OHS Code Explanation Guide

Published Date: July 01, 2009
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Part 10 Fire and Explosion Hazards

Section 162.1 Classification of work sites

The hazard assessment required by Part 2 of the OHS Code will help an employer to determine if there are one or more locations at a work site where there exists or where there is the potential for an explosive atmosphere to exists. If such locations are present, they are considered to be a “hazardous” location.

A “hazardous location” is further described in Part 1 of the CEC. According to the CEC, a hazardous location is a location, building, or parts thereof, in which

(a) an explosive gas atmosphere is present, or may be present, in the air in quantities that require special precautions for the construction, installation and use of electrical equipment;
(b) combustible dusts are present, or may be present, in the form of clouds or layers in quantities to require special precautions for the construction, installation, and operation of electrical equipment; or
(c) combustible fibres or flyings are manufactured, handled or stored in a manner that will require special precautions for the construction, installation, and operation of electrical equipment.

An explosive atmosphere is a mixture with air, under atmospheric conditions, of flammable substances in the form of gas, vapour, or mist in which, after ignition, combustion spreads throughout the unconsumed mixture.

Explosive concentrations of gas, vapour or dust might be present temporarily as a result of flammable materials being brought into an area. As a result, the area might not be classified under this section. Special precautions, including the features, design and installation of electrical equipment must still be taken in these areas to ensure that ignition of the flammable gas, vapour or dust is prevented (see section 169(1)(b)). An example of one of these situations would be the use of a solvent, near or above its flash point, in an enclosed area.

Flammable gases and vapours, and flammable or combustible liquids, can burn and explode. Some dusts can also burn and explode. Examples include grain dust, sugar dust, cardboard dust, and metal dust.

If a flammable or combustible dust (or ignitable fibres) is suspended in air at a high enough concentration, a source of ignition such as a spark, open flame, or hot surface can trigger a fire and explosion. The minimum concentration in air of suspended dust that can burn and explode is approximately 100,000 milligrams/cubic metre (0.1 ounces/cubic foot). This amount is 10,000 times greater than the occupational exposure limit permitted for nuisance dusts. The exact concentration varies from substance to substance and depends on factors such as particle size and oxygen concentration.

While most dust clouds with a sufficiently high concentration of particles occur within process equipment, dust clouds can be formed by the mechanical disturbance of an accumulated layer of dust. Often the mechanical impact that disturbs the dust also creates an incendive spark resulting in an explosion that raises more dust, thereby creating a series of violent explosions. In areas where the fine dust particles accumulate, the frequency of cleaning may determine whether or not the area is classified as a hazardous location.

Ignitable fibres and flyings are materials cast off into the air that normally fall to the ground because of their size and weight. By example, small particles of sawdust that remain suspended in the air are dust. Wood chips created by a chain saw or planer are flyings. While fibres are not generally a threat to cause an explosion, fibres collecting on heat-producing equipment can be the source of serious fires.

Subsection 162.1(1) Classification of hazardous locations

Except for the situations noted in subsections 162.1(1)(b) and 162.1(1)(c), a professional engineer experienced in such classifications (or a competent person authorized by a professional engineer) must carry out the classification in accordance with Section 18 of the CEC.

Rule 18-004 of the CEC classifies hazardous locations into three Classes as follows based on the degree of hazard in the location:

(a) Class I – locations which are hazardous because of the presence of gas or vapour in the air;
(b) Class II – locations which are hazardous because of the presence of combustible or electrically conductive combustible dust; and
(c) Class III – locations which are hazardous because of the presence of easily ignitable fibres or flyings.

Currently the CEC requires that Class I hazardous locations be divided into three zones based on the frequency and duration of the occurrence of an explosive gas atmosphere. Facilities that were in existence prior to the 1998 edition of the CEC may continue to use the two Division method of classification for Class I hazardous locations. Class II hazardous locations are currently divided into two Division based on the probability that flammable concentrations of dust may be in the air or could be thrown into the air as a result of the failure of equipment or apparatus. Class III hazardous locations are currently divided into two Divisions based on the likelihood of an accumulation of fibres or flyings.

The intended purpose of the classification system from the perspective of the CEC is to help define the design characteristics of electrically powered equipment used within these zones. In the more hazardous zones or Divisions, equipment must be of a higher level of design to ensure it will not become an ignition source. However, the classification system can also be used for such purposes as assessing hazards, establishing control areas, and defining where personal protective equipment may be required.

