3.0. Hazardous Area Classification and Electrical Safety
Use of electricity in households as well as in industry has been available since the 19th century. Electricity has also been in use in coal and other mining applications. Using electricity in potentially explosive areas has been a concern from very early days. This has prompted developments of methods to use electricity in hazardous areas. After a number of initial accidents, developmental work has been undertaken to take care of safety. In this modern era, the number of accidents caused by electrical ignition sources is low. So, expenditure on development, manufacturing, and statutory regulations has proven to be successful. Today, many codes and standards toward safe use of electricity in hazardous areas have been developed. Like process hazards, it is also the duty of the designers to follow codes and standards during engineering processes so that quantified risks are well maintained within acceptable limits. This means the risks associated with use of electrical energy in hazardous/flammable areas are first assessed and quantified. Based on this, designers need to follow the recommendations of codes and standards to develop their design and engineering documentation. Also O&M personnel need to operate and maintain their plants accordingly. Obviously, the starting point of the discussions will be identification of the sources of release of flammable gas or vapor and/or accumulation of dusts. Such situations may arise from constant activity; from time to time in normal operation; or as the result of some unplanned event [22]. Also in a process, inside the equipment may be a source of hazard where both flammable materials and air are present, without actual release. Naturally, it is important to understand the properties of the materials and their impact on an explosion. In brief it is necessary to have a clear understanding of the principles of explosion and how people, property, and the environment can be protected. To have a standardized approach there have been a number of standards developed. Most internationally recognized standards have been developed by the IEC generally in line with CENELEC. After issuance of Directive 1999/92/EC, it has now become a precondition for setting up and operating a potentially explosive facility in Europe. These standards made it possible to select systems, equipment, and components with regard to explosion protection and to install, operate, maintain, and eventually repair them as required by the standards. At this point it is worth noting that loss of containment of a vessel, that is, vessel rupture or line rupture, is not part of area classification discussions and these should be treated as part of PHA discussed in previous chapters. In this clause, discussions shall cover mainly the following topics:
• Explosion discussions
• Electrical area classification (EAC)
• A few relevant standards
• Combustible gas detection
• Explosion protection
• Enclosure class
• Intrinsically safe circuits
During the discussions the main topic will be instrumentation and associated electronic parts, because these are mainly related to safe instrumentation.
Starting with explosion principles, now is the time to look at the details of the system.
3.1. Explosion Discussions
Flammable gases, vapors, mists, dusts, and fibers escape during production, processing, transportation, and storage of flammable substances in many industries. These flammable gases, vapors, mists, and dusts form an explosive atmosphere with oxygen or air. When this explosive atmosphere comes in contact with energy such as ignitions, sparks, hot surfaces, etc., explosions may take place and cause severe damage/harm to people, property, and the environment.
3.1.1. Explosion of Flammable Substances
According to the HSE.UK “Explosive atmospheres can be caused by flammable gases, mists or vapors or by combustible dusts. If there is enough of a substance, mixed with air, then all it needs is a source of ignition to cause an explosion.” This means chemical explosions involve an oxidation and reduction reaction. However, all oxidation and reduction reactions do not cause explosions, for example, the rusting of iron. An explosion is a sudden, violent change of potential energy, which transfers to its surroundings in the form of a rapidly moving rise in pressure. Therefore combining the two, it is seen that an explosion is a sudden reaction involving a rapid physical or chemical oxidation reaction or decay, generating an increase in temperature or pressure or both simultaneously. So, in an explosion caused by a chemical reaction of a flammable substance with oxygen, there will be a simultaneous release of high energy. The following are major issues involved in explosions:
• Triangle: Similar to a fire triangle, there is also a triangular requirement for flammable materials that cause an explosion, as shown in Fig. X/3.1.1-1.
So, for explosions to happen in atmospheric air, all three of the following factors must be present [23]:
• Flammable material (in ignitable quantities)
• Oxygen (in the air)
• Ignition source/source of energy
In workplaces, hazardous areas can develop wherever the first and third preconditions for an explosion are fulfilled. In a workplace there has to be air and oxygen. Typical hazardous areas are created by flammable gases and vapors from chemical/petrochemical factories, refineries, offshore plants, tank facilities, loading areas for flammable materials, and/or boiler areas of natural gas-based power plants, and also dusts in milling plants, pulverizers in power plants, etc. This means that emphasis will be on flammable materials and sources of energy for ignition because oxygen cannot be dispensed with. Joint efforts from manufacturers, users, and established standards offer protection against such explosions as shown in the figure, with special emphasis on excluding the ignition source.
• Flash point, flammable liquid classifications: As indicated earlier, it is important to understand various properties of the flammable materials for the study of explosions involving flammable materials. Out of various characteristic properties of flammable materials, flash point is a very important safety consideration. According to the Occupational Safety and Health Administration (OSHA): “Flash point is the minimum temperature at which a liquid gives off vapor within a test vessel in sufficient concentration to form an ignitable mixture with air near the surface of the liquid. The flash point is normally an indication of susceptibility to ignition.” When the flash point of a flammable liquid is well above the maximum operating temperature or hazardous atmosphere temperature, then an explosive atmosphere cannot be formed. The flash point of a mixture of various liquids may be lower than that of the individual components. As per OSHA, combustible liquids are defined as “any liquid having a flash point at or above 100°F (37.8°C).” Different nomenclatures for flammable and combustible liquids have been presented in Table X/3.1.1-1.
Similarly, as per Council Directive 98/24/EC, flammable liquid can be divided as per Table X/3.1.1-2. Boiling point is one of the parameters to be considered here.
Table X/3.1.1-1
Flammable Liquid Classification (US Based)
| Standards/Flash Point (Below) | OSHA | NFPA 30 | ANSI |
| Flash point <−7°C | Flammable | Class I | Extremely flammable |
| Flash point −7°C–38°C | Flammable | Class I | Flammable |
| Flash point 38°C–60°C | Combustible | Class II | Flammable |
| Flash point 60°C–66°C | Combustible | Class III | Combustible |
| Flash point 66°C–93°C | Combustible | Class III | Combustible |
Table X/3.1.1-2
Flammable Liquid Classification as per Directive 98/24/EC
| Designation of the Flammable Liquid | Flash Point and Boiling Point (°C) |
| Highly flammable | Flash point <0°C and boiling point <35°C |
| Easily flammable | Flash point <0°C and boiling point >35°C or 0°C < flash point < 21°C |
| Flammable | 21°C < flash point < 55°C |
3.1.2. Range of Explosion
To form an explosive atmosphere, the flammable substance must be present in a certain concentration, as shown in Fig. X/3.1.2-1. Therefore lower explosive limit (LEL) and upper explosive limit (UEL) are two extreme limits and play an important role in determining the cause of an explosion. As seen in the drawing, if the concentration of flammable material is too high or too low (beyond the limits), then no explosion will occur on account of lack of oxygen and lack of flammable materials, respectively. Instead, there is just a steady-state combustion reaction or none at all.
It is generally assumed that a volume of 10 L of an explosive mixture in an enclosed space can cause damage—particularly to people. For this reason, any area in which such a volume of an explosive mixture can collect is described as a potentially explosive atmosphere. It is only in the range between the LEL and UEL that the mixture reacts explosively when ignited. So, from a safety point of view, it is better to keep this concentration around 20% below LEL. Also see Clause 3.4.1 for further explanations. LEL and UEL are defined in Fig. X/3.1.2-2. The LEL and UEL of a few important gases are also shown in Fig. X/3.1.2-1 with associated gas group as per zone system (and NEC 505).
3.1.3. Flammable Substances
There are three kinds of flammable materials. These are gases/vapors, liquids, and solids in the form of dust or flying.
• Flammable gases/vapors: A flammable gas may be in the form of an element such as H2 or often compounds of carbon and hydrogen (hydrocarbon—e.g., natural gas), which can react with oxygen with very little additional energy. Also oxidation reaction of carbon (in complete combustion) like CO can cause explosion. A vapor is the proportion of a liquid that has evaporated into the surrounding air as a result of the vapor pressure above the surface of the liquid, for example, around a jet or droplets of the liquid. When spraying a flammable liquid, a mist can form consisting of very small droplets with a very large overall surface area. These mists can explode and in cases of fine mists a flash point is formed, as discussed in Clause 3.1.1. Flash points for mists are not that important because mists are behaviorally nearer a vapor than a liquid. So, it is necessary to consider mists as highly flammable material [21].
• Flammable liquids: Flammable liquids are mostly hydrocarbon compounds, for example, petroleum spirit. At room temperature some of these can even change into the vapor phase in sufficient quantities to form an explosive atmosphere near the surface. Many other liquids can form such an atmosphere near the surface at increased temperatures. So, the flash point discussed in Clause 3.1.1 is important. From that discussion, it is clear that flammable liquids with a high flash point are less dangerous than those with a flash point at room temperature or below. Fine mists are formed from a flammable liquid and this is included (covered) above because of its behavior toward explosion.
