Potentially explosive atmospheres are classified into zones in order to facilitate the selection of appropriate electrical equipment as well as to design suitable electrical installations. The zone classification reflects the probability of the occurrence of an explosive atmosphere.

The maximum risk potential for each area must be taken into account when dividing the potentially explosive areas into zones and determining the necessary protective measures. If the company does not have a suitable person to carry out the risk assessment and determine suitable measures, an authority must be deployed for it.

Zone 0* defines an area in which a hazardous explosive atmosphere as a mixture of air and combustible gases, vapours or mists is present continuously, for long periods or frequently.

Zone 1* describes an area in which a hazardous explosive atmosphere as a mixture of air and combustible gases, vapours or mists is likely to occur under normal operating conditions.

Zone 2* refers to an area in which a hazardous explosive atmosphere as a mixture of air and combustible gases, vapours or mists is normally not present, or is only present for a short period of time, under normal operating conditions.

Zone 20* defines an area in which a hazardous explosive atmosphere in the form of a cloud containing combustible dust in air is present continuously, for long periods or frequently.

Zone 21* describes an area in which a hazardous explosive atmosphere in the form of a cloud containing combustible dust in air is likely to occur under normal operating conditions.

Zone 22* refers to an area in which a hazardous explosive atmosphere in the form of a cloud containing combustible dust in air is normally not present, or is only present for a short period of time, under normal operating conditions.
 

^{* Definitions from the German Ordinance on Hazardous Substances (Gefahrstoffverordnung, GefStoffV)}


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The various safety requirements for the equipment used depend on the probability of the occurrence of an explosive atmosphere. The safety level for equipment is coordinated with the risk potential in the various zones.

In Europe, explosion-protected equipment is classified in categories in accordance with 2014/34/EU (ATEX). At an international level, the IEC 60079-0 standard determines the EPL (Equipment Protection Level). Equipment should be designed with explosion protection measures of various levels of protection depending on the equipment category or EPL.

Categories 1 (very high level of safety), 2 (high level of safety) and 3 (required level of safety) are defined for equipment for use in potentially explosive atmospheres – except in mines susceptible to firedamp.

The additional letters "G" and "D" refer to the use of equipment in potentially explosive gas atmospheres (G = gas) or for atmospheres with combustible dust (D = dust).

For equipment used in mines susceptible to firedamp, a distinction is made between the two categories: M1 (very high level of safety) and M2 (high level of safety).

In accordance with IEC 60079-0, equipment for potentially explosive atmospheres is classified into three protection levels: EPL Ga or Da (very high level of protection), EPL Gb or Db (high level of protection) and EPL Gc or Dc (enhanced level of protection).

For equipment in mines susceptible to firedamp, two protection levels are defined: EPL Ma (very high level of protection) and ELP Mb (high level of protection).

The additional letters "G" and "D" are used here to indicate the use of equipment in potentially explosive gas atmospheres (G = gas) or for atmospheres with combustible dust (D = dust).

Application of equipments in specific category/EPL in hazardous areas is shown in the table below.
 

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Gases, vapours and dusts are subdivided into various groups according to their characteristics, which are assigned to individual material groups, that are designed for correct selection of equipment. The equipment groups therefore describe the level of protection and the application range of the equipment.

Equipment Group I applies to equipment intended for use in underground parts of mines as well as parts of surface installations of mines susceptible to firedamp and/or combustible dusts. This equipment group is subdivided into the categories M1 and M2.

Equipment Group II applies to equipment intended for use in other industries susceptible to explosive gas or dust atmospheres. Depending on the occurrence of a hazardous atmosphere, the Equipment Group II is subdivided into categories 1, 2 and 3 and is marked according to the explosive gas (G) or dust (D) atmosphere.

In addition, electrical equipment in Group II (Gas) is, subdivided into Groups IIA (Propane), IIB (Ethene) and IIC (Hydrogen).

In 2007, the IEC 60079-0 international standard divided Group II again, into the following three equipment groups: 

• Group I:   Equipment for mines susceptible to firedamp
• Group II:  Equipment for all potentially explosive gas atmospheres except for mines susceptible to firedamp
• Group III: Equipment for potentially explosive dust atmospheres except for mines

The equipment in Group III is intended for use in potentially explosive dust atmospheres. Electrical equipment in Equipment Group III is subdivided according to the type of dust into Groups IIIA (combustible flyings), IIIB (non-conductive dust) and IIIC (conductive dust).

