When engineers are determining if the site conditions for their new project should be considered &#;hazardous&#;, they need to consult their copy of the National Electric Code and remember a few definitions before they create their requests for bids.
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The National Electric Code (also titled NFPA code, number 70) defines whether an area is considered &#;hazardous&#;. Hazardous areas are at risk of explosion from a spark in a combustible atmosphere. Wiring in these areas require special methods including seal fittings, purged panels, or cast iron enclosures.
The code classifies an area with a &#;class&#;, and a &#;division&#;. Class I is for areas that could have flammable gases or combustible vapors. A Class II area are those that are hazardous because of the presence of combustible dust. A Class III area is hazardous because of the of easily ignitable fibers. Once a Class is determined, a Division is assigned. Division 1 is given if the area has the combustible material from the Class under normal operation. Division 2 is given if the area has the combustible &#;&#;only in case of accidental rupture or breakdown of such containers or systems or in case of abnormal operation of equipment.&#; For example, an area that would experience a potentially explosive atmosphere of gas upon an accidental rupture would be considered a &#;Class I, Division 2&#; environment.
The code has requirements defined for electrical devices being installed in classified areas, but the common objective is obvious&#;keep the explosive atmosphere from contacting a source of ignition. This source is typically a spark or &#;arc&#;. Some arcing devices are a relay, a motor starter, a light switch, or anything with contacts. These need to be in an enclosure to prevent contact with an explosive environment. Enclosures for devices in hazardous areas are typically one of two types&#;&#;Explosion-proof&#; or &#;Purged and Pressurized&#;.
Explosion-proof enclosures typically have two features; when closed, they seal out the environment they are designed for and they are constructed to contain an explosion should an explosive atmosphere enter the enclosure and ignite. These enclosures are usually cast iron and the larger the enclosed volume, the greater the wall thickness. The rating used is NEMA 7 for Class I, and NEMA 9 for Class II.
The other type of enclosure for hazardous area is called purged and pressurized. When these enclosures are closed, the operator engages a valve on the purge system to introduce air into the panel and vent any potential combustibles. Once purge is complete, the purge system will pressurize the panel to prevent combustibles from entering the enclosure. The larger the enclosure, the more practical it is to use a purge system. The standard defining the requirements for these purge systems is NFPA 496.
If the NEC defines a Class I, Division 2 area as an area where gas &#;&#;can escape only in case of accidental rupture or breakdown of such containers or systems or in case of abnormal operation of equipment.&#;, then one could say any area where natural gas flow through anything but a pipe could be a hazardous area. That&#;s every home with a gas furnace.
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However, for 250HP boilers and greater, NFPA-85 clarifies:
A.5.3.8.1 Locations at which natural gas, propane, or fuel oil systems are installed in compliance with this code normally are not considered hazardous locations for electrical equipment as defined in NFPA 70, National Electrical Code.
This helps when discussing boiler installation with an engineer.
Once an area is determined to be hazardous as defined by the NEC, you probably don&#;t need a 500 pound cast iron enclosure to comply with code.
· A purged enclosure can create an atmosphere where average electrical components installed inside can operate in a hazardous environment.
· If a specification calls for an &#;explosion-proof&#; enclosure, it is calling for a cast iron enclosure rated NEMA 7 or NEMA 9. Purging does not make an enclosure &#;explosion-proof&#;.
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