• For solids, particle fineness, surface area, and chemical properties are important. Generally, solids require more energy than do gases and vapors to activate the explosion in air. However, once combustion starts, the energy released by the reaction can produce high temperatures and pressures. Dust reacts very differently, depending on whether it is in a deposited layer or whether it is in a suspended dust cloud [23]. Normally, smoldering first starts in dust layers on hot surfaces (secondary combustion in a hot furnace is an example), whereas in a dust cloud when ignited locally or through contact with a hot surface, it can explode immediately. Dust explosions could be a consequence of smoldering dust layers. When such layers are stirred up (say, as an extinguishing effort) they can lead to a dust explosion. A gas or vapor/air explosion can also stir up the dust. In such cases, first is the gas explosion, followed by the dust explosion. In deep coal mines, methane explosions often cause coal dust explosions.
3.1.4. Air/Oxygen
For any chemical reaction to take place it is known that definite quantities of constituents are necessary. The quantity of oxygen available in the air near flammable material can only oxidize/burn a certain quantity of the flammable material. A commonly known term, “stoichiometric” ratio, is responsible for determining the quantity of oxygen necessary to react with available flammable materials. Naturally, when the quantity of the flammable material and the available atmospheric oxygen are near to the stoichiometric ratio, the reaction will be near completion and cause an explosion with increase in temperature and pressure. The explosion will be violent. When the quantity of flammable material is too small, combustion cannot spread and may cease. The situation is similar when the quantity of flammable material is too large, because the lack of the required quantity of O2 means that the reaction cannot proceed further. As indicated in an earlier clause (Fig. X/3.1.2-1), all flammable materials have their explosion ranges and limits: LEL and UEL. It may be possible to dilute flammable materials in excess air, but it is very difficult to create a situation where there is a dearth of oxygen because of the work force, hence this is only applicable inside equipment.
3.1.5. Sources of Ignition
With reference to Fig. X/3.1.1-1, we now need to look in depth at the third requirement for explosion—the sources of ignitions. There are many sources from where energies may come to ignite an explosive atmosphere. In this connection it is better to refer to EN 1127-1:2007. It is also important to consider the ignition properties of the explosive atmosphere such as listed here (see Clause 4.3 of the aforesaid standard):
• Minimum ignition energy requirement
• Minimum temperature for ignition of dust layer
Also classification of ignition sources is useful. Ignition sources can be classified as follows (Clause 5.3.1 of the said standard):
• Likelihood of occurrences:
• Continuously/frequently
• Rare situation
• Very rare situation
• Operation/equipment protection:
• During normal operation
• Result of malfunction
• Result of rare malfunction
The basic features of these sources are itemized in the following. The reference indicated in parentheses against each source corresponds to the clause in EN 1127-1:2007.
• Hot surface (5.3.2): Heated surface (e.g., heater, metal cuttings), dust layers, etc. can cause ignition when in contact with an explosive mixture. The capability of a heated surface causing an ignition is dependent on the type and concentration of the particular substance in the explosive mixture. Also the capability of causing an ignition is dependent on the temperature of the surface, surface area size, and shape of the heated body as well as how long it is in contact with the heated surface. In the event of a malfunction, for example, with overloading or tight bearings, etc., there will be a higher temperature to cause the ignition. Technical equipment, for example, tight bearing, resistor etc., must always be assessed.
• Flames and hot gases including hot particles (5.3.3): Even very small flames are among the most effective sources of ignition. Flames, their hot reaction products, or otherwise highly heated gases can ignite an explosive atmosphere. Therefore it is necessary to prevent flame propagation, for example, welding beads with a very large surface area during welding or cutting.
• Mechanically generated sparks (5.3.4): These could be generated as a result of friction, impact, or abrasion processes, for example, in grinding particles are hot because of the energy required for separation. These particles may be oxidized to reach higher temperatures and can be the cause of sparks to ignite flammable materials. In deposited dust, smoldering can be caused by the sparks and this can be a source of ignition for an explosive atmosphere. Cracks in rotating parts and sliding materials one over the other without lubrication can cause similar sparks, for example, a rusty hammer in contact with other tools.
• Electrical apparatus (5.3.5). These are always considered as a sufficient source of ignition. Electric sparks can be generated:
• When electric circuits are opened and closed
• On account of loose connections
• Because of stray currents
It is to be noted that low voltages (<50 V) are created against personal protection not against explosion. However, lower voltages can still produce sufficient energy to ignite an explosive atmosphere. For this reason, intrinsic safety (IS) circuits are used in low power instrumentation. Very low energy sparks with minimal energy, say microwatt seconds, may be regarded as too weak to start an explosion.
• Stray electric current cathodic corrosion protection (5.3.6): Stray currents can flow in electrically conductive systems or parts of systems:
• Return currents: In the vicinity of electric railways and large welding systems, conductive electrical system components such as rails and cable sheathing laid underground lower the resistance of this return current path–could be a source.
• Result of short circuit or short circuit to earth because of faults in electrical installations
• Result of magnetic induction
A highly conductive connection to all the electrically conductive parts of the equipment should be provided to reduce potential difference to a safe level. An equipotential bonding (discussed later) must always be provided to meet the situation.
• Static electricity (5.3.7): The discharge of charged, insulated conductive parts can easily lead to incendive sparks, that is, electrical sparks can be caused by static electricity. This means that the stored energy can be released in the form of sparks that can act as an ignition source. Cone discharges from bulk material and cloud discharges can also occur. Friction is one of the possible causes of the generation of static electricity. Brush discharges can ignite almost all explosive gas and vapor atmospheres. The ignition of explosive dust/air atmospheres with extremely low minimum ignition energy by brush discharges is also a possibility. Sparks, propagating brush discharges, cone discharges, and cloud discharges can ignite all types of explosive atmospheres, depending on their discharge energy. Static electricity must be prevented from becoming an ignition source by taking appropriate measures, for example, synthetic enclosure.
• Lightning (5.3.8): When lightning strikes in an explosive atmosphere, ignition will always occur. Also there can be ignition of an explosive mixture because of high temperature caused by lightning. Large currents flow from where the lightning strikes and these currents can produce sparks in the vicinity of the point of impact. Even in the absence of lightning strikes, thunderstorms can cause high induced voltages in equipment, protective systems, and components.
• Radiofrequency electromagnetic waves from 104 Hz to 3 × 1012 Hz (5.3.9): Systems that generate and use radiofrequency electrical energy, such as radio transmitters or radiofrequency generators for heating, drying, hardening, welding, and cutting, emit radiofrequency waves. All conductive materials in this radiofrequency field functions as receiving aerials. When the field is powerful, the receiving energy is sufficiently large. So, these conductive parts can cause ignition in explosive atmospheres. The quantum of energy received is a function of the distance between transmitters and receivers and the dimension of the receiver. Suitable protective measures need to be taken against this.
• Ionizing radiation (5.3.11): This is generated, for example, by X-ray tubes, and radioactive substances can ignite explosive atmospheres (especially explosive atmospheres with dust particles) as a result of energy absorption.
• Ultrasonic (5.3.12): In the use of ultrasonic sound waves, a large proportion of the energy emitted by the electroacoustic transducer is absorbed by solid or liquid substances. As a result, the substance exposed to ultrasonic sound waves warms up so that, in extreme cases, ignition may be induced.
• Two other types are adiabatic compression and shock waves (5.3.13) and exothermic reactions (5.3.14) including self-ignition of dusts.
With this brief knowledge about explosions, we will now focus on area classifications, which are the basis for explosion protection.
3.2. Electrical Area Classification (EAC)
Basically, area classification is for hazardous area classification. Since the main focus is on instrumentation systems, it has been designated as EAC to indicate that attention will be given to selection of instruments and associated enclosures, etc. The EAC process is quite rigorous and critical. The main objective of this classification is to divide the entire plant area according to the degree of presence of explosive environments [21]. Here degree mainly refers to the degree of severity. There are various codes and standards available to provide suitable guidance for selection, building up, and installation of electrical equipment in a particular area. Here, two important points are worth noting; first, that such classifications have been made in two ways:
• Class and division method: Mainly followed in North America and
• Zone division method: Mainly followed by Europe and other parts of the world. However, with introduction of IEC standards this is somewhat similar to the second type and is mainly followed globally; even the NEC has included zone divisions.
