Classified areas wiring

The mere fact that voltage, current, or even both, are at low levels does not guarantee a circuit to be intrinsically safe, even though intrinsically safe circuits do utilize relatively low voltage and current levels. Intrinsically safe systems employ electrical barriers to assure that the system remains intrinsically safe. The barriers limit the voltage and current combinations so as not to present an ignition hazard should a malfunction develop. Typically, devices upstream of barriers are not intrinsically safe and are installed in control rooms or other unclassified locations. All devices and wiring on the downstream side of the barriers are intrinsically safe and can be installed in classified areas. 

The process designer and project engineer should classify the various areas of a plant following NEPA-70, Article 50528.29 order to advise the electrical and other project team members of the degree of electrical hazards anticipated. The appropriate equipment (motors, instruments, conduit, wiring, etc.) should be specified according to NEPA-70, Article 500 and others as applicable, the ASME Code and the API Code as appropriate. See NEPA-497A and... 

NFPA 497A—Recommended Practice for Classification of Class I Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas contains recommendations for wiring requirements in an enclosed area where lighter-than-air flammable gases may be released [5.12]. While these recommendations are for chemical processing areas, the releases are similar in the hazard they present to what would be found for natural gas released in repair garages. These recommendations are also illustrated in Figure 5-1. 

Sieves are used to separate particles into fractions with different size ranges. The particles are classified according to their ability or inability to pass through an aperture with a controlled size. Sieves are constructed with wire mesh with openings between 20 pm and 10 mm, which are characterized by a mesh size and a corresponding aperture size. The wire mesh has square apertures, whose size is determined by the number of wires per linear dimension and the diameter of the wire. The mesh size is equal to the number of wires per inch linearly of the sieve screen, which is the same as the number of square apertures per inch. The relationships among mesh number M, aperture width a, wire diameter w, and the open area A can be described by the following equations ... 

It is desirable to classify the particles before they are used in further operations. When the particles are very small the surface area increases and thus the amount of resin required to wet the surface increases. Oversized particles can adversely affect the quality of the final product because of internal flaws in the particles. While some classification is accomplished using air streams, screen classification methods are the most common. In screen classification, the particles are fed over a vibrating flat screen, or a series of screens. The screens may be wire cloth, plates with holes or slots, or plates set on edge. 

Some variations of the basic coupon specimens can provide rapid material corrosivity evaluations. The helical coil adopted in the ISO 9226 methodology is a high surface area/weight ratio coupon that gives a higher sensitivity than panel coupons of the same material. The use of bimetallic specimens in which a helical A91100 aluminum wire is wrapped around a coarsely threaded bolt may provide additional sensitivity and forms the basis of the Classify Industrial and Marine Atmospheres (CLIMAT) coupon [11 18]. 

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