Class I – Flammable gases and vapours

  • locations in which flammable gases or vapours such as natural gas or gasoline vapour are or may be present in the air in quantities sufficient to produce explosive gas atmospheres
  • gas or vapour could be ignited if an electrical or other source of ignition was present
  • electrically powered equipment in such locations must meet the explosion-proof enclosure requirements of section 18 of the CEC
  • examples of typical Class I locations are:

    • petroleum refineries and gasoline storage and dispensing areas
    • dry cleaning plants where vapours from cleaning fluids can be present
    • aircraft hangars and fuel servicing areas
    • utility gas plants and operations involving the storage and handling of liquefied petroleum gas or natural gas

Class II – Combustible dusts

  • locations that are hazardous because of the presence of combustible or electrically combustible dusts
  • electrically powered equipment in such locations must meet the dust-tight enclosure requirements of section 18 of the CEC
  • examples of typical Class II locations are:

    • flour and feed mills
    • plants that manufacture, use or store magnesium or aluminum products
    • producers of plastics, medicines and fireworks
    • producers of starch or candies
    • spice-grinding and sugar plants
    • coal preparation plants and other carbon handling or processing areas

Class III – Easily ignitable fibres and flyings

  • locations that are hazardous because of the presence of easily ignitable fibres or flyings
  • the fibres and flyings are not likely to be suspended in the air but can collect around machinery or on lighting fixtures
  • heat, a spark, or hot metal can ignite the fibres or flyings
  • electrically powered equipment in such locations must meet the requirements of section 18 of the CEC e.g. totally enclosed motors and dustproof enclosures
  • examples of typical Class III locations are:

    • textile mills
    • flax processing plants
    • plants that shape, pulverize or cut wood and create sawdust or flyings.

Compared to previous editions of the CEC, classification terminology has changed. Class I locations have changed from the Division system of classification to the Zone system of classification. This provides users with the option of using either the North American approach (electrical equipment approved and marked with the location (Division) in which it can be used) or the IEC (International Electrotechnical Commission) approach (equipment approved and marked with the methods of protection used).

Class I locations further subdivided

Class I locations are further divided into three Zones (see Figure 10.4) as described in Rule 18-006 of the CEC. This subdivision of the Class is based on the frequency and duration at which concentrations of a gas exceed the gas’s lower explosive limit (LEL). Providing adequate ventilation in buildings to dilute the normal (fugitive) emissions to safe levels, and providing means to eliminate large (abnormal) emissions within a short period may reduce the hazardous location within buildings to Zone 2. In general, outdoor (unenclosed) areas above grade are considered to be adequately ventilated.

Area classification is done for normal and abnormal operating conditions, but does consider the conditions associated with catastrophic failure. During normal operation, the gases or vapours present are the result of an accumulation of “fugitive emissions”. Fugitive emissions are the continuous flammable gas and vapour releases that are relatively small compared to releases due to equipment failure. These releases occur during normal operation of closed systems from components such as pump seals, valve packing, and flange gaskets. Abnormal operating conditions in which flammable concentrations (>100% LEL) of gas or vapour could be present may be the result of events such as valve or pump packing failures. Catastrophic events such as well blowouts or process vessel rupture are unpredictable events and are not considered for classification of hazardous locations. Such events require the use of emergency measures.

Figure 10.4 Class I locations subdivided

 

Classification examples

Some locations in which flammable materials are present in closed piping and containers at very low pressure may be unclassified. This is often the case with heating boilers or natural-gas-fuelled engines where the gas pressure relative to the piping design pressure is low. For example, CSA Standard CAN/CSA-B149.1 Natural Gas and Propane Installation Code allows pressure as high as 450 kPa (66 psig) in industrial buildings. This assumes that the piping and containment systems meet the requirements of that code. Proper area classification requires an understanding of the various parameters such as normal and abnormal process conditions and ventilation.

A common rule of thumb used by industry is:

  • Zone 0 hazardous locations are those where flammable concentrations of gas or vapour (100% LEL or greater) are present for more than 1,000 hours per year
  • Zone 1 hazardous locations are those where flammable concentrations of gas or vapour are present for less than 1,000 hours and more than 10 hours per year
  • Zone 2 hazardous locations are those where flammable concentrations of gas or vapour are present for less than 10 hours per year

Some typical examples are:

  • Zone 0 – the vapour space above a flammable liquid in a tank vented to atmosphere
  • Zone 1 – the interior of an inadequately ventilated natural gas compressor building
  • Zone 2 – the interior of an adequately ventilated natural gas compressor building where there are means to limit the time flammable concentrations of gas can be present to a short time.

For the purpose of a Zone 2 area classification, buildings may be considered adequately ventilated when the ventilation rate is four times the rate required to dilute the normal (fugitive) emissions to concentrations below 25% LEL.