Second, the discussions will be focused mainly on instrumentation and control parts. The specifics concerning other electrical equipment such as motors, etc. will not be covered unless called for as part of instrumentation discussions. In general, the guiding factors for EAC, hence hazardous area classification, shall include but not be limited to the following:
• Type of hazards
• Type of flammable material present
• Flammable material properties and concentration
• Likelihood of the presence of flammable material
• Flammable materials present as normal or because of malfunction
• Area/location temperature
• Autoignition temperature of flammable material
• Sources and types of ignition energy
From NEC 497/499 and IEC 60079-10 it may be inferred that hazardous (classified) areas are those where fire and explosion hazards exist because of the presence of flammable materials (gas, vapor, liquid, dust). Area classification is a method of analyzing and classifying the environment where an explosive atmosphere may occur so as to facilitate the proper selection and installation of apparatus to be used safely in the environment, taking into account various groups and temperature classes. In practice, in places where there exist flammable materials, it is very difficult to ensure that (1) an explosive atmosphere will never exist or (2) apparatus will never give rise to a source of ignition. Therefore reliance is placed on using apparatus with low likelihood of creation of a source of ignition. All cases will not provide the same solution, however. In some cases, a suitable enclosure, say an explosion-proof enclosure (see Clause 3.6), will suffice, whereas in some cases suitable means should be chosen to limit the energy release, say an IS circuit (see Clause 3.7). Two kinds of area classification are shown in Fig. X/3.2-1.
3.2.1. Class Division System
According to this system, as shown in Fig. X/3.2-1, the entire hazardous area is divided into three classes based on the general nature or properties of the flammable materials such as gas/vapor, dusts, fibers, etc. Based on the likelihood of the presence of flammable materials in the atmosphere, the areas are further classified into divisions. Grouping of flammable materials is done based on type of hazardous materials present. Also there is categorization as per temperature.
• Class: Classes are presented in Table X/3.2.1-1.
Table X/3.2.1-1
Class—General Nature of Hazardous Materials
| Class | Area |
| I | Area/location in which flammable gases or vapors1 may (or may not2) be present in sufficient quantities to produce explosive or ignitable mixtures. 1: important is gas/vapor 2: may not: qualifies as sufficient to indicate chances of explosion. |
| II | Area/location in which combustible or conductive dusts1 (in suspension intermittently, or periodic) may (or may not2) be present in sufficient quantities to produce explosive or ignitable mixtures. 1: important is combustible or conductive dusts 2: may not: qualifies as sufficient to indicate chances of explosion. |
| III | Area/location in which ignitable fibers1, not likely to be in suspension, may (or may not2) be present in sufficient quantities to produce explosive or ignitable mixtures. For this class, grouping does not have any relevance. 1: examples: wood chips, cotton, nylon, etc. 2: may not: qualifies as sufficient to indicate chances of explosion. |
Table X/3.2.1-2
Division—Likelihood of Hazardous Material Present
| Division | Presence of Hazardous Materials Present |
| I | The substance referred to by class is present continuously, intermittently, or periodically or present because of normal operation, and has high probability of producing explosive or ignitable mixtures. |
| II | The substance referred to by class is present only in abnormal conditions (such as a container failure or system breakdown) or for a short duration and has low probability of producing explosive or ignitable mixtures. |
Table X/3.2.1-3
Grouping of Hazardous Materials
| Group | Hazardous Materials | Applicable |
| A | Acetylene. | Gases/vapors Class I |
| B | Hydrogen, fuel, and combustible process gases containing more than 30% hydrogen by volume or gases of equivalent hazard such as butadiene, ethylene, oxide, propylene oxide, and acrolein. | |
| C | Cyclopropane, CO, ether, hydrogen sulfide, morpholine, ethyl ether, and ethylene or gases of equivalent hazard. | |
| D | Acetone, ammonia, benzene, butane, cyclopropane, ethanol, gasoline, hexane, methanol, methane, vinyl chloride, natural gas, naphtha, propane, or gases of equivalent hazard. | |
| E | Combustible metal dusts: aluminum, magnesium, and their commercial alloys or other combustible dusts whose particle size, abrasiveness, and conductivity present similar hazards in connection with electrical equipment. | Dusts and flying Class II |
| F | Carbonaceous dusts, carbon black, coal black, charcoal, coal/coke dusts (>8% total entrapped volatiles). | |
| G | Dusts not included in E and F such as flour dust, grain dust, flour, starch, sugar, wood, plastic, and chemicals. |
• Division: Area classification based on likelihood is presented in Table X/3.2.1-2.
• Group: Hazardous materials in the surrounding areas are grouped. Their grouping and applicability are given in Table X/3.2.1-3.
• Temperature class: As discussed in Clause 3.1.5, flammable materials especially gases may ignite by coming in contact with a hot surface. Many factors, such as size, shape, type and surface quality, gas concentration, etc. have an influence on the ignition temperature. In all standards (say, IEC 60079-20-1), gases and vapors are divided into temperature classes. Approved equipment receives a temperature code indicating the maximum surface temperature limit of the equipment. This means that surface temperatures in explosion protected equipment and other technological objects are controlled in such a way that ignition by the surface is not possible [23]. Table X/3.2.1-4 presents the ignition temperatures as per NEFA 497 article 500 and ATEX (atmosphere explosives). There are subdivisions in T2, T3, and T4. Permissible temperatures for electrical apparatus are also given in the table.
As shown in Fig. X/3.2-1, there is another way of classifying hazardous area in terms of zones and gas group. However, prior to this it is necessary to have some knowledge of ATEX directives, which is discussed in the following clause. Later, the zone type of classification is addressed.
3.2.2. ATEX Directives
The “Directive on Equipment and Protective Systems Intended for use in Potentially Explosive Atmospheres” (94/9/EC) came into force on March 1, 1996. All manufacturers of mechanical/electrical equipment, as well as protective systems, intended for safe operation in and around potentially explosive atmospheres in the European Union (others also follow) need to follow this directive. It is popularly known as ATEX 95. This directive is meant for equipment (electrical in relation to this text) to be used in potentially explosive atmospheres. This should not be confused with workplace area classification, which is covered by a separate directive known as ATEX 137. In this directive there are two distinct groups: Group I meant for mines and Group II meant for surface installations with potential explosive atmospheres. Essential classification as per ATEX Directive 95 is elaborated in Table X/3.2.2-1.
Table X/3.2.1-4
Surface Temperature Division and Electrical Apparatus Temperature Limit
| Temperature Code | Ignition Temperaturea in °C (°F) | Ignition Temperatureb Range in °C | Permissible Temperature Electrical Apparatus in °Cc |
| T1 | 450 (842) | >450 | 450 |
| T2 | 300 (572) | >300 to ≤450 | 300 |
| T2A | 280 (536) | ||
| T2B | 260 (500) | ||
| T2C | 230 (446) | ||
| T2D | 215 (419) | ||
| T3 | 200 (392) | >200 to ≤300 | 200 |
| T3A | 180 (356) | ||
| T3B | 165 (329) | ||
| T3C | 160 (320) | ||
| T4 | 135 (275) | >135 to ≤200 | 135 |
| T4A | 120 (248) | ||
| T5 | 100 (212) | >100 to ≤135 | 100 |
| T6 | 85 (185) | >85 to ≤100 | 85 |
• Group I: Meant for mining applications, has the following categories as per the table:
• Category M1: Equipment in this category should remain functional in an explosive atmosphere (in mining). Required protection level “Very High.”
• Category M2: Equipment required to be deenergized in an explosive atmosphere (in mining). Required protection level “High.”
• Group II: Meant for equipment to be used in locations (other than mine use) endangered by potential explosive atmospheres. Here both gas and dust systems are considered and the markings are distinguished by “G” or “D” to indicate gas and dust, respectively. This group has the following categories as per Table X/3.2.2-1:
• Category 1: Equipment in this category is intended for use in areas in which explosive atmospheres caused by mixtures of air and gases/vapors/mists or by air/dust mixtures are present continuously, for long periods or frequently. Equipment or protective systems need to guarantee a “very high level” of protection and are duly certified by a notified body (e.g., CESI/IMQ/TUV). See Table X/3.2.2-1.
• Category 2: Equipment in this category is intended for use in areas in which explosive atmospheres caused by mixtures of air and gases/vapors/mists or by air/dust mixtures are likely to occur. Equipment or protective systems need to guarantee a “high level” of protection with due certification from a notified body. See Table X/3.2.2-1.
Table X/3.2.2-1
Area Classification for Equipment in Explosive Atmospheres (ATEX 95)
| Group/Category | Group I: Equipment in Mines Below or Above Ground | Group II: Equipment in Other Locations, Endangered by Explosive Atmospheres |
| Category 1 | Category M1 (Cat M1) | Category 1 (Cat 1) |
| Category 2 | Category M2 (Cat M2) | Category 2 (Cat 2) |
| Category 3 | Not applicable | Category 3 (Cat 3) |
• Category 3: Equipment in this category is intended for use in areas in which explosive atmospheres caused by gases, vapors, mists, or air/dust mixtures are unlikely to occur or, if they do occur, are likely to do so only infrequently and for a short period only. Equipment or protective systems need to guarantee “a normal level” of protection. Self-certification (internal control) is allowed in this category if manufacturers have sufficient testing facilities. See Table X/3.2.2-1.
