Three factors

Flammable material

Flammable material can be either gaseous, a vapour from liquid or solid. For a general discussion relevant to work places, their reactivity with atmospheric oxygen is considered.

Flammable gases

A flammable gas may be an element such as hydrogen which can be made to react with oxygen with very little additional energy. Flammable gases are often compounds of carbon and hydrogen. These flammable gases and vapours require only small amounts of energy to react with atmospheric oxygen. A vapour is the proportion of a liquid - if talking about the explosion protection of flammable liquids - which has evaporated into the surrounding air as the result of the vapour pressure above the surface of the liquid, around a jet of that liquid or around droplets of the liquid. Mist is a special type, which because of its explosion behaviour, can be included with the vapours, for the purposes of fulfilment of safety considerations.

Flammable liquids (actually the vapour only)

Flammable liquids are often hydrocarbon compounds such as ether, acetone or petroleum spirit. Even at room temperature, sufficient quantities of these can change into the vapour phase so that an explosive atmosphere forms near their surface. Other liquids form such an atmosphere near their surface only at increased temperatures. Under atmospheric conditions this process is strongly influenced by the temperature of the liquid. For this reason the flash point, or rather the flash point temperature, is an important factor when dealing with flammable liquids. The flash point relates to the lowest temperature at which a flammable liquid will, under certain test conditions, form a sufficient quantity of vapour on its surface to enable an effective ignition source to ignite the vapour air mixture. The flash point is important for the classification of potentially explosive atmospheres. Flammable liquids with a high flash point are less dangerous than those with a flash point at room temperature or below.When spraying a flammable liquid, a mist can form consisting of very small droplets with a very large overall surface area, as is well-known from spray cans or from car paint spraying stations. Such a mist can explode. In this case the flash point is of lesser importance. For a fine mist - made from a flammable liquid - the behaviour relevant to safety can be roughly derived from the known behaviour of the vapour.

Flammable solids (actually dust only)

Flammable solids in the form of dust or flyings can react with atmospheric oxygen and produce disastrous explosions. Normally more energy is required for activating the explosion in air than with gases and vapours. However, once combustion starts, the energy released by the reaction produces high temperatures and pressures. In addition to the chemical properties of the solid itself, the fineness of the particles and the overall surface area, which increases with increasing fineness, play an important role. The properties be determined by processes which take place immediately at the surface of the solid particles. Igniting and extinguishing a paraffin wax candle provides a demonstration of a series of processes undergone by a solid material within a short period of time which cannot easily be presented in a simplified form. An experiment shows that when the wick of a candle is lit, the paraffin wax melts and then vaporises and that this vapour feeds the flame. After extinguishing the candle, the paraffin vapour can still be smelled, the melted paraffin wax solidifies and the paraffin vapours disperse. Now the paraffin wax candle is once again a harmless object. Dust reacts very differently, depending on whether it is in a deposited layer or whether it is in a swirled dust cloud. Dust layers are liable to begin smouldering on hot surfaces, while a dust cloud which has been ignited locally or through contact with a hot surface can explode immediately. Dust explosions are often the consequence of smouldering dust layers which become swirled up and already carry the ignition initiation. When such a layer is stirred up, for example by mechanical cleaning methods during transportation or inappropriate extinguishing attempts, this can lead to a dust explosion. A gas or vapour/air explosion can also swirl up the dust, which then often turns from the first, the gas explosion, into the second, the dust explosion. In deep coal mines methane/firedamp explosions often have triggered off coal dust explosions whose consequences were more serious than those of the original firedamp explosion.


The quantity of oxygen available in the air can only oxidise/burn a certain quantity of the flammable material. The ratio can be determined theoretically, it is called the stoichiometric mixture. When the quantity of the flammable material and the available atmospheric oxygen are near to the optimum (most ideal) ratio, the effect of the explosion - temperature and pressure increase - is most violent. If the quantity of flammable material is too small, combustion will only spread with difficulty or will cease alltogether. The situation is similar when the quantity of flammable material is too great for the amount of oxygen available in the air. All flammable materials have their explosive range, which also depend on the available activation energy. This is usually determined by igniting the mixture with an electric spark. The explosive range is bounded by the lower flammable (previous referred to as explosive) limit and the upper flammable (previous referred to as explosive) limit. This means that below and above these limits, explosions will not happen. This fact can be utilised by sufficiently diluting the flammable substances with air or by preventing the ingress of air/oxygen into parts of the equipment. The latter option is, however, not or only with restrictions possible in environments where people regularly work (inerting means danger for suffocation) and must therefore be reserved for technological equipment only.

