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Physical and technical background

An explosion is a sudden chemical reaction of a combustible substance with oxygen under release of high energy. These combustible substances can be gases, mists, vapours or dusts. An explosion can only proceed if three factors come together: a combustible substance (in appropriate distribution and concentration), oxygen (in the air) and an ignition source (e.g. electric spark).

It is important, therefore, to avoid an ignition or to minimise the effects of an explosion to a safe extent. For this, all operating equipment used in potentially explosive atmospheres must be designed, manufactured and marked according to the applicable regulations (ATEX product directive 2014/34/EU, IECEx regulations). The equipment is classified into groups and categories according to ATEX product directives or EPL according to IECEx standards in dependence on their area of use or the safety level of the protective measures and the frequency with which a potentially explosive atmosphere occurs. The highest possible potential hazard must be considered in this regard in each case.

Only explosion-proof equipment may be used in areas in which explosive atmospheres can occur in spite of all measures to avoid them. This equipment is constructed in various ignition protection classes in accordance with the corresponding construction regulations (IEC/EN 60079 standards series and ISO 80079-36/EN ISO 80079-36 standards series). Which ignition protection class the manufacturer uses depends on the type and function of the equipment. All standardised ignition protection classes in a category are equivalent. The manufacturer certifies in the EU declaration of conformity contained in the technical documentation that the product complies with the ATEX directives.

Regarding the exact markings of electrical and non-electrical equipment, you can download an information sheet as well as the poster Expertise in explosion-proof crane technology.

Integrated explosion protection

The principle of integrated explosion protection requires that all explosion-protection measures are implemented in a fixed order. A distinction is drawn between primary, secondary and tertiary (constructional) protection measures.

Classification into zones

According to the definitions of the German Ordinance on Hazardous Substances (GefStoffV), potentially explosive areas are subdivided into zones to facilitate selection of appropriate and suitable equipment and electrical installations. The zone classification indicates the probability of a potentially explosive atmosphere occurring.

Gases

Zone 0: is an area in which hazardous, potentially explosive atmospheres prevail as a mixture of air and combustible gases, vapours or mists continuously, over long periods of time or frequently.
Zone 1: is an area in which a hazardous, potentially explosive atmosphere can form as a mixture of air and combustible gases, vapours or mists occasionally in normal operation.
Zone 2: is an area in which a hazardous, potentially explosive atmosphere as a mixture of air and combustible gases, vapours or mists does not normally occur in normal operation, and if it does, then only rarely and for a short time.
Dusts

Zone 20: is an area in which hazardous, potentially explosive atmospheres prevail in the form of a cloud of combustible dust contained in the air continuously, over long periods of time or frequently.
Zone 21: is an area in which a hazardous, potentially explosive atmosphere can occur in the form of a cloud of combustible dust contained in the air occasionally in normal operation.
Zone 22: is an area in which a hazardous, potentially explosive atmosphere in the form of a cloud of dust contained in the air does not normally occur in normal operation, and if it does, then only rarely and for a short time.

Ignition protection classes for electrical equipment

Overview of ignition protection classes

General requirements for design, testing and marking of electrical equipment and components.
- Standard: IEC 60079-0 | EN 60079-0 | UL 60079-0
Increased safety “e”

This involves measures to prevent the possibility of impermissibly high temperatures and generation of sparks and arcs inside and on outer parts of electrical equipment in which these do not occur in normal operation through an increased level of safety.
- Symbol: e, eb – Zone 1 | ec – Zone 2
- Standard: IEC 60079-7 | EN 60079-7 | UL 60079-7
- Main application: terminals and junction boxes, control boxes for Ex-components (protected in another ignition protection class), squirrel cage motors, light fittings, installation materials, inductive ballasts, transformers
Flameproof enclosure “d”

Parts that can ignite an explosive atmosphere are housed in an enclosure that in the event of an explosion of an explosive mixture inside withstands the pressure and prevents transmission of the explosion to the atmosphere surrounding the enclosure.
- Symbol: da – Zone 0 | d, db – Zone 1 | dc – Zone 2
- Standard: IEC 60079-1 | EN 60079-1 | UL 60079-1
- Main application: switchgear and switchgear assemblies, control and signalling equipment, control systems, motors, transformers, heating equipment, light fittings, variable resistors
Pressurised enclosure “p”

The formation of an explosive atmosphere inside an enclosure is prevented by an ignition protection gas that keeps the pressure inside higher than that of the surrounding atmosphere; if necessary, the inside of the enclosure is supplied continuously with ignition protection gas to dilute combustible mixtures.
- Symbol: px, pxb – Zone 1, Zone 21 | py, pyb – Zone 1, Zone 21 | pz, pzc – Zone 2, Zone 22
- Standard: IEC 60079-2 | EN 60079-2 | UL 60079-2
- Main application: switch and control cabinets, analysers, large motors, slip ring and collector motors
- Old symbols for dust: pD21, pD22
Intrinsic safety “i”

The operating equipment used in potentially explosive atmospheres is equipped merely with intrinsically safe circuits. A circuit is intrinsically safe when sparks or thermal effects that can cause ignition of an explosive atmosphere do not occur under defined test conditions (which cover normal operation and certain failure conditions).
- Symbol: ia – Zone 0, Zone 20 | ib – Zone 1, Zone 21 | ic – Zone 2, Zone 22
- Standard: IEC 60079-11 | EN 60079-11 | UL 60079-11
  Intrinsically safety systems IEC 60079-25 | EN 60079-25 | UL 60079-25
- Main application: instrumentation and control technology, fieldbus technology, sensors, actuators
- [Ex ib] = associated electrical apparatus – installation in the safe area
- Old symbols for dust: : iaD = use in Zone 20, Zone 21, Zone 22 | ibD = use in Zone 21, Zone 22
Oil immersion “o”

