scope:

This part of IEC 60079 describes a test method intended for the measurement of the maximum experimental safe gaps for gas- or vapour-air mixtures under normal conditions of temperature¹ and pressure so as to permit the selection of an appropriate group of flameproof enclosures.

The method does not take into account the possible effects of obstacles on the safe gaps² .

¹ An exception if made for substances with vapour pressures which are too low to permit mixtures of the required concentrations to be prepared at normal ambient temperatures. For these substances, a temperature 5 °C above that needed to give the necessary vapour pressure is used.

² The design of the test apparatus for safe gap determination, other than that used for selecting the appropriate group of enclosure for a particular gas, may need to be different to the one described in this standard. For example, the volume of the enclosure, flange width, gas concentrations and the distance between the flanges and any external wall or obstruction may have to be varied. As the design depends on the particular investigation which is to be undertaken, it is unpracticable to recommend specific design requirements, but for most applications the general principles and precautions indicated in the clauses of this standard will still apply.