Class II locations further subdivided

Class II locations are further divided into two Divisions (see Figure 10.5) as described in Rule 18-008 of the CEC. Class II locations are those which are hazardous because of the presence of combustible or electrically combustible dusts.

Division I locations are those in which

  • explosive concentrations of dust are present in the air during normal operation,
  • accumulation of dust is sufficient that explosive quantities of dust may be thrown in the air as a result of abnormal operation or failure of equipment, or
  • electrically conductive, combustible dust may be present.

Division 2 locations are those in which

  • explosive concentrations of dust may be present in the air as a result of the infrequent malfunction of handling or processing equipment, but such dust would be present in quantities insufficient to produce explosive or ignitable mixtures, except for short periods of time and would be insufficient to interfere with the normal operation of electrical or other equipment.

Combustible dust that blankets electrical equipment can cause overheating because the dust layer acts as insulation and prevents the release of heat from heat producing equipment such as electric motors. Sparks or fire from the equipment can ignite the combustible dust layer, and may throw dust into suspension in the air in sufficient quantity to form an explosive mixture, which in turn would be ignited by the sparks or fire. Combustible dusts include dusts produced in the handling and processing of materials such as coal, agricultural materials, resins, and combustible metallic dust. Finely pulverized material, suspended in the air, can cause as powerful an explosion as one occurring at a petroleum refinery.

Some metallic dusts, such as magnesium and aluminum, are both combustible and conductive. Nonmetallic dusts, such as pulverized coal, carbon black, and coke dust, are not considered to be conductive for the purposes of area classification. For information about classifying dusts, readers are referred the CEC Handbook, referenced at the end of this section.

Figure 10.5 Class II locations subdivided

 

Class III locations further subdivided

Class III locations are further divided into two Divisions (see Figure 10.6) as described in Rule 18-010 of the CEC. Class III locations are those which are hazardous because of the presence of easily ignitable fibres or flyings, but in which such fibres or flyings are not likely to be in suspension in air in quantities sufficient to produce ignitable mixtures.

Figure 10.6 Class III further subdivided

 

Unlike Class I and Class II hazardous locations, the hazard in Class III areas does not include explosions. Rather it is the danger of fires resulting from a buildup of fibres on electrical or other heat producing equipment.

Equipment for hazardous locations

Electrical equipment has three recognized sources of ignition:

(1) arcs and sparks – produced by normal equipment operation. Motor starters, contactors, and switches can ignite a hazardous location atmosphere;
(2) high temperatures – equipment such as lamps and lighting fixtures can produce heat. If this heat exceeds the ignition temperature of the hazardous material that is present, a flammable atmosphere can be ignited; and
(3) electrical equipment failure – the shorting of an electrical terminal, set of contacts, or failure of a motor winding or power cable could create a spark that ignites an explosive atmosphere.

Electrical equipment used in hazardous locations is specially designed and constructed. The type of equipment allowed in each Zone is based on the frequency and duration of the occurrence of explosive concentrations of gas or vapour in the Zone.

In areas classified as Zone 0, explosive concentrations of gas or vapour are present for very long periods i.e. more than 1,000 hours/year. As a result, the only electrical equipment allowed is equipment that does not contain sufficient energy to ignite the specific gas or vapour in the area i.e. intrinsically safe electrical equipment.

In areas classified as Zone 1, explosive concentrations of gas may be present during normal operation i.e. more than 10 and less than 1,000 hours/year. As a result, all electrical equipment that produces incendive arcs or sparks during normal operation must be contained in explosion-proof or flameproof enclosures. Equipment that does not produce arcs or sparks must be enclosed in explosion-proof or flameproof enclosures or be certified as using an “increased safety” method of protection and be enclosed in “safety enclosures”.

In areas classified as Zone 2, explosive concentrations of gas occur only as the result of the abnormal operation of equipment and the concentrations are allowed to persist for a short time only (less than 10 hours per year). Equipment that arcs and sparks during its normal operation must still be enclosed in explosion-proof or flameproof enclosures. Equipment that does not produce arcs or sparks, such as lighting fixtures and 3-phase motors is allowed to be contained in a variety of non-explosion-proof or non-flameproof enclosures.

In Class I areas it is assumed that gas or vapour will eventually enter electrical equipment if the equipment is exposed to explosive concentrations of gas or vapour for a sufficiently long period. Therefore devices that produce incendive arcs or sparks must be contained in explosion-proof or flameproof enclosures that are sufficiently strong to prevent rupture when gas or vapour inside the enclosure is ignited. Gases that are released along the “flame paths” (around covers, conduits and cable entries) are cooled to the point that they will not ignite the gas outside the enclosure. For the flame paths to function properly, it is important that the equipment is properly assembled and that the flame paths are not damaged by corrosion or other mechanical means.