The ignition temperature of an explosive gas atmosphere or dust cloud is the lowest temperature of a heated surface at which, under specified conditions, either a combustible in the form of gas or vapour, or as a dust/air mixture, ignites.

The ignition temperature of gases, vapours and dusts is therefore a crucial criterion for the selection of equipment in potentially explosive atmospheres.

Combustible gases and vapours are classified in temperature classes according to their inflammability. The maximum surface temperature of electrical equipment must always be lower than the ignition temperature of the gas/air or vapour/air mixture in which it is used. Equipment which correspond to a higher temperature class (e.g. T5) are, also permitted for use in applications in which a lower temperature class is required (e.g. T2 or T3). Note: In North America, there is a system incorporating further classification into temperature subclasses.

Combustible dusts are not divided into temperature classes and so the ignition temperature of the dust cloud must be compared with the maximum permissible surface temperature of the equipment. However, since dusts can build up on the equipment, the ignition temperature of the dust layer (glow temperature) must also be taken into account. In both cases, the specified safety factors must be considered. Higher layer thicknesses also increase the thermal insulation, which is why maximum permissible surface temperature on the equipment must be further reduced. Further considerations are needed here, which we explain in our "Essential Explosion Protection" brochure.

In areas in which an explosive atmosphere is likely to occur only explosion-protected equipment should be used. Electrical equipment for potentially explosive atmospheres can be designed in various types of protection in accordance with the building regulations of the IEC 60079 series of standards. Types of protection for non-electrical equipment are defined in the ISO 80079 series of standards. The latter replaces the EN 13463 series of standards which is currently valid.

The type of protection which the manufacturer applies to equipment largely depends on the type and function of the equipment. Some types of protection are available in different protection levels. These correspond to the equipment categories in accordance to ATEX Directive 2014/34/EU and/or the equipment protection level (EPL) in accordance with IEC 60079-0. From a safety point of view, all standardised types of protection within a category and/or an equipment protection level should be considered equally.

The following tables provide an overview of the standardised types of protection and describe the basic principle and the normal applications. By integrating the types of protection for potentially explosive dust atmospheres in the IEC 60079 series of standards, the symbols for the types of protection are also simplified.

If necessary, combinations of different types of protection can also be applied in order to provide an optimum solution from the safety, functional and economical point of view. Modern explosion-protected equipment generally combine with multiple types of protection. By using different types of protection on one item of equipment, smaller and more cost-effective designs can be implemented on the one hand. On the other, it means that construction types are possible that make operation and maintenance easier for the user.

Our "Essential Explosion Protection" brochure clearly sets out detailed information on types of protection, equipment groups, physical and technical bases, legal specifications and standards as well as erecting and operating systems in potentially explosive atmospheres.

Equipment produced for potentially hazardous atmospheres is marked accordingly. The marking requires certain details depending on the certification and country-specific use.

The marking for electrical equipment is set out in IEC 60079-0. In addition to the name of the manufacturer or their trademark, the type designation, serial number and the test body with certificate number, a special coding is required which describes the intended use of the equipment:

• Ex symbol.
• Symbol of any type of protection (e.g. q) or protection level (e.g. ib) used.
• Equipment Group IIA, IIB or IIC for potentially explosive gas atmospheres and Equipment Group IIIA, IIIB or IIIC for potentially explosive dust atmospheres.
• Temperature class for potentially explosive gas atmospheres and max. surface temperature in degrees Celsius for potentially explosive dust atmospheres.
• Equipment Protection Level (EPL).

The marking for non-electrical equipment is specified in ISO 80079-36. It largely identical to that of electrical equipment. However, instead of various symbols for the protection types, the letter “h” is always used.

In Europe, the marking of electrical equipment in accordance with the IEC standard must meet additional requirements in accordance with ATEX Directive 2014/34/EU and must include the following data:

• Manufacturer's address.
• CE marking (where applicable, with identification number of the notified body).
• Symbol and Equipment Group (e.g. II) as well as Category 1, 2 or 3 and the letter G (gas) or D (dust).

The marking of non-electrical equipment has now been adapted even more closely to that of electrical equipment. In Europe, the EN ISO 80079 series of standards has been applicable since 2016. An "h" is always specified as a symbol for the type of protection.

In addition to the normal data (manufacturer, model, serial number and electrical data), any data relating to explosion protection must be shown on the equipment marking. The specification for this is laid out in the NEC for the USA, in the CEC for Canada and in the relevant building regulations of the test bodies.

 You can find further information in our "Essential Explosion Protection" brochure.

Overview of marking of electrical and non-electrical equipment