Let's Clear the Air on Hazardous Location Requirements and CodesNovember 1, 2005 By: Dave Wilson, Minco Sensors
Hazardous location codes are intended to prevent fires or explosions caused by electrical equipment malfunction in the presence of flammable gases, combustible dust, or ignitable fibers.
This article presents both a brief explanation of hazardous location requirements and an attempt to unravel the codes used to identify them. It is not intended for engineers designing the actual electrical equipment, but rather for those who need enough basic information to properly interpret and specify the right electrical equipment for installation on their machinery.
Division and Zone Systems
There are two basic systems for classifying hazardous locations and equipment requirements. The Division system has been used in the U.S. and Canada for many years and is defined in the National Electrical Code (NEC) Article 500 and the Canadian Electrical Code (C22.2).
Figure 1. Likelihood of Hazardous Atmosphere
The international, or Zone, system, based on International Electrotechnical Commission IEC 60079 specifications, has been adopted and modified by a number of organizations including the Canadian Standards Association (C.S.A.), Atmosphere Explosive (ATEX), and NEC Article 505. It is gaining popularity in all parts of the world including the U.S. and Canada. In Canada, all new installations must be classified using this approach.
Figure 2: Gas Groups
Hazardous locations can contain gas, dust (e.g., grain, metal, wood, or coal) or flying fibers (textiles or wood products). Hazardous gas locations are by far the most commonly studied, primarily due to the petroleum process industry, and will be our focus here.
Figure 3. To satisfy the Flameproof requirements, an apparatus must be housed in such a way as to contain internal explosions and to prevent their igniting external gases.
Likelihood of Presence
Hazardous locations are broken down in several different ways. The first is by the likelihood of a hazard's being present. In the Division system, this is separated into two divisions; in the Zone system, it is segmented into three (see Figure 1).
Figure 4. An Intrinsically Safe device must not store enough electrical energy to ignite its ambient atmosphere. A safety barrier is therefore required that under a fault condition limits the voltage and current to safe levels.
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