A hazardous location is defined as a location where fire or explosion hazards exist due to the presence of flammable gases, vapors, or liquids, combustible dusts or ignitable fibers. Fire explosions can occur in a wide range of industrial environments e.g. refineries, petrochemical plants, oil rigs, chemical processing facilities, grain silos, etc. Fire or explosions can be started by a wide range of ignition sources; although sparks or heat from electrical, instrumentation and control equipment are the most common causes.
An electric arc or spark can have sufficient energy to cause ignition depending on the composition of the explosive atmosphere. Even if this equipment does not produce an arc or spark, it may produce enough heat to cause ignition. Therefore, the surface temperature of the equipment could pose special consideration when installed in a hazardous location.
In this article, we discuss the classification systems used in North America (classes/divisions and modified zones) and in Europe (zones and categories) for hazardous locations.
Classifying hazardous locations helps engineers design equipment or enclosure for specific conditions of the location.
Contents
Key Classification Factors
The classification of hazardous locations depends on the physical characteristics of the area and the nature of the flammable material, the key factors considered area:
Explosive limits
Flammable gases and vapors have the lower and upper limits of explosibility. The lower explosive limit (LEL) is the volume of the gas or liquid in the air below which it will not explode. The upper explosive limit (UEL) is the volume of the gas or vapor in the air above which it will not explode.
Flash point
Flash point is the lowest temperature at which a liquid will form an ignitable gas-air mixture. Elevated temperatures also increase the rate of evaporation of a flammable liquid thereby increasing the amount of vapor present in the air.
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Containment
Containment is the keeping an ignitable gas-air mixture in a controlled or contained area. Containing the mixture minimizes the size of the hazardous locations.
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Grades of release
There are three grades of release:
- Continuous
- Primary
- Secondary
Continuous is an ongoing release or one that occurs over long periods of time.
A primary release occurs periodically during normal operation.
A secondary release is one that is not expected to occur in normal operation or occurs infrequently for a short period.
Molecular
Molecular weight relates to the density of a gas or weight vapor. Density affects the direction, distance and speed the gas or vapor travel. Gas or vapors with molecular weight less than air will rise, dissipate faster and travel at a greater speed than the gases or vapors with molecular weights greater than the air. Besides, gases and vapors that are heavier than air will collect in low areas such as sumps and pits and lead to special concerns when classifying these areas.
Ventilation
There are 2 types of ventilation:
- Natural ventilation
- Artificial ventilation
Natural ventilation is the movement of air due to the effect of wind or temperature gradients.
Artificial ventilation is air movement due to the operation of fans or blowers.
Source of the release
The first factor that is considered when determining the classification of a hazardous location is to fix the point from which the flammable material is released into the atmosphere into the atmosphere to form an explosive mixture. Classifying a location begins at the source and extends for a certain distance from the source. The distance depends on the frequency, duration, and the rate of release and the local ventilation.
North America Division System
Hazardous locations classifications in the US are based on the National Electrical Code (NEC), NFPA 70 and in Canada, the Canada Electrical Code (CEC), C22.1, part 1 applies. Both countries follow a system where hazardous locations are categorized by the form, degree, and type of flammable material present.
The division system consists of three components namely:
- Class
- Division
- Group
Class describes the physical form of the flammable material in the atmosphere.
Division describes the likelihood or degree of a hazardous atmosphere.
Group describes the type of flammable material in terms of chemical composition and ignition energy.
Classes
In North America, hazardous locations are categorized into 3 classes depending on the form of flammable material present:
Class I
Flammable gases, vapors or liquids are present in the air in quantities sufficient to produce an explosive atmosphere.
Class II
Combustible dust or powder suspended in the air in quantities sufficient to produce an explosive atmosphere.
Class III
Flammable fibers or flyings are suspended in the air in quantities sufficient to produce an explosive atmosphere.
The natural gases and petroleum gases/vapors/liquids are examples of Class I hazards.
Grain and coal dust suspended in high concentrations in the air are examples of a class II hazards.
Suspended fibers, such as those present in the textile and paper manufacturing are examples of Class III hazards.
Division
In the United States, division classifications are defined in the National Electrical Code, NFPA 70, Articles, 500 through 504. In Canada, division classifications are defined in the Canadian Electrical Code C22.1, part 1.
Division 1
Division 1 hazardous locations are those where an explosive atmosphere exists continuously or intermittently under normal condition, during repair or maintenance or when a fault or breakdown of equipment occurs.
