A household light switch may emit a small, harmless visible spark when switching. In an ordinary atmosphere this arc is of no concern, but if a flammable vapor is present, the arc might start an explosion. Electrical equipment intended for use in a chemical factory or refinery either is designed to contain any explosion within the device or is designed not to produce sparks with sufficient energy to trigger an explosion.
Many strategies exist for safety in electrical installations. The simplest strategy is to minimize the amount of electrical equipment installed in a hazardous area, either by keeping the equipment out of the area altogether or by making the area less hazardous by process improvements or ventilation with clean air. Intrinsic safety, or non-incendive equipment and wiring methods, is a set of practices for apparatus designed with low power levels and low stored energy. Insufficient energy is available to produce an arc that can ignite the surrounding explosive mixture. Equipment enclosures can be pressurized with clean air or inert gas and designed with various controls to remove power or provide notification in case of supply or pressure loss of such gases. Arc-producing elements of the equipment can also be isolated from the surrounding atmosphere by encapsulation, immersion in oil, sand, etc. Heat producing elements such as motor winding, electrical heaters, including heat tracing and lighting fixtures are often designed to limit their maximum temperature below the autoignition temperature of the material involved. Both external and internal temperatures are taken into consideration.
As in most fields of electrical installation, different countries have approached the standardization and testing of equipment for hazardous areas in different ways. As world trade becomes more important in distribution of electrical products, international standards are slowly converging so that a wider range of acceptable techniques can be approved by national regulatory agencies.
Area classification is required by governmental bodies, for example by the U.S. Occupational Safety and Health Administration and compliance is enforced.
Documentation requirements are varied. Often an area classification plan-view is provided to identify equipment ratings and installation techniques to be used for each classified plant area. The plan may contain the list of chemicals with their group and temperature rating, and elevation details shaded to indicate Class, Division(Zone) and group combination. The area classification process would require the participation of operations, maintenance, safety, electrical and instrumentation professionals, the use of process diagrams and material flows, MSDS and any pertinent documents, information and knowledge to determine the hazards and their extent and the countermeasures to be employed. Area classification documentations are reviewed and updated to reflect process changes.
Soon after the introduction of electric power into coal mines, it was discovered that lethal explosions could be initiated by electrical equipment such as lighting, signals, or motors. The hazard of fire damp or methane accumulation in mines was well known by the time electricity was introduced, along with the danger of suspended coal dust. At least two British mine explosions were attributed to an electric bell signal system. In this system, two bare wires were run along the length of a drift, and any miner desiring to signal the surface would momentarily touch the wires to each other or bridge the wires with a metal tool. The inductance of the signal bell coils, combined with breaking of contacts by exposed metal surfaces, resulted in sparks which could ignite methane, causing an explosion.
In an industrial plant such as a refinery or chemical plant, handling of large quantities of flammable liquids and gases creates a risk of leaks. In some cases the gas, ignitable vapor or dust is present all the time or for long periods. Other areas would have a dangerous concentration of flammable substances only during process upsets, equipment deterioration between maintenance periods, or during an incident. Refineries and chemical
lants are then divided into areas of risk of release of gas, vapor or dust known as divisions or zones.
The process of determining the type and size of these hazardous areas is called area classification. Guidance on assessing the extent of the hazard is given in the NFPA 497 or NFPA 499 standards published by the National Fire Protection Association for explosive gas or dust atmospheres respectively, or RP 500 and RP 505 standards published by the American Petroleum Institute for explosive gas or dust atmospheres respectively, and IEC 60079-10-1 or IEC 60079-10-2 standards published by the International Electrotechnical Commission for explosive gas or dust atmospheres respectively.
The National Electric Code (NEC), NFPA 70, as published by the National Fire Protection Association, defines area classification and installation principles. The principles of the NEC Division and Zone classification systems are applied in countries around the globe, such as in the United States.
Specifically, Article 500 describes the NEC Division classification system, while Articles 505 and 506 describe the NEC Zone classification system. The NEC Zone classification system was created to provide multinational companies with a system that could be harmonized with IEC classification system and therefore reduce the complexity of management.
Canada has a similar system with the Canadian Electrical Code defining area classification and installation principles. Two possible classifications are described in Canadian Standards Association (CSA) C22.1 Canadian Electrical Code (CEC) Section 18 (Zones) and Appendix J (Divisions).
Typical gas hazards are from hydrocarbon compounds, but hydrogen and ammonia are common industrial gases that are flammable.
Class I, Division 1 classified locations
An area where ignitable concentrations of flammable gases, vapors or liquids can exist all of the time or some of the time under normal operating conditions. A Class I, Division 1 area encompasses the combination of Zone 0 and Zone 1 areas.