For Class II equipment, explosive dust must be kept away from equipment housed within an enclosure. This prevents an internal explosion from occurring and does away with the need for heavy explosion-containing construction or gas escape routes. This difference explains why Class I, Zone 0 equipment is called “explosion-proof” and Class II equipment is called “dust-ignition proof”.

Class II equipment must seal out dust and be able to operate with a blanket of dust covering it without the temperature of the enclosure increasing to the point that the dust layer is ignited.

Class III equipment is very similar in design to Class II equipment. Class III equipment must minimize the entrance of fibres and flyings; prevent the escape of sparks, burning material or hot metal particles resulting from failure of equipment, and operate at a temperature that prevents the ignition of fibres that have settled on the equipment.

For more information
American Petroleum Institute RP500. Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class 1 and 2.
 
American Petroleum Institute RP505. Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Zone 0, Zone 1, and Zone 2.
 
Bossert, J. CSA Special Publication PLUS 2203, Hazardous Locations: A Guide for the Design, Testing, Construction, and Installation of Equipment in Explosive Atmospheres. Canadian Standards Association, Rexdale, Ontario.
 
CSA Handbook to the Canadian Electrical Code, Part 1, 2002. Canadian Standards Association, Rexdale, Ontario.
 
International Electrotechnical Commission (IEC). IEC Standard 60079-10, Area Classification.
 
Institute of Petroleum (British). Model Code of Safe Practice – Part 15: Area Classification Code for Petroleum Installations.
 
National Fire Protection Association (NFPA). NFPA Standard 497 (1997 Edition), Recommended Practice for Classification of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas.
 
Canadian Electrical Code: Section 18 – Hazardous Locations 
Alberta Municipal Affairs. STANDATA Electrical Safety Information Bulletin; October 2001

Code for Electrical Installations at Oil and Gas Facilities

Area classification of most Class I hazardous locations requires qualified individuals to classify these areas in accordance with the definitions in Rule 18-006 of the CEC. Individuals carrying out area classifications of Class I hazardous locations will typically be guided by the principles and recommendations in the Standards and Recommended Practices listed above. Individuals classifying hazardous locations using the three zone method will typically follow classification methods outlined in API RP505 and individuals classifying hazardous locations using the two Division method will typically follow recommended practice API RP500. The remaining publications are most frequently used for additional reference material.

However there are two publications that define the area classification of specific types of installations. These are

(1) Alberta’s “Code for Electrical Installations at Oil and Gas Facilities”, and
(2) Section 20 of the CEC titled “Flammable Liquid and Gas Dispensing and Service Stations, Garages, Bulk Storage Plants, Finishing Processes, and Aircraft Hangers”.

The Alberta Code, commonly referred to as the “Oilfield Code”, is updated on the same cycle as the CEC. The Alberta Code applies to electrical installations used in the search and transmission of oil, natural gas and related hydrocarbons. It does not apply to

(a) petroleum refineries,
(b) petrochemical facilities, or
(c) gas distribution systems distributing gas to consumers.

The Alberta Code allows professional engineers to establish the area classification, typically by using the procedures outlined above. Where this is not done, the Alberta Code mandates the area classification for specific installations.

Section 20 of the CEC specifies the area classification of the following installations:

  • Gasoline dispensing and service stations
  • Propane dispensing, container filling, and storage
  • Compressed natural gas refueling stations and compressor and storage facilities
  • Commercial garages – repairs and storage
  • Residential storage garages
  • Bulk storage plants
  • Finishing processes
  • Aircraft hangers

In addition to specifying the area classification of these facilities, section 20 also contains some additional installation requirements in addition to those outlined in section 18 of the CEC.

Area classification of Class II and Class III hazardous locations requires them to be classified in accordance with the definitions in Rules 18-008 and 18-010 of the CEC. Unlike Class I, there are far fewer standards available to assist the user in determining the classification of the Class II and Class III hazardous locations. Two standards that are available for classification of Class II hazardous locations are

(1) ISA-12.10 Area Classification in Hazardous (Classified) Dust Locations, and
(2) IEC Standard 61241-10 Electrical Apparatus for Use in the Presence of Combustible Dust – Part 10: Classification of Areas Where Combustible Dusts Are or May Be Present.

Note that the IEC standard uses a three zone approach to the classification of hazardous dust areas. However, the principles involved are similar to the North American two division approach.

Subsection 162.1(2)

No explanation required.