Product certification methods: In line with ATEX Directive 95, Fig. X/3.2.2-1 briefly describes the acceptability criteria for products. In all categories, the manufacturer needs to prepare a full technical file for the product(s) that demonstrates their conformity with the requirements of the ATEX Directive 94/9/EC.
The meaning of various terms in this regard may be noted:
• EC Type Examination: Examination by a notified body for compliance of documentation and a sample of the product to be certified
• Production Quality Assurance: The manufacturer's quality control system for production, inspection, and testing shall be approved and assessed regularly by a notified body.
• Product Verification: Each unit produced shall be assessed by a notified body.
• Conformity to Type: The manufacturer shall take all necessary steps to ensure that products are manufactured in compliance with the directive and to the same design and quality as the sample type tested. Each product manufactured will be subjected to testing approved by a notified body.
• Internal Control of Production: Manufacturers shall prepare technical documentations that allow the conformity of a product to be assessed. In some cases a copy of these documentations shall be presented to a notified body for review. Here it is important to note that there are two categories of CE certifications, namely, CE marking with or without identification number, as shown in Fig. X/3.2.2-1. Surface temperature details are available in Table X/3.2.1-4. There is another ATEX directive (99/92/EC), which is known as the “ATEX workplace directive” or ATEX 137. This is meant for the duty of employees to minimize the risk from explosive atmospheres. See also Clause 3.2.3.
3.2.3. Zone System
In line with the directive, employers need to classify into zones the areas in which explosive atmospheres may be present, and to ensure that the directive is observed. In the zone system there are clear divisions between gas and dust. For gas systems, zones are classified as zone 0, 1, and 2, whereas for dust they are designated as zone 20, 21, and 22, as detailed in Table X/3.2.3-1.
In the zone system the gases are further divided into Groups I and II. Again Group II is further subdivided as Groups IIA, IIB, and IIC. Classification criteria are the maximum experimental safe gap (MESG) and the minimum ignition current (MIC). See Fig. X/3.2.3-1 for MESG and MIC definitions. Also there are symbolic representations of these zones, typical symbols for gas have been depicted in Fig. X/3.2.3-2.
Grouping details are found in Table X/3.2.3-2 where one may note that ignitability increases from top to bottom.
For surface temperature, details in connection with ATEX and the zone system, including apparatus temperature limits, Table X/3.2.1-4 may be referred to.
The discussion on classification will be concluded by trying to marry both directives of ATEX, namely, ATEX 95 and 137, and linking them with IEC 60079-0 in Table X/3.2.3-3.
Table X/3.2.3-1
Zone Classification (Gas and Dust)
| Material | Zone | Explanation |
| Gas | 0 | A location in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapor, or mist is present continuously or for long periods or frequently. |
| Gas | 1 | A location in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapor, or mist is likely to occur in normal operation occasionally. |
| Gas | 2 | A location in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapor, or mist is not likely to occur in normal operation but, if it does occur, will persist for a short period only. |
| Dust | 20 | A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods or frequently. |
| Dust | 21 | A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally. |
| Dust | 22 | A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only. |
Table X/3.2.3-3
Relation Between ATEX 95,137, and IEC 60079-0
| Material Type | ATEX 137 Zone | ATEX 95 Group | IEC 60079-0 Group (Equivalent) | Equipment Category (ATEX 95) | EPL IEC 60079-0 |
| Gas/vapor | Zone 0 | II | II | 1G | Ga |
| Gas/vapor | Zone 1 | II | II | 2G, 1G | Gb, Ga |
| Gas/vapor | Zone 2 | II | II | 3G, 2G, 1G | Gc, Gb, Ga |
| Dust | Zone 20 | II | III | 1D | Da |
| Dust | Zone 21 | II | III | 2D, 1D | Db, Da |
| Dust | Zone 22 | II | III | 3D, 2D, 1D | Dc, Db, Da |
| CH4/C-dust (mine) | – | I | I | M1 | Ma |
| CH4/C-dust (mine) | – | II | II | M2, M1 | Mb, Ma |
In IEC there is an equivalence of category and it is termed the equipment protection level (EPL). EPL stands for the level of protection assigned to the equipment. Like categories in ATEX they have different designations. From Table X/3.2.3-3 it is seen that in the ATEX category of protection, IEC-EPLs are designated by 1, 2, 3 and a, b, c, respectively. In both cases, gases are designated by “G” and dusts by “D” for Group II and all Group I items in both cases by M. In the ATEX category, designation is followed by G/D/M, whereas in IEC it is the reverse. To have a better understanding it is important to draw a comparison with US (NEC 506)/Canadian (CEC Annex J) systems of area (and material) classifications with respect to the foregoing (see Tables X/3.2.1-1 and X/3.2.3-3).
• Zone 0 and/or zone 1 in gas/vapor is equivalent to division 1 class I.
• Zone 0 and/or zone 1 in dust is equivalent to division 1 class II.
• Zone 0 and/or zone 1 in fiber (not separately shown) is equivalent to division 1 class III.
• Similarly, zone 2 gas/dust in the table is equivalent to division 2 classes I and II, respectively. Also there is division 2 class III for ignitable fiber.
3.3. Discussions on Miscellaneous Standards
A short discussion is presented here on various standards so that different approaches can be understood. Major standards in these connections shall include but not be limited to what has been listed below. It is worth noting that the standards more related to electrical and instrumentation systems have been listed. There could be other standards such as National Fire Protection Association (NFPA) 50A, 59, etc.
• NFPA:
• NFPA 30: Flammable and Combustible Liquids Code (cabling installations)
• NFPA 70: National Electrical Code (Chapter 5 article 500 series)
• NFPA 497: Classification of Flammable Liquids, Gases, or Vapors
• NFPA 499: Classification of Combustible Dusts
• ANSI C2: National Electrical Safety Code: section 127
• Factory mutual
• Underwriter's laboratory standard
• OSHA
• IEC standards
• IEC 60079-0 Explosive atmosphere
• IEC 60079-1: Gas—Flameproof enclosures “d”
• IEC 60079-2: Gas—Pressurized enclosures “p”
• IEC 60079-5: Gas—Powder filling “q”
• IEC 60079-6: Gas—Oil immersion “o”
• IEC 60079-7: Gas—Increased safety “e”
• IEC 60079-10: Electrical apparatus for explosive gas atmosphere
• IEC 60079-10-1 Explosive atmosphere (classification—explosive dust atmosphere)
• IEC 60079-11: Gas—Intrinsic safety “i”
• IEC 60079-15: Gas—Type of protection “n”
• IEC 60079-18: Gas and dust—Encapsulation “m”
• IEC 61241-1: Dust—Protection by enclosures “tD”
• IEC 61241-2 (IEC 61241-4): Dust—Pressurization “pD”
• IEC 61241-11: Dust—Intrinsic safety “iD”
• IEC 60079-14 for electrical installations
• IEC 60079-17 for electrical apparatus maintenance
• ATEX Directives
• ATEX 95
• ATEX 137
Major features of these standards will be outlined as guidelines only. It is extremely important to note that for application of any of these standards, it is necessary to consult the latest version of the relevant standard so that it may be interpreted correctly before its implementation.
3.3.1. Area Classification Principles/Procedures
Very briefly, the gist of classification procedures in various standards is enumerated in the following. It is necessary that the applicable code be consulted:
• IEC (60079-10): Usually, this process is undertaken after basic documents such as a piping and instrumentation diagram (P&ID), layout, and other related documents for pertinent properties of flammable materials are available. The following are the major steps toward this:
• Identification of source of release and its grade. It is better to consider all the applicable equipment (individually) caused by loss of containment.
• Selection of type of zones mainly based on the likelihood of the presence of gas, which is very much dependent on ventilation and grade of release.
• Decision about extent of zone, that is, distance from source (in all directions) up to which an explosive atmosphere exists before the material is dispersed. Rate of release (velocity, size, and geometry of sources), ventilation, and relative density of flammable material (mainly gas) are important considerations.
• Other influencing factors are topology of the area including barriers, etc. and climatic conditions.
• NFPA: This is also basically the same as what was discussed previously. The major steps are:
• Assembling all pertinent information such as P&ID, layout, flammable materials, operating description safety procedure, etc.
• Determination of class based on the properties of the flammable materials present.
• Based on likelihood of occurrence, divisions 1and 2 are determined.
• Extent of hazardous area is determined including boundary walls, etc.
• Documentation of EAC is determined.
• Documentation during classification procedure: The general documentations during area classification shall include but not be limited to the following. These are mainly based on ATEX directives but are generally applicable to others:
• Identification of person responsible
• Structural and geographical details about plant site
• Description of process and operational details
• Material data
• Risk assessment (risk analysis as in ATEX 137)
• Protection concept and principles
• Organizational measures, for example, training
3.3.2. Discussions on Zone Systems
Zone systems are followed in area classification by ATEX and IEC, and even NFPA has included zone systems of classification in their standard. The zone classification system probably has the edge over the class/division system because many consider the zone classification system to be more flexible and safer in hazardous locations. However, zone systems do not take care of the consequence of release. In case the consequences are important, as per the advice of the HSE.UK, it is better to go for a more conservative equipment selection. Although not officially accepted yet, various sources have tried to place time limits on these zones. The most commonly used values are as shown in Table X/3.3.2-1 based on probability and duration of explosive atmosphere.
