How-to-decipher-hazardous-location-definitions-in-NEC-and-ATEX

How to decipher hazardous-location definitions in NEC and ATEX

Dec. 10, 2021

I’m always amazed at how many different standards exist for the same things in this world and the resulting differences between them.

When traveling abroad, we Americans have to be conscious of voltage and plug differences for our electronics, metric vs. imperial measurements, differences in how we write calendar dates and a multitude of other things.

I have a very nice binder that some friends in Germany gave me, but I cannot really use it because I don’t have a supply of A4 paper, and I can’t simply put letter-size paper in it because the A4 notebook-paper standard for binder holes is not three-hole-punched as we use in the United States, but two-hole.

Is there any wonder we have issues with more important things when we can’t agree on how many holes our notebook paper have?

But I digress. So, it goes without saying that different regions in the world will have different standards for electrical equipment which must be followed.

Specifically, when specifying or designing equipment for hazardous locations (Hazlocs) containing flammable or explosive gasses or dust, it’s necessary to understand the regional standards into which the equipment will be installed and the standards that particular location or country follows.

There are several standards to be concerned with, depending on where the equipment will be installed in the world. The United States follows the National Electric Code (NEC); Canada follows the Canadian Electric Code (CEC); the European Union (EU) follows the Atmosphères Explosibles (ATEX) directives; and the remainder of the world has adopted the global International Electrotechnical Commission (IEC) standards.

The CEC follows the NEC very closely, and the ATEX is derived from the IEC standards, so, for simplicity, we will be briefly comparing the NEC standards and ATEX directives.

It’s important to keep in mind that the applicable standard is the installation location, not the manufacturer location of the equipment, so, in our global economy, we must be able to be able to follow the local directives and standards for Hazlocs.

Class/Division system

The NEC has historically defined Hazlocs by a system of classes and divisions, as defined in NEC, Article 500.

Classes account for the type of explosive substance expected to be present:

  • Class I—gas/vapor
  • Class II—dust
  • Class III—fibers.

And divisions signify the frequency of the presence of the flammable substance:

  • Division 1—under normal operation/continuously/for long periods
  • Division 2—rarely or under abnormal circumstances.

Additionally, groups A-G account for the specific explosive substance:

  • Group A—acetylene
  • Group B—hydrogen
  • Group C—ethylene
  • Group D—propane
  • Group E—metallic dusts
  • Group F—carbon-based dusts (coal and charcoal)
  • Group G—nonconductive dusts (flour, wood/sawdust, plastic dust).

Zone system

Differing from the NEC’s Class/Division system, the ATEX defines Hazlocs by a system of three zones for gas and vapor (zones 0, 1, 2) and three zones for dust (zones 20, 21, 22), which account for both the type and expected frequency of the substance of concern:

  • Zone 0 or 20—continuously or long periods during normal operation
  • Zone 1 or 21—likely under normal operation
  • Zone 2 or 22—rarely or under abnormal circumstances.

The ATEX then defines equipment groups for where the equipment will be installed:

  • Group I—intended for mines
  • Group II—intended for explosive gas atmospheres
  • Group III—intended for explosive dust atmospheres.

And then there are subgroups for the gas and dust groups, which define the specific hazardous substance:

  • Group IIA—environment contains propane or flammable gas with similar properties
  • Group IIB—environment contains ethylene or flammable gas with similar properties
  • Group IIC—environment contains acetylene, hydrogen or flammable gas with similar properties
  • Group IIIA—environment contains flammable flyings
  • Group IIIB—environment contains non-conductive dust
  • Group IIIC—environment contains conductive dust.

NEC changes to allow zone definitions

In the spirit of globalization, in 1996 the NEC adopted a version of the IEC/ATEX zone definitions in NEC, Article 505. I say a “version of” because, instead of simply referencing the IEC or ATEX standards, the NEC has its own text, which was intended to closely match the ATEX standard, but since changes to each are made at different frequencies, the two standards will likely diverge unless an exceptional effort is made to keep them in sync.

But the advantage is that U.S. equipment manufacturers should be able to certify to a U.S. standard, which also meets European requirements.

As you may have been able to gather from the descriptions of the different definitions of Hazlocs, the Class/Division and Zone systems do not have a one-to-one correlation in defining hazardous areas. Specifically, the NEC, Class I, Div. 1 definition encompasses both Zone 1 and Zone 0 areas of the IEC/ATEX definitions. So, those two areas are equivalent from the perspective of the NEC.

Some see this as a drawback with the Class/Division system, since equipment, sensors and devices in Div. 1 (both Zone 1 and Zone 0) have to be specified according to the highest requirements, whereas using the ATEX system, devices in Zone 1 would be somewhat less expensive than Zone 0.

About the Author

Joey Stubbs | contributing editor

Joey Stubbs is a former Navy nuclear technician, holds a BSEE from the University of South Carolina, was a development engineer in the fiber optics industry and is the former head of the EtherCAT Technology group in North America.