HAZARDOUS AREA CLASSIFICATION

 HAZARDOUS AREA CLASSIFICATION

A Hazardous Area is defined by three main criteria, these being:

1. The type of hazard (groups)

2. The auto-ignition temperature of the hazardous material (temperature or “T” rating)

3. The likelihood of the hazard being present in flammable concentrations (zones)

Type of Hazard:

Type of hazard will be in the form of either a gas or vapour or a dust or fibre.

The classification of these hazardous zones is divided into two groups depending on whether it is in a mining or above surface industry. These are defined below:

Group I -electrical equipment for use in mines and underground installations susceptible to firedamp Group II and Group III -electrical equipment for use in surface installations.

Groups II gases are grouped together based upon the amount of energy required to ignite the most explosive mixture of the gas with air. 

Groups  III dusts are subdivided according to the nature of the explosive atmosphere for which it is intended.

AUTO IGNITION TEMPERATURE OR “T” RATING

The hazard level of the gases increases from gas group IIA to IIC with group IIC being the most severe. Substances in this group can be ignited very easily with Hydrogen being the most at risk to ignition. 

Gases/Dusts are divided in sub groups:

IIC-   Gases     Hydrogen, Acetylene, Carbon Disulphide

IIC-   Dusts     Conductive dusts          (Graphite Powder, Toner)

IIB-   Gases     Ethene, Dimethyl ether

IIB-   Dusts     Non-conductive dusts (Milk Powder, Powdered Sugar)

IIA-   Gases     Propane, Petrol, Ethanole, Acetone, Acetaldehyde

IIA-   Dusts     Fibers and Flyers (Tobacco, Saw Dust)

The Likelihood of Hazard

The likelihood of the hazard being present in a concentration high enough to cause an ignition will vary from location to location. For most installations this risk is dependent upon how long the hazard (gas, vapour, dust or fibre) is present.These areas are referred to as Zones. For gases and vapours and dusts and fibres there are three zones.

Zone 0/20 - A hazardous atmosphere is highly likely to be present and may be present for long periods of time (>1000 hours per year) or even continuously.

Zone 1/21 - A hazardous atmosphere is possible but unlikely to be present for long periods of time (>10 <1000 hours per year).

Zone 2/22 - A hazardous atmosphere is not likely to be present in normal operation or infrequently and for short periods of time (<10 hours per year).