3.3.3. ATEX Directives—Explanations
As discussed earlier, the European Union has adopted two harmonized directives on health and safety, known as ATEX 94/9/EC (also ATEX 95) and ATEX 99/92/EC (also ATEX 137). ATEX Directive 94/9/EC focuses on the essential safety requirements (SRSs) for products and protective systems for use in potentially explosive atmospheres and the respective conformity assessment procedures. ATEX Directive 99/92/EC, on the other hand, looks after the minimum health and safety requirements for workplaces with a potentially explosive atmosphere. In ATEX 95, entire systems have been divided into groups and categories as already discussed. In Directive 137, workplace area has been divided into zones. Use of various categories of equipment in various zones is elaborated in Table X/3.2.3-3. The basic purposes of each of these directives are elaborated in Fig. X/3.3.3-1. This figure also shows the correlation between the two directives so readers can have better understanding of the directives with their distinctive features.
3.3.4. Protection Selection
Protection selections in accordance with various standards shall be discussed here to help in electrical equipment selections.
• ATEX: The ATEX concept of protection is elaborated in Table X/3.3.4-1. This may be compared with Table X/3.3.4-2 for IEC.
• EPL IEC: So far as IEC is concerned the various protection types and associated IEC standard references have been elaborated at the beginning of Clause 3.3 (under IEC standard). The EPL for gas with applicable zone and protection principles is elaborated in Table X/3.3.4-2 to help in the selection process. For EPL symbols see Clause 3.2.3. Ingress protections (IPs) mentioned in the table are covered in Clause 3.6 of this chapter.
Table X/3.3.4-1
| Concept | Code | Zone | Category | Principle of Protection |
| Increased safety | Ex e | 1, 2 | 2, 3 | No arc, spark, or hot surface |
| Nonsparking | Ex nA | 2 | 3 | No arc, spark, or hot surface |
| Flame proof | Ex d | 1, 2 | 2, 3 | Contains the explosion. Quenches the flame |
| Enclosed break | Ex nW | 2 | 3 | |
| Quartz/sand filled | Ex q | 1, 2 | 2, 3 | Contains the explosion. Quenches the flame |
| Intrinsic safety | Ex ia | 0, 1, 2 | 1, 2, 3 | Limits both the energy of spark and the temperature |
| Intrinsic safety | Ex ib | 1, 2 | 2, 3 | |
| Energy limitation | Ex nL | 2 | 3 | |
| Pressurize | Ex p | 1, 2 | 2, 3 | Keeps the flammable gas from hot surface and ignition-capable equipment |
| Simplified pressurization | Ex nP | 2 | 3 | |
| Encapsulation | Ex m | 1, 2 | 2, 3 | Keeps the flammable gas from hot surface and ignition-capable equipment |
| Oil immersion | Ex o | 1, 2 | 2, 3 | Keeps the flammable gas from hot surface and ignition-capable equipment |
| Restricted breathing | Ex nR | 2 | 3 | |
| Special | Ex s | 0, 1, 2 | 1, 2, 3 | Any proven method |
The EPL for dust with applicable zone and protection principles is elaborated in Table X/3.3.4-3 to help in the selection process.
• Protection concept NFPA: NFPA 70 (NEC) article 500 (500-7) 2012 specifies various explosion equipment for electrical apparatus as listed in Table X/3.3.4-4, where sequential details as per NFPA 70 article 500-7 A through L are given.
The discussion on hazardous area standards is concluded with the recommendation that the applicable standard part must be read thoroughly before starting any equipment selection. Since area classification is mainly carried out by mechanical and safety engineers, good coordination is helpful. Gas detection systems for hazardous locations is another area of concern for instrumentation and control engineers so brief discussions on combustible gas detection systems will be dealt with in the next clause.
3.4. Combustible/Flammable Gas Detection
Combustible liquid and flammable liquids are actually not same although the two terms are frequently used interchangeably. Some of the differences between combustible liquid and flammable liquids are given in Table X/3.4-1.
Table X/3.3.4-2
IEC EPL Protection Techniques (Gas) (See Also Appendix II)
| Method | Code Ex | EPL | Zone | IECa | Principle of Protection |
| Increased safety | e | Gb | 1, 2 | 7 | No arc, spark or hot surface. IP54 or better |
| Type n (nonsparking) | nA | Gc | 2 | 15 | |
| Flame proof | d | Gb | 1, 2 | 1 | Contains the explosion. Quenches the flame |
| Type n (enclosed break) | nC | Gc | 2 | 15 | |
| Quartz/sand filled | q | Gb | 1, 2 | 5 | Quenches the flame |
| Intrinsic safety | ia | Ga | 0, 1, 2 | 11 | Limits energy of spark and surface temperature |
| Intrinsic safety | ib | Gb | 1, 2 | 11 | |
| Intrinsic safety | ic | Gc | 2 | 11 | |
| Type n (energy limiting) | nL | Gc | 2 | 15 | |
| Pressurized | p, px | Gb | 1, 2 | 2 | Keeps the flame gas out |
| py | Gb | 1, 2 | 2 | ||
| pz | Gc | 2 | 2 | ||
| Type n (hermetic sealing) | nC | Gc | 2 | 15 | |
| Type n (restricted breathing) | nR | Gc | 2 | 15 | |
| Type n (simple pressurized) | nZ | Gc | 2 | 15 | |
| Encapsulation | ma | Ga | 0, 1, 2 | 18 | Keeps the flame gas out |
| Encapsulation | mb | Gb | 1, 2 | 18 | |
| Oil immersion | o | Gb | 1, 2 | 6 | Keeps the flame gas out |
Table X/3.3.4-3
IEC EPL Protection Techniques (Dust) (See Also Appendix II)
| Method | Code Ex | EPL | Zone | IECa | Principle of Protection |
| Enclosure | t | Da, Db, Dc | 20 | 31 | Standard protection for dust-tight enclosure |
| Intrinsic safety | i | Da, Db, Dc | 21 | 11 | Same as above, some relaxation for intrinsic circuit |
| Encapsulation | m | Da, Db, Dc | 22 | 18 | Protection by encapsulation of incendive part |
| Pressurized | p | Db, Dc | 21, 22 | 2 | Protection by pressurization of enclosure |
Table X/3.3.4-4
Protection Concepts Class Division System
| Method | Class | Division | Remarks |
| Explosion proof | I | 1, 2 | |
| Dust ignition proof | II | 1, 2 | |
| Dust tight | II | 2 | |
| III | 1, 2 | ||
| Purged and pressurized | – | – | Classified area for which it is identified |
| Intrinsic safety | I | 1, 2 | |
| II | 1, 2 | ||
| III | 1, 2 | ||
| Nonincendive circuit | I | 2 | |
| II | 2 | ||
| III | 1, 2 | ||
| Nonincendive equipment | As above | ||
| Nonincendive component | As above | ||
| Oil immersion | I | 2 | |
| Hermetically sealed | I | 2 | |
| II | 2 | ||
| III | 1, 2 | ||
| Combustible gas detector | Means of protection in industry with restricted public access elec | ||
| Inadequate ventilation | Classified as class 1 division 1 for inadequate ventilation. Electrical apparatus class 1 division 1 | ||
To develop an appropriate strategy to combat the hazards of combustible/flammable vapor or gas it is essential to have a better understanding of the basic properties of combustible materials. So, discussions on combustible/flammable gas detection systems start with the general properties necessary for determining a material's hazardous potential, then attention will turn to detection systems and their location, etc.
Table X/3.4-1
Differences Between Combustible and Flammable Liquids
| Characteristics | Combustible (Vapor) Liquid | Flammable (Vapor) Liquid |
| Flash point | Flash point >37.8°C | Flash point <37°C Vapor pressure <276 kPa |
| Typical nature | When heated shows many similar characteristic of flammable gas | Volatile vapor heavier than air |
| Nature of vapor | Remains near source till atmospheric temperature > flash point | Flows down in a slope and is collected at low point |
3.4.1. General Properties of a Combustible/Flammable Gas System
To gather knowledge about the properties of combustible/flammable gases it is necessary to address a few terms associated with creating combustible/flammable atmospheres:
• Flash point: Discussions on flash point were covered in Clause 3.1.1. Many hazardous liquids have flash points at or below room temperature and are very dangerous. These liquids are covered by a layer of flammable vapors that will ignite immediately if exposed to an ignition source. Open cup and closed cup are testing methods for flash points of liquids. The former system is less accurate but useful. Acetylene, ammonia, hydrogen, and LPG are examples of common explosive gases. However, there are many other explosive gases encountered in industries.
