Consulting-Specifying Engineer - July 2008 - (Page 50)

For example, a Class I, Division 1 location could be inside a vessel that has flammable gas around its vent(s), where flammable vapors can be present. A Division 2 location may be the area immediately surrounding the tank that does not normally contain sufficient quantities of gas or vapors but could in the event of rupture or leak. Class II locations (NEC Articles 500 and 502) have combustible dusts (or have the potential) in high enough concentrations within the atmosphere that the potential for fire or explosion exists. In Class II, Division 1 areas, combustible dusts normally are suspended in the air. Examples of Class II, Division 1 areas are inside an elevator leg or a grain receiving area with no aspiration. In Class II, Division 2 areas, the con- tions within industrial and commercial facilities. Mechanical breakdown of equipment, open flame, cigarettes, or sparking from sources such as dropping tools, excessive temperatures, and electric arcing or sparking are all potential sources. For the purpose of this article, attention will be focused on electric arcing or sparking. Industries that typically are at risk for hazardous classified locations include, but are not limited to, occupancies such as gasoline dispensing and service stations, paint-finishing process plants, petroleum and petrochemical processing plants, wood processing facilities, agricultural grain handling and processing facilities, coal pulverizing plants, textile mills, and facilities where metal dusts and/or powders are processed. The EGC must be electrically permanent, sufficiently sized, and have a low impedance path from the source to the power supply. centration of combustible dusts is normally not present within the atmosphere, but it could be under adverse conditions, such as mechanical breakdown or abnormal equipment operation. The interior of a typical feed mill would be an example of a Class II, Division 2 area. Class III locations (NEC Articles 500 and 503) contain ignitable fibers or flyings that are not likely to be suspended in air but are in concentrations high enough to produce a fire or flash explosion. The Class III location categories are divided differently from those in Class I and II. Class III, Division I locations primarily contain fibers and flyings due to manufacturing materials such as rayon or cotton, while Class III, Division II locations contain ignitable fibers and flyings that are handled and stored but are not part of the manufacturing process. While Class I and II locations are subdivided into several groups based on their material composition, Class III locations are not. Various sources of ignition can prompt fire or explosions due to hazardous locaWith the guidelines for hazardous “classified” locations covered in summary detail, the next step is to define grounding and bonding and discuss how they are used in hazardous and nonhazardous areas. Grounding and bonding There are many different types of grounding systems. Equipment (safety) grounds, grounding for lightning protection, static grounds, system grounds, and electronic system grounding are all examples. Equipment grounding consists of three primary considerations: personnel safety, proper operation of protective devices, and mitigation of electromagnetic interference (EMI). In regard to grounding and bonding in hazardous locations, engineers mainly are interested in equipment grounding, and within equipment grounding, engineers primarily are concerned with personnel safety and operation of protective devices. Therefore, EMI and the other types of grounding systems mentioned above will not be covered in this article. In order to understand grounding and bonding, one must know the differences between them. Grounding is the physical connection of conductive material to earth in order to limit the voltage imposed by lightning or unintentional contact with a higher voltage line, and also to limit the voltage to ground on normally noncurrent carrying materials. An example is the green wire pulled back from a light fixture to a panel board. Bonding, on the other hand, is defined in Article 250 of the NEC as “The permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and the capacity to conduct safely any current likely to be imposed.” An example of bonding would be bare copper wire connecting a processing pipe to nearby building steel. Why is bonding necessary? Article 250 of the NEC requires that all conductive material be electrically connected (bonded) together to limit the potential difference between one piece of equipment and another. As mentioned above, personnel safety and proper operation of protective devices are two primary concerns with equipment grounding. Each of these is attained through the proper use of an equipment grounding conductor (EGC). The EGC is defined by Article 100 of the NEC as “the conductor used to connect the noncurrent carrying metal parts of equipment, raceways, and other enclosures to the system grounded conductor, the grounding electrode conductor, or both, at the service equipment or at the source of a separately derived system.” Noncurrent carrying metal parts are bonded together and then tied to the ground (grounding) using an EGC. A properly designed and installed equipment grounding system can prevent fire and explosions in hazardous areas. The EGC, or fault return path, aims to accomplish three tasks. First, the EGC must be electrically permanent and continuous from any point in the system back to the source of power for that circuit. Second, the EGC must be sufficiently sized to handle any fault 50 Consulting-Specifying Engineer • JULY 2008

Table of Contents Feed for the Digital Edition of Consulting-Specifying Engineer - July 2008

Consulting-Specifying Engineer - July 2008 Contents Viewpoint Letters News M/E Roundtable 40 Under 40 Using Demand-Based Reset Strategies VFDs and Motors: Making the Right Match Grounding and Bonding Practices for Hazardous Areas Codes & Standards Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - July 2008

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