Explosive atmosphere sign (Ex)

How to read it

The sign is set out in Annex III of the EU’s ATEX workplace directive, 1999/92/EC: a triangle with black letters on a yellow background and black edging, the yellow at least half the sign’s area. The directive’s drawing has the letters EX. Under Article 7, places where explosive atmospheres may occur in amounts that endanger workers are marked with the sign at their points of entry, where necessary. The directive defines an explosive atmosphere as a mixture with air, under atmospheric conditions, of flammable gases, vapours, mists or dusts in which, once ignited, combustion spreads to the entire unburned mixture. Great Britain has the same sign in Schedule 4 of the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR), which the Health and Safety Executive (HSE) says substantially reproduces the directive’s annex. Member states may add other explanatory text to the sign.

Also searched as: ATEX sign, DSEAR sign, EX in a triangle, explosive atmosphere symbol, EX sign, Ex symbol.

The zones behind the sign

Places with an explosive atmosphere are classified into zones by how often and how long the atmosphere is present. Under DSEAR, zones 0, 1 and 2 are for gas, vapour or mist. In zone 0 the explosive atmosphere is present continuously, for long periods or frequently; in zone 1 it is likely to occur occasionally in normal operation; in zone 2 it is not likely in normal operation and, if it does occur, persists for a short period only. Zones 20, 21 and 22 are the same three levels for a cloud of combustible dust in air.

Where the sign goes

HSE’s approved code of practice for DSEAR (L138) describes the sign at points of entry, alerting people to places where an explosive atmosphere may occur and where special precautions apply. Its examples include signs facing outward on a site’s boundary and at fire service entry points, and signs that add to painted lines on the ground around large open-air plant.

Look-alikes

On equipment, an Ex inside a hexagon is a different mark. HSE says equipment built for use in explosive atmospheres carries the explosion protection symbol in a hexagon, with an equipment category number (1, 2 or 3) and the letter G for gas or D for dust, or both. The hexagon marks equipment made for such places; the yellow triangle marks the place itself. The British regulations also have a separate warning triangle for explosive material, and on a chemical’s label explosives carry the GHS01 pictogram, the exploding bomb.