• Autoignition temperature: Also known as spontaneous ignition temperature, autoignition temperature is defined as the minimum temperature for self-sustained combustion of a substance, independent of heating. See also Clause 3.2.1 and Table X/3.2.1-4.
• Explosion range, LEL and UEL: See Clause 3.1.2 for detailed discussions and explanations. A few characteristics regarding LEL and UEL are important:
• LEL: Actual LEL for different gases varies widely. LEL is measured as a percentage by volume in air and typical values are 1.4–5% by volume. When the temperature is increased, less energy is necessary, hence the LEL percentage by volume decreases.
• UEL: In the case of an oxygen-enriched condition, there will be a rise in UEL of gas and its rate and propagation intensity will also increase. A high level of O2 increases the flammability of any material.
• Vapor density: The vapor density of gas is the density of the gas in comparison to air = 1. So vapor density can be defined as the weight ratio of a volume of flammable vapor compared to an equal volume of air. [Vapor density <1 (air) will rise, and vapor density >1 will fall.] This is important in the placement of detectors. Major flammable vapors are heavier than air. CH4 (0.55), CO (0.6), and NH3 (0.6) are lighter than air, whereas petroleum vapor (3.0), H2S (1.45), and propane (2.48) are heavier than air.
• The relation between vapor pressure and boiling point is noted in the following:
• Vapor pressure: Some of the molecules at the surface of a liquid have enough kinetic energy to escape to the surrounding atmosphere. These molecules naturally exert pressure at the wall of the closed container. Depending on temperature, this pressure of escaped molecules will increase up to a certain threshold. When this threshold is reached, the space is considered to be saturated. At saturation, rate of molecules escape equals they return. Vapor pressure is the pressure exerted by these escaping molecules when the molecules leave the surface at the same rate at which they return. Naturally, when the intermolecular forces in the liquid are less, then the molecules can easily escape and the liquid will have a higher vapor pressure. The reverse is noted when vapor pressure is low. Low vapor pressure means there are fewer molecules of the substance to ignite, so there is generally less of a hazard present. However, from an instrumentation point of view, this is more challenging, and a highly sensitive instrument will be required as there are fewer molecules present to be detected. If the vapor pressure is higher, then there will be more molecules to ignite, hence the hazard will be greater. Vapor pressure is temperature dependent. The higher the temperature, the higher will be the vapor pressure. Also the nature of dependency on temperature varies with the gas types.
• Boiling point: The boiling point of a liquid may be defined as the temperature at which it changes state from a liquid to a gas throughout the bulk of the liquid. Also the boiling point of a liquid may be defined as the temperature at which the vapor pressure of the liquid equals the external environmental pressure surrounding the liquid. This is because at that point, bubbles of vapor that form below the surface of the liquid contain vapor at a pressure that matches the external pressure. Therefore the bubbles are not crushed by the surrounding liquid and their buoyancy causes them to rise through to the surface of the liquid and give the familiar appearance of a boiling liquid.
• Interrelation: If there are high intermolecular forces, the molecules will be strongly attracted to each other, so fewer will enter the gas phase, and the vapor pressure will be low. This means that more heat will be required to separate the molecules; hence the boiling point will be higher. On the contrary, if the liquid has a high vapor pressure, little heat will have to be added to separate the molecules and the boiling point will be low. Therefore it may be concluded that the higher the vapor pressure of a liquid at a given temperature, the lower the normal boiling point of the liquid.
Now let us explore combustible/flammable gas detection systems and their associated technology.
3.4.2. Combustible/Flammable Gas Detector
Combustible gas detectors are required whenever there is a possibility of a hazard to life or property caused by the accumulation of combustible gases. There are mainly two types of combustible/flammable gas detectors that are often used in most industries. These are the low-cost catalytic bead sensor and infrared sensor [26,27].
• Catalytic bead-type detector: This is a low-cost detection system used to detect combustible gases. The system uses a catalytic bead to oxidize combustible gases. On account of combustion there will be a change in temperature, which is detected by resistance with the help of a Wheatstone bridge. Typical sensor and measuring principles are shown in Fig. X/3.4.2-1.
Figure X/3.4.2-1 Catalytic bead flame detector. (A) Catalytic bead sensor details, (B) catalytic bead sensing operation.
The system consists of a very small sensing element popularly known as a “bead.” Beads are made of platinum wire coil, covered first with a ceramic base like alumina and then with a final outer coating of palladium or rhodium catalyst (Fig. X/3.4.2-1A). When a combustible gas/air mixture passes over the hot catalyst surface, combustion occurs and the heat evolved increases the temperature of the “bead.” This in turn alters the resistance of the platinum coil. The change is measured by a Wheatstone bridge stated earlier. The resistance change is then directly related to the gas concentration in the surrounding atmosphere. As is typical in a Wheatstone, imbalance current can be sent to the transmitter or controller for further control and display. The necessary requirements of design safety mean that the catalytic type of sensor has to be mounted in a strong metal housing behind a flame arrestor to prevent propagation of flame outside. This slightly slows down the response. Typical response time is around 20–30 s. To ensure temperature stability under varying ambient conditions, the best catalytic sensors use thermally matched beads, as shown in the figure. The instrument sensitivity is affected by poisoning and blockage of the flame arrester. These are calibrated in terms of % LEL. For calibration and its implications refer to EN 60079-29-2.
• Infrared sensors: Although these are costlier with respect to catalytic bead sensors they offer a number of advantages such as faster response (∼10 s), low maintenance, self-checking with the help of microprocessor circuitry, and measurement does not depend on O2 to detect the gas. It may be designed in such a way to be unaffected by poisonous gases. This system can be developed with a single or dual beam, as shown in Fig. X/3.4.2-2.
The technique operates on the principle of dual wavelength infrared absorption. Light passes through the sample mixture at two wavelengths, one of which is set at the absorption peak of the gas to be detected, while the other is not. The two light sources are pulsed alternatively and guided along a common optical path to emerge via a flameproof “window” and then through the sample gas. The beams are subsequently reflected again by a retroreflector, returning once more through the sample and into the unit. Here a detector compares the signal strengths of sample and reference beams and by subtraction can give a measure of the gas concentration [28]. Internal compensation eliminates the chances of drift.
On account of gas leaks, the cloud created could be a relatively stationary or readily dissipated type. This depends on factors like wind, rate of leak, density of leaked gas, and the structural environment around the leak. Gas leak clouds normally have the following characteristics:
• The highest gas concentration is at the source and decreases toward the edge.
• The shape of the gas cloud is dependent on air current.
• Outdoor environment gas clouds dissipate faster and hence have low gas concentrations.
Open path detection is very important in applications like floating production storage and offloading vessels, loading/unloading terminals, pipelines, perimeter monitoring, offshore platforms, and liquid natural gas storage areas. The conventional method of gas leak detection is point type detection. A number of individual sensors are used to cover an area or perimeter. Over time, however, instruments have become available that make use of infrared and laser technology in the form of a broad beam (or open path), which can cover a distance of several hundred meters [28]. Open path applications are affected by fog and rain. In today's instruments, necessary means have been incorporated to compensate for these effects. A brief comparison between both types of instruments is presented in Table X/3.4.2-1.
So, it is necessary to assess the site risk and prime objective, then to identify the gas to be detected, and finally, based on requirement, choose the detector taking into account the financial aspect.
Table X/3.4.2-1
Comparison Between Catalytic Bead and Infrared Instruments
| Points of Comparison | Catalytic Bead Sensing | Infrared Sensing |
| Inert atmosphere | Requires O2 (combustion) | Yes |
| Susceptibility to poison | No | No but less affected |
| Diatomic gas, e.g., H2 | Yes | No |
| Dust and drift immunity | Yes with proper protection | Yes with proper protection |
| O2-enriched condition | Yes | Yes |
| Speed of response | 20–30 s (typical) | <10 s (typical) |
| Cost | Less costly | Higher capital investment |
| Maintenance requirement | High | Low |
3.4.3. Combustible Detector Placements
The placement of detectors should be determined following the advice of experts having sound knowledge of both gas dispersion and process/equipment and safety. All the agreed locations must be recorded. Obviously, detectors shall be located where the gas is most likely to be present. In this instance, Clause 3.6 of Chapter VIII may be referred to. Major issues in this regard shall include but not be limited to the following:
• To detect gases that are lighter than air (e.g., NH3, CH4), detectors should be mounted at high level and preferably use a collecting cone, whereas gases heavier than air (e.g., SO2) should be mounted at a low level as discussed.
• To study the behavior of escaping gas against natural or forced air currents, detectors may be mounted in ventilation ducts if appropriate.
• Process condition is an important consideration. Also it is better to locate the detector away from high pressure parts to avoid the formation of clouds.
• Ease of access for functional testing and servicing is recommended.
• To avoid dust, it is better to locate the detector at a suitable location with the detector pointing downward.
