National Electric Code: The 2017 edition of the National Electrical Code (NEC) devotes 13 Articles spread over more than 69 pages to Hazardous Classified Locations. This constitutes more than ½ of the requirements for Special Occupancies, and nearly 10% of the requirements contained within the entire NEC. So, what is a Classified Area?
Classified Areas: Installations are “Classified” hazardous because of the presence, or the potential presence, of a flammable or combustible “material”. This material can be either gas/vapor, dust, or fiber/flyings. While each type of material has unique hazardous events to consider, they all present a risk of explosion. An explosion can directly cause several types of hazards including fire and the expelled energy. An explosion can also indirectly lead to additional hazards such as injury from flying shrapnel or the shockwave itself.
Explosion Triangle: The basic elements needed to cause an explosion are found in a diagram commonly referred to as the “explosion triangle”. All 3 elements of the triangle must be present to achieve an explosion.
Preventing an Explosion: If any one of the legs of the explosion triangle is removed, an explosion is not possible. Which means that to prevent an explosion, at least one leg of the explosion triangle has to be eliminated. The preferred method depends on the application. However, most protection techniques are based on removing the ignition source.
A) Fuel: There are two basic ways to remove the fuel leg:
B) Oxygen: There is one basic way to remove the oxygen leg:
C) Ignition Source: Most protection techniques are based around removing this leg of the explosion triangle. There are two basic ignition sources to consider when eliminating the ignition source – both must be eliminated:
Conclusion: On paper, removing the explosion hazard is as simple as removing one leg of the triangle. However, in the real world, this is not so easy to accomplish. For example, removing the fuel leg is typically accomplished by using other electrical equipment. However, this adds electrical equipment in the hazardous classified area that must also comply with the hazardous equipment requirements. And the added equipment can’t protect itself during start-up conditions, so it must be protected by removing a separate leg of the explosion triangle locally.
And things don’t always go according to plan. Moving parts wear out, seals leak, human error is inevitable. We can’t design for safety without considering reliability, expected wear, failure modes and the need for redundant or fail-safe features - topics which we will discuss in future whitepapers.
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