This article was originally published on July 11, 2022, and updated on April 23, 2025.
In operations that involve a risk of fire or explosion, using the wrong pressure instruments can result in severe consequences, including operator injury, equipment damage or worse. To reduce the risk of anything catastrophic happening, pressure switches and other instruments used in these environments must adhere to strict class and division codes.
During my extensive career in pressure and temperature measurement, my expertise in this area is a requirement. I write about it often to help others know what to look for when purchasing instruments for their hazardous location applications.
Read this article to learn about the different class and division codes and which ones will apply to your specific needs. You will also be directed to additional resources designed to deepen your knowledge even further about pressure instruments and hazardous locations.
What are explosion-proof class and division codes?
Class and division codes originated from the need to ensure safety in environments where there is a risk of fire or explosion due to the presence of explosive atmospheres or mixtures. These environments are referred to as hazardous locations, as defined by the National Fire Protection Association (NFPA) in the United States.
The NFPA established the National Electric Code (NEC) standard (or code) NFPA 70 to provide U.S. guidelines for the design and use of electrical equipment, such as Ashcroft's explosion-proof pressure switches, in hazardous locations to prevent or contain potential fires or explosions. In Canada, the Canadian Electric Code (CEC) serves a similar function as the NEC.
Globally, the International Electrotechnical Commission (IEC) uses the Zone system to define hazardous locations.
Class codes
Class codes are categorized by the type of flammable material present.
Figure 1: Class codes for hazardous locations
Division codes
Division codes are based on the NEC 500 codes to indicate the possibility of fuel being present. Division 1 is the most hazardous location with hazards present under normal conditions. Division 2 is an area where the hazard is not present in normal conditions but could be present in abnormal operations such as changes to the surrounding air around a gas station. (See Figure 3.)
Figure 2: Division codes for hazardous locations
Figure 3: Division and 2 code examples
How the IEC defines the zone system
As mentioned previously, the Zone system is based on the IEC standards and used in the NEC505, ATEX, IECEx and INMETRO systems. (See Figures 4 and 5.)
Figure 4. The Zone system chart
Figure 5: Zone system example
Who enforces explosion-proof codes for hazardous locations?
Several individuals are given the authority to enforce these codes, including These include a local inspector, insurance underwriter’s representative and municipal authority (i.e., Fire Marshal or Electrical Inspector). Each is responsible for approving a specific installation.
A nationally recognized testing lab (NRTL) can certify your product. NRTLs are organizations empowered by OSHA to certify compliance with recognized standards. These include:
- Underwriter's Laboratories (UL)
- Factory Mutual Research Corporation (FM)
- Canadian Standards Association (CSA)
All of these NRTLs test to the NEC or CEC codes following their own test procedures or standards. They can also test to the IEC standards and, through their own international associations, issue ATEX or IECEx approvals.
NOTE: UL, FM, and CSA offer approvals for other safety requirements, so you need to be sure of which specific approval the instrument has. The instrument label will identify any hazardous location approvals and requirements.
All equipment being used must be able to meet the specific division or zone requirements and be clearly marked with its ratings. (See Figure 6 ).
Figure 6: Code markings on an Ashcroft product
How to prevent an explosion from occurring in a hazardous location
To have a fire you need to have all three of these components present: flammable material (fuel), air (oxygen) and an ignition source.
Figure 7: The fire triangle
If you remove any one of the components you will not have a fire. The method used for instruments to prevent a fire or explosion is to eliminate the ignition source by either containment or by limiting energy.
Methods of protection used in hazardous locations
Explosion-proof or flame Proof
This means containing a defined explosion, as these enclosures have specially designed flame paths, which allows the internal explosion to vent without a flame escaping the enclosure.
Intrinsically safe
In this method, energy applied to an instrument is limited using an approved barrier to prevent explosions. The barrier acts as a buffer between hazardous and non-hazardous locations. This is the safest and only approved method for Zone 0 applications.
Nonincendive
This method involves limiting the energy that is applied to an instrument to prevent an explosion from occurring without the use of a barrier. The design must limit the energy to the instrument. This can only be used in division 2 hazardous areas.
Simple apparatus
This device cannot generate nor store energy like a dry contact microswitch. These simple apparatuses can be used with an approved barrier in intrinsically safe applications as long as the barrier’s approval includes the use of simple apparatuses.
Ready to learn more?
Understanding the hazardous area classification requirements of your application is critical in selecting instruments that will operate safely. Know who the code enforcing authority is and/or which approval type or agency is required. Substitutions or alternative methods may not be acceptable.
Also, make sure all associated wiring is performed following the method of protection type you are using, and the wetted materials of the instrument are compatible with the process media.
For more information about pressure switches and working in hazardous locations, here are a few related articles that may interest you:
- When Should You Use an Electronic Pressure Switch?
- Choosing a Pressure Switch: 9 Factors to Consider
- What is SIL Certification for Pressure Switches?
Feel free to contact us anytime to speak with an industry expert and get your questions answered.
In the meantime, check out our guide to learn the hazards and safe operation of pressure and temperature instrumentation in hydrogen systems: