How to Select Explosion-Proof Equipment | Rockwell Automation | US

By Daniel Craner, Product Specialist, POSITAL-FRABA Inc.

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Accidental explosions in industrial and commercial installations, triggered by flammable gases or dust, have a tragic history of causing severe damage and loss of life. To reduce the risk of these accidents, industrial safety authorities in many parts of the world have mandated the use of explosion-proof electrical equipment.

It's important for both equipment manufacturers and end users to understand the conditions that can create hazardous environments, the measures that can be taken to reduce risks, and the systems designed to help regulate the use of electrical equipment in these environments.

About Hazardous Environments

“Hazardous environments” are defined in industrial safety regulations as areas where there is a risk of dangerous concentrations of combustible gases or dust (this is sometimes referred to as “explosive atmospheres”).

Hazardous environments involving gases or fumes can occur in oil and gas refineries, chemical plants and fuel transfer facilities. Dangerous concentrations of flammable dust can occur in underground mines, grain handling facilities, milling operations and woodworking facilities. Emerging industries, such as hydrogen production, might also face risk of accidental explosions.

The term “explosion-proof” refers to equipment that has been designed to minimize the risk of starting an explosion or igniting a fire in a hazardous environment.

Designing to Reduce Risk

According to industry experts, there are three basic principles for managing the risk of explosions from combustible gases or dust.

Rule 1: Avoid. A good place to start is to design facilities and equipment to minimize leakage of combustible gases. Where gases or dust will be present, good practice calls for these areas to be well-ventilated, reducing the risk of dangerous concentrations. If explosive atmospheres are unavoidable, equipment must be designed and tested for explosion-proof properties.

Rule 2: Reduce. If there is a possibility of an explosive atmosphere due to leaks, equipment malfunction or other extraordinary circumstances, reduce the risk by installing equipment that has been certified as explosion proof.

Rule 3: Isolate and contain. To deal with any residual risk, facilities should be designed to keep the damage from an explosion as localized as possible.

Explosion-Proof Equipment

In most jurisdictions, electrical codes and industrial safety regulations require the use of explosion-proof equipment and design practices wherever dangerous concentrations of combustible gases or airborne solids can occur.

Equipment operated in these environments is required to be certified, typically by a competent third-party testing service or lab. These bodies will issue a certification code that identifies the conditions where the equipment can be operated safely.

The most widely used regulatory systems for explosion-proof electrical equipment are:

ATEX: In the European Union, ATEX directives address safety in workplaces where explosive atmospheres might occur. ATEX 153 defines requirements for protecting workers potentially at risk from explosive atmospheres, while ATEX 114 deals with equipment operating in these environments. ATEX directives are mandatory in the European Union and ATEX certification is required for equipment sold in these markets. ATEX certification is also recognized in some other countries.

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UL Solutions: In the United States, Canada, Brazil and some other countries, UL defines standards for explosion-proof and dust-ignition-proof electrical equipment for installation and use in hazardous locations. This certification verifies that the equipment meets the requirements of the U.S. National Electrical Code (NEC) and the Canadian Electrical Code (CEC).

China Compulsory Certification (CCC): This system requires manufacturers of explosion-proof products to obtain the CCC mark before they can be sold in China. Obtaining a CCC mark involves both product testing and audits of the production facilities.

IECEx: Developed by the International Electrotechnical Commission, IECEx is an international framework of standards and certification for equipment used in explosive atmospheres.

While equipment certified for a particular hazardous environment according to one standard, such as ATEX, is likely to be safe for use in similar environments elsewhere in the world, local authorities may require a specific form of certification.

Designers, builders and operators must make sure they understand local regulations of the jurisdiction where the equipment will be operated. Manufacturers can often provide products that have been certified for multiple jurisdictions.

Classifying Explosive Atmospheres: Divisions & Zones 

Certification systems for explosion-proof equipment recognize that not all explosive atmospheres are the same. In some situations, explosive gases or flammable airborne dust may be present almost continuously. In others, these will occur only occasionally, for example, an accidental leak. Equipment intended for use in riskier environments must meet higher safety standards. 

Figure 1 outlines the classification systems used in North America, Europe and other parts of the world. 

Another important consideration is the type of gas or explosive material that might be present. Safety regulations typically have different standards for explosive gases and flammable dust or fibers. They also classify the gases and airborne solids according to how easily they can be ignited (see Figure 2). 

Understanding these classification systems is important when it comes to specifying certification requirements for equipment for hazardous locations.

Certification Procedures

In most parts of the world, government authorities define the requirements for explosion-proof equipment, while independent testing services such as UL, TuV, CSA or Intertek verify products meet these requirements and issue certificates of compliance. This typically involves testing sample devices, plus periodic factory inspections to verify manufacturing standards are being maintained. 

A certification service will check for several elements when examining a device for certification. These include:

• Is the equipment intended for use in atmospheres that might contain explosive gases or for flammable dust or fibers? While the design features that will make equipment safe to use in either environment are similar, the certification process treats these as different situations.
• Is the enclosure or housing robust enough to isolate internal sparking or hotspots and to contain any fires or explosions that occur inside the housing?
• Are openings or shaft seals adequate to protect internal components? This is important for dusty environments, where IP6x-rated seals are needed to ensure that dust particles won’t infiltrate the housing and catch fire.
• Are external surface temperatures low enough to avoid starting a fire? Certification systems define several temperature classes, recognizing that different gases have different ignition temperatures.

Once a product has been examined and tested, the certification service will award a certification code that verifies what standards it meets and under what conditions it would be safe. A sample of a certification code for ATEX-certified POSITAL encoders is shown in the sidebar, along with a breakdown of what each part of the code means.

Sourcing Explosion-Proof Position Sensors

Explosion-proof sensors, including position control sensors such as rotary encoders or inclinometers, have often been regarded as niche products, essential in certain industries but too specialized for general use. The costs associated with certification have discouraged some manufacturers from updating product lines with newer technologies. As a result, buyers may face limited choices when they need certified equipment for new projects or while maintaining older systems.

Fortunately, newer sensors based on modular design concepts can be adapted to meet explosion-proof requirements. For example, these specifications can include heavy-duty external housings, high-performance seals and energy-efficient electronic components that run at lower temperatures. The result is sensors that meet UL, ATEX, IECEx and CCC requirements for use in oil and gas installations, agriculture, chemical plants, milling operations and mines.

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