• When installing open path infrared devices it is important to ensure that there is no permanent obstruction or blocking of the infrared beam. Short-term blockage from vehicles, site personnel, birds, etc. can be accommodated [28].
• Structures that open path devices are mounted to should be sturdy and not susceptible to vibration [28].
We will now concentrate on explosion prevention/protection principles.
3.5. Explosion Protection
The main aim of explosion protection in instrumentation is to prevent ignition sources or the coming together of such sources with potentially explosive atmospheres. The other two factors of explosion (Fig. X/3.1.1-1)—the oxygen in the air and often the flammable substance—cannot be reliably and permanently ruled out in workplaces. Therefore the only way left is to isolate the source of ignition from the explosive atmosphere. In primary explosion protection, it is best to substitute something for the flammable substances or the atmospheric oxygen, or reduce their quantities to the point where there is no danger of an explosive mixture forming. Increased air circulation is another way of achieving this. These are achieved by structural measures, for example, the open layout of filling stations. Removal/replacement of atmospheric oxygen cannot be an option for areas where people work. For such locations the options available shall include:
• Avoidance or restriction of flammable substances
• Avoidance or restriction of release of flammable substances by:
• Limiting their concentration
• Using enclosures filled with an inert substance—natural or artificial ventilation
• Concentration monitoring by means of a gas detection system.
3.5.1. Basic Protection Principles (Fig. X/3.5.1-1)
Protection principles focus on the ways and means to exclude equipment and components as ignition sources. Ignition sources are prevented in explosion protected equipment by selecting appropriate materials and by constructive measures. Another way is to limit the energy level from the equipment to prevent explosion. All these must be verified and confirmed by the appropriate tests.
Generally, four protection principles as detailed in Fig. X/3.5.1-1 are used to prevent equipment from any explosion by protecting it and limiting its energy so that it cannot become a source of ignition. In all the protection principles depicted in Fig. X/3.5.1-1, it is necessary to ensure that an explosive atmosphere must not be able to reach nonpermitted temperatures with respect to the ignition temperature of substances present in the surrounding atmosphere. The alternative is that all propositions may not hold good. This also shows the relevance of the ignition temperature in all protection principles. Universal uniform requirements have been formulated by international standardization committees to combat hazards from flammable material emerging because of physical/chemical processes. Manufacturers and operators are required to adhere to these, and where there are increased protection requirements, they are monitored by notified bodies and authorities [23]. For wiring methods and cable standards in explosive atmospheres, guidelines from IEC session 13 [35] may be referred to. Explosion protection principles are applicable to fieldbus systems also, and this is where our attention will now turn.
3.5.2. Explosion Protection for Fieldbus
Conventional systems are mainly based on two aspects: each device has a separate barrier and they shall be wired separately from other field circuits. Probably the first definition of the required parameter for intrinsic application in a fieldbus system was done in the FOUNDATION fieldbus. Fieldbus devices are low-power devices, so IS is very much suitable for fieldbus installations in hazardous areas. The following are distinct features found in fieldbus intrinsic applications:
• There can be several devices connected through a single barrier; similarly, several barriers may be used for a single device.
• The same segment may have IS as well as other devices.
• Alike, conventional system, a fieldbus has the same philosophy of limiting energy.
• Fieldbus IS has more flexibility and is cost effective.
There are several approaches for fieldbus explosion protection:
• IS barrier for fieldbus
• Entity model
• Fieldbus intrinsic safety concept (FISCO)
• High-power trunk (HPT) concept
• Dynamic arc recognition and termination (DART)
• Fieldbus nonincendive concept (FNICO)
Only brief conceptual discussions on these are presented because of limited space. For more detail, standard literature on fieldbus systems may be consulted.
• IS (multiple) barriers in fieldbus system: A typical fieldbus IS connection is shown in Fig. X/3.5.2-1A. The major points are:
• Avoidance of single point failure
• Integration of shielding and grounding
• Current and power limitations to field device
• Distance limitation of cable length
A trunk and spur topology is used. In a typical fieldbus installation, 10–12 fieldbus devices are connected via one cable with length up to 1900 m. This single cable needs to support power and communications. It is recommended to use short-circuit protection and energy limitation to isolate a fault condition from negatively affecting device communications on that segment. As seen in the figure, there are a few fieldbus barriers being fed by one power supply. Each of the spurs connected to one field barrier is Ex i. From the ignition curve described in Fig. X/3.5.2-3 it is clear that based on voltage there will be a limit in the maximum current drawn from each of the field barriers. Typically, 110 mA are allowed in gas groups A and B and 235 mA in groups C and D [31]. Again various devices (at 24 V power supply) like pressure/differential pressure transmitters (20 mA), temperature transmitters (16 mA), valves (25 mA), and flow meters (10 mA) have different current consumptions (typical current consumption indicated in parentheses). So, while designing a system with a number of these devices in use, it is necessary to combine them under each barrier so that the maximum allowed current is not exceeded. Maximum cable length, from a voltage drop point of view, is also an important consideration. This discussion applies to typical IS in a fieldbus.
Figure X/3.5.2-1 Fieldbus intrinsic safety. (A) Multiple barriers fieldbus connection (typical), (B) FISCO concept, (C) HPT concept. HPT, high-power trunk.
Figure X/3.5.2-2 Dynamic arc recognition and termination (DART) and fieldbus nonincendive concept (FNICO) for fieldbus protection. (A) Dart with charactistic curves, (B) FNICO connection system (for zone 2).
• Entity model: The entity model defined in IEC 60079-1 is meant to use intrinsically safe parameters for validation of IS of a fieldbus. The entity model considers that all electrical parameters of the wire are concentrated at the point of a fault and electrical wire is a source of energy. In this method, 83 mA maximum current and 18.4 V maximum voltage are allowed. This model does not enjoy much popularity. Only a few power supply-conforming entity models are available. At best, these power supplies support two to three devices [32].
• FISCO: A typical FISCO model is depicted in Fig. X/3.5.2-1B. In this, various field devices are connected to an active junction box with Ex i spurs and Ex i FISCO trunk cable; also the topology provides isolation for intrinsically safe area equipment, as shown in the figure. In the topology of FISCO, there shall be isolation between safe and hazardous areas. Active junction boxes have segment protectors (current limiting devices). There are active junction boxes with foldback short-circuit protection. Suitable selection is important. These isolate a single short from affecting the entire network. Limitation is 200 mA @ 17 V for group IIB.
• High-power trunk concept: In the foregoing, FISCO power supply redundancy is not possible, whereas in the case of a high-power trunk redundancy is possible. In this method, Ex e has been considered for the trunk connection cable. This topology provides the highest possible cable length and at the same time supports the largest number of field devices per segment. However, spur cables are the same as FISCO. Typical current limitation is higher at 500 mA @ 31 V.
• DART: This is a newer concept in IS of the fieldbus system. To understand the system it is better to look at the curve showing how an arc is developed, as shown in Fig. X/3.5.2-2A. According to IEC 60079-11 a circuit is considered intrinsically safe when “electrical energy within the apparatus and of interconnecting wiring exposed to the potentially explosive atmosphere is restricted to a level below that which can cause ignition by either sparking or heating effects.” DART detects the characteristic behavior of a spark, especially the sharp current change di/dt in the initial phase and extinguishes the spark before it becomes incendive. It uses Ex i in both trunk and spur. Redundant power supply is possible. The maximum current limit is 35 mA @ 24 V.
• FNICO: This is applied in zone 2/division 2 hazardous areas, where the explosion hazard is expected to exist only in abnormal circumstances. At these locations, application of IS is not a compulsion. The type of protection “n” (see Clause 3.6) for zone 2 and nonincendive protection for division 2 are well-established techniques. The financial benefits of a zone 2 approach for instrumentation are seen in reduced capital expenditure and lower operating costs during the life of the plant [30]. Here, Ex nL is used and is applicable for zone 2 applications. A typical comparison of Ex i and Ex nL is presented in Table X/3.5.2-1.
Table X/3.5.2-1
Comparison of Fieldbus Protection by Ex i and Ex nL
| Protection | Code | Fault Tolerance | Applicable Zone |
| Energy limited (see Clause 3.6) | Ex e | 0 | Zone 2 |
| Intrinsic safety ib (see Clause 3.7) | Ex ib | 1 | Zones 1 and 2 |
| Intrinsic safety ia (see Clause 3.7) | Ex ia | 2 | Zones 0, 1, and 2 |
Typical FNICO is shown in Fig. X/3.5.2-2B. The major components are FNICO power supply, field cable, and field cable termination devices. Power supply connections to the host control system are made at its “safe area” terminals, and those to the field trunk at its “hazardous area” terminals. It also includes the necessary functions for a reliable fieldbus with those of an energy-limited interface. This incorporates a repeater function, connections for 24 VDC supply input, and a switchable terminator [30]. Field terminators take the form of a DIN rail-mounted terminal module. In an FNICO system, the requirements are simple: the wiring hub and its enclosure must be certified for zone 2 or division 2 as appropriate, and be suitable for the environment.
