Dynamic elaboration on the Art & Science of Hazardous Area Protection
The minimum current required from an inductive spark discharge to ignite the MOST EASILY IGNITABLE MIXTURE of a GAS, MIST or DUST in air …..relative to that which is required to ignite a METHANE-AIR mixture . (Under the same test conditions)
Reference from IEC
Minimum current in resistive or inductive circuits that causes the ignition of the explosive test mixture in the spark-test apparatus according to IEC 60079-11.
(A bit of technical details) : An Inductor stores Electrical Energy in its Magnetic Field. An energized inductive circuit, if interrupted (by opening a switch or accidentally cutting the connecting wire) then this current, which is flowing in the circuit will ultimately become ZERO in a very short time. This rate of change of current will develop a voltage across the point of interruption. A large inductive effect can generate high voltage. This voltage will cause a spark across the point of interruption.
Similarly , a charged capacitor, if allowed to discharge through a connecting wire will generate spark at the point of contact , since the process of shorting the terminals is in progress and ultimately the terminals will physically meet ,the spark will terminate.
The main difference between the SPARKS of OPENING OF INDUCTIVE CIRCUIT and CLOSING OF CAPACITIVE CIRCUIT is as follows:
The spark due to inductance will not extinguish by itself unless the insulation of the medium restores its initial value, where as the spark due to capacitor will ultimately disappear as soon as the physical contact is complete.
These two parameters are used to establish GAS GROUPS.
Gases and vapors may be classified according to the ratio of their minimum igniting currents (MIC) with the ignition current of laboratory methane. The standard method of determining MIC ratios shall be with the apparatus described in IEC 60079-11.
Group II equipment is subdivided and, for the purpose of classification of gases and vapors, the MIC ratios are:
Group IIA: MIC > 0,8.
Group IIB: 0,45 ≤ MIC ≤ 0,8.
Group IIC: MIC < 0,45.
Classification according to MESG and MIC
For most gases and vapors, it is sufficient to make only one determination of either MESG or MIC ratio to classify the gas or vapor.
One determination is adequate when:
Group IIA: MESG > 0,9 mm, or MIC > 0,9.
Group IIB: 0,55 mm ≤ MESG ≤ 0,9 mm, or 0,5 ≤ MIC ≤ 0,8.
Group IIC: MESG < 0,55 mm, or MIC < 0,5.
Determination of both the MESG and MIC ratio is required when:
for IIA: 0,8 ≤ MIC ≤ 0,9 need to confirm by MESG,
for IIB: 0,45 ≤ MIC ≤ 0,5 need to confirm by MESG,
for IIC: 0,5 ≤ MESG ≤ 0,55 need to confirm by MIC.