When doing research for our whitepaper on ‘NEMA Ratings of Enclosures Explained’, I kept on running into the terms ‘Explosion-Proof’, and ‘Intrinsically Safe’. I thought I had the general idea of what these terms meant, but once I looked into it a bit more I realized how important these types of products are to industries of all kinds.
As far as the electrical industry is concerned, the term ‘Explosion-Proof’ applies to electrical products (including, but not limited to enclosures, fittings, light-fixtures and receptacles) designed and constructed to contain and control potential flames or explosions that may occur within them due to arcs or flashes. Therefore, explosion-proof does not mean that it is able to withstand an exterior explosion, but instead it refers to its ability to prevent an internal spark from causing a larger blast that might be harmful to both the equipment and its operating engineers.
An arc, as defined by OSHA, is, “a phenomenon where a flashover of electric current leaves its intended path and travels through the air from one conductor to another, or to ground. The results are often violent and when a human is in close proximity to the arc flash, serious injury and even death can occur.” (OSHA) Arcs can be caused by faulty installation, material failure, corrosion, or even something as small as an accumulation of condensation or dust.
To protect engineers from potential harm, the National Electric Code (NEC aka NFPA 70), was adopted and enforced throughout the United States as a standard to regulate electrical installations. The NEC is updated and maintained by the National Fire Protection Association (NFPA), and while it is not considered a law outright, it is largely adopted by most states as such. The NEC was originally created by the NFPA in 1897 as a combination of various electrical codes being used at the time into one uniform standard. “This draft was then send to over 1,000 reviewers from all over the world, who provided comments and suggested additions and changes…[and]…the final result was the National Electrical Code of 1897.” (Beach, Emily)
The NEC separated Explosion-classified areas into three different classes, two different divisions, and seven different groups. All of these designations combine into one classification, IE: A ‘Class I, Div 1, Group A’ enclosure is one designed for a location where Acetylene is present in the atmosphere in continually present quantities sufficient to produce an explosion. The aforementioned designations are explained here:
Class I – Is for locations in which flammable gasses or vapors are or may be present in the air in quantities sufficient to produce explosive or ignitable mixtures.
Class II – Is for locations that are considered hazardous due to the presence of combustible dust.
Class III – Is for locations that are considered hazardous due to the presence of easily ignitable fibers or flyings, but in which such ignitable materials are not likely to be suspended in the atmosphere in quantities likely to produce an explosion.
Division I (Div 1) – Is for locations in which hazardous concentrations in the air exist continually, intermittently, or periodically under normal operating conditions.
Division II (Div 2) – Is for locations in which hazardous concentrations are handled, processed, or used but are normally within closed containers or closed systems from which they can escape only in case of accidental rupture or breakdown.
Group A (Class I) – Atmospheres containing Acetylene
Group B (Class I) – Atmospheres containing Hydrogen and/or Manufactured Gas
Group C (Class I) – Atmospheres containing Diethyl Ether, Ethylene, and/or Cyclopropane
Group D (Class I) – Atmospheres containing Gasoline, Hexane, Butane, Naphtha, Propane, Acetone, Toluene and/or Isoprene
Group E (Class II) – Atmospheres containing Metal Dust (Aluminum, Magnesium, and their commercial alloys)
Group F (Class II) – Atmospheres containing Carbon Black, Coal, and/or Coke Dust
Group G (Class II) – Atmospheres containing Flour, Starch, and/or Grain Dust
When dealing with the NEMA classification of certain boxes, you may come across something called an IP rating. “The IP rating is set out in standard IEC EN 60529 and rates electrical enclosures by the level of ingress protection against solids (1st digit) and against liquids (2nd digit). The higher number for each digit, the better the protection” (Rotork). These IP ratings directly correspond to NEMA classifications for enclosures meant for non-hazardous locations only, and work almost like a cheat sheet once you know what information correlates to each digit.
On top of these classifications, the NEC requires that all explosion-proof products meet certain temperature requirements for each specific location. For example, if you have a motor in an enclosure that is being put in a hazardous location, the operating temperature of the motor and its enclosure has to be lower than the temperature that the potentially hazardous dust ignites at.
Testing centers like Underwriters Laboratories, CSA Group, and Intertek test a representative sample of each piece of equipment that is designed for installation in a hazardous location to see if it meets the criteria/requirements specified by the NEC. These testing centers use, “marks to denote that the products they have tested conform to the standards set by the (NFPA) and by other international standards organizations. These marks, which include UL, CSA, ETL, and others, can be looked for to determine compliance with the standards.” (Specific Systems)
These certifications only apply to the U.S. and Canada, as there are differing regulations and requirements in Europe and the U.K. that dictate how products are designed, manufactured, and certified in their region with their electrical codes and procedures. This means that it is not possible to claim a product that was certified at intrinsically safe or explosion-proof in the U.S. is also considered intrinsically safe or explosion-proof in Germany.
But is a piece of equipment that is rated as ‘Explosion-proof’, intrinsically safe by default? In short, no. An intrinsically safe component is a tested piece of equipment of low power that cannot cause an explosion in a hazardous environment. Conversely, an explosion-proof product has no limitations on energy, and this protection method can be used for high-voltage or high-current applications, as well as for devices with high levels of inductance or capacitance. The basic principle of an explosion-proof device lies in its ability to prevent an internal spark from causing a larger blast that might be harmful to both the equipment and its operating engineers.
“Understanding Arc Flash.” OSHA. The Occupational Safety and Health Administration, U.S. Department of Labor, 2007. Web. 23 July 2015.
“What Does Explosion Proof Mean?” Specific Systems. Web. 23 July 2015.
“Enclosures Explained – Understanding Hazardous and Non-Hazardous Actuator Enclosures” Rotork. Web. 23 July 2015.