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.

Why Fume Extraction is Important!

A Few Key Reasons Why Proper Weld Fume Extraction is Important, & A Few Comments On Possible Solutions

1. Looking to enhance profits? An unhealthy work environment can severely impact your bottom line.

2. A single welder can produce 20 – 40 g fumes per hour, or upwards of 70 kg a year!

3. The haze you see in a lot of metal fabrication departments is the result of the vaporization of metal and flux. The majority of this haze originates from the welding consumable, with the rest being attributed to the base metal. As it cools the vapor condenses and bonds with oxygen in the air to form very fine particles. As a rule of thumb, the smaller the particle the greater the danger: particles larger than 5 µm are deposited in the upper respiratory track, while those from 0.1-5 µm penetrate deep into the lungs.

4. Once common concern for welders is exposure to Hexavalent chromium Cr(VI); a known carcinogen. These particles can be as small as 0.01 µm. See https://www.cdc.gov/niosh/topics/hexchrom/default.html - “NIOSH considers all Cr(VI) compounds to be occupational carcinogens. Cr(VI) is a well-established carcinogen associated with lung, nasal, and sinus cancer. Some of the industries in which the largest numbers of workers are exposed to high concentrations of airborne Cr(VI) compounds include electroplating, welding, and chromate painting.”

5. Manganese is another common concern: long term exposure at high enough concentrations can damage the nervous system (Parkinson’s Manganism). From https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4515672/ “ . . . excessive and prolonged inhalation of Mn particulates in mining, welding and industries results in its accumulation in selected brain regions that causes central nervous system (CNS) dysfunctions and an extrapyramidal motor disorder, referred to as manganism. Prolonged and chronic occupational exposure to Mn (>1 mg/m3) represents a risk factor Parkinson’s disease.”

6. Pneumoconiosis - a chronic respiratory disease caused by inhaling metallic or mineral particles; in particular siderosis, a type of pneumoconiosis, related to inhaling iron oxide http://www.ccohs.ca/oshanswers/occup_workplace/welder.html

7. From WorkSafe magazine January/February 2017 – “More than eight out of ten welders — there are around 100,000 in Canada — are said to be exposed to lead, which can cause stomach, lung, kidney, and brain cancers, while five out of ten are exposed to nickel, which can lead to nasal and lung cancer.”

8. One of the best solutions available on the market provides for the extraction of these harmful fumes at the source (such as a properly located extraction arm, or a welding torch with an integrated “on-torch” fume extractor). Hoods placed over workbenches aren't optimal as the smoke inevitably contaminates the general airflow. They can also impact energy conservation as they have a tendency to extract large amounts of heated (or cooled) air from the workplace.

9. If you’ve elected to use an extraction arm as your means of protection, best practice stipulates that the hood should be positioned close to and above the arc at about a 45° angle, with the welder positioning his or her head outside of the extraction zone.

10. If your welders are required to work inside of restricted areas that won’t permit the use of a conventional extraction arm, weld fumes can be effectively removed with the on-torch method mentioned above. They are also common for robotic welding applications.

Looking for a new Dust Collector?

If you’re investigating the use of a dust collection system, one of the first things you’re going to want to do is determine the explosive characteristics of the dust you’re dealing with, as well as how severe such an unmitigated explosion may be. With this information in hand, you’ll be able to properly spec in the associated equipment, and make certain that you and your coworkers are properly protected.

With that in mind we’ve highlighted a few key terms and phrases that will aid you in your discussions with your team members, and prospective vendors. Naturally, we’d like to be the vendor of choice for such projects, but we understand the need to do a little due diligence.

So, in a nutshell, here are a few things to consider:

Kst Values, Pmax, and ST Class

You’re bound to hear these terms more than once during your consultations. In short, these are explosive properties as measured in a laboratory environment to quantify the severity of a potential dust explosion.

The explosion indices test follows BS EN 14034-1:2004 - to determine the maximum explosion pressure (Pmax) of a dust cloud; and BS EN 14034-2:2006 – to determine the maximum rate of explosion pressure rise of a dust cloud (Kst).
The tests are caried out in a 20-litre sphere, which reproduces a high state of turbulence with a mind to simulate a worse-case scenario.

Here is how it works. A weighted quantity of combustible dust is placed into the dust container. The main explosion chamber is then evacuated to 0.4 bar absolute, followed by an automatic test sequence to pressurize the dust container to 20 bar gauge. At this point a fast-acting valve on the dust container outlet is opened to allow material to enter the explosion chamber. A rebound nozzle ensures an even distribution of the dust within this chamber, while the control system activates two 5 KJ chemical igniters at the centre of the sphere 60 ms after the dust has been properly dispersed.

Explosion pressures are measured for a range of dust concentrations using piezo-electric pressure transducers. The tests are carried out over three series to ensure a firm understanding of the explosive properties of the dust being tested. From here the arithmetic mean of the maximum values (both maximum pressure & the maximum rate of pressure rise) is determined.
The Kst value is calculated as the equivalent pressure in a 1 m³ sphere from the cube law (Kst value = cube root of volume x explosion pressure rise).

The ST Class is based on the Kst values as follows:
ST Class 0 – Kst value = 0
ST Class 1 – Kst value less than 200 bar m/sec
ST Class 2 – Kst value between 200 and 300 bar m/sec
ST Class 3 – Kst value greater than 300 bar m/sec

These results are essential for validating the protection design of the equipment you’re looking to install. This may include such things as explosion venting, explosion suppression, as well as explosion containment.

Say, for example, you’re dealing with aluminum dust. Just how explosive is it? Aluminum dust often comes with an ST3 designation, which make it very explosive. For those applications you’re going to want to follow strict safety guidelines, as well as employ proper explosion mitigation and containment as it pertains to dust collection for your facility.

There is a lot more that can be said about Pmax, Kst, and ST Ratings, but this will give you a pretty good framework to get started. Should you have any questions please feel free to reach out. We’re here to help.