Division 2
Division 2 hazardous locations are those adjacent to a Division 1 location or where an explosive atmosphere exists when a fault or breakdown of equipment occurs.
Consider an application example of a vented fuel storage tank illustrating the NEC/CEC division classification below:
Assuming the tank does not have the Nitrogen blanket and the fuel-air mixture is within the explosive limits, then the space within the storage tank is an example of a Division 1 hazardous location while the areas surrounding the tank are Division 2 hazardous locations.
North American Modified Zone System
Although the division system is still used in North American, a modified zone concept was adopted in the US in 1996 and in Canada in 1998. In 1996 the U.S. adopted a modified zone system based on the International Electrotechnical Commission (IEC) standards. Canada adopted a similar system in 1998, CAN/CSAE areas. Presently (at the writing of this post) this system covers Class I hazardous locations containing gases or vapors and Class II (dust). Class III (fibers or flyings) atmosphere aren’t covered.
North American Industries have historically classified hazardous areas as either Division 1 or Division 2 based on NEC or CEC standards. The problem with this system is that the rest of the world uses a different, zone based system. Since manufacturers want to harmonize with the international standards, so that a plant built in one country can use the same equipment and installation standards as one built in another country, they modified the hazardous classification system to cater for this.
In 1996, Europe adopted the ATEX category-zone concept for classifying hazardous locations. North America has made changes in its standards in order to incorporate the international zone classification.
Under this modified zone system, areas are divided into three zones instead of the two divisions based on the probability of an ignitable concentration being present as illustrated in the table below:
Table 1 North American Division-Zone System
European Category Zone System
IEC developed a classification system that is based on zones for hazardous-location safety issues (IEC 60079-10). The IEC zone system is international and many countries prefer to use this standard as the basis for installation and production classifications. European countries have adopted a classification system developed by CENELEC and modified where necessary to meet individual country national codes.
In much 1996, the European commission adopted a revised CENELEC classification system defined by ATEX Directive 94/9/EC. The standard established under this directive became mandatory on July, 1, 2003 and cover the design, manufacture and installation of items used in hazardous locations.
The key elements in the Directive are the separation of zone 0 and zone 1, the definitions of category 1, 2, and 3 and the new requirements for production quality and the contents of installation instructions.
The change in classification brought by ATEX Directive is that, all approved equipment be divided into categories, which determines in which location hey can be used.
The category system provides the standards for the equipment used in hazardous locations. These categories are for Group II equipment which is equipment that is used above the ground.
Category 1
The equipment that is suitable for use in zone 0, where explosive atmosphere is continuously present or present for long periods in normal operation.
Category 2
The equipment that is suitable for use in zone 1, where explosive atmosphere is intermittently present in normal operations.
Category 3
The equipment that is suitable for use in zone 2, where an explosive atmosphere is unlikely to occur in the normal operation.
The following diagram illustrates the ATEX category classification for a vented fuel tank.
ATEX Zone Classification System
Gases, Vapors, and Mists
Zone 0 – continuous or frequent presence of ignitable or combustible concentrations of gases and vapors during typical operations.
Zone 1 – occasional presence of concentrations of gases and vapors during typical operations.
Zone 2 – seldom that gases and vapors will be present in ignitable or combustible concentrations during typical operations or the levels may only be present for short durations.
Dusts
Zone 20 – continuous or frequent presence of combustible dusts or ignitable fibers and flyings during typical operations.
Zone 21 –occasional presence of combustible or ignitable concentrations of dust or fibers and flyings during typical operations.
Zone 22 –unlikely that combustible dusts or ignitable fibers and flyings will be present in high sufficient concentrations during typical operations or can only be present for short durations like during a malfunction.
Cross References of Classifications
Most countries are using standards based on IEC zone system for classifying hazardous location. Europe has adopted ATEX category system. The table below shows across-reference of classifications.
Table 2 Hazardous locations classification cross references
| System | Continuous Hazard | Intermittent Hazard | Abnormal Conditions |
| International (IEC) | Zone 0 | Zone 1 | Zone 2 |
| EU countries (ATEX) | Zone 0 (Category 1) | Zone 1 (Category 2) | Zone 2 (Category 3) |
| United States (NEC 500) | Division 1 | Division 1 | Division 2 |
| United States (NEC 505) | Zone 0 | Zone 1 | Zone 2 |
The Ignition Energy
To guarantee the safe use of electrical equipment in hazardous locations, the heat generated or the energy of any arc, or spark produced must be considered. The surface temperature of the equipment operating under the worst conditions must be less than ignition temperature of the surrounding hazardous atmosphere. The energy of an arc or spark must be less than the ignition energy of the surrounding hazardous atmosphere.
