Gas groups defined:
The gas groups of the Ex certified equipment for use in potentially explosive atmospheres, have been recognised as generally described in the table here.
Previously called a gas group it now has the term “equipment group” as described in the standard IEC 60079-20-1. Gases used in the above ground industry are often mixtures of more than one gas. For practical reasons the flammable gases have been subdivided into three subdivisions based on the various properties of the flammable gases. This subdivision is based on the maximum experimental safe gap (MESG) or the minimum ignition current ratio (MIC ratio) of the explosive gas atmosphere in which the equipment may be installed and identify the different levels of risks for the gases. These terms are generally described as below.
There are calculations and tests to establish the gas properties. These tests are carried out in controlled conditions to establish the properties of a gas mixture. The determination of the properties of a gas are tested by using test methods identified in the relevant standard.
MESG - Maximum experimental safe gap
Maximum gap between the two parts of the interior chamber which, under the test conditions, prevents ignition of the external gas mixture through a 25 mm long flame path when the internal mixture is ignited, for all concentrations of the tested gas or vapour in air. The test equipment looks something like this.
MIC - Minimum igniting current
Minimum current in resistive or inductive circuits that causes the ignition of the explosive test mixture in the spark-test apparatus, example as shown. A motor driven rotating contact is in a small chamber filled with a gas mixture .The wire makes contact and the current supply is increased to a point where the spark will ignite the gas in the chamber. The gases are classified based on their relation to Methane.
There are other ways this relationship can be observed as with what is called the ignition energy. This is measured in joules and can also show a similar relationship to how less energy can be required to ignite some gases and so are classified most easily ignitable. Most of the gases are classified by one of these methods, other gases may be determined by their chemical similarities.
When we look at the equipment protection methods such as Ex d and Ex ia for example. These require tests to establish whether the equipment design is suitable for the appropriate group and gas sub division. Where others such as Ex e and certain Ex n protection methods were given the marking II to indicate they were suitable for all subdivisions. In the recent standards this has now been revised so that equipment group will indicate the gas subdivision, either IIA, IIB or IIC.
When the electrical equipment is for use in a particular gas in addition to being suitable for use in a specific group of electrical equipment, the chemical formula will follow the group and be separated with the symbol “+”, for example, “IIB + H2” as shown on the label here.
Certified Equipment Label
The table here shows the equipment group in relation to the gas area where it can be used.
The equipment marked IIB is suitable for applications requiring group IIA equipment. Similarly, equipment marked IIC is suitable for applications requiring group IIA and group IIB equipment.
Note: This table is only to show the simple relationship, Refer to technical authorities for detailed gases in the area.
Some protection types have limitations, for example, the equipment protection type Ex d – Flameproof. The equipment group and sub division of this protection type become very important when identified and installed correctly in accordance to IEC 60079-14, which shows the limits for this type of protection. This would involve consideration of the types of flamepath joint, the size of the enclosure, an obstructions distance from the flamepath and whether grease tape application is allowed.
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