Hazardous area classification is the process of identifying where flammable gases and vapors, combustible dusts, or ignitable fibers can exist at high enough concentrations to ignite or explode. It is commonly used to determine the appropriate electrical fixtures that are required to be installed within the process areas, per NFPA 70 – National Electric Code. Various systems are used nationally and internationally to categorize these environments and ensure safe electrical equipment installation. In all cases, area classification principles guide how locations are assessed and equipment is selected.
Every system aims to describe a few core concepts:
- Hazard type: gas/vapor, dust, or fibers/flyings.
- Likelihood and duration: how often the explosive atmosphere exists during normal vs. abnormal conditions.
- Material group: separating substances by ignition energy, flame propagation, and gap characteristics.
- Maximum surface temperature: limit for the maximum temperature allowed before equipment ignites.
- Protection technique: what kinds of features equipment must have to prevent ignition in these areas.
Though the terminology used may differ from one organization to the next, many are comparable and, in some cases, directly convertible, ensuring consistent area classification across regions.
The Class/Division System
Traditionally used in North America per the NFPA 497 and NFPA 499 standards, this classification method uses Class to identify what can burn and Division to determine how likely it is to be present.
- Class I: Flammable gases, vapors, or liquids.
- Class II: Combustible dusts.
- Class III: Ignitable fibers or flyings (ex. Textile or woodworking fibers).
- Division I: The hazard is present under normal operating conditions.
- Division 2: The hazard is present only under abnormal conditions.
This system also uses Groups to further refine the classification based on how easily a substance can be ignited and how the explosion propagates. Groups A-D fall under Class I, and Groups E-G fall under Class II.
- Groups A-D: Various gases and vapors
- Group E: Metal dusts
- Group F: Carbon black and coal dust
- Group G: Grains, flour, wood, and similar
Temperature codes, also called T-codes, list the maximum external surface temperature for a piece of equipment. A lower T-code allows for a higher surface temperature: T1 has a max surface temperature of 842 degrees F, while T6 has a maximum temperature of 185 degrees F.
For example, a piece of equipment labeled “Class I, Division 2, Group D, T4” would signify that it is suitable for a location where ignitable concentrations of gas or vapor aren’t expected in normal operation, with propane-type gases, and the equipment’s surface temperature won’t exceed the T4 limit of 275 degrees F.
This framework is designated in the U.S. by NFPA 70, the National Fire Protection Association’s National Electrical Code (NEC) and isn’t often used in other countries or many zone classifications, unless it’s within the chemical industry. As part of hazardous area classification, it provides clear area classification parameters for equipment selection.
The Zone System
The International Electrotechnical Commission (IEC) and ATmosphères EXplosibles (ATEX) in the EU, on the other hand, classify hazards by Zones. This classification gives finer resolution of how long an explosive atmosphere is present than Divisions, because it distinguishes between constant and intermittent hazards. It includes classifications for gas (Zones 0-2) and dust (Zones 20-22).
- Zone 0/20: A hazard is present continuously or for long periods
- Zone 1/21: A hazard is likely to occur under normal operating conditions
- Zone 2/22: A hazard is unlikely to occur or will only exist for a short period of time
In this system, equipment uses an EX marking that combines the protection technique, group, and temperature class. You may also see an Equipment Protection Level (EPL) and an ATEX category on the nameplate. Groups include:
- IIA: Includes propane and butane. Requires basic protection
- IIB: Includes ethylene and similar. Requires moderate protection.
- IIC: Includes hydrogen and acetylene. Requires the highest protection.
For example, a piece of equipment labeled “II 2 G Ex db eb IIC T4 Gb” is surface industry equipment in category 2 for gas, provides flameproof protection for gas group IIC, with a max surface temperature of 275 degrees F, and has a high protection level.
The rankings are relatively equivalent across systems. For example, Class I, Division 1 is generally equivalent to Zone 1, while Class I, Division 2 is typically comparable to Zone 2. This helps align hazardous area classification practices globally and supports consistent area classification decisions.
Equipment Selection
Next, let’s examine the organizations who set standards for electrical rating requirements. You’ll typically see the following:
Ingress Protection (IP)
Though IEC technically defines the standard as International Protection marking, it’s widely referred to as Ingress Protection to describe how well a device is sealed against dust and liquids.
The first digit of an IP rating refers to the protection against solids and ranges from 0-6, and the second digit distinguishes the degree of protection against liquid and ranges from 0-8.
For example, a device with an IP 65 would be dust tight and protect against water jets from any angle, while an IP 23 would designate equipment that is protected against a solid object greater than 12.5mm and water spray at a 60-degree angle.
National Electrical Manufacturers Association (NEMA)
measures environmental protection against ingress of dust and liquids, plus corrosion and ice.
These ratings are self-declared by the manufacturer and range from 1-13. Some are designated for indoor or outdoor use, submersible, or explosion-proof. A few include additional K, P, R, S, or X designation, signaling additional capabilities against water submersion, ice, corrosion, etc.
For example, NEMA 4 protects against windblown dust, rain and hose-directed water, whereas 4X can also withstand corrosion. NEMA 4, 4X, 12 or 12K switches are typically seen in Class II, Division 2 locations (where combustible dust is typically only found under abnormal conditions).
Underwriters Laboratories, Inc. (UL)
UL is primarily known for being a Nationally Recognized Testing Laboratory (NRTL) that is a leader for this in the US for conducting testing for certification of equipment and fixtures/enclosures for use in hazardous locations. For example, NEMA would issue the standard for certification and UL would conduct the certification testing. However, UL does also write standards and specifications for dust-ignition-proof equipment itself.
Like NEMA ratings, UL ratings explain durability in different environments. However, UL ratings require rigorous testing by an independent third party and often prioritize safety code compliance for safety, fire prevention, and electrical hazards.
UL ratings also range from 1-13, though they don’t include type 3X, 3RX or 3SX like NEMA does. . UL ratings are based on both the level of protection from ingress of liquids, gases, or dust, as well as the testing method utilized for verification of the ingress protection. Most ratings correlate directly to a Class, Division, and Group outline in NFPA 70, making UL ratings preferred for those who must comply with certain standards.
Essentially, hazardous area classification is all about identifying risk and selecting the appropriate equipment. Conversion Technology, Inc. (CTI) has decades of experience in identifying and reducing hazards at industrial facilities. Contact CTI to learn how our licensed professionals can audit existing processes to determine compliance and keep your employees safe.