The ignition temperature (auto-ignition temperature) is the minimum temperature required to ignite or cause self-sustained combustion of a solid, liquid or gas independently of the ignition source or heating element.
Ignition energy is the minimum electrical or thermal energy in the form of electric spark or heat source that is necessary to cause ignition of the most easily ignited concentration of a flammable gas or combustible dust.
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Groups Classification for North America
Group classifications for North America categorize the type of flammable material present in terms of chemical composition and ignition energy.
Class I hazardous locations are divided into 4 groups based on the type of gas or vapor.
Class II locations are divided into three groups based on the type of dust or powder present.
Class III where combustible fiber or flyings are present is not divided into groups.
Class I
Group A – atmospheres containing acetylene.
Group B – atmospheres containing hydrogen, butadiene, ethylene oxide, propylene oxide, and similar gases or vapors.
Group C – atmospheres containing ethylene, acetaldehyde, hydrogen sulfide and similar gases or vapors.
Group D – atmospheres containing propane, acetone, ammonia, benzene, butane, cyclopropane, ethanol, gasoline, hexane, methanol, methane, natural gas, naphtha, and similar gases or vapors.
Class II
Group E – atmospheres containing metal dusts (aluminium, magnesium, bronze, powder, etc.) or other combustible dusts.
Group F – atmospheres containing carbonaceous dusts (carbon black, charcoal, coal or coke dusts) or other combustible dusts.
Group G – atmospheres containing non-conductive dusts not included in Group E and F such as flour, grain, wood, plastic, and chemicals.
Class III
This is for atmospheres containing fibers and flyings. No groups here.
Groups under Zone System
Group I
Mines which are susceptible to naturally occurring flammable gas mixtures i.e. fire damps.
Group II
Explosive gas: These are non-mine locations that are prone to the occurrence of fire damps. The atmosphere contains gases such as petrol and methane (Group IIA), ethylene and ethyl ether (Group IIB), and hydrogen or carbon disulphide (Group IIC).
Group III
Explosive dusts: These are atmospheres that contain non-gaseous hazards such as combustible dusts and ignitable fibers or flyings e.g. combustible flyings (Group IIIA). Non-conductive dusts with high electrical resistivity such as cement dust (Group IIIB) and conductive dusts such as aluminum dust (Group IIIC).
Placing Equipment in Explosive Atmospheres
In North America and Europe, equipment is certified on the basis of its design and construction characteristics. Equipment classified for a specific surface temperature class can be used in the presence of all gases with an ignition temperature higher than the temperature class of the equipment.
To properly select and install an Instrument in a hazardous location, all the potential gases, vapors, and dusts that may come into contact with the heat-producing device must be listed. Each flammable material has its own temperature classification. The material with the lowest ignition temperature is used to determine the required T-classification for the equipment or location.
Surface Temperature
The surface temperature of the equipment is the measure of the amount of heat produced. In order for an equipment to be safely installed in hazardous location, it is important to know the maximum surface temperature that the equipment generates during the normal operation and under fault condition. An explosion can occur if the surface temperature of the equipment is greater than the ignition temperature of the gas in the vicinity.
The maximum surface temperature of equipment is the temperature the equipment may reach at its worst operating or fault condition. Electrical equipment are tested and labelled using a temperature classification system in accordance with applicable standards. The international standards are set by IEC, European standards are set by CENELEC, Canadian standards are set by CSA and U.S. standards are set by NFPA.
The maximum surface temperature of equipment is the basis for the classification. Each heat producing equipment is assigned a T-number based on the highest temperature it produces under the most severe operating conditions. The maximum surface temperature is measured in relationship to a reference ambient temperature. Normally the ambient temperature 40 °C is used as the starting point unless otherwise stated on the equipment labelling.
Surface temperatures are divided into six classes T1 to T6.
North American Surface Temperature Classifications
North American temperature classifications are divided into six major classes: T1, through T6; classes T2, T3, and T4 are further subdivided. The surface temperature classifications are set by NEC (NFPA) and CEC (CSA). The table 3 is based on NEC 500 standards.
Table 3 North American Surface Temperature Classifications
European Surface Temperature Classifications
Temperature classifications used in Europe are also divided into six classes, T1 through T6. Unlike the North American System, classifications T2, through T4 are not subdivided. The surface temperature classifications are set by IEC and CENELEC. Table 4 is based on IEC 60079-8
Table 4 European Surface Temperature Classifications
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