Zone 0 classified locations
An area where ignitable concentrations of flammable gases, vapors or liquids are present continuously or for long periods of time under normal operating conditions. An example of this would be the vapor space above the liquid in the top of a tank or drum. The ANSI/NEC classification method consider this environment a Class I, Division 1 area. As a guide for Zone 0, this can be defined as over 1000 hours/year or >10% of the time.
Zone 1 classified location
An area where ignitable concentrations of flammable gases, vapors or liquids are likely to exist under normal operating conditions. As a guide for Zone 1, this can be defined as 10–1000 hours/year or 0.1–10% of the time.
Class I, Division 2 or Zone 2 classified locations
An area where ignitable concentrations of flammable gases, vapors or liquids are not likely to exist under normal operating conditions. In this area the gas, vapor or liquids would only be present under abnormal conditions (most often leaks under abnormal conditions). As a general guide for Zone 2, unwanted substances should only be present under 10 hours/year or 0–0.1% of the time.
Also known as non-hazardous or ordinary locations, these locations are determined to be neither Class I, Division 1 or Division 2; Zone 0, Zone 1 or Zone 2; or any combination thereof. Such areas include a residence or office where the only risk of a release of explosive or flammable gas would be such things as the propellant in an aerosol spray. The only explosive or flammable liquid would be paint and brush cleaner. These are designated as very low risk of causing an explosion and are more of a fire risk (although gas explosions in residential buildings do occur). Unclassified locations on chemical and other plant are present where it is absolutely certain that the hazardous gas is diluted to a concentration below 25% of its lower flammability limit (or lower explosive limit (LEL)).
An explosion of dust at this grain elevator in Kansas killed five workers in 1998.
Flammable dusts when suspended in air can explode. An old system of area classification to a British standard used a system of letters to designate the zones. This has been replaced by a European numerical system, as set out in directive 1999/92/EU implemented in the UK as the Dangerous Substances and Explosives Atmospheres Regulations 2002
The boundaries and extent of these hazardous locations should be decided by a competent person. There must be a site plan drawn up of the factory with the divisions or zones marked on.
Class II, Division 1 classified locations
An area where ignitable concentrations of combustible dust can exist all of the time or some of the time under normal operating conditions.
Class II, Division 2 classified locations
An area where ignitable concentrations of combustible dust are not likely to exist under normal operating conditions.
Class III, Division 1 classified locations
An area where easily ignitable fibers or materials producing combustible flyings are handled, manufactured or used.
Class III, Division 2 classified locations
An area where easily ignitable fibers are stored or handled.
Zone 20 classified locations
An area where ignitable concentrations of combustible dust or ignitable fibers/flyings are present continuously or for long periods of time under normal operating conditions.
Zone 21 classified location
An area where ignitable concentrations of combustible dust or ignitable fibers/flyings are likely to exist under normal operating conditions.
Zone 22 classified locations
An area where ignitable concentrations of combustible dust or ignitable fibers/flyings are not likely to exist under normal operating conditions.
Also known as non-hazardous or ordinary locations, these locations are determined to be neither Class II, Division 1 or Division 2; Class III, Division 1 or Division 2; Zone 20, Zone 21 or Zone 22; or any combination thereof.
Explosive atmospheres have different chemical properties that affect the likelihood and severity of an explosion. Such properties include flame temperature, minimum ignition energy, upper and lower explosive limits, and molecular weight. Empirical testing is done to determine parameters such as the maximum experimental safe gap (MESG), minimum igniting current (MIC) ratio, explosion pressure and time to peak pressure, spontaneous ignition temperature, and maximum rate of pressure rise. Every substance has a differing combination of properties but it is found that they can be ranked into similar ranges, simplifying the selection of equipment for hazardous areas.
Flammability of combustible liquids are defined by their flash-point. The flash-point is the temperature at which the material will generate sufficient quantity of vapor to form an ignitable mixture. The flash point determines if an area needs to be classified. A material may have a relatively low autoignition temperature yet if its flash-point is above the ambient temperature, then the area may not need to be classified. Conversely if the same material is heated and handled above its flash-point, the area must be classified.
