The filling, which may be quartz or glass particles, achieves safety by what is known as “suppression of flame propagation”. The flammable gas or vapour may permeate the granules and reach the parts producing arcs/sparks or hot surfaces, but the quantity of gas will be too small to support an explosion within the inert powder.
Tests specified I the construction standard provide a safe correlation between the depth of granules and the duration & level of the arc current produced by the components.
This method of protection is suitable for use in all group II gases or vapours.
