The requirement that the enclosure must also prevent transmission of any flame from an internal explosion to any explosive atmosphere surrounding the enclosure is tested by repeating the explosion test at least five times with the machine in a chamber, which is filled with the same explosive mixture. If the mixture in the chamber is not ignited, then the second set of tests is considered to have been passed & a Flameproof Ex d certificate is granted. Re-certification is needed for any modification of the casing.
Explosive mixtures vary in their ability to force flame through a gap of given dimensions; machines are grouped according to the design of the joint with respect to the length of the flame path, maximum gap & dimensions. Gases are generally listed for a given enclosure group.
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