MINIMUM IGNITION ENERGY (≤ 1 J)
[EN 13821, ASTM E 2019]
This value can possibly determine the extent and hence the cost of protective measures.
The minimum ignition energy is the lowest energy value of a high-voltage capacitor discharge required to ignite the most ignitable dust-air mixture.
A modified Hartmann tube made of glass with a volume of 1,2-ℓ is used as the explosion vessel.
The dust dispersion system at the base of the tube is of the "mushroom-shaped" type around which the sample is loosely scattered.
A blast of compressed air at seven bar is used to disperse the dust in the glass cylinder where it is ignited by a spark between two electrodes.
The energy just sufficient to ignite the dust under investigation is determined.
This ignition energy is then successively halved with variation of the dust concentration and the ignition delay time (turbulence) in a series of tests until no ignition takes place in at least 10 successive experiments.
The MIE lies between the lowest energy value (E2) at which ignition occurred and the energy (E1) at which in at least 10 successive experiments no ignition was observed.
The energy range thus determined is called the minimum ignition energy of a combustible dust in a mixture with air.
However, for simplification purposes, often only the lower limit value (E1) is specified as the MIE.
E1 < MIE < E2
To assess the ignition hazard for dust/air mixtures due to operational spark discharges, especially electrostatic discharges, the MIE must be determined with a purely capacitive spark discharge (without inductance) by the method described above.