Selection Criteria

If you plan to use electrical equipment in areas where there is the potential for an explosive atmosphere to form, you need to be sure that the equipment selected is suitable to prevent ignition of that explosive atmosphere.

The integrity of all explosion protection for electrical equipment is dependent upon it being suitable for the explosive atmosphere that could form, is correctly installed, inspected, tested, operated and maintained in compliance with engineering regulations and standards.

To determine the protection characteristics required for your installation it is important to know 4 key attributes of the area in which equipment is to be installed:

  • The probability of the presence of a flammable atmosphere – THE ZONE
  • The lowest ignition temperature of the substances that could be present – THE T- CLASS
  • The lowest ignition energy of the substances that could be present – THE ‘GAS or DUST GROUP’
  • The required level of protection against solids and water.

The first three of those will be determined by an area classification assessment. ATEX directive 1999/92/EC requires that employers conduct an area classification assessment of the likelihood of the presence of a flammable atmosphere and the nature of the installation, substances used, processes and their interactions. Such an area classification should be conducted by suitably competent persons experienced in the application of the requirements of IEC 60079 part 10 and Energy Institute Model Code of Safe Practice EI-15

Likelihood of Presence of Explosive Atmosphere

Areas where flammable substances are present are classified into Zones according to the probability of an explosive atmosphere being formed.There are 6 Zones, 3 for Gas and 3 for Dust as below:

Zone 0:
place in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas or vapour is present continuously or for long periods or frequently

Zone 20:
area in which an explosive atmosphere in the form of a cloud of dust in air is present continuously, or for long periods or frequently

Zone 1:
place in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas or vapour is likely to occur in normal operation occasionally.

Zone 21:
area in which an explosive atmosphere in the form of a cloud of dust in air is likely to occur, occasionally, in normal operation

Zone 2:
place in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas or vapour is not likely to occur in normal operation but, if it does occur, will persist for a short period only

Zone 22:
area in which an explosive atmosphere in the form of a cloud of dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only
 

Ignition Energy and Explosive Energy of the Explosive Atmosphere

Some protection types are designed to contain an internal ignition of an explosive atmosphere without allowing it to ignite the atmosphere outside the enclosure or to reduce the energy present in the system to a level below that required to cause an ignition. These types require consideration of the intensity of the internal explosion that would occur if there were to be an ignition or the energy levels required to ignite the substance present.

Gases, vapours and dusts are grouped into Subdivisions or ‘Gas/Dust Groups’ I, II (IIA, IIB, IIC), IIIA, IIIB and IIIC according to the energy needed to ignite the atmosphere and the intensity of the explosion produced. The table below shows some examples.

Gas/Dust Groupings
Group Example Gas/Dust
I (mining) methane (firedamp)
IIA industrial methane, propane, aviation fuel
IIB ethylene, hydrogen sulphide
IIC hydrogen, acetylene, carbon disulphide
IIIA combustible flyings
IIIB non-conductive dust
IIIC conductive dust

Equipment certified for a particular group can also be installed in areas requiring a less onerous certification as shown below:
 

Location gas/vapour or dust group Permitted equipment group
I I
IIA II, IIA, IIB or IIC
IIB II, IIB or IIC
IIC II or IIC
IIIA IIIA, IIIB or IIIC
IIIB IIIB or IIIC
IIIC IIIC

Note: Equipment is also available certified for IIB+H2. This is certified for all IIB gases and also for Hydrogen
 

Ignition Temperature of the Explosive Atmosphere

Different flammable materials have different temperatures at which they will ignite. The design and construction of equipment must be such that, even under fault conditions, no surface ever reaches the temperature at which the explosive atmosphere will ignite and sustain combustion. A system of temperature ratings, often referred to as T-Class or T-Rating, identifies the certified maximum surface temperature of equipment.

Temperature Rating

Temperature Rating (often referred to as T rating or temperature class)
 

T-Rating required T rating Ignition temperature range of the explosive gas-air mixture Maximum surface temperature of equipment
T1 greater than 450ºC 450°C 450°C
T2 between 301°C and 450°C 300°C 300°C
T3 between 201°C and 300°C 200°C 200°C
T4 between 136°C and 200°C 135°C 135°C
T5 between 101°C and 135°C 100°C 100°C
T6 between 86°C and 100°C 85°C 85°C

 

Equipment certified for a particular T-class can also be installed in areas requiring a less onerous T-Class as shown below:
 

T-Rating required Permitted Equipment T-Rating
T1 T1,T2,T3,T4,T5 or T6
T2 T2,T3,T4,T5 or T6
T3 T3,T4,T5 or T6
T4 T4,T5 or T6
T5 T5 or T6
T6 T6