Minimum ignition current ratio (MIC), Maximum experimental safe gap (MESG)
Ignition on a hot surface occurs in a relatively large „macroscopic“ part of the mixture volume. In contrast, the ignition from a spark spreads in a relatively small „microscopic“ part of the mixture volume. The interruption of a predefined resistive/inductive electric circuit can be used for classifying gases and vapours according to their ease of ignition in the microscopic part of the mixture volume (in the µJ range).
For the assessment of the ignition of gases and vapours in a circuit using an intrinsically safe equipment as defined in EN-IEC 60079-11, a comparative value with methane as reference in a standardised circuit is used. This comparative value is the minimum ignition current ratio, MIC. It is the means used for classifying gases and vapours within explosion group II in the subgroups IIA, IIB and IIC, where methane becomes ratio 1.
An analogous grading is done when the ignitability of a hot gas jet escaping from a gap is used for the classification. In EN-ISO/IEC 80079-20-1 „Method of test for the maximum experimental safe gap“, a test apparatus is agreed in which a spherical gas volume of 20 cm³ is formed by two hemispheres. These have a 25 mm wide flange. This ball-shaped object is placed into a larger vessel and both spaces are filled with the mixture for which the safe gap is to be determined. The gap between the 25 mm wide flanges for which ten ignitions inside the ball volume just fail to ignite the mixture in the outer vessel is a value specific to the mixture and is called the maximum experimental safe gap, MESG.
The processes involved in the prevention or spread of the explosion in the gap are very complex. Classifying the gases and vapours by the safe gap results approximately - with a small overlap - in the same classification as that obtained with the minimum ignition current ratio. EN-ISO/IEC 80079-20-1 provides an overview of the classification using the two measuring methods MESG and MIC.
The safe gap value is of considerable importance for designs of protection type „Flameproof enclosure“; the value for the minimum ignition current ratio is important for those of protection type „Intrinsic safety“. For these two types of protection, the subgroups IIA, IIB and IIC for gases and vapours are relevant. The information on gases and vapours can also be applied approximately to mists.
For the assessment of conditions concerning Intrinsic safety, the MIC ratio values are not commonly used. As there is to deal with Voltages as well, it is preferable to use the M I E minimum Ignition Energy [in Joule] of gases and vapours where a general accepted, but indicative, table is available for Group IIA, IIB or IIC, as follows:
– IIA MIE = 180 µJ
– IIB MIE = 60 µJ
– IIC MIE = 20 µJ
Equipment sub-groups for combustible Dusts
From the point of view of electrical engineering, it is not possible to classify dust as precisely as the chemically defined gases and vapours. For that reason, it is considered sufficient to divide the dust according to type and conductivity. EN-ISO/IEC 80079-20-2 contains the test method to determine the specific electrical resistance of dust. Dust is divided into 3 sub-groups:
– IIIA combustible flyings
– IIIB non-conductive combustible dust, specific electrical resistance > 10³ Ω • m
– IIIC conductive combustible dust, specific electrical resistance ≤ 10³ Ω • m
The minimum ignition energy, a parameter similar to the minimum ignition current, is determined in accordance with EN-ISO/IEC 80079-20-2 for combustible dusts or flyings and is in the mJ range (a factor 1000 higher than the MIE for gases and vapours).