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ATEX Directive

ATEX directive 1999/92/EC is about minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres (ATEX).

Hydrogen is a flammable gas which can form an ATEX where mixed with air (such an ATEX is defined by the directive as a mixture in which, after ignition occurred, combustion spreads to the entire unburned mixture).

An employer who runs facilities where hydrogen is processed has the duty to meet the requirements of the directive, because his workers are potentially at risk from the effects of explosions which may be produced by ignition of an ATEX which can be formed.

The employer has a general obligation to provide protection to his workers potentially at risk from ATEX.

In order to provide this protection, the employer shall take technical and/or organisational measures appropriate to the nature of the operation, in order of priority and in accordance with the following basic principles:

  • the prevention of the formation of ATEX or, where the nature of the activity does not allow that,
  • the avoidance of the ignition of ATEX, and
  • the mitigation of the detrimental effects of an explosion so as to ensure the health and safety of workers.

The directive indicates the different obligations which the employer shall carry out, the main of them being detailed underneath.

In carrying out its obligations, the employer shall assess the specific risks arising from ATEX, taking into account at least of :

  • the likelihood that ATEX's will occur and their persistence,
  • the likelihood that ignition sources, including electrostatic discharges, will be present and become active and effective,
  • the installations, substances used, processes, and their possible interactions, the scale of the anticipated effects.

A place in which an ATEX may occur in such quantities as to require special precautions to protect the health and safety of the workers concerned is deemed to be hazardous within the meaning of this directive.

The employer shall classify hazardous places where ATEX's may occur into zones on the basis of the frequency and duration of the occurrence of an ATEX and in accordance with the following definition:

  • Zone 0 : a place in which an ATEX is present continuously or for long periods or frequently.
  • Zone 1: a place in which an ATEX is likely to occur in normal operation occasionally.
  • Zone 2: a place in which an ATEX is not likely to occur in normal operation but, if it does occur, will persist for a short period only.

The employer shall ensure that a document, hereinafter referred to as the "explosion protection document", is drawn up and kept up to date. The explosion protection document shall demonstrate in particular:

  • that the explosion risks have been determined and assessed,
  • that adequate measures will be taken to attain the aims of this directive,
  • those places which have been classified into zones as defined above.

The formation of a hydrogen-air ATEX is governed by the following characteristics :

  • the high ability of hydrogen to combust (a quite large explosion range and a rather low LEL (see ) can be seen as corresponding to this characteristic),
  • the very small size of the molecule, which explains the very low density of the gas (hydrogen is about 14,5 times lighter than air) and its high diffusion coefficient.

Hydrogen has explosivity characteristics which show a high reactivity:

  • its fundamental burning velocity is high (see ),
  • at some concentrations the mixture with air requires only a low ignition energy to react,
  • inside elongated geometries (tubes or pipelines!) a flame acceleration will rapidly occur and a DDT is very likely,
  • its combustion releases a high energy: its TNT equivalent is high (about 30g TNT for 1g of H2).

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Page last modified on December 03, 2008, at 12:08 PM