Electrical equipment or parts of electrical equipment are immersed in a protective liquid (e.g. oil) such that an explosive atmosphere above the surface or outside the enclosure cannot be ignited.
- Symbol: o, ob – Zone 1 | oc – Zone 2
- Standard: IEC 60079-6 | EN 60079-6 | UL 60079-6
- Main application: sensors, display units, electronic ballasts, transmitters, transformers, capacitors, heating conductor connection boxes
Powder filling “q”

By filling the enclosure of an electrical apparatus with a fine-grained powder, it is prevented that an arc generated inside the enclosure during designated use can ignite an explosive atmosphere surrounding the enclosure. Neither ignition by flames or ignition by increased temperatures on the enclosure surface may occur.
- Symbol: q, qb – Zone 1
- Standard: IEC 60079-5 | EN 60079-5 | UL 60079-5
- Main application: sensors, display units, electronic ballasts, transmitters, transformers, capacitors, heating conductor connection boxes
Encapsulation “m”

The parts that could ignite an explosive atmosphere are embedded in casting compound such that the explosive atmosphere cannot be ignited.
- Symbol: ma – Zone 0, Zone 20 | mb – Zone 1, Zone 21 | mc – Zone 2, Zone 22
- Standard: IEC 60079-18 | EN 60079-18 | UL 60079-18
- Main application: Low-power switchgear, control and signalling equipment, display units, sensors
- Old symbols for dust: maD – Zone 20, Zone 21, Zone 22 | mbD – Zone 21, Zone 22
Ignition protection class “n”

Electrical equipment is not able to ignite a surrounding explosive atmosphere (in normal operation and under defined abnormal operating conditions)
- Symbol: nA, nAc – Zone 2 | nC, nCc – Zone 2 | nR, nRc – Zone 2
- Standard: IEC 60079-15 | EN 60079-15 | UL 60079-15
- Main application: All electrical equipment for Zone 2.
- nA – non-sparking apparatus | nC – sparking apparatus in which the contacts are protected in a suitable manner | nR – restricted breathing enclosure
Optical radiation “op”

Suitable measures are taken to avoid optical radiation igniting an explosive atmosphere.
- Symbol: op – Zone 0, Zone 20 | op – Zone 1, Zone 21 | Zone 2, Zone 22
- Standard: IEC 60079-28 | EN 60079-28
- Main application: op is – intrinsically safe optical radiation | op pr – protected optical radiation | op sh – optical radiation interlock
Protection by enclosure “t”

The enclosure is sealed tight enough to prevent penetration of dust or restrict penetration of dust to a safe level. Flammable apparatus can thus be built into the enclosure. The temperature at the enclosure must be limited and must not ignite the surrounding atmosphere.
- Symbol: ta – Zone 20 | tb – Zone 21 | tc – Zone 22
- Standard: IEC 60079-31 | EN 60079-31 | UL 60079-31
- Main application: switchgear and switchgear assemblies, control, junction and terminal boxes, motors, light fittings
- Old Symbols: tD A21 – under procedure A for Zone 21 | tD B21 – under procedure B for Zone 21

Ignition protection classes for non-electrical equipment

Overview of ignition protection classes

General requirements for design, testing and marking of non-electrical equipment and components.
- Standard: ISO 80079-36 | EN ISO 80079-36
Constructional safety “c”

In types of equipment that do not have an ignition source in normal operation established technical principles are applied to reduce the risk of mechanical failures that could lead to the development of flammable temperatures and sparks to a minimum.
- Symbol: hc
- Standard: ISO 80079-37 | EN ISO 80079-37
- Main application: couplings, pumps, gear drives, chain drives, conveyors, direct current motors
- Old symbol: c to EN 13463-5
Control of ignition source “b”

Sensors are built into the equipment so that potentially hazardous conditions can be detected and countermeasures initiated at an early stage of the disturbance. The measures applied can be initiated automatically by direct connections between the sensors and the ignition protection system or manually by emission of a warning to the operator of the equipment.
- Symbol: hb
- Standard: ISO 80079-37 | EN ISO 80079-37
- Main application: pumps, conveyors, gearboxes, mechanical seals
- Old symbol: b to EN 13463-6
Protection by liquid immersion “k”

Potential sources of ignition are rendered ineffective by immersion in a protective liquid or by constant wetting with a film of protective liquid.
- Symbol: hk
- Standard: ISO 80079-37 | EN ISO 80079-37
- Main application: submersible pumps, gearboxes
- Old symbol: k to EN 13463-8
Flameproof enclosure “d”

Parts that can ignite an explosive atmosphere are housed in an enclosure that in the event of an explosion of an explosive mixture inside withstands the pressure and prevents transmission of the explosion to the atmosphere surrounding the enclosure.
- Symbol: h
- Standard: IEC 60079-1 | EN 60079-1
- Main application: brakes, couplings
- Old symbol: d to EN 13463-3
Protection by enclosure “t”

The enclosure is sealed tight enough to prevent penetration of dust or restrict penetration of dust to a safe level. Flammable apparatus can thus be built into the enclosure. The temperature at the enclosure must be limited and must not ignite the surrounding atmosphere.
- Symbol: h
- Standard: IEC 60079-31 | EN 60079-31
- Main application: equipment exclusively for areas subject to dust explosion
Pressurised enclosure “p”

The formation of an explosive atmosphere inside an enclosure is prevented by use of an ignition protection gas to maintain a higher pressure inside the enclosure than that of the surrounding atmosphere. If necessary, the inside of the enclosure is supplied continuously with ignition protection gas to dilute combustible mixtures.
- Symbol: h
- Standard: IEC 60079-2 | EN 60079-2
- Main application: pumps