Designing for rail: The importance of specifying the right connectors

While these are huge projects, there are plenty of less glamorous ones involving the refurbishment and modification of existing facilities and rolling stock. Between them, they offer many opportunities for the UK supply chain. But they also present challenges, particularly for those designing the electronics and connectivity solutions for them. Whether it's designing new trains or updating old ones, there is a host of systems that add to the complexity of connectivity required. One of the big themes is energy efficiency, but there are also driver advisory systems being installed, Wi-Fi to meet passenger demands and the roll-out of ERTMS and ETCS. ERTMS, the European Rail Traffic Management System, aims to improve procurement and interoperability by creating a single European standard for train control and command systems. A key component of this is ETCS, the European Train Control System. This is the standardised control equipment is installed in the train cab and which is being widely rolled out in the UK. There are plenty of standards in this market; unsurprising, given the consequences of failures in safety systems. There are two main standards that engineers, looking to design products for the rail sector, must be aware of. Mike Brookes, regional manager of Harting, explained: "The common standard, and the one to which Harting works, is what used to be called the EN50155. It covers reliability, heat, humidity, shock, vibration, EMC and so on. That has been superseded by EN50467, which is just about to be released, that is more about the electrical qualification for the connector." The other main considerations are the flammability and fire standards, something that is relevant to the underground rail infrastructure since the King's Cross fire in 1987. Brookes said: "What they are looking for in nearly every case are materials with low smoke, zero halogen – the LSOH approach. The most stringent of the flammability standards are BS6853, DIN5510-2, NF F 16-101 and NF F 16-102. But the way in which LSOH is tested for is all over the place. Some standards differ from others, which is why work is in progress to develop a 'Euronorm', so everyone is singing from the same hymn sheet." This harmonised European version is EN45545-2, which is coming soon. Designers can use 'grouping factors' to satisfy the requirements of such standards. Brookes noted: "What will be looked at is how the connector is packaged and what material is used. We sometimes have to provide customers with material weights. If there are 10 or 12 connectors in a certain area, they will take the mass of materials and write a concession around that." On top of that are fire barrier tests, where manufacturers have to prove their connectors will withstand fire from one side of a bulkhead. It all points, unsurprisingly, to designers needing to choose connectors from the top end of the market. "If, for example, you use a D-Sub for rail applications, it will feature metal shells with good EMC screening properties, rather than plastic hoods and housings," said Brookes. Contacts will turned, rather than stamped and formed, and will be crimped, rather than soldered. He continued: "At one time, designers were reluctant to use anything other than crimp technology, because it is a controllable process, whilst IDC (insulation displacement connectors) and solder are not. Gradually, IDC is being accepted in some areas but, in the UK, designers still want to use crimped connectors, rather than anything else." Rail communications is increasingly turning to Ethernet and designers will look to use off the shelf components. But again, says Brookes, these will not be RJ45 (8P8C connectors). "They will use M12 and they will be crimped to accommodate shock and vibration." Other applications have specific requirements. "On the Bombardier units for MITRAC, we use F body DINs," commented Brookes. "The reason is that, because it is a PCB mounted product, there are issues with track distances; designers don't like some of these miniature connectors which feature very small distances between tracks. They prefer to have a reasonable spacing, due to harmonics and similar factors." The MITRAC Energy Saver reduces energy consumption in light rail vehicles by up to 30% and by up to 35% in multiple diesel units. The system's ultracapacitors store the energy released each time a vehicle brakes and reuses it during acceleration or operation. Harting has designed a coatless zinc die cast housing that provides the necessary EMC protection through the labyrinth design of the casing shell and cover. However, the connectors used on the MVB (multi vehicle bus) and wire train bus – the typical bus systems that are used to control and monitor doors, HVACs, safety related RMTR on train monitoring units with black boxes and so on – tend to be standard DIN connectors. It is no surprise that the area in which Harting sees the most growth in is in communications. "It means higher bandwidths, more data and shared networks, which means you have products that may require VLAN compatibility," said Brookes. This also means having systems that operate 'inter car' with Ethernet, rather than just 'in car', and some of the dynamics involved with CAT 6, and particularly the higher bandwidth CAT 7 Ethernet cables, can present problems when run across cars. Harting uses MIRA to undertake dynamic testing of such solutions to ensure their integrity. The majority of cables are copper, but a number of companies are introducing fibre optic options with associated connectors – and some are looking to combine the two. Brookes said: "It is an area where Harting is strong because our heavy duty connectors are designed using a modular concept. We are not restricted to a particular shell size with a certain amount of pin outs, then having produce a special design when the customer requires it. We can modularise the inserts to include what is required – it can be power, fibre, signal, Ethernet – and we can swap them around." With around 50 modules in the range, and with six modules in each connector, the number of combinations is enormous and some redundancy can be built in to cater for future expansion, typically with comms cabling. "It is a system that is coming into its own," Brookes concluded.