Technical Principles

Zone Classification

Hazardous areas are classified into zones to facilitate the selection of appropriate electrical apparatus as well as the design of suitable electrical installations. Information and specifications for the classification into zones are included in IEC 60079-10.

The greatest potential risk has to be taken into account when classifying the potentially explosive areas into zones and determining the necessary protective measures.

If there is no expert (skilled person) available in the company to verify the risk of explosion and to determine the necessary measures, it is recommended that a competent authority be turned to.

The equipment used in the defined hazardous zone must meet the requirements of the relevant assigned category.

Minimum Ignition Energy and Explosion Group

The minimum ignition energy is the minimum energy just sufficient to ignite the most ignitable mixture. This characteristic has to be considered when selecting the apparatus. The measured value of the minimum ignition energy is indicated for dusts. Gases are divided into explosion groups.

Explosion groups

Apparatus are divided into two groups:

• Group I:
  Electrical apparatus for mines endangered by firedamp
  
  
• Group II:
  Electrical apparatus for other places liable to be endangered by explosive atmospheres

In the case of electrical apparatus in Group I (mining), it is assumed that the only flammable gas that can occur is methane, but combined with coal dust. Other flammable gases, which can also occur in these areas, must be further classified as shown in Group II.

Electrical apparatus of Group II used in explosive gas atmospheres are further classified into explosion groups.

Classification criteria are the Maximum Experimental Safe Gap (MESG) and the “Minimum Ignition Current (MIC)”. The MESG and MIC are determined for the various gases and vapour according to a stipulated testing arrangement. The maximum experimental safe gap is the gap of the test apparatus with a width of flameproof joint of 25 mm at which an internal ignition of an explosive mixture is not propagated to the exterior (IEC 60079-1-1). The minimum ignition current relates to the minimum ignition current for laboratory methane.

An overview of the maximum experimental safe gaps and minimum ignition currents for the various explosion groups is shown in following table.

Explosion Groups:

*rel. to methane = 1

The dangerousness of the gases increases from explosion group IIA to IIC. The requirements for the electrical apparatus increase accordingly to these explosion groups. For this reason, the marking of the electrical apparatus must show to which explosion group it belongs. Electrical apparatus approved for IIC may also be used for all other explosion groups.

Ignition Temperature and Temperature classes

The ignition temperature of a flammable gas, vapour, or combustible dust is the lowest temperature of a heated surface at which the gas/air or vapour/air mixture ignites. It represents virtually the lowest temperature at which a hot surface can ignite a respective explosive atmosphere.

Flammable gases and vapours are classified into temperature classes according to their inflammability. The maximum surface temperature of electrical apparatus should always be lower than the ignition temperature of the gas/air or vapour/air mixture in which it is used. Of course, equipment classified in a higher temperature class (e.g. T5) may also be used for applications in which a lower temperature class is required (e.g. T2 or T3). In North America there is a system incorporating further classification according to temperature subclasses.

Combustible dusts are not divided into temperature classes. The minimum ignition temperature of the dust cloud has to be compared with the max. surface temperature of the apparatus. In doing so, a safety factor has to be considered. The max. surface temperature of the apparatus must not exceed 2/3 of the minimum ignition temperature of the dust cloud. Since dust can also deposit on apparatus, the minimum ignition temperature of the dust layer must also be taken into account. This temperature is the lowest temperature of a hot surface on which a dust layer of 5 mm can be ignited. The max. surface temperature of the apparatus has to be adjusted using a safety factor of 75 K. The thicker the layer, the higher the heat insulation. For this reason, the dust layer is already ignitable at low temperatures, which is why a reduced surface temperature is admitted on the apparatus. It is determined according to the scheme (fig. 4) (EN 61241-14).

Determination of the max. Surface Temperature of Dust Layers of 5 mm to 50 mm

If the layer is thicker than 50 mm, the ignition temperature has to be determined by laboratory tests. This applies also to layers thicker than 5 mm when the ignition temperature at 5 mm is lower than 250 °C. Laboratory testing is also necessary when the apparatus are completely covered with combustible dust.

Types of Protection

Only explosion protected equipment may be used in areas in which an explosive atmosphere may still be expected despite the implementation of prevention measures. Electrical, explosion protected equipment can have various types of protection according to the construction regulations of the standards series EN 60079, former EN 50014 and following. If electrical equipment shall be used in areas with combustible dust, the standards series EN 61241 is applicable. The type of protection employed by the manufacturer depends mainly on the kind and function of the apparatus. Various safety levels exist for some types of protection. These correspond to the equipment categories as defined in the 94/9/EC Directive. The Ex ia version relative to intrinsic safety can be classified as category 1. It can be installed in Zone 0. The Ex ib version corresponds to category 2 which suits Zone 1. From a safety point of view, all standardized types of protection should be seen as being equal.

Types of Protection for Electrical Apparatus in Explosive Gas Atmosphere, Part 1:

Type of protection in accordance with IEC, EN, UL, FM and NFPA	Representation (diagram)	Basic principle	Main application
Increased safety “e” EN 60079-7 UL 60079-7 IEC 60079-7 FM 3600		Additional measures are applied to increase the level of safety, thus preventing the possibility of excessive temperatures and the occurance of sparks or electric arcs within the enclosure or on exposed parts of electrical apparatus, where such ignition sources would not occur in normal service.	Terminal and connection boxes, control boxes for installing Ex-components (which have a different type of protection), squirrelcage motors, light fittings
Flameproof enclosure “d” EN 60079-1 UL 60079-1 IEC 60079-1 FM 3600		Parts which can ignite a potentially explosive atmosphere are surrounded by an enclosure which withstands the pressure of an explosive mixture exploding inside the enclosure, and prevents the transmission of the explosion to the atmosphere surrounding the enclosure.	Switchgear and control gear and display units, control systems, motors, transformers, heating equipment, light fittings
Intrinsic Safety “i” EN 60079-11 UL 60079-11 IEC 60079-11 FM 3610		Apparatus used in a potentially explosive area contain intrinsically safe electric circuits only. An electric circuit is intrinsically safe if no sparks or thermal effects produced under specified test conditions (which include normal operation and specific fault conditions) is not capable of causing ignition of a given explosive atmosphere.	Measurement and control technology, communication technology, sensors, actuators ia = use in Zone 0, 1, 2 ib = use in Zone 1, 2 [Ex ib] = associated apparatus – installation in safe area
Protected by enclosures “tD” EN 61241-1 IEC 61241-1		Thanks to the tightness, dust cannot ingress the apparatus at all or its quantity is limited to a safe degree. For this reason, ignitable apparatus can be mounted into the enclosure. The surface temperature of the enclosure must not ignite the surrounding atmosphere.	Switchgear and control gear, control, connection, and terminal boxes, motors, light fittings td A21 = according to method A for Zone 21 td B21 = according to method B for Zone 21