Enclosure rating

Most sensors have a specification called enclosure rating, environmental rating, degree of protection, or protection class. This indicates the kind... 

Table XI3.6.8-1 NEMA Enclosure Rating With Interpretation ...

The minienvironment approach to contamination control has been increasing in use. A minienvironment is a localized environment created by an enclosure that isolates the product wafer from contamination and people (48). Another approach is using integrated processing, where consecutive processes are linked in a controlled environment (32). Both requite in situ sensors (qv) to measure internal chamber temperatures, background contamination, gas flow rates, pressure changes, and particularly wafer temperature (4). 

Injection-Molded Products. Numerous housings, electrical enclosures, and cabinets are injection-molded from rigid PVC. These take advantage of PVC s outstanding UL flammabiUty ratings and easy mol ding into thin-waHed parts. PVC has developed melt flow capabiUties to the point where it competes with essentially any other flame-retarded engineering thermoplastic and molds easier than most. 

Circulating fluidized-beds do not contain any in-bed tube bundle heating surface. The furnace enclosure and internal division wall-type surfaces provide the required heat removal. This is possible because of the large quantity of soflds that are recycled internally and externally around the furnace. The bed temperature remains uniform, because the mass flow rate of the recycled soflds is many times the mass flow rate of the combustion gas. Operating temperatures for circulating beds are in the range of 816 to 871°C. Superficial gas velocities in some commercially available beds are about 6 m/s at full loads. The size of the soflds in the bed is usually smaller than 590 p.m, with the mean particle size in the 150—200 p.m range (81). 

The explosion-proof enclosure is designed such that an explosion in the interior of the enclosure containing the electronic circuits will be contained. The enclosure will not allow sufficient flame to escape to the exterior to cause an ignition. Also, a surface temperature rating is given to the device. This rating must indicate a lower surface temperature than the ignition temperature of the gas in the hazardous area. 

NEMA/MG-1/1993 NEMA/MG-2/1989 Motor and generators ratings, construction, testing and performance Safety Standards (enclosures) for construction and guide for selection, installation and use of rotating machines ... 

For small brake power, resistance unit is small and can be located within the main enclosure. But for higher power that may call for large resistance units and have to dissipate excessive heat, it is mounted as a separate unit. The resistance units are shod-time rated depending upon the duty they have to perform. 

The test is carried out by washing down the test enclosures in every direction by means of a standard hose nozzle of 6.3 mm inside diameter, as illustrated in Figure 11.11, held at 3 m from the enclosure with a water pressure equal lo a head of nearly 3 m of water (— 30 kN/iiF), enough to give a delivery rate of 12.5 litres/min. The duration of Ihe lest will be determined at 1 min/m" of the surface area under test, subject lo a minimum of 3 minuies. 

Type tests are conducted on the first enclosure of each voltage, current rating and fault level to demonstrate compliance with electrical and constructional design parameters. The tests provide a standard reference for any subsequent enclosure with similar ratings and constructional details. The following tests may be conducted to demonstrate verification of the following ... 

For such ratings, enclosure of non-magnelic material alone is recommended due to high iron losses in a magnetic material. 

As discussed later, the enclosure of an IPB may carry induced currents up to 95% of the current through the main conductors. Accordingly, the enclosure is designed to carry longitudinal parasitic currents up to 90-95% of the rated current of the main busbars. The cross-sectional area of the enclosure is therefore maintained almost equal to and even more than the main conductors to account for the dissipation of heat of the main conductors through the enclosure only, unless an additional forced cooling system is also adopted. The outdoors part of the enclosure exposed to atmospheric conditions is also subjected to solar radiation. Provision must be made to dissipate this additional heat, from the enclosure. 

The continuous current rating of a bus system can be defined by the current at which a steady-state thermal condition can be reached. It is a balance between the enclosure and the conductor s heat gain and heat loss. If this temperature is more than the permissible steady-state thermal limit it must be reduced to the desired level by increasing the size of the conductor or the enclosure or both, or by adopting forced cooling. Otherwise the rating of the bus system will have to be reduced accordingly. 

The cross-sections of the conductor and the enclosure arc then checked for their adequacy to dissipate the heat generated. The cross-section is then adjusted suitably by permuialions and combinations to arrive at the most appropriate size, keeping in mind the available extruded sections, accounting for all such factors that may affect the rating or the fatilt level of the enclosure and the conductor, such as tap-offs, which are subjected to a cumulative faull level, openings in the conductor for heat dissipation or the enclosure for inspection windows that can infhience their ratings. A calculation in Example 31.1 will clarify the procedure to establish the size of the... 

We have estimated the likely heat that may be generated by a particular size of conductor and enclosure for a certain current rating and then have counterchecked whether the conductor and the enclosure so chosen can dissipate this heat by radiation and natural convection, and reach a state of thermal stability within permissible limits or we may have to increase the size of the conductor... 

Current rating varies with the surface area of a conduelor and its thickness (annulus). In our sample calculation, to establish the basic parameters of the conductor and the enclosure, we have considered the current density for both as 400 A/inch. ... 

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