In areas at risk of explosion, a flammable substance (gas, liquid, dust) and an oxidizer are present in specific concentrations, so that contact with an ignition source can cause an explosion.
Relation to UL
Classifying the hazardous area is of fundamental importance for designing and manufacturing both the machine and the control cabinet in accordance with the correct standard and its requirements!
What is a potentially explosive atmosphere?
For an area to pose an explosion hazard, a flammable substance must be present in a certain minimum concentration; below this so-called ignition limit, there is no hazard, even if an ignition source is present. Furthermore, an oxidizer—in practice, usually oxygen—is required. In addition to gases, vapors, and mists, clouds of solid-air mixtures can also ignite if the individual particles or fibers are small enough and present in the mixture in sufficient concentration. Well-known examples of this include flour dust and coal dust explosions.
How are potentially explosive areas classified in the United States?
All areas where these conditions exist are referred to as “potentially explosive” or, in North America, as “hazardous locations” (or “HazLoc” for short). In practice, of course, the classification is more complicated. In the U.S. and Canada, there are even two systems for classifying HazLocs, which makes the whole thing even more confusing.
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Potentially Explosive Areas According to NEC 500
In the U.S., classifying explosion hazards into three zones based on frequency—as implemented here in the ATEX Directive 1999/92/EC —was uncommon for a long time. Instead, in the U.S., areas at risk of explosion are classified into two categories, known as “divisions.” However, this classification is based not on the frequency of the explosion hazard, but on its probability.
The classification of these two areas is defined in Article 500 of the National Electrical Code and is easy to understand. All areas where an explosion hazard exists only during malfunctions or other unusual conditions are classified as Division II. If, on the other hand, an explosive atmosphere is expected even during normal operation, the area in question is classified as Division I.
Differences from Europe
The key difference from the European ATEX Directive is that the frequency of the presence of a potentially explosive atmosphere does not play a role in the U.S. classification system. Thus, an area is classified as Division I whether the risk of explosion during normal operation exists for only 5 minutes per year or is present continuously.
Further Classification of Potentially Explosive Areas
In addition to the divisions, NEC 500 distinguishes three classes based on the form of the explosive substances. Class I includes gases, mists, and vapors; Class II includes dusts; and Class III includes fibers. Within Classes I and II, further differentiation is made based on the likelihood of explosion into various groups (A through D and E through F). A gas in Group A, for example, explodes more readily than one in Group B, and so on. However, the frequency of the explosion hazard is again irrelevant.
Ex Zones According to NEC 505 – The U.S. Is Aligning with the ATEX Directive
As part of the effort to align with international standards, the 1996 amendment to the National Electrical Code in the United States introduced a classification system for hazardous locations based on the frequency of the hazard. This was done through Article 505 of the NEC. But please note: Article 500 is not thereby superseded; rather, both definitions coexist. NEC 504 rounds out the picture by incorporating the requirements for intrinsically safe systems used in hazardous locations.
Zone 2 or 22: Rare risk of explosion
If an explosive atmosphere is expected to be present for less than 50% of the operating time and does not last longer than half an hour, this is referred to as a rare and short-term risk.
Occasional risk of explosion in Zones 1 and 21
If the risk of an explosion exists for no more than half of the operating time, but for more than 30 minutes per year, this is referred to as an occasional occurrence .
Zone 0 or 20 indicates a frequent risk of explosion
If an area is at risk of explosion for more than 50% of its operating time, this is classified as “frequent,” and the area falls under Zone 0 or 20.
What is the practical significance of areas at risk of explosion?
Explosion protection naturally plays a central role in the design of systems and equipment intended for use in hazardous areas. However, the practical implementation of these requirements is governed in part by other articles of the NEC, as well as by other regulations and standards.
Safety in Control Cabinets for Hazardous Areas
For us as control cabinet manufacturers, UL 1203 plays a central role in the design of equipment for potentially explosive atmospheres. This standard sets forth numerous requirements for explosion protection, all of which are ultimately aimed at ensuring that no ignition source is present in the equipment at any time . It is also important that only components approved for use in hazardous areas are used in the construction of control cabinets (see the article on Category Code Number NNNY for more information).
How does explosion protection work in the United States?
Which type of ignition protection is used in practice depends on the specific circumstances of each case. For example, the control cabinet can be flooded with Intergas (“ignition-protection gas”) to prevent explosive substances from entering from the surrounding environment and coming into contact with a potential ignition source. Alternatively, the control panel enclosure be designed in such a way that it can withstand the pressure of an internal explosion and prevent air from escaping to the outside.
Zusammenfassung
Potentially explosive atmospheres are locations where flammable gases, liquids, dusts, or airborne particles are present in a certain minimum concentration. In the U.S. and Canada, two different definitions of hazardous areas coexist, with NEC and CEC 505, respectively, based on the ATEX Directive. The measures that must be taken to prevent explosions are outlined in guidelines and standards such as UL 1203.
