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That transformer tank on the pole by the road, the little box your computer keyboard is connected to, the panelboard in the basement-ll must be supported, protected, isolated, yet accessible
THE TERM ELECTRICAL ENCLOSURE APPLIES to any physical structure containing some electrical equipment or circuitry. From the little "black box" to the walk-in switchgear cubicle, all such enclosures serve at least two or three of the following purposes: Support. For the internal components, conduit fittings, instruments, indicating lamps, or whatever else is part of the assembly (Figure 1).
Protection/isolation. Keeping safely away from the interior any contaminants that may be present in the surroundings; the prying fingers of untrained or unwary trespassers; and sometimes electromagnetic interference, which may need to be confined within as well as excluded without. Access. Whatever is inside the enclosure must be accessible for servicing, testing, and removal/replacement (Figure 2).
- Ventilation. Many enclosures contain heat-producing components that must be cooled (Figure 3). Anyone familiar with electric motor construction and standards will recognize that all these functions are built into the enclosure of a typical motor, whatever its size or rating. Motor enclosures are well understood in the electrical industry. For general purpose machines, only one set of standards applies in the U.S. (per NEMA MGI), quite sim
ENCLOSURES continued from previous page ilar to international (IEC) standards that govern elsewhere. Some applications are dealt with by modifying documents, such as IEEE Standard 841 for petrochemical industry service, or Standard No. 19 of the Baking Industry Sanitation Standards Committee, for washdown duty motors used in food processing.
Complex enclosures
In contrast, enclosing other kinds of electrical apparatus can be far more complex and confusing. One reason is that switchboxes, panelboards, and similar enclosures located in or around process equipment may see varieties of abuse that many motors never encounter. Furthermore, internal motor construction is fairly simple: stator, rotor, bearings. Even the presence of brush rigging, commutator, or collector rings adds little to basic function. The same is true for space heaters or other common accessories. Almost all the electrical circuitry is fully insulated.
Other electrical enclosures, however, often contain bare terminals and exposed contacts. Corrosion or dirt can quickly render devices inoperable or cause catastrophic faults. The variety (and...