The casing for flameproof protection of an electric motor or other type of rotating machinery must be strong enough to withstand pressure during an internal explosion. The strength of a prototype of the casing is put to the test by the spark plug ignition of an explosive mixture containing a specified gas. The pressure developed by the explosion is found and may then be used as the basis for a further static or dynamic test at 1.5 times the explosion pressure or 3.5 bars, whichever is greater. For the protection to be deemed satisfactor;, the casing should not have been damaged or deformed (it is the casing or enclosure which is tested by the explosion and which must remain intact, but there is no stipulation about internal damage).