Sources of ignition

With the use of technical equipment a large number of ignition sources are possible. In the following overview the numbers given behind the ignition sources refer to the appropriate clauses of the basic standard: EN 1127-1: 2019 “Explosive atmospheres - Explosion prevention and protection- Part 1: Basic concepts and methodology.”

Hot surfaces (5.1)
arise as a result of energy losses from systems, equipment and components during normal operation. In the case of heaters they are desired. These temperatures can usually be controlled. In the event of a malfunction - for example with overloading or seized bearings - the energy loss, and therefore the temperature, increases unavoidably. Technical equipment must always be assessed as to whether it is stabilizing - for example whether it can attain a final temperature, or whether non-permissible temperature increases are possible which need to be prevented by taking appropriate measures. Examples: coils, resistors or lamps, hot equipment surfaces, brakes or overheating bearings
Flames and hot gases (including hot particles) (5.2) 
can occur inside combustion engines or analyser equipment during normal operation and when a malfunction has occurred. Protective measures are in such case required which are able to permanently prevent them from leaving the enclosure. Examples: exhausts from internal combustion engines or particles which are formed by the switching sparks of power switches eroding material from the switch contacts
Mechanically generated sparks (5.3)
are produced for example by grinding and cutting devices during normal operation and are therefore not permitted in a potentially explosive atmosphere. Cracks in rotating parts, or parts sliding over each other without sufficient lubrication or similar situations can generate such sparks when malfunctioning. Specific requirements to the materials used to produce enclosures serve to reduce the risks from such ignition sources. Examples: tools such as a rusty hammer and chisel in contact with light alloys or the metal fork of a fork lift truck
Electrical equipment and components (5.4)
must normally be regarded as a sufficient ignition source. Only very low energy sparks with energies of only a few micro Joules (= micro Watt seconds) may be regarded as too weak to start an explosion. For this reason, suitable measures must be adopted to prevent these ignition sources. Examples:switching sparks, sparks at collectors or slip rings
Stray electric currents, cathodic corrosion protection (5.5)
which then may result in a potential difference between different earthing points. This is why a highly conductive connection to all the electrically conductive parts of the equipment must be provided so that the potential difference is reduced to a safe level. It is not relevant whether the conductive equipment is electrical or non-electrical parts of the installation, as the cause of the current may be found outside of the equipment. An equipotential bonding shall always be provided, irrespective of whether or not such currents are expected or whether its sources are known. Examples: Electric railways and other earthed voltage supplies for example for electric corrosion protection of equipment
Static electricity (5.6)
Independently of whether or not there is an electrical voltage supply, electrical sparks can be caused by static discharges. The stored energy can be released in the form of sparks and function as an ignition source. Because this ignition source can arise quite independently of an electrical voltage supply, it must also be considered with non-electrical devices and components. It is connected with separation processes; therefore these cases must be assessed where this ignition source needs to be taken into account. Friction during normal operation can be the cause of electrostatic charging. For example, portable devices cannot - due to their portability - either be earthed or connected to potential equalization. When interacting with the clothes of the user, static charging can occur during normal operation. Static electricity must be prevented from becoming an ignition source by taking appropriate measures. Examples: Transmission belts made from plastic materials, enclosures of portable devices, synthetic clothing material. Separation processes when rolling out paper or plastic film, plastic transport tubing systems
Lightning (5.7)
and the impact of lightning can result in the ignition of an explosive atmosphere. Lightning always results in the ignition of an explosive atmosphere, so there is a need for lightning distraction. However, there is also a possibility of ignition due to the high temperature reached by lightning distraction routes. Large currents flowing from where the lightning strikes can produce sparks in the vicinity of the point of impact.
Radio frequency (RF)
electromagnetic waves from 104 kHz to 300 GHz are not the only ignition sources where radiation energy enters the explosive mixture, the following needs to be listed:
Electro-magnetic radiation - radio RF waves (5.8),
Electro-magnetic radiation - IR, visible and UV light (5.9),
Ionising radiation - röntgen and gamma (5.10),
Ultrasonic (5.11).
Systems, devices and components that use radiation may be set up and operated in the Ex area if their parameters are limited permanently and reliably and this equipment is checked. Examples: transmitting and receiving equipment, mobile telephones, photoelectric barriers and scanners
Adiabatic compression and shock waves (5.12) inside tube-shaped structures operated at negative pressure can also become a source of ignition. Examples: transport tubes with narrow passage, breakage of a long fluorescent tube in a hydrogen/ air atmosphere
Exothermic reactions (5.13)
are together with self-ignition of dusts the finally defined possible type of ignition sources.


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