troughing was found to be practical but raised the question of how to mark the location.

Using stump-mounted disconnection boxes is preferred for the connection from an intermediary cable to a tail cable that connects to the end equipment. This has allowed standard tail cable lengths of 25 metres to be used. The number of possible interfaces grew all the time resulting in an over-engineered scheme with too many cable types being put forward for approval. Working hand in glove with the signalling design team and the cable route contractor is a must for future applications.

Despite the problems, the Cardiff scheme has shown that 95% of on-site termination work can be eliminated, cable wastage is minimised and automated testing of the preformed cables saves time by showing up faults and defects more quickly.

Combining with other equipment

The Ely to Norwich re-signalling project has seen two other new technologies deployed in addition to plug-coupled cables: modular signalling and the automation of four-barrier level crossings using obstacle detection. Combining all these together was always going to be a challenge and John Woolley from Signalling Solutions gave an insight into some of the problems faced. The control of the 42 route miles is from Cambridge using a DeltaRail Scalable IECC linked to an SSL Smartlock interlocking. Lineside architecture is based around ‘signalling islands’ contained within Re-locatable Equipment Buildings (REB) at periodic intervals, connected back to Cambridge via the FTN telecom network. Lineside location cases do not exist, all end devices being connected via plug-coupled cables to the REBs. This has resulted in extended tail cables being needed but no lineside power feeds.

The claimed advantages of plug- coupled cables have been proven in principle in that pre-testing and installation were all made easy with functional testing and lineside work kept to a minimum. However, the introduction of so much new technology led to many design changes during the implementation phase and this resulted in alterations to some external equipment with consequential changes to cable lengths and new cables having to be made.

Other problems encountered were similar to those experienced at Cardiff, particularly with getting accurate cable lengths and cables installed the wrong way round. Undertrack crossings (UTX) emerged as a problem. If the pipe was not big enough or insufficient space was available in the pipe, then pulling through a cable and coupler is a big problem. Similarly in REBs, a bigger than normal hole in the floor was needed to pull through the coupler so maybe a side entry system would be better.

IMG_1936 Credit Unipart [online]All of this resulted in the anticipated savings in both time and cost not being achieved. The big lesson is to freeze any design changes once the manufacture, pre-testing and installation work has started. Another lesson is to find ways of accommodating excess cable in perhaps a turning chamber.

Similar findings have been experienced by Invensys (now Siemens Rail Automation) in their initial application of plug-coupled cables. Joe Coll presented some other factors that might impact on design.

  • Is there mileage in having a partial plug coupled system with some cables remaining with a free end for conventional termination to an end device? This would ease the cable length criticality but would take more time.
  • Will plug coupled cables have any relevance for stageworks should these be necessary? Perhaps some standard cable lengths connected to additional disconnection boxes might be of value and capable of re-use once the project is completed.
  • Security remains an issue in two areas: what will prevent a plug ended cable being connected to the wrong socket in a location. Good labelling and colour marking is part of the answer but should there be some form of key coding to prevent human error. It is likely that if a wrong connection is made, the resulting circuitry will fail safe but can we be absolutely sure of this? Secondly, is tampering by outside parties made easier? Anyone with the right keys can access a lineside location and unscrewing a cable connection is a simple task.

The supply base and the future

It is still early days but much has already been learnt. Standardisation and application rules are making good progress with the Signal Works and Signal Maintenance Testing Handbooks having been updated accordingly. Co-operation between the various parties is good and a natural supply base for the various component parts is emerging, those firms specialising in trackside cabling equipment having perhaps the biggest impact to make.

Re-design of point machines, signal heads, AWS magnets, and barrier machines is taking place to accommodate plug connections. New designs of lineside location cases will be required and an associated thrust to remove the need for concrete bases for cabinets and connection boxes
is happening in parallel. Getting all this through the Network Rail product approval systems will hopefully be made easier and quicker.

Without doubt, plug coupled cables are here to stay. The advantages are just too great to be ignored. The measuring, installation and design processes have to be refined and some re-orientation of mindsets needs to happen but, this said, efficiency gain, cost reduction and safety improvement are there for the taking. Maybe the technology will extend to power cables but ‘playing’ around with 650 volts will bring its own set of challenges.

It is good that Network Rail is allowing everyone an early look at the technology and is being open about the problems.

Photos courtesy of Unipart