Zoning in on Explosive Atmospheres

Everybody wants to wake up in the morning and know they’re going to a safe workplace. A major safety concern among certain industries is the occurrence of fires and explosions. This makes for some of the most expansive safety codes and standards. This article is aimed at explaining hazardous area classification in simple terms for easy comprehension. Before we begin to classify hazardous areas, it is crucial to define what they are.

What is a hazardous area?

A hazardous area is a place in which an explosive atmosphere may occur in quantities requiring the implementation of special precautions to protect the health and safety of workers. Hazardous areas are classified into two major categories: gases/vapors and dusts.

Classification of hazardous areas

Both categories are further divided into three ATEX Zones, as directed by the European Union for protection against explosive atmospheres. Each zone indicates the frequency and duration an explosive atmosphere may be present. Hazardous areas involving gases/vapors are classified as follows:

    • Zone 0 is an area where an explosive atmosphere consisting of a mixture of air with flammable substances in the form of gas, vapor, or mist is present continuously for long periods or frequently (continuous hazard)
    • Zone 1 is an area where an explosive atmosphere is likely to occur in normal operation occasionally (intermittent hazard)
    • Zone 2 is an area where an explosive atmosphere is unlikely to occur in normal operating conditions, and if it does occur, it is likely to do so for a short period only (possible hazard)

Similarly, dusts classify into three different zones: Zones 20,  21, and  22, each representing identical meanings as their gas/vapor code counterparts, respectively. The gas station example below offers a real-world picture of these hazardous zones.

Gas station hazard zonesThe vessels containing the fuel underground and on the truck are classified as Zone 0 because these areas are continuously holding flammable substances. The gas pumps and any valve or opening into the gas containers are classified as Zone 1 because gas will be passing through intermittently — when a customer uses the pump or an employee fills the tanks. Zone 2 is the natural space or the natural environment. While fuel should not be exposed to the natural environment under normal operating conditions, it is possible. Spills, for example, can create a possible hazard for a short duration.

It’s important to note the European Union ATEX directive is not compliant with OSHA standards in the United States. While similar, the U.S. has its own classification system for identifying hazardous zones called the NEC Zone Classification System. See how the two systems compare here.

Safety

The more frequent an explosive gas or dust cloud is present, the more dangerous the zone. Therefore, companies practice ATEX zone reduction by implementing safety measures.

In areas with risk of explosion, accurate and reliable position detection is often relied on to complete tasks. Examples include monitoring hydraulic and pneumatic cylinders, checking hydraulically and pneumatically controlled valves, and level detection.

The Difference Between Intrinsically Safe and Explosion Proof

The difference between a product being ‘explosion proof’ and ‘intrinsically safe’ can be confusing but it is vital to select the proper one for your application.

Both approvals are meant to prevent a potential electrical equipment malfunction from initiating an explosion or ignition through gases that may be present in the surrounding area. This is accomplished in both cases by keeping the potential energy level below what is necessary to start ignition process in an open atmosphere.

What does this mean?

The term “intrinsically safe” describes a protection technique that limits the electrical and/or thermal energy of electrical equipment used in potentially explosive areas such that there is insufficient energy to ignite the hazardous gases or dust.

‘Explosion proof’ applies to an encased apparatus that is capable of withstanding a material explosion. Which means, if combustible gases entered the explosion proof housing and were ignited by the electrical energy within the housing, the resultant “explosion” would be contained inside the housing. The energy from the explosion would then be dissipated through the large surface of flanges or threads paths of the enclosure. By the time the “explosion” exits the housing, there is insufficient energy remaining to ignite the surrounding atmosphere.

How do I know which to choose?

Zone classification is one method for defining the level of risk in a hazardous area and determining which level of protection is required.

Zone 0: Area with permanent risk of explosive atmosphere of air and gas

Zone 1: Area with occasional risk of explosive atmospheres

Zone 2: Area of rare risk of explosive atmospheres of air and gas, only for short periods

Zone 20: Like Zone 0 except atmosphere of air and dust

Zone 21: Like Zone 1 except atmosphere of air and dust

Zone 22: Like Zone 2 except atmosphere of air and dust

Additional certifications and classifications used to determine both explosion proof and intrinsically safe approvals, including more in-depth divisions that explore application and environment specifics, can be found here.