The use of electrical instruments and control devices in certain areas where explosion hazards are present carries with it the potential for disaster unless specific preventive measures are taken. Hazards exist in the form of escaped flammable gases such as acetylene, hydrogen, propane and others. Various kinds of dusts – metal, coal, flour – as well as some fibers suspended in air are capable of being ignited, causing fires with destructive consequences.
In order to have a fire or explosion, the three components –air, a flammable material in ignitable mixture and source of ignition – must be present as shown in the fire triangle below:
As air is readily available (one leg of the fire triangle) and facilities that handle flammable or combustible materials can provide another leg of the triangle, the third leg –an ignition source – is all that is required to cause an explosion or a fire. In most cases, electrical and control equipment could readily provide the required source of ignition to complete the fire triangle.
Since electrical equipment is a possible source of ignition, Electrical hazardous area classification methods that prevent the electrical equipment from serving as an ignition source have been developed. These methods allow the safe placing of electrical equipment in areas where a flammable material could be present. The first requirement in using this method is to determine the nature, potential and extent of the flammable material hazard.
What is Electrical Hazardous Area Classification?
This is a method used to classify area hazards in regard to using electrical equipment in areas classified as hazardous. Article 500, 501 – 555 of the National Electrical Code (NEC) deals extensively on the use of electrical equipment in hazardous areas. You can get the NEC code here
Hazardous Area Classification
Area classification is divided into three designators: Class, Group and Division. Each hazardous area is classified according to Class, Group and Division. Areas that are not classified are considered unclassified or non hazardous.
Class identifies the physical nature of the hazard. Class designation is often used by North American approval agencies such as FM and CSA in their certifications. The class designators are as follows:
|Class I||This is a location in which flammable gases or vapours are or may be present in the air in quantities sufficient to produce explosive or ignitable mixtures|
|Class II||This is a location where combustible dusts are present|
|Class III||This is a location where easily ignitable fibres or flyings are present.|
The group designator identifies the chemical nature of the hazard. The group designators with their representative chemicals are as follows:
Chemicals or Materials
|B||Hydrogen, ethylene oxide, propylene|
|C||Ethylene, acetaldehyde, carbon dioxide, methyl ether|
|D||Gasoline, methane, ethane, propane, propylene|
|E||Combustible metal dusts with resistivity of less than 100 ohm-centimeter|
|F||Combustible dusts: carbon black, charcoal, coal coke dust with a resistivity of 1,000 - 100,000,000 ohm-centimeter|
|G||Combustible dusts with a resistivity of greater than 100,000 ohm-centimeter, corn, wheat, polypropylene, polyethylene|
Groups A – D are regarded as Class I chemicals
Groups E – G are class II chemicals or materials
Class III does not have any groups.
The Division designators identify the probability and extent that the flammable or combustible mixture will exist in the area at any given time. There are two different division designators as used in North America and the IEC in Europe. These Division designators are as follows:
|Zone 0||Ignitable concentrations present most of the time under normal conditions|
|Zone 1||Ignitable concentrations present under normal conditions for short periods|
|Division 2||Zone 2||Ignitable concentrations present only under fault conditions|