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Classification Basics

What Is A Classified Area?

Operating Methods

Some areas are unclassified even though flammable or combustible gases, liquids, vapors or dusts may be present. In order for classification of an area to make sense for electrical installation purposes, there must be electrical components present and the operating electrical components or equipment must provide an ignition source. Some of the considerations that may make such areas unclassified include:

Non-electrical ignition sources are normally present. Example ExampleA gas furnace with a standing pilot light is a common example. Hot surfaces may also be ignition sources. Mixtures in air are not ignitable. Example ExampleLiquids kept below their flash point will not produce vapor to produce an ignitable mixture. The diesel generator shown in the Helpful Hint illustrates this point. Also, air must be mixed with vapors, gases, or dusts in such a way that an ignitable mixture can form. For instance, a flooded gasoline engine will not start because there is not enough air in the mixture to form an ignitable concentration. In other cases there may be too much air and not enough fuel - either because of ventilation or because small quantities of fuel are available such as where only small containers are stored. Only pyrophoric materials are present. Example ExamplePyrophoric materials will burn on contact with air, so the electrical system is not a significant ignition source. Phosphorus metal is an example of a pyrophoric material. High explosives are manufactured or used. Example ExampleSome accidents in high explosive manufacturing and use have been caused by such things as static charges produced by inappropriate clothing, so special electrical equipment may do little good in such areas. Also, although special electrical enclosures are often used because they afford additional protection not offered by ordinary enclosures, they are not tested for use with high explosives. Experience shows that no practical hazard exists. Example ExampleAreas that include ammonia refrigeration systems and adequate mechanical ventilation are usually unclassified. Ammonia is not easily ignited and experience has shown that hazardous leaks are rare. Household gas appliances also fit this condition as do many woodworking facilities with good dust collection and housekeeping methods. Only a catastrophic failure would create the hazard. Example ExampleThe NEC does not generally provide rules for catastrophic events as no design could withstand every possible catastrophe. In a gas processing plant or a refinery, some events can be reasonably anticipated, such as the failure of a valve. Other events, such as an aircraft or asteroid striking the facility cannot be reasonably considered.
Code Examples

Helpful Hint
NOTE: This is an unformatted excerpt from our online fire protection training library.

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