Enclosures
temperature range in the cabinet, different options can be brought into play. A thermal output of up to 240 Watts can typically be dissipated by natural convection alone, at an average temperature in the cabinet of 90°C and an outside temperature of 80°C. This is the result of a thermal analysis using numerical methods, the result of which is a heat transfer value of 20 to 24 W/K based on the temperature under the roof or the average temperature inside the cabinet (environment according to standard IEN 61969-3, Class 1). A standard feature of an Outdoor Modular Cabinet for railway applications is supporting active heat dissipation using a fan kit. Typically, this kit consists of two fans with a thermostat, for modified cooling capacity. Using the two integrated 24V DC fans that draw cold air through the doors and blow heated air out through the external top, a thermal simulation provides for a thermal transfer of 160 W/K and an air flow capacity of 540 m³/h. Speed-controlled, air-filtered fans with thermostats represent an economic and effective means of climate control in cabinets. This active cooling through fans is an open circuit that transports ambient air into the interior of the cabinet to cool the electronics. The air supplied via air-filtered fans must be adapted to the air requirements of the components. Filter mats reliably protect the interior of the cabinet from dust. If air- filtered fans are used at very low outside temperatures, a heating unit may be required in the cabinet. The prerequisite for the use of air-filtered fans is that the ambient temperature is below the maximum possible internal cabinet temperature. As a rule of thumb, a temperature difference of at least 5 K can be assumed. Direct air cooling is extremely reliable and has a long service life with low energy consumption.
Air-conditioning cooling units offer the highest cooling capacity and enable cooling of the cabinet interior independent of the ambient temperature. An air conditioning compressor, used in the cabinet area, is tasked with transporting the heat from the cabinet to the outside, thus keeping the internal temperature in the cabinet at the desired level. The fan responsible for the internal circuit permanently circulates the cabinet air. The temperature is monitored by a thermostat. This thermostat activates the compressor and the external circuit fan upon reaching the required cabinet temperature. This activates the cooling circuit and stops only when the temperature is reduced by the value of the thermostat. An air conditioner can only be operated on a closed, waterproof electronic cabinet. Otherwise, part of its output would be used
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for dehumidification instead of cooling. The selection of an air conditioning unit should be based on the maximum power dissipation in the cabinet and the desired operating point. When using an air conditioner, the air duct in the cabinet must be designed in such a way that no air short-circuits occur, where the air that has just been cooled is almost directly drawn in again by the unit instead of cooling the components. Various AC air conditioners are available providing cooling capacities of approx. 1,000 to 2,000/3,500/5,500 W.
Protection against environmental influences
Outdoor use exposes electronic systems to extreme conditions such as temperature fluctuations from 30 to 80°C, high humidity as well as rain, snow, hail, ice, intense solar radiation, lightning strikes, fire, storms, rockfall, dirt such as sand and dust, and electromagnetic radiation. Passing trains cause sudden differences in air pressure as well as vibrations and shocks which should not be underestimated. Depending on the speed, enormous pressure forces can develop due to the fluctuations in air pressure and affect the cabinet. For outdoor cabinets to withstand these mechanical influences, aspects like reinforcements on the cabinet rack must be taken into account in the design. In particular, protection against shock and vibration must be ensured at a distance of 1 to 3 m from the track. For this purpose, an outdoor cabinet must be certified according to EN 50125-3. In addition, an earthquake resistance up to Bellcore Zone 4, which corresponds to a magnitude of over 6 on the Richter scale, is of great importance, depending on the installation site. Wind resistance with values from 180 km/h up to 250 km/h, which corresponds to a Category 4 hurricane, must also be guaranteed by the structure of the cabinet. A weather resistant powder coating or an anti-graffiti coating is typical to protect the cladding parts from mechanical damage. Protection against unwanted electromagnetic radiation on sensitive electronics also plays a central role in railway systems. The environment must be protected from emitted radiation, while the electronic systems must be protected from penetrating radiation. Appropriate measures for grounding and lightning protection are also necessary.
Security and anti-intrusion protection
In freely accessible outdoor applications, there is also the risk of vandalism and unauthorized access. Therefore, a high degree of robustness and stability are standard features of
Components in Electronics
outdoor cabinets. A stable cabinet frame and a double-walled design with modified wall thicknesses are effective measures when it comes to security and anti-burglary protection. It is also important that handles are protected against vandalism, and that screws for removing the roof, side walls, and base panels are only accessible from the inside, making access to the interior of the cabinet considerably more difficult for unauthorized persons.
Scalability and flexibility
For different areas within rail applications, it is crucial that outdoor cabinets are available in different sizes. Ideally, the cabinets are designed to be scalable in height, width, and depth, and can be easily adapted to the desired applications and future proofed. An extensive range of accessories with elements for interior installation, cable routing, and cable entry, is also helpful and ensures a high degree of flexibility. Easy access to the inside of the cabinet from all sides is important for performing maintenance as well as mounting and installing electronics within the cabinet. The flexibility of Modular Outdoor Cabinets is supported by the scalability of the cabinet design and a wide range of compatible 19” accessories.
Summary
Customers’ requirements for cabinets often include the integration of additional components such as display elements, monitoring, cabling, or power supplies. These
components can either be supplied by the customers or purchased and assembled via the cabinet supplier according to the specifications submitted. Outdoor cabinets containing a suitably configured air conditioning system to cool built in electronics guarantee reliability and durability for railway applications. In addition to certified protection against environmental influences and vandalism, the flexibility and scalability of the cabinet is necessary to set up a solution modified to the customer’s application.
The nVent SCHROFF Modular Outdoor Cabinet range available from Foremost Electronics offers all the necessary features in this respect. This cabinet range has been tested and certified for all relevant railway applications on the trackside and thus offers safety and reliability for the proper functioning of the application over a long service life. Features also include protection against environmental influences through a robust and tight design; a well-thought-out cooling concept consisting of a double-walled construction for optimal use of natural, free convection; and additional cooling possibilities via filter fan or air-conditioning unit. The protection measures against intrusion and vandalism, combined with its scalability and flexibility for adaptation to customer-specific applications make the nVent SCHROFF Modular Outdoor Cabinet an ideal, certified solution for outdoor railway trackside applications.
www.4most.co.uk www.cieonline.co.uk
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