Explosive Atmospheres Zones, Groups, Categories, & Classes Explained


If you’re working in an environment with potentially explosive materials, you’ve probably discovered that areas within the plant have been divided into Zones; and the equipment used in each into Groups & Categories.  Here’s what it all means.

This is applicable for facilities that may have combustible elements present: namely gases, mists, vapors, and dust.

Areas with potentially explosive atmospheres are divided into six zones, based on the probable frequency, and the associated timeframe, a potentially explosive atmosphere exists.

For gases, mists, and vapors Zones 0, 1, and 2 exist.  The requirements for equipment used in these Zones increases inversely with the Zone number.  Equipment in Zone 0, for example, must ensure that even if a type of protection fails, or if two faults occur, sufficient explosion protection is guaranteed.

For dust atmospheres Zones 20, 21, and 22 exist.  Much like Zones 0, 1, and 2, the requirements for the equipment increases as we move from 22 to 20.  Equipment used in Zone 20 and Zone 21 require special approval.

Equipment Groups

Equipment Groups determine in which Zones the equipment may be installed.  Here too there are six categories.

Categories 1G, 2G, and 3G are classifications for gas explosion protection.  Hence the suffix G.

1G equipment is suitable for use in Zones 0, 1, and 2.  2G is suitable for use in Zones 1 and 2, and 3G is suitable for use in Zone 2.

Categories 1D, 2D, and 3D are classifications for dust explosion protection.  Hence the suffice D.

1D equipment is suitable for use in Zones 20, 21, and 22.

2D for Zones 21 and 22.

3D equipment is suitable for use in Zone 22.

Explosion Groups

While the equipment groups and categories determine in which Zones the equipment can be installed, the explosion groups and classes determine what mediums inside each zone are permitted.

Explosion protected equipment for gases, mists, and vapors is divided into three explosion groups: IIA, IIB, and IIC.  This is a measure of the ignitability of the gases, and the requirements increase as you move from IIA to IIB to IIC.

Temperature Classes

There are 6 temperature classes: T1, T2, T3, T4, T5, and T6.

For Temperature Classes the deciding factor – contrary to conventional understanding – is not the operating temperature of the equipment.  Instead, the determinant is the maximum surface temperature of the equipment, in relation to +40° C ambient temperature.  This temperature cannot be exceeded at any time, and must always remain below the ignition temperature of the surrounding medium.

Location Classes

Class I – Gas & Vapour Environments

Locations deemed hazardous due to the presence of gases or vapours in the air that are in sufficient quantity to produce an explosion.

Division 1 – a Class I location where the hazard is anticipated to be present on a continuous, intermittent, or periodic basis.

Division 2 – A Class I location in which volatile liquids or gases are handled, processed, or used; but for which they would be normally confined to closed containers, or closed systems.  For Division 2 scenarios, the only way the potentially volatile material would escape would be in the event of an accidental rupture of the container, or breakdown of the system.

Class II – Dust

Class II locations are deemed hazardous due to the presence of combustible dust, and that the quantities present are present in sufficient quantities for a fire or an explosion to occur.  It is important to understand what, exactly, constitutes dust.  For a material to be considered “dust” it must exist as a finely divided solid of 420 microns or less.

Division 1 – a location where combustible dust is suspended in the air in sufficient quantities to ignite.  This also includes scenarios where electrically conductive dust, while not suspended in the air, has instead settled on equipment, permitting the electrically conductive dust particles to penetrate the openings in the equipment creating the potential for electrical failure.

Division 2 – locations where combustible dust is not normally present in the air in sufficient enough quantities to produce an explosion, & dust accumulations are not in sufficient enough quantities to interfere with the normal operation of electrical equipment.