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I NTRODUCT I ON HAZARDOUS LOCATION DATA

In Canada, the Canadian Electrical Code (CE Code), Part 1 – Section 18 contains the installation Rules for Explosive Atmospheres and mandates the use of the Zone system for all new construction. Facilities constructed before the introduction of the Zone System, are permitted to continue using the CE Code, Part 1 – Annex J18 that addresses the Class/Division System. The introduction of the Zone System into both the NEC and CE Code identified areas using “Class Zone” to align with “Class/Division”. In 2015, the CE Code removed the term “Class” when referring to the “Zone” System, the NEC followed in 2020. Both Codes continue to use “Class” when referring to Class/Division locations. Class/Division System – Material Definition Class I - locations are those in which flammable gases, flammable liquid–produced vapors, or combustible liquid–produced vapors are or may be present in the air in quantities sufficient to produce explosive or ignitable mixtures. Class II - locations that are hazardous because of the presence of combustible dust. Class III - location in which easily ignitable fibers/ flyings are handled, manufactured, or used. Zone System – Material Definition Explosive Atmospheres – mixture with air, under atmospheric conditions, of flammable substances in the form of gas, vapor, dust, fibers, or flyings which, after ignition, permits self-sustaining propagation. Explosive Gas Atmospheres – a mixture with air, under atmospheric conditions, of flammable substances in the form of gas, vapor, or mist in which, after ignition, combustion spreads throughout the unconsumed mixture. Explosive Dust Atmosphere – mixture with air, under atmospheric conditions, of flammable substances in the form of dust, fibers, or flyings which, after ignition, permits self-sustaining propagation.

The table below provides a simple comparison between the “Class/Division” System and the “Zone” System. EXPLOSIVE ATMOSPHERE CLASS/DIVISION SYSTEM

ZONE SYSTEM

Explosive Gas Atmospheres, Zones 0, 1 and 2 Explosive Dust Atmospheres, Zones 20, 21 and 22

Gases and Vapors

Class I

Combustible Dusts

Class II

Easily Ignited Fibers & Flyings

Class III

General Properties of Hazardous Location Materials Simply because hazardous location materials are present does not mean that the conditions necessary for an explosion to occur also exist. With explosive materials several other factors must occur simultaneously to result in an explosion. Larger dust particles are referred to as fibers and flyings. While there is no serious risk of an explosion with fibers and flyings, if sufficient material is present, there is the risk of a flash fire, which spreads at near explosive speeds. Although there is the risk of an explosion with both explosive gas and dust materials, the factors required for that to occur are somewhat different. In both cases the material needs to mix with air (to provide the oxygen required), be in specific fuel to air concentrations (flammable limits), and then encounter an ignition source with sufficient energy to start an explosion. The physical properties of gases and vapors allow them to easily mix with air whereas dusts tend to settle on surfaces. If gas is released into the atmosphere it can rise, settle, or linger in the air around, based on the vapor density, and dispersion depends on the air movement in the area. For a dust to form an explosive cloud it needs to be suspended. This can take place in process equipment, leaking equipment, or by strong air movement which could be caused by a smaller explosion.

NOTE: The Zone System for Explosive Dust Atmospheres incorporates Class III easily ignitable fibers/flyings.

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