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ATEX – Basic principles: let’s get some clarity

ATEX – Basic principles: let’s get some clarity

The term “ATEX” derives from the words ATmosphères EXplosives, – explosive atmosphere. An explosive atmosphere is defined as a chemical reaction that occurs in the presence of:

  • flammable substances like gases, vapors, mists or dust,
  • Oxygen (air)
  • Ignition source (such as sparks, electric arcs, electrostatic and atmospheric discharges, electromagnetic waves, flames, high surface temperatures …).

The above can be effectively represented in a graphical manner by the well-known triangle, which indicates the three conditions necessary for the explosive reaction to occur. If one of these 3 factors is missing, the explosion cannot take place.

An atmosphere capable of transforming into an explosive atmosphere due to the conditions mentioned above is called a potentially explosive atmosphere. The products based on ATEX directive are intended to be applied in this type of atmosphere.

Like the previous ones, also the current ATEX directive, 2014/34/UE, divides products into groups and categories.

Group I includes products for underground work in mines.

Group II includes devices intended for use in surface environments where an explosive atmosphere is likely to occur.

The ATEX Directive also classifies products into categories, in relation to the level of protection.

The mine products are divided into 2 categories:

category M1: equipment or systems that guarantee a very high level of protection;

category M2: equipment or systems that guarantee a high level of protection; they must be able to be switched off in the presence of gas.

For group II there are 3 categories, depending on the level of protection (area of use); these categories are followed by the letter G (Gas) or D (Dust).

  • category 1: equipment or systems that guarantee a very high level of protection;
  • category 2: equipment or systems that guarantee a high level of protection;
  • category 3: equipment or systems that guarantee a normal level of protection.

For some protection modes, group II is divided into IIA, IIB, IIC. This subdivision classifies the gases against which the user must protect himself by using a product with an adequate protection system.

Some examples of gases:

  • IIA group: propane,
  • group IIB: ethylene,
  • IIC group: hydrogen / acetylene.

There is also a further classification of hazardous areas, depending on the frequency and time of presence of the explosive substance.

GAS

Zone 0 place in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapour or mist is present continuously or for long periods or frequently

Zone 1 place in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapour or mist is likely to occur in normal operation occasionally

Zone 2 place in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapour or mist is not likely to occur in normal operation but, if it does occur, will persist for a short period only

DUST

Zone 20 place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods or frequently

Zone 21 area in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur, occasionally, in normal operation

Zone 22 area in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only

Furthermore, there are different types of protection methods depending on the solution the device pursues to contain the explosion.

  1. Ex d – Flameproof: In this method, the energized electrical circuits can be in contact with the explosive atmosphere. However, they have to be enclosed within an enclosure specially designed to withstand the pressure caused by a possible explosion occurred inside the case itself and to prevent the spread of flames outside the enclosure that could ignite the external explosive atmosphere. The main feature is a strong and robust construction, which guarantees a high reliability for a long time.
  2. Ex e – increased safety: This protection method applies measures in order to prevent the formation of arcs, sparks or temperatures which can ignite the explosive mixture, therefore guaranteeing a high safety coefficient. The equipment is designed to prevent sparks even in irregular operating conditions.
  3. Ex n – simplified: This protection method uses other protection methods in a simplified way. It’s application to electrical equipment makes them unable to cause the ignition of an explosive atmosphere even during the normal operation. It can be used only in Zone 2. The equipment is divided into “non-sparking” and “sparkling” equipment that can produce sparks, arcs and cause high surface temperatures during normal operation. Based on this classification, it will therefore be possible to have the Ex nA method for non-sparking equipment, Ex nC method for sparking equipment, and Ex nR method for rrestricted breathing enclosures.
  4. Ex i – Intrinsic Safety. In this protection method the electrical components are of such a limited power that, in any operating condition, the explosion cannot occur.
  5. Ex-m, Ex-p, Ex-q, Ex-o- isolative principle of the explosion. In these protection methods the electrical materials that could cause the explosion are insulated in order to make the explosion impossible.

Explosion-proof equipment can also be classified according to the temperature class, which is linked to the ignition temperature of the dangerous substances present in the environment.

The surface temperature of the electrical equipment must not exceed the ignition temperature of these substances.

The aim of this article is to state the main ATEX principles to clarify some important issues that represent the starting point for the design and production of efficient explosion-proof products, able to guarantee the safety of the plant and of the people who work there. For Nuova ASP, security is a continuous spur for research and development of increasingly reliable solutions.

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