Each chemical gas or vapour used in industry is classified into a gas group.
|NEC Division System Gas & Dust Groups|
|Class I, Division 1 & 2||A||Acetylene|
|Class II, Division 1 & 2||E (Division 1 only)||Metal dusts, such as magnesium (Division 1 only)|
|F||Carbonaceous dusts, such as carbon & charcoal|
|G||Non-conductive dusts, such as flour, grain, wood & plastic|
|Class III, Division 1 & 2||None||Ignitible fibers/flyings, such as cotton lint, flax & rayon|
|NEC & IEC Zone System Gas & Dust Groups|
|Zone 0, 1 & 2||IIC||
Acetylene & Hydrogen
(equivalent to NEC Class I, Groups A and B)
(equivalent to NEC Class I, Group B)
(equivalent to NEC Class I, Group C)
(equivalent to NEC Class I, Group D)
|Zone 20, 21 & 22||IIIC||
Conductive dusts, such as magnesium
(equivalent to NEC Class II, Group E)
Non-conductive dusts, such as flour, grain, wood & plastic
(equivalent to NEC Class II, Groups F and G)
Ignitible fibers/flyings, such as cotton lint, flax & rayon
(equivalent to NEC Class III
|Mines susceptible to firedamp||I (IEC only)||Methane|
Group IIC is the most severe Zone system gas group. Hazards in this group gas can be ignited very easily indeed. Equipment marked as suitable for Group IIC is also suitable for IIB and IIA. Equipment marked as suitable for IIB is also suitable for IIA but NOT for IIC. If equipment is marked, for example, Ex e II T4 then it is suitable for all subgroups IIA, IIB and IIC
A list must be drawn up of every explosive material that is on the refinery/chemical complex and included in the site plan of the classified areas. The above groups are formed in order of how explosive the material would be if it was ignited, with IIC being the most explosive Zone system gas group and IIA being the least. The groups also indicate how much energy is required to ignite the material by energy or thermal effects, with IIA requiring the most energy and IIC the least for Zone system gas groups.
Another important consideration is the temperature classification of the electrical equipment. The surface temperature or any parts of the electrical equipment that may be exposed to the hazardous atmosphere should be tested that it does not exceed 80% of the auto-ignition temperature of the specific gas or vapor in the area where the equipment is intended to be used.
The temperature classification on the electrical equipment label will be one of the following (in degree Celsius):
Continuous - Short Time
|T1 - 450||T3A - 180||T1 - 450||G1: 360 - 400|
|T2 - 300||T3B - 165||T2 - 300||G2: 240 - 270|
|T2A - 280||T3C - 160||T3 - 200||G3: 160 - 180|
|T2B - 260||T4 - 135||T4 - 135||G4: 110 - 125|
|T2C - 230||T4A - 120||T5 - 100||G5: 80 - 90|
|T2D - 215||T5 - 100||T6 - 85|
|T3 - 200||T6 - 85|
The above table tells us that the surface temperature of a piece of electrical equipment with a temperature classification of T3 will not rise above 200 °C.
The auto-ignition temperature of a liquid, gas or vapor is the temperature at which the substance will ignite without any external heat source. The exact temperature value determined depends on the laboratory test conditions and apparatus. Such temperatures for common substances are:
|Carbon disulfide||102 °C|
The surface of a high pressure steam pipe may be above the autoignition temperature of some fuel/air mixtures.
The auto-ignition temperature of a dust is usually higher than that of vapours & gases. Examples for common materials are:
To ensure safety in a given situation, equipment is placed into protection level categories according to manufacture method and suitability for different situations. Category 1 is the highest safety level and Category 3 the lowest. Although there are many types of protection, a few are detailed
Equipment construction is such that it can withstand an internal explosion and provide relief of the external pressure via flamegap(s) such as the labyrinth created by threaded fittings or machined flanges. The escaping (hot) gases must sufficiently cool down along the escape path that by the time they reach the outside of the enclosure not to be a source of ignition of the outside, potentially ignitable surroundings.
|IEC/EN 60079-1||Zone 1 if gas group & temp. class correct||Motors, lighting, junction boxes, electronics|
|Increased Safety||e||Equipment is very robust and components are made to a high quality||IEC/EN 60079-7||Zone 2 or Zone 1||Motors, lighting, junction boxes|
|Oil Filled||o||Equipment components are completely submerged in oil||IEC/EN 60079-6||Zone 2 or Zone 1||switchgear|
|Sand/Powder/Quartz Filled||q||Equipment components are completely covered with a layer of Sand, powder or quartz||IEC/EN 60079-5||Zone 2 or Zone 1||Electronics, telephones, chokes|
|Encapsulated||m||Equipment components of the equipment are usually encased in a resin type material||IEC/EN 60079-18||Zone 1 (Ex mb) or Zone 0 (Ex ma)||Electronics (no heat)|
|Pressurised/purged||p||Equipment is pressurised to a positive pressure relative to the surrounding atmosphere with air or an inert gas, thus the surrounding ignitable atmosphere can not come in contact with energized parts of the apparatus. The overpressure is monitored, maintained and controlled.||IEC/EN 60079-2||Zone 1 (px or py), or zone 2 (pz)||Analysers, motors, control boxes, computers|
Any arcs or sparks in this equipment has insufficient energy (heat) to ignite a vapour
Equipment can be installed in ANY housing provided to IP54.
|'ia': Zone 0 &
'ib': Zone 1
'ic: zone 2
|Instrumentation, measurement, control|
Equipment is non-incendive or non-sparking.
