Warning: Explosion hazard – one spark is enough

To prevent an explosion, at least one of the factors that lead to ignition must be eliminated or minimised. This can be achieved in a number of ways, for instance by keeping the concentration of the combustible content low using extraction. Alternatively, the hazard can be eliminated completely, for instance by releasing combustible substances.

Eliminating ignition sources prevents hazardous explosive atmospheres from being ignited. This can be achieved by initially analysing potential sources of ignition. Open fires, flames or embers can ignite an explosive atmosphere; hot surfaces, mechanical or electrical sparks, and electrostatic discharge are also potential sources of ignition. Less obvious sources of ignition such as those caused by repair work or carelessly throwing away a cigarette butt must also not be forgotten. The required scope of protective measures is then determined and suitable equipment is selected. 

In some cases, the processes described above cannot be implemented reliably because it is impossible to avoid generating dust clouds or to eliminate certain sources of ignition. In these cases, measures that limit the hazardous effect of any ignited explosions to an insignificant extent must be implemented. "Constructive explosion protection" includes the use of explosion-proof structures, explosion pressure relief, explosion suppression and explosion barriers.

However, there is no "one size fits all" solution for dust explosion protection. No two systems are the same – they are uniquely designed to meet the specific needs of their operators and production. Analysing the potential hazards can be made simpler by classifying dust hazardous areas in zones. In this process, they are assessed according to the frequency and duration of the occurrence of hazardous explosive atmospheres. The higher the frequency and the longer the duration of the explosion, the stricter the requirements are for the devices used in the area. The definitions of the zones can be found in the following table.

Dividing dust hazardous areas into zones is a method that has been used to great success in explosion protection for gas atmospheres.

Equipment that is to be used in a hazardous area is classified using device categories according to the hazardous explosive zone. Devices in category 1 must guarantee a high degree of safety; this means that even if two errors occur on the device, no ignition can occur even if the faults occur rarely. These devices can be used in Zone 20. Equipment in category 2 must be designed so that it guarantees the required degree of safety in the event of frequently occurring faults that are expected to occur in normal operation. This equipment can be used in Zone 21. Devices which generate no sources of ignition can be used in Zone 22. These devices belong to category 3.Dividing dust hazardous areas into zones is a method that has been used to great success in explosion protection for gas atmospheres.

Technical measures must be implemented in order to ensure that no sources of ignition can arise in accordance with the safety-related key figures of an assumed explosive mixture. According to current standards, four types of protection for dust exist. These can be found in the table below. The "t" type of protection – protection by enclosure – is seen as the most important version by many experts. It is based on the principle of limiting the maximum surface temperature of the enclosure and restricting dust ingress through the use of dust-tight and dust-protected enclosures. The "p" type of protection – pressurised enclosure – enables "non-Ex"-approved devices to be used in hazardous areas and could, for instance, be used to protect cabinets in dust hazardous areas. The surrounding atmosphere is prevented from entering the enclosure of electrical equipment because an ignition protection gas is stored inside the enclosure at a pressure higher than that of the surrounding atmosphere. Intrinsically safe equipment with the "i" designation only contains intrinsically safe circuits. This refers to electrical circuits with very low energy levels, in which no sparks or thermal effects can cause a dust-air mixture to ignite. In the case of the "m" type of protection – encapsulation – parts that could ignite an explosive atmosphere as a result of sparks or heating are embedded in a compound to prevent the explosive atmosphere from being ignited.

Abbreviation	Principle	Type of protection	IEC/EN
t		Protection by enclosure	IEC/EN 60079-31
p		Pressurised enclosure	IEC/EN 60079-2
i		Intrinsic safety	IEC/EN 60079-11
m		Encapsulation	IEC/EN 60079-18

Types of protection for dust according to current standards.

Once the requirements for equipment to be used in dust hazardous areas have been determined, the equipment can be selected. This process requires careful attention. Only machines and components that have been developed for dust explosion protection and bear the corresponding equipment designations may be used. If equipment is to be used in an explosive atmosphere, its maximum temperature must not reach the ignition temperature of either the dust cloud or dust layer. Within this context, certain safety factors must be considered.