Explosion protection

Any assessment regarding the presence of explosion hazards, i.e. whether or not hazardous explosive atmospheres can develop, must be related to the individual case at hand. Explosions with hazardous consequences can occur when the following four preconditions are fulfilled simultaneously:

• High degree of dispersion of flammable substances

• Concentration of flammable substances in the air within their explosion limits

• Hazardous quantity of explosive atmosphere

• Effective source of ignition

The applicable EU directive regarding explosion protection is 99/92/EG on minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres.

As a basis for evaluating the extent of applicable requirements, areas exposed to explosion hazards are divided up into zones according to quality and probability of the development of hazardous explosive atmospheres (see table). Regarding flammable gases, vapours and mists, zones 0, 1 and 2 apply; for flammable dusts, zones 20, 21 and 22 apply.

Zone 0: Areas where an explosive atmosphere consisting of a mixture of air and flammable gases, vapours or mists is present permanently, over extended periods of time or frequently. Usually, conditions for zone 0 are only present inside of containers or installations (evaporators, reaction vessels etc.); under some conditions, however, they can be present close to openings for ventilation or other purposes.

Zone 1: Area where an explosive atmosphere consisting of a mixture of air and flammable gases, vapours or mists can develop occasionally in standard operation. This can include, among others: close surroundings of zone 0; immediate surroundings of infeed openings; immediate surroundings of highly fragile equipment or tubing made of glass, ceramics or the like, except for cases where the content is too insignificant to generate hazardous explosive atmospheres; immediate surroundings of insufficiently insulating compression glands, e.g. at pumps and slides; the inside of installations such as evaporators or reaction vessels.

Zone 2: Area where an explosive atmosphere consisting of a mixture of air and flammable gases, vapours or mists normally does not develop in standard operation, or short-term only. Zone 2 can include, among others: areas surrounding zones 0 or 1. Areas surrounding tubing carrying flammable substances exclusively in permanently technically sealed tubing, however, are not exposed to explosion hazards.

Zone 20: Areas where an explosive atmosphere consisting of a cloud of flammable dust contained in the air is present permanently, over extended periods of time or frequently. These conditions generally only appear on the inside of containers, tubing, equipment etc. This usually only includes the inside of installations (mills, dehumidifiers, mixers, conveyor pipes, silos etc.) where mixtures containing explosive dust in hazardous quantities can develop permanently, over extended periods of time or frequently.

Zone 21: Areas where an explosive atmosphere consisting of a cloud of flammable dust contained in the air can develop occasionally in standard operation. This includes, among others, areas immediately surrounding e.g. dust removal or filling stations as well as areas where dust deposits can form and occasionally generate explosive concentrations of flammable dust mixed with air in standard operation.

Zone 22: Areas where an explosive atmosphere consisting of a cloud of flammable dust contained in the air normally does not develop in standard operation, or short-term only. This can include, among others: areas surrounding installations containing dust, if dust is emitted through leakages and form deposits in hazardous quantities.

The selection of equipment for areas exposed to explosion hazards depends on the respective zone classification as well as the flammable substances’ temperature classes and explosion groups. Regarding flammable dusts, ignition and glow temperature must be considered.

Zones 0, 1 and 2 require specially tested and certified Ex luminaires. Luminaires intended for use in zone 20 (formerly zone 10 and zone 21), suitability must be identified in the type examination certificate as well as on the device.

Luminaires intended for use in zone 22 (formerly zone 11) should be labelled accordingly on the type plate. The manufacturer must specify the respective suitability, e.g. in manufacturer lists, as well as the operational surface temperature if it exceeds 80°C. Significant properties of such luminaires are:

• minimum protection ratings of IP5x for non-conductive and IP6x for conductive dusts;

• impact-resistant luminaire covers;

• limited surface temperatures. The temperature must not exceed 2/3 of the ignition temperature of the dust/air mixture in question. Surfaces which do not prevent hazardous deposits of dust capable of glowing from forming must not develop surface temperatures exceeding the glow temperature of the dust in question reduced by 75°C. Luminaires must be labelled with the maximum occurring surface temperature (control gear unit failure¹) if said temperature exceeds 80°C.

• For fluorescent luminaires, the use of electronic control gear (ECG) is recommended. When using inductive control gear, versions with thermal fuses should be used.