Pressurized enclosures

We chose a NEMA 4x enclosure and pressurized it enough so that if a leak did occur, the air inside the enclosure would move outward, and not allow hazardous gases to enter. Since the unit always had an operator in attendance to observe critical pressure changes with a dedicated pressure sensor, this was a viable alternative for short term testing of the system. The operator shuts down the system as soon as a problem is observed. 

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Ignition of flammable Provide safe separation distances release resulting in fire. Develop appropriate area electrical or explosion. classification Provide ignition source control Place ignition sources in positive pressure enclosure and buildings Provide adequate ventilation API RP500 BS 5345 BS 5958 NFPA-70 NFPA-77... 

NFPA 496 Standard for Purged and Pressurized Enclosures for Electrical Equipment,... 

For hazardous areas flameproof enclosures alone are recommended, except in areas with moderate intensity of contamination and where such assemblies are located away from the affected area and in a separate well-ventilated room, when pressurized enclosures may also be. safe. The reason for this precaution is that frequent arcing takes place within the enclosure on each switching of a contactor, switch, breaker or an OCR etc. and also during operation of power and auxiliary contactors. 

Since it is not practical to manufacture a llameproof enclosure due to its size and bulk and the number of knockouts and openings on the doors for switches, metering, indicators, and pushbuttons (PBs) etc., it is common practice to locate the.se assemblies some distance from the affected area in a separate well-ventilated room. Depending upon the location and intensity ol contamination, it may be permissible to meet the requirement by using a pressurized enclosure by maintaining a positive pressure inside the enclosure similar to that for motors (Section 7.1.3..3). When there arc many switchgear assemblies, the room itself can be pressurized, which is safer and easier. Small enclosures, however, such as a PB station, switch or a switch fuse unit or an individual starter unit etc., which can be easily made of MS plates or cast iron, as discussed in Section 7.13, can be mounted in the hazardous area while the main MCC can be installed in the control room, away from the contaminated area and from where the process can be monitored. 

While some video display screens such as liquid crystal, gas plasma or vacuum fluorescent displays do not present the same charged screen hazards as CRTs, this does not imply that they are safe for use in hazardous locations. This requires special design and certification for use with a given flammable atmosphere. Non-certified equipment used in locations classified as hazardous under Article 500 of NFPA 70 National Electrical Code require a purged or pressurized enclosure to control ignition hazards as described in NFPA 496 Standard for Purged and Pressurized Enclosures for Electrical Equipment. The screen in this case is located behind a window in the enclosure. 

Standard for Purged and Pressurized Enclosures for Electrical Equipment in Hazardous (Classified) Locations Recommended Practice for Classification of Class I Hazardous Locations for Electrical Installations in Chemical Plants... 

Purged and Pressurized Enclosures for Electrical Equipment, NFPA 496, National Fire Protection Association, Quincy, MA, 1982 Approval Standard Intrinsically Safe Apparatus and Associated Apparatus for Use in Class I, II and III, Division 1, Hazardous Locations, Class No. 3610, Factory Mutual Research, Norwood, MA, Oct. 1979... 

National Fire Protection Association (NFPA), NFPA 496. Standard for Purged and Pressurized Enclosures for Equipment. NFPA Quincy, MA, 1993. 

Acceptable protection techniques for electrical and electronic valve accessories used in specific class and division locations include explosion-proof enclosures intrinsically safe circuits nonincendive circuits, equipment, and components dust-ignition-proof enclosures dusttight enclosures purged and pressurized enclosures oil immersion for current-interrupting contacts and hermetically sealed equipment. Details of these techniques can be found in the National Electrical Code Handbook, available from the National Fire Protection Association. 

The rule described above forms a basis for the secondary type of explosion protection, which results in the avoidance of ignition sources, e.g. by using either a flameproof enclosure of a commutator motor or the pressurized enclosure of a gas analyser, or by limiting the electrical values in intrinsically safe circuits. 

In the case of a pressurized enclosure outside, a flameproof enclosure according to EN 50018 may by used in addition to those listed above. 

This includes apparatus generally marked Group II, e.g. a pressurized enclosure EEx p IIT... 

An enclosure whose interior is filled with air (without any combustibles) or inert gas, e.g. nitrogen, argon, carbon dioxide or sulphur hexafluoride showing a positive pressure differential to the environmental potentially explosive atmosphere, indicates the very general principle of pressurization. The same concept is applied to clean rooms in semiconductor manufacturing or in pharmaceutical production. Gases, vapours, mist or even dust are prohibited to penetrate into the interior of such a clean room or pressurized enclosure. 

PY = pressurization that reduces the classification within the pressurized enclosure from zone 1 to zone 2... 

Safety devices, control and monitoring units for pressurized enclosures, general safety aspects... 

In the following, a fully pneumatically operated control and monitoring unit as a safety device is described for a Group I pressurized enclosure, e.g. for a frequency convertor. 

Table 6.8 Typical pneumatic data for Group I pressurized enclosures (according to [17] and [31])...

Figure 6.22 Schematic assembly of a pressurized enclosure for a large-volume Group I energy distribution unit [31],...

The next step of component integration has been achieved with walk-in pressurized enclosures [17], [31] which enable the personnel admittance via an airlock. 

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