Explosion Proof Devices and the Code

There are stringent codes governing the use of electrical equipment and 
components in potentially flammable atmospheres. But, let's start with a
 few definitions.<br><br>Two terms that sometimes get confused are 
explosion-proof and intrinsically safe. Though similar, they are not 
identical. While the intent of both is to prevent ignition or explosion 
of flammable atmospheres, how they go about it is handled differently. Appleton Group / Emerson Electric

Steve Maurer, IME

There are stringent codes governing the use of electrical equipment and components in potentially flammable atmospheres. But, let's start with a few definitions.

Two terms that sometimes get confused are explosion-proof and intrinsically safe. Though similar, they are not identical. While the intent of both is to prevent ignition or explosion of flammable atmospheres, how they go about it is handled differently.

The main difference is that of containment versus prevention. An explosion-proof device is enclosed in a housing that will contain any internal explosions of flammable gases or vapors. This prevents the ignition of any external surrounding vapors and/or gases caused by any sparks, sparks, or explosions inside the device.

Additionally, the exposed, external parts of the device or enclosure cannot run so hot as to ignite the surrounding atmosphere.

Intrinsically safe devices are different. With this type of device, the circuitry is designed to be incapable of igniting surrounding hazardous atmospheres. So, as you can see, explosion-proof devices contain while intrinsically safe devices prevent.

These definitions are outlined in the NFPA Publication, The National Electrical CodeĀ® in Articles 500 to 506. These Articles establish the quantity of hazardous substances needed to cause explosion or ignition.

The Code also establishes three area classifications based on the hazardous substances found in them.

  • Class I locations contain flammable gases or vapors.
  • Class II hazardous locations contain combustible dust.
  • Class III hazardous locations contain fibers or flying's that are easily ignitable.
These location classes are further broken down into Divisions and Zones. Any electrical devices or components installed in these areas must conform to manufacturing standards required for the class/division/zone intended.

For example, a Class I device designed for gases and vapors cannot be used Class II, even though both are designed to be explosion proof.

Add to the mix the Class designations that the NEC uses to define hazardous vapors, gases, and dusts according to their ignitable or combustible properties, and you can clearly see that choosing the right device is a precise and critical task.

OSHA regulation 29 CFR 19 10.307(c)(2)(ii) sets forth the requirement for proper labeling of the devices according to class, group and operating temperature or temperature range for which it is approved.

Once the characteristics of the potentially hazardous atmosphere are defined, is then a matter of matching the labeled device to the location.

Obviously, choosing the correct device is a detailed, exacting process. However, this is necessary to ensure that you maintain compliance with regulations.


Photo courtesy of Appleton Group / Emerson Electric