Meeting the rigours of rail
28 February 2012
With safety the paramount concern in the railway industry, it is essential for electronic systems to be protected from extreme environmental conditions. Keith Reynolds looks at the particular demands placed on enclosures in this sector
The role of an enclosure is to house and protect sensitive electronic systems so that they continue to function properly regardless of where they are located. Few environments are as challenging as those associated with railway applications, yet any damage to installed equipment can have potentially catastrophic consequences. It is therefore vital for railway systems to be housed in cabinets, subracks and cases that are capable of withstanding the harshest of conditions.
These environmental conditions are as diverse as the locations in which the enclosures are used: stationary in indoor applications, outdoors at the trackside or mobile on board a train. In all cases, particular attention must be paid to mechanical robustness and resistance to shock and vibration. Other important factors include IP and EMC protection, space and weight restrictions, and thermal management.
Shock and vibration
On a moving train, all components – including cabinets and subracks mounted in it – are subjected to sometimes substantial shock and vibration loading. Acceleration forces of up to 5g are not uncommon. (The gravitational acceleration, g, is 9.81m/s2 at sea level.) For comparison, some roller-coasters passing through a complete loop develop centrifugal forces of up to 4g.
Shock forces occur in situations such as shunting, when the rolling stock of a train is being assembled. A 19in. cabinet and any installed subracks and components must be able to withstand such negative accelerations without damage. Significantly more important still are the vibration loads to which the entire train, including cabinets and subracks, is exposed. Over a number of years and in unfavourable circumstances these vibrations can lead to fatigue and component failure. New high-speed routes are generally less critical in this respect than older lines in emerging nations.
The requirements and test conditions for vibration resistance are documented in the IEC 61587-1 and -2 and EN 50155 international standards. Additional requirements have been formulated by the railway operators of specific countries; for example, SNCF standards NF-F 67-012/60-002 and Deutsche Bundesbahn standard BN 411002.
One product that has been thoroughly tested by an independent test house in accordance with all of these standards is Schroff’s europacPRO 19in subrack range. Featuring a robust aluminium construction, this has proved itself capable of withstanding severe shock and vibration.
In the most extreme conditions of shock and vibration, accessories such as Card-Lok PCB retainers can also be used to ensure that printed-circuit boards remain firmly in place. Manufactured by Calmark, one of Schroff’s sister companies within the Pentair Technical Products Group, these devices provide the additional benefit of helping to transfer heat away from the board.
IP and EMC
With both ingress protection (IP) and electromagnetic compliance (EMC), the question arises: retrofit or new design? Where a train is being designed from scratch, the cabinets - mostly only cabinet frames - are positioned in separate sections in the train and protected from these influences by wall elements and/or internal cladding. Because of this, no additional steps need to be taken with regard to cabinet IP and EMC protection.
If the enclosures are retrofitted into a train, however, they must themselves provide all necessary protection against particles and moisture - usually to IP 54. EMC protection is achieved either at board/sub-assembly level using suitable EMC gaskets between the plug-in units or at cabinet level, depending on the prevailing circumstances. Any material used for gaskets must be carefully selected in order to minimise the risk of galvanic corrosion, which can occur when two dissimilar metals are in contact in a moist environment and the voltage differential between them exceeds 0.6V.
For protection at cabinet level, Schroff’s Varistar product range combines both EMC shielding and ingress protection. The cabinet’s frame profile features an outer surface set at 45 degrees and fitted with a conductive textile gasket. When the side panels, back panel, top cover, base and door are attached, they are all interconnected to create a Faraday-cage that prevents any induced interference currents from reaching the frame. As well as achieving the requisite level of EMC shielding, the conductive textile gasket also serves as an environmental seal, enabling the cabinet to provide IP55 protection as standard.
Varistar satisfies the requirements of a wide variety of railway applications, including use in the particularly harsh environments found in underground locations, which demand high levels of both IP and EMC protection. For these applications, IP55 protection is a standard specification.
Space and weight
For on-board applications, whether an existing train is being retrofitted or a new train developed, the space required for installing electronics enclosures is normally restricted. On a new design, the space for cabinets and subracks can be more easily planned, whereas for a retrofit, a compact wall-mounted case may be a good space-saving option. Schroff’s Conceptline series, for example, provides IP 66 protection and is available in a range of sizes from 300 x 250 x 150mm to 800 x 600 x 420mm.
Weight is also a key consideration for equipment used on trains, and designers are therefore keen to use the lightest possible structures and materials in their construction. It follows, therefore, that any electronics cabinets and subracks should be optimised in terms of their weight, but they must nevertheless satisfy the requirements for robustness, as discussed above.
Although the continuing trend towards miniaturisation of electronic and electrical components is generally welcome, it can in fact be problematic when it comes to the question of cooling. With the higher density of electronic equipment has come the need to dissipate heat more effectively.
For an on-board retrofit, cooling without fans is preferable to avoid unnecessary maintenance outlays. Indeed, the power supply system is quite likely to be operating close to its limit, anyway, and therefore incapable of powering additional equipment such as fans.
To address this, Schroff offers energy-neutral methods such as conduction cooling. In the case of a new train, however, cabinets and subracks are included in the overall thermal design and positioned so that the train’s own air conditioning system provides sufficient cool air to dissipate heat away from installed electronic systems.
For trackside cabinets, the best approach to thermal management is to make use of a double-wall design that provides natural convection between the outer and inner walls, thereby enabling heat to be dissipated without additional cooling systems. Outdoor cabinets such as Schroff’s Modular and Unibody series both utilise this technique. Furthermore, the double walls serve to minimise the amount of heat generated in the enclosure due to solar radiation and also protect the installed components from mechanical damage.
Where natural convection is insufficient, additional thermal management systems such as fans, heat exchangers or compressor-assisted cooling units can be installed, and this is done is such a way as to ensure that the security of the cabinet is not compromised in any way.
Whether equipment needs to be located in control rooms, on platforms, in signal boxes, beside the track or on-board trains, Schroff has a wide range of standard enclosures available to satisfy the requirements of the rail industry. Detailed information on these products can be found on the company’s new microsite, schroff.co.uk/railway.
Keith Reynolds is with Schroff UK
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