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

Identifying Materials & Equipment

Liquids, Vapors & Gases


The Maximum Experimental Safe Gap (MESG) is the maximum clearance between two parallel metal surfaces that has been found, under specified test conditions This definition indirectly describes a test apparatus which has a chamber within a chamber. The inner chamber has a flat ground joint that is adjustable in fine increments and a means of igniting the contained mixture. The surrounding chamber is filled with the same mixture. The mixture in the internal chamber is ignited, and by adjusting the gap and doing repeated tests, the MESG for that mixture is determined., to prevent an explosion in a test chamber from being propagated to a secondary chamber containing the same gas or vapor at the same concentration.

MESG information is critical to the design of electrical equipment that will be used in a classified area. Some electrical equipment is very difficult and impractical to make for environments where materials with a very small MESG Hydrogen and acetylene are examples of materials with very small MESGs of 0.28 mm and 0.25 mm respectively. Compare these values to the MESGs for ammonia or methane which are 3.17 mm and 1.12 mm respectively. Enclosures for use in a hydrogen environment require mating surfaces with a much closer fit than enclosures for methane, so some equipment, such as a motor with a rotating shaft may be available for methane but prohibitively expensive to make for hydrogen. will be present.
A modern version of the Davy Lamp.

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

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