Introduction

In chemical and petrochemical industries, industrial processes, oil platforms, military installations, etc., there are substances which are stores, processed, or produced in areas where an explosive atmosphere could be generated.

In these cases it is necessary to take preventive measures to reduce the risk of these materials exploding.

These preventive measures are based on three principles which should be applied in the following order:

• Substitution
• Control
• Reduction

Substitution means replacing flammable material with other materials that are less so.

Control means reducing the amount of flammable materials, avoiding or minimising or controlling leaks, stopping the formation of an explosive atmosphere, collecting and confining leaks, avoiding ignition sources, etc.

Reduction means reducing the number of people exposed, taking measures to avoid the propagation of the explosion, reducing or suppressing the pressure of the explosion, providing protective equipment to personnel, etc.

Once the substitution and control principles have been applied, any remaining dangerous locations will be classified in areas according to the likelihood that an explosive atmosphere is present.

Said classification allows the protection levels of the material to be determined and therefore what the suitable protection methods are for each place.

For an explosion to happen there needs to be an explosive atmosphere and an ignition source.

The purpose of protective measures in the Electricfor ATEX resistors is to reduce to an acceptable level the likelihood that they might become a source of ignition, both due to their surface temperature or electric arcs.

Definition of an explosive atmosphere Acc. Directive

According to Directive 2014/34/UE, an explosive atmosphere is defined as a mixture with air, under atmospheric conditions, of flammable substances the form of gases, vapours, mists, or dusts where, after ignition, combustion spreads through the entire unburned mixture.

According to the standard, normal atmospheric conditions exist when:

• The temperature is within the range -20° C and +40° C
• The pressure is within the range 0.8 bar to 1.1 bar
• The air's oxygen content is normal (usually 21%)

The use of electrical equipment in atmospheric conditions outside this range requires special consideration and may require further evaluation and testing.

The REX range of Electricfor has been certified to work between a range of ambient temperature of  -40 ºC ≤ Ta ≤ +60 ºC.

Material groups Acc. Directive

The electrical equipment of group I is intended for use in mines susceptible to firedamp.

The electrical equipment of group II is intended for use in places with an explosive gas or dust atmosphere other than mines susceptible to firedamp. We assimilate it to industry.

Category of equipments Acc. Directive

• Category 1: Equipment designed to ensure a very high level of protection and intended for use in areas in which explosive atmospheres caused by mixtures of air and gases, vapours or mists or by air/dust mixtures are present continuously, for long periods or frequently.

• Category 2: Equipment designed to ensure a high level of protection and intended for use in areas in which explosive atmospheres caused by gases, vapours, mists or air/dust mixtures are likely to occur occasionally.

• Category 3: Equipment designed to ensure a normal level of protection and intended for use in areas in which explosive atmospheres caused by gases, vapours, mists, or air/dust mixtures are unlikely to occur or, if they do occur, are likely to do so only infrequently and for a short period only.

Gas groups Acc. standard EN 60079-0

The electrical equipment of group II (Industry) is subdivided according to the nature of the explosive gas atmosphere for which it is intended

This subdivision is based on the maximum experimental safe gap (MESG) or the ratio of minimum ignition current (MIC ratio) of the explosive gas atmosphere in which the material can be installed.

Examples of classifications of a few representative gases:

	IIA	IIB	IIC
T1	Propane, methane, ethane, benzene, ammonia, acetic acid, carbon monoxide, methanol, toluene	Acrylonitrile	Hydrogen
T2	Ethanol, amyl acetate, butane	Ethylene	Acetylene
T3	Gas oil, hexane	Sulfuric acid
T4	Acetaldehyde	Diethyl ether
T5
T6			Carbon disulphide

The material labelled IIC is also suitable for applications requiring material from groups IIA and IIB.

The material labelled IIB is suitable for applications that require material from group IIA.

Dust groups Acc. standard EN 60079-1

The electrical equipment of group II (Industry) is subdivided according to the nature of the explosive dust atmosphere for which it is intended

IIIA	IIIB	IIIC
Fibres	Non-conducting dust	Conductive dust

Material labelled IIIC is also suitable for applications requiring material from groups IIIA and IIIB.

Material labelled IIIB is suitable for applications requiring group IIIA material

EPL (Equipment Protection Level) Acc. standard EN 60079-0

This is the level of protection assigned to the material according to its risk of becoming an ignition source. It also distinguishes between the explosive gas atmospheres, explosive dust atmospheres and explosive atmospheres in mines susceptible to firedamp

• Mines: Ma and Mb

• Gases: Ga, Gb, and Gc

• Dusts: Da, Db, and Dc

Classification of zones Acc. standard  EN 60079-10

The explosive atmospheres are classified in zones. The classification into zones depends on the temporal and spatial probability of a hazardous explosive atmosphere appears.

Gases	Dusts	Criterion
Zone 0	Zone 20	Presence of the explosive atmosphere continuously or for long periods or frequently
Zone 1	Zone 21	Presence of the explosive atmosphere occasionally
Zone 2	Zone 22	Presence of the explosive atmosphere is not likely to occur in normal operation but, if it does occur, will persist for a short period only

Classification of equipment

The following table illustrates the interaction of the various concepts outlined above.

You can always use a level of protection greater than that required for the area

Protection modes

Protection modes are constructive and electrical measures taken in the material to protect against explosions in potentially explosive atmospheres.

 Relationship between modes of protection and the EPLs