Secondary explosion protection - avoiding effective ignition sources

Typical sources of ignition according to EN 1127-1 include:

• Hot surfaces
• Flames and hot gases
• Mechanically generated sparks
• Electrical installations
• Electrical compensating currents
• Static electricity
• Lightning
• Electromagnetic fields
• Optical radiation
• Ionising radiation
• Ultrasound
• Adiabatic compression
• Chemical reactions

All sources of ignition will always generate energy, either in the form of heat or in the form of sparks. The sources of ignition are assessed based on the likelihood of an explosive atmosphere forming.

In Zone 0 and Zone 20, sources of ignition that may be continuously, frequently, occasionally or rarely present must be eliminated. In Zone 1 and Zone 21, sources of ignition that may be continuously, frequently or occasionally present must be eliminated. In Zone 2 and Zone 22, sources of ignition that may occur during normal operation of the devices must be eliminated.

The minimum ignition temperature and minimum ignition energy of the combustible materials are important parameters to consider when assessing whether sources of ignition are effective or not.

Examples: Minimum ignition temperature of combustible gases and vapours. 
 

Examples: Minimum ignition energy of combustible gases and vapours. 

For instance, hydrogen has an ignition temperature of 560 °C. This means that this substance will ignite on contact with a hot surface if the temperature of the surface is at least 560 °C. In contrast, the ignition energy of hydrogen is very low, which makes this substance dangerous, especially in the vicinity of electrically or mechanically generated sparks.

Examples: Minimum ignition temperature of combustible dusts. 
 

Examples: Minimum ignition energy of combustible dusts. 

The ignition temperature of the dust cloud and that of the dust layer must be compared with the maximum permissible surface temperature of the equipment. When doing so, safety factors must be taken into consideration. The maximum surface temperature of the device must only be two-thirds of the dust cloud ignition temperature; similarly, the maximum surface temperature of the device cannot be higher than the ignition temperature of the dust layer minus a safety allowance (75 °C for a 5-mm dust layer).