A special standard for instrumentation is IEC/EN 60079-27, describing requirements for FieldbusNon-Incendive Concept (FNICO) (zone 2)
|Zone 2||Motors, lighting, junction boxes, electronic equipment|
|Special Protection||s||This method, being by definition special, has no specific rules. In effect it is any method which can be shown to have the required degree of safety in use. Much early equipment having Ex s protection was designed with encapsulation and this has now been incorporated into IEC 60079-18 [Ex m]. Ex s is a coding referenced in IEC 60079-0. The use of EPL and ATEX Category directly is an alternative for “s” marking. The IEC standard EN 60079-33 is made public and is expected to become effective soon, so that the normal Ex certification will also be possible for Ex-s||IEC/EN 60079-33||Zone depending upon Manufacturers Certification.||As its certification states|
The types of protection are subdivided into several sub classes, linked to EPL: ma and mb, px, py and pz, ia, ib and ic. The a subdivisions have the most stringent safety requirements, taking into account more the one independent component faults simultaneously.
Many items of EEx rated equipment will employ more than one method of protection in different components of the apparatus. These would be then labeled with each of the individual methods. For example, a socket outlet labeled EEx'de' might have a case made to EEx 'e' and switches that are made to EEx 'd'.
In recent years also the Equipment Protection Level (EPL) is specified for several kinds of protection. The required Protection level is linked to the intended use in the zones described below:
|Group||Ex risk||Zone||EPL||Minimum type of protection|
|II (gas)||explosive atmosphere > 1000 hrs/yr||0||Ga||ia, ma|
|II (gas)||explosive atmosphere between 10 and 1000 hrs/yr||1||Gb||ib, mb, px, py, d, e, o, q, s|
|II (gas)||explosive atmosphere between 1 and 10 hrs/yr||2||Gc||n, ic, pz|
|III (dust)||explosive surface > 1000 hrs/yr||20||Da||ia|
|III (dust)||explosive surface between 10 and 1000 hrs/yr||21||Db||ib|
|III (dust)||explosive surface between 1 and 10 hrs/yr||22||Dc||ic|
The equipment category indicates the level of protection offered by the equipment.
All equipment certified for use in hazardous areas must be labelled to show the type and level of protection applied.
Mark for ATEX certified electrical equipment for explosive atmospheres.
In Europe the label must show the CE mark and the code number of the certifying body (Notified Body). The CE marking is complemented with the Ex mark, followed by the indication of the Group, Category and, if group II equipment, the indication relating to gases (G) or dust (D). For example: Ex II 1 G (Explosion protected, Group 2, Category 1, Gas) Specific type or types of protection being used will be marked.
Industrial electrical equipment for hazardous area has to conform to appropriate parts of standard EN 60079 and in some cases, certified as meeting that standard. Independent test houses (known as Notified Bodies) are established in most European countries, and a certificate from any of these will be accepted across the EU. In the United Kingdom, the DTI appoint and maintain a list of Notified Bodies within the UK, of which Sira and Baseefa are the most well known.
In North America the suitability of equipment for the specific hazardous area must be tested by a Nationally Recognized Testing Laboratory. Such institutes are UL, MET, FM, CSAor Intertek (ETL), for example.
The label will always list the Class(es), Division(s) and may list the Group(s) and temperature Code. Directly adjacent on the label one will find the mark of the listing agency.
Some manufacturers claim "suitability" or "built-to" hazardous areas in their technical literature, but in effect lack the testing agency's certification and thus unacceptable for the AHJ (Authority Having Jurisdiction) to permit operation of the electrical installation/system.
All equipment in Division 1 areas must have an approval label, but certain materials, such as rigid metallic conduit, does not have a specific label indicating the Cl./Div.1 suitability and their listing as approved method of installation in the NEC serves as the permission. Some equipment in Division 2 areas do not require a specific label, such as standard 3 phase induction motors that do not contain normally arcing components.
Also included in the marking are the manufacturers name or trademark and address, the apparatus type, name and serial number, year of manufacture and any special conditions of use. The NEMA enclosure rating or IP code may also be indicated, but it is usually independent of the Classified Area suitability.