• Other important issues: From these discussions it is clear that each of the systems has some limitations. Such limitations are compared in Table X/3.5.2-2.
The parameters in the table are standard values; however, on account of power supplies available in the market, in reality the values are far less than standard values. Since the foregoing discussions are mainly based on IS and energy limiting devices it is better to look in detail at the various enclosures to cover the various protection principles described (in Fig. X/3.5.1-1) in Clause 3.6 and then look at the details of IS circuits in Clause 3.7.
Table X/3.5.2-2
Comparison of Various Fieldbus Protection Concepts [29]
Fieldbus Parameter | IS Entity | FISCO | HPT | FNICO | ||
| Gr. IIB | Gr. IIC | Gr. IIB | Gr. IIC | |||
| Maximum current (mA) | 80 | 265 | 120 | 500 | 320 | 180 |
| Maximum devices | 4 | 13 | 6 | 25 | 16 | 9 |
| Maximum trunk length (ma) | 1900 | 1900 | 1000 | 1900 | 1900 | 1000 |
| Maximum spur length (ma) | 120 | 60 | 60 | 120 | 60 | 60 |
| Hazard zone | 0 and 1 | 1 | 1 | 0 and 1 | 2 | 2 |
3.6. Enclosure Class
From Fig. X/3.5.1-1 it is seen that the majority of explosion protections are undertaken with the help of various kinds of enclosures. In the figure there is another method of protection, namely, IS, which is extremely important for instrumentation, and this will be dealt with greater detail in the following Clause 3.7. While discussing enclosure class, it is obvious that ingress protection (IP) is very important. So, short discussions on this have been included also. During discussions, necessary references will be touched upon. Also it is necessary to note that nonelectrical protections are out of the scope of this discussion. Within electrical protection, the main focus will be on instrumentation. For each of the enclosures, applicable standards and broad areas of application are mentioned in the corresponding figures.
3.6.1. Flameproof Enclosure
See Fig. X/3.6.1-1 for a flameproof enclosure. It is marked as Ex d.
• Standards and application: Refer to the figure.
• Principles: Parts that can ignite a potentially explosive atmosphere are located inside an enclosure, which can withstand the pressure of an explosive mixture inside. Also enclosure prevents the transmission of the explosion to the atmosphere surrounding the enclosure. So, the major issue is related to the gap during manufacturing so that these are either sealed or explosion propagation is decelerated.
• Design considerations: The following points may be considered:
• Mechanical strength in line with the safety factor considered for internal explosion pressure: Depending on applicability this could 1.5 times.
• Gap: When a gap is not sealed after manufacture, then it shall (if at all) be very narrow and long to retard the propagation of explosion to the outside atmosphere. Gap parameters vary with gas groups and are stringent for group IIC.
3.6.2. Increased Safety
See Fig. X/3.6.2-1 for increased safety. It is marked as Ex e.
• Standards and application: Refer to the figure.
• Principles: Additional measures are applied to increase the level of safety protection to ensure prevention of unacceptably high temperatures and sparks or electrical arcs, both on the internals of the enclosure and on exposed parts of electrical apparatus, where such ignition sources would not occur in normal service.
• Design considerations: The following points may be considered:
• Protective requirements for the uninsulated, live parts
• Wider air gap and special requirements for IP (discussed later)
• Minimum cross-sections for winding wires
3.6.3. Pressurized Enclosure
See Fig. X/3.6.3-1 for pressurized enclosure. It is marked as Ex p (Ex pD for dust application).
• Standards and application: Refer to the figure.
• Principles: In relation to the surrounding atmosphere, positive internal pressure of protective gas (air, inert, or a different suitable gas) is maintained inside the enclosure to prevent formation of a potentially explosive atmosphere. As necessary, a flow of protective gas, which acts to dilute any combustible mixtures, is supplied.
• Design considerations: The following points may be considered.
• Strength to withstand at least 1.5 times overpressure
• Alert when flushing gas fails
3.6.4. Oil Immersed
See Fig. X/3.6.4-1 for oil immersed. It is marked as Ex o.
• Standards and application: Refer to the figure.
• Principles: Electrical apparatus or parts of electrical apparatus, which might ignite an explosive atmosphere, are immersed in oil or other nonflammable, insulating liquid so that a potentially explosive atmosphere above the oil level and/or outside the enclosure cannot be ignited by electric arcs or sparks generated.
• Design considerations: The following points may be considered:
• Protection of oil/liquid from contamination and moisture
• Means of monitoring oil level
• Not applicable for portable apparatus
3.6.5. Powder Filling
See Fig. X/3.6.5-1 for powder filling. It is marked as Ex q.
• Standards and application: Refer to the figure.
• Principle: By filling the enclosure with a finely grained powder, any arc within the enclosure under normal conditions is unable (with correct use) to ignite a potentially explosive atmosphere surrounding the enclosure. There should not be any risk of ignition by flames, nor by increased temperatures at the surface of the enclosure.
• Design considerations: Filling is normally done with sand or glass balls, which should not leave the enclosure under any circumstances.
3.6.6. Encapsulated
See Fig. X/3.6.6-1 for encapsulated. It is marked as Ex m for gas and mD for dust.
• Standards and application: Refer to the figure.
• Principle: Parts that are capable of igniting an explosive atmosphere by either sparking or heating are encapsulated in a compound in such a way as to avoid ignition of a dust layer or cloud. This compound is normally resistant to physical—especially electrical, thermal, and mechanical—and chemical influences.
• Design considerations: The following points may be considered:
• High breakdown strength
• Low water absorption
• Resistant to chemical influence
• Casting compound penetrated only by the cable entries
3.6.7. Type of Protection “n-”
See Fig. X/3.6.7-1 for type of protection “n-.” This consists of a number of types of protection, as listed in the figure.
• Types, standards, and application: Refer to the figure.
• Principles: In this case the electrical apparatus cannot ignite potentially explosive atmospheres surrounding them in normal and defined abnormal situations. These are applied for zone 2 applications:
• nA: Unacceptably high temperatures and sparks or electrical arcs, on both the internal and external parts of electrical equipment are prevented by construction. Also normal operation does not call for high temperatures, sparks, etc. These are achieved by applying special protective layers on uninsulated live parts, specifying the proper air gap.
• nC: It is so constructed that an explosive atmosphere cannot enter. It is hermetically sealed—sealing by welding/solder, glass fusion, etc. Contacts of some process switches offer this kind of sealing.
• nL: Energy limiting apparatus: These are very much in use in fieldbus systems in zone 2 discussed earlier (FNICO). These are circuits in which there is no spark or thermal effect occurring under normal operation and certain fault conditions stipulated in the standard. However, they can ignite the explosive atmosphere of subgroups IIA, IIB, and IIC or of an air/dust mixture only when energy level exceeds stipulated value. The permissible currents or voltages exceed those stipulated for the IS type of protection. Also the requirements to be fulfilled by the circuit and the loads on the components are lower than those for the IS type of protection.
• nR: Restricted breathing: The construction does not permit ingress of explosive gases.
• nZ: Purging pressurization: It is protected by purging and pressurization.
3.6.8. Ingress Protection
Enclosure types: Mainly two standards, namely, IEC 60529 and National Electrical Manufacturer Association (NEMA), are used in industries for enclosures pertinent to electrical systems. Both these standards are covered briefly during the discussions. Discussions will be concluded by providing a comparison between them (Fig. X/3.6.8-2):
• IEC: According to IEC 60529 definitions for degrees of protection provided by enclosures of electrical equipment as regards protection cover:
• Access by persons to hazardous parts inside the enclosure
• Ingress of solid foreign objects
• Harmful effects caused by the ingress of water
The standard also puts distinct designations for these degrees of protection and requirements for each designation. The tests to be performed to verify that the enclosure meets the requirements of this standard are also prescribed in the standard. The standard recommends the necessary measures to be taken to protect the enclosure and equipment inside against external influences like:
• Mechanical impact
• Corrosion
• Corrosion solvent
• Fungus
• Vermin
• Solar radiations
• Moisture
• Icing
• Explosive atmospheres
Various IP code designations are depicted in Fig. X/3.6.8-1—developed based on IS/IEC 61529:2001.
Thus from the figure it is clear that the first letter code indicates protection against the back of hand, finger, tool, and wire. Also it indicates protection of equipment from foreign bodies of different sizes and dust. Detailed explanations of protections are important and it is recommended to consult the standard prior to application. For protection against people and foreign bodies, Tables 1 and 2 of the standard may be referred to. Similarly, Table 3 of the standard defines protection against water. The harmful effect of water and protection against the same is represented by the second letter. In all the cases, relevant test method references are also indicated. So, an enclosure of IP 66 stands for protection against wire, and equipment in the enclosure