Explosion protection categories

Following the historical development of electrical engineering and explosion protection, zone classification was the objective of national standards and installation rules. Most of the leading industrial countries established an installation practice for chemical plants and the oil and gas industry with two or three zones for areas hazardous due to gas- or vapour-air mixtures and two zones for areas with hazardous dust-air mixtures. Apart from this philosophy, the coal mining industry in most countries tends to avoid an area classification and defines only one category of explosion protection ( firedamp-proof ). More recent standards or directives present a three-zone concept for areas endangered by combustible gas- (vapour-, mist-) air mixtures and dust-air mixtures in industrial plants (other than coal mines). 

Some years ago, Directive 94/9/EC (the so-called ATEX 100a Directive ), dated 1994-03-23 (following the title in French Atmospheres explosibles. .. and referring to article 100a of the Treaties of the European Community), introduced categories of explosion protected apparatus to give a guideline for their installation with respect to the risks that may be expected in different locations exposed to combustible atmospheres (Table 2.3). 

Until 1999, there exists no international standard for explosion protected electrical apparatus intended for the application in zone 0. Intrinsically safe circuits according to IEG 60079-11 and EN 50020, category ia, seemed to be an appropriate type of protection for zone 0 for some time. In the same way, for coal mines the first international standard describing the requirements for electrical apparatus that are intended for operation in the presence of methane/coal dust-air mixtures was prepared in 2000. 

Explosion protected electrical equipment for zone 1 (Group II, category 2 G) and the classical firedamp-proof apparatus for coal mines (Group I, category M2) are covered by the B module (EC-type examination) and by the C module (conformity to type) or the E module (product quality assurance). Alternatively, with respect to the B module, the notified body issues an EC-type examination certificate (in German EG-Baumusterpriifbescheinigung), whose marking mainly includes ... 

For maintenance or repair of explosion protected apparatus, three categories may be differentiated ... 

Table 6.6 Overview about the assignment between equipment categories and zones for the implementation of the explosion protection for the equipment group II.

Vacuum pumps of category 2 which pump from zone 1 have to be subjected by the manufacturer to an internal control of production according to annex Vlll of the directive. In addition, the manufacturer has to deposit a technical documentation for the vacuum pump at a Notified Body, including a risk assessment, ignition hazard assessment and notes for the implemented measures to maintain the explosion protection. 

Depending on the results of the risk assessment and ignition hazard analysis the measures for explosion protection for the wanted equipment category must be realized. 

The Ex marking for protective systems and equipment of the category 1 is normally determined by the Notified Body according to the result of an EC-type examination. The Ex marking for non-electrical equipment of the categories 2 or 3, for example explosion-protected vacuum pumps, is determined by the manufacturer or by the supplier of this equipment. 

Before vacuum pumps are put into operation, the operating company of a vacuum pump has to examine and assess the explosion risks which can be caused in the connection with the installation site and the processes applied. He must prevent or limit the formation of explosive areas. Remaining explosive areas have to be assigned to zones. Possible ignition sources in the zones must be avoided. The work equipment for the required equipment categories, for example explosion-protected vacuum pumps of the respective category, has to be selected. The results of the risk assessment, the applied protective measures, the zone classification and the remaining risks have to be recorded in the explosion protection document. 

The common Fire and Explosion Protection (FEP) approaches can be differentiated into two categories hazard and risk analyses. 

Explosion characteristics include the values of average and maximum rate of pressure rise and maximum pressure produced by the explosion (Table 3.59). Effective explosion suppression requires getting sufficient amounts of chemical to the trouble area in a very short time. Water and C02 are not generally utilized for explosions. Halogenated compounds, mostly methane derivatives, are popular suppressants. The hardware for explosion suppression falls into three categories (1) detectors (2) control units, which initiate the corrective action and (3) the actuated devices, which blanket the protected area with the suppressant. 

EPA. 1976. Explosives manufacturing point source category. U.S. Environmental Protection Agency. Code of Federal Regulations. 40 CFR 457.11. 

According to the NFPA Fire Protection Handbook, there are two general categories of explosions, physical and chemical. In a physical explosion, the high-pressure gas is produced by mechanical means, i.e., even if chemicals are present in the container, they are not affected chemically by the explosion. In a chemical explosion, the high-pressure gas is generated by the chemical reaction that takes place. 

There are probably more stories about lab incidents that caused, or nearly caused, injury to eyes than any other category of incident. Not surprisingly, splashes and explosions of chemicals can occur in any lab at any time. Because of this, perhaps the single most important safety rule in every chemistry lab is wear appropriate eye protection at all times. This section discusses exactly why this is so important, and how and why this rule sometimes gets violated. Like the cat in Dr. Seuss tales, keeping your eyes open (and working ) in chemistry labs is important ... 

Category Ml Equipment in this category should remain functional in an explosive atmosphere (in mining). Required protection level Very High. ... 

Category 1 Equipment in this category is intended for use in areas in which explosive atmospheres caused by mixtures of air and gases/vapors/mists or by air/dust mixtures are present continuously, for long periods or frequently. Equipment or protective systems need to guarantee a very high level of protection and are duly certified by a notified body (e.g., CESI/IMQ/TUV). See Table X/3.2.2-1. 

Non-electrical equipment of the equipment group II, category 2 for explosive atmosphere formed from air and gases, vapours and/or mists, type of protection flame-proof enclosure, permissible explosive atmosphere of the Ex-Group IIB, T-Class T4, ambient temperature (T ) with 0°Cdiffering from the standard values -20 °C < T. -t-TO C ... 

As a further complication, the ATEX Directive, enacted in the UK as The Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations 1996 (see Chapter 7) introduced the concept of equipment categories, which will have to be adopted at the end of the transition phase in 2003. In very broad terms these categories are ... 

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