Requirements for electronic equipment cabinet properties have been changing continuously. For instance, programmable controllers are installed in cabinets for automation purposes, and, increasingly, corresponding control and protection systems are added as well. Electronic equipment cabinets (fig. 1) are designed for heavy operation in industrial areas and have all the attributes required for ensuring operational reliability of the final product: high mechanical durability, water- and dust-resistant fillers, a very high degree of EMC-shielding, grounding connections as per VDE standard, and an appropriate cooling system. Advantages and benefits from this application are ensured by flexible arrangements with various accessories and components.
Single Platform as Basis for Various Applications
There is a number of cabinet types on the market suitable various applications. When requirements or applications change, in most cases a new suitable cabinet is required. New accessories and components are required when various cabinet types are not compatible with each other. This results in customer spending more time and money. An optimal cabinet would be the one that, on the hand, ensured a high technical standard, and on the other hand – allowed to resist growing price pressure. It must be versatile and flexible to accomodate various applications. One solution to this problem could includer various cabinet types on the same basis. This basic model will allow to create various options for different target and user markets. With this model, the manufacturer and the user will be able to save considerable amounts of money.
Schroff Company has developed such a concept for a platform to be used in electronic equipment cabinets of the new generation. Even when requirements in individual market sectors vary particularly greatly, a common denominator with convenient solutions still exists. The new platform for VARISTAR electronic equipment cabinets, designed for application in automation systems, instrumentation and many other areas, is specifically intended for difficult environments, and is quite versatile when applications require variability due to technical or economic reasons. Therefore, it becomes possible to create standard custom-made solutions for users, which could have been earlier implemented only in the form of expensive specialised versions. Varistar cabinet platform comprises a rack to be implemented in two options, with various lining elements and accessories, and components from an extensive manufacturing program. Both rack options are compatible with 19” equipment. Installation dimensions of both racks coincide, therefore, one can install them side by side. Any lining element, for instance, doors, lateral, rear, top and bottom panels can be used in both rack versions. Appropriate accessories designed for heavy loads, for instance, extra stable shelves, guides etc., are provided for use of especially heavy equipment. They serve as a basis for assembling the cabinet that meets particular requirements.
High Stability
During shipping or installation electronic equipment cabinets are exposed to various dynamic and static loads, which often require especially high stress and vibration strength, as well as seismic stability of cabinets. Varistar electronic equipment cabinet platform meets such requirements due to a specially designed rack profile (fig. 2).
Fig. 2 Multifunctional Rack Profile Designed for Heavy Loads
This allows this type of cabinet platforms to withstand the highest loads described in respective standards. In order to meet significantly varying requirements welded cabinet racks are implemented in two options. The so-called Slim-Line light-weight version withstands maximum static load of up to 400 kg. A specially designed hollow chamber-type profile used in this product meets DL5 and DL6 requirement levels as per IEC 61 587-1 standard concerning resistance to static and dynamic loads with minimum material consumption. The DL5 requirement level refers to stress and vibration loads typical for road and railroad alarm systems, and operation in the vicinity of rotary machinery. DL6 maximum requirement level is effective for loads occurring on commercial ships and military equipment of low load class. In case resistance to higher static load or seismic resistance is required, the second rack option is applied. A heavy-duty reinforced version is comprised of a similar hollow chamber-type profile, but extended in main directions of load application, therefore making it more resistant. The heavy-duty option is capable of withstanding the maximum static load up to 1000 kg. Seismic resistance tests conducted as per IEC 61 587-2 standard have proven seismic resistance of cabinet platforms up to 3 Bellcore zone, and up to 4 Bellcore zone with stiffening angle pieces welded on cabinet corners.
Optimal Environmental Protection
Electronic instrumentation and automation systems are of great significance for operational reliability of plants and machinery in automation systems. For instance, spurious radiation effects on operation of controllers or measurement instruments can have fatal consequences. Therefore electronic equipment cabinets in which such components are installed must have an effective protective concept against mechanical disturbances and high-frequency emission. Common EMC-shielding concepts at cabinet level are based, as a rule, on conductive contact between the rack, the inner components and lining elements. This requires applying special coatings , for instance, galvanised chromatized (yellow) surfaces. In order to ensure protection against high-frequency emissions, Varistar electronic equipment cabinet platform offer a brand-new shielding concept, which provides emission offset via lining elements without penetrating inside the rack. An important role here is performed by the aforementioned frame hollow chamber-type profile. The rack profile is equipped with symmetric 45-degree bevel along the perimeter, which is the base of new sealing concept for protection against electromagnetic interference and water/dust ingress. Sealant for protection against external effects (protection degree up to IP 55) or combined sealant for protection against electromagnetic interference and external effects shall be fastened directly on rack 45-degree bevel. In this case electromagnetic interference is conducted from lining element via the sealant immediately to adjacent lining element (fig. 3).
Fig. 3. EMC-shielding: Faraday cage principle (1 – rack; 2 – air-tight sealant; 3 – conductive shielding sealant; 4 – rear panel; 5 -lateral panel)
The rack itself stays inactive, and the Faraday cage effect occurs. Such a shielding concept allows using a cost-effective typical rack, which significantly improves environmental safety according to RoHS European Directive. Conductive textile packing material is used as sealant. The new shielding concept has been proven by EMS tests in certified laboratories. It establishes new electromagnetic interference protection standards (fig. 4).
Fig. 4. Diagram: electromagnetic interference attenuation for Schroff cabinets in comparison; 20 dB at 1 GHz stands for 10-fold, 40 dB at 1 GHz – 100-fold and 60 dB at 1 GHz – 1000-fold wave intensity attenuation.
A common shielding concept shall also include interference to be transmitted by wires. Therefore special considerations must be given to cabinet cable input. In case of separate cable inputs, as a rule, PG contacting threaded connectors are used. They ensure maximum protection against external effects (IP 66), and have good parameters for electromagnetic interference attenuation. Schroff Company has developed a patented concept for protecting wire group inputs in cabinets. First of all, when a large number of cables is used, such a concept allows for especially easy and quick maintenance. This cable input contributes in electromagnetic interference attenuation up to 70 dB at 1 GHz frequency and maximum humidity and contamination protection level (IP 55). Single or several openings are provided in the cabinet bottom to fasten the oblong cable chamber running parallel to the floor. Shielded wires with stripped sheath of any diameter are placed in this chamber and fastened using cable links. Cables sealed with foamed material elements are in reliable contact almost along the entire chamber length, thus ensuring required electromagnetic interference attenuation. As compared to other solutions available on the market, users save 10-15% in price, while the shielding parameters remain the same.
Effective Cooling
In many applications cooling of installed components is provided by natural convection. When this is insufficient, fans, heat exchangers and even air conditioners are used for forced ventilation and cooling. Accurately defined air exchange in cabinets is the matter of importance (mainly for common air cooling), i.e. the way air comes into the cabinet, how it is supplied to components to be cooled and how the warm air is evacuated from the cabinet. Schroff Company creates tailored cooling concepts to be tested in detailed computer simulations both for standardised and individual custom-made systems. This includes not only the cooling components (heat exchangers, air conditioners, fans). Often, it is more important to have a know-how method that allows detecting and accurately localising concern areas in various applications, for instance, ‘hot spots’, and find optimal cooling solutions.
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