One of the most challenging tasks design engineers can face is designing measurement or control systems to operate safely in hazardous environments. These environments may contain flammable or explosive gas, dust, or fiber — and involve oil and gas extraction, mining, spray painting, or chemical, grain, or textile processing.

Specially designed and rated equipment is required in these locations; more specifically, encoders are often needed for speed or position feedback, and must be designed and operated within the correct hazardous area rating.

To ensure the safety of systems operating in hazardous locations, governments worldwide have established regulatory bodies that have developed testing and certification standards and product marking requirements. Some countries have coordinated the certifying process, creating reciprocity agreements with the different agencies, and allowing certifying bodies of one country to test and certify to standards of another country.

Even so, with various classifications of hazardous locations, different nations have developed standards for equipment installed in these environments — and a patchwork of requirements has evolved.

So, as an engineer needing to specify an encoder amid the maze of regulations, where should one begin? Start by understanding how a hazardous location is defined, and then determine what rating is required for the installation at hand.

Classifying a hazardous location

An area can be classified as hazardous based on the following factors: First, is there a possible presence of an explosive or flammable agent? If so, what is that agent? Second, is the explosive atmosphere present under normal operating conditions, or only if there is a malfunction or unusual occurrence that might release an explosive agent into the area? Third, in what part of the world will the equipment be installed?

In fact, the United States (ANSI/NFPA), Canada (CEC), Europe (CENELEC), and much of the rest of the world (IEC) use this approach to area classification.

Once engineers have answered the above questions, they can determine both the hazardous area rating needed and the certifying body that must approve the equipment or installation.

Two main classification systems exist: The Class, Division, and Group system is used to classify hazardous locations in the U.S., Europe, and other parts of the world use the Class and Zone system, also called the CENELEC system. Let's explore both of these classification systems in more detail.

Types of hazardous agents

Once the possible presence of an explosive agent is determined, the engineer must classify that material into one of three categories — to provide the top level rating necessary for the encoder to be integrated. Class I includes flammable gasses, liquids, and vapors; Class II covers combustible dusts; Class III encompasses ignitable fibers and flyings.

Materials within these classes have different levels of ignitability, so within each Class/Division or Class/Zone there are Groups and Temperature Codes that correspond to the ignition temperatures of the flammable agents.

With only minor exceptions, Zone 1 in the CENELEC classification system is the same as Division 1 in the U.S., and Zone 2 CENELEC is virtually the same as Division 2 in the U.S.

Determining exposure levels

The conditions under which exposure to the hazardous agent is expected also affect encoder rating requirements. If the explosive agent will be present continuously or intermittently under normal operating conditions, it is defined as a Division 1 location in the U.S. and a Zone 0 or Zone 1 location in other parts of the world. Equipment in Division 1/Zone 0 and 1 environments must pass stringent testing and cannot create an ignition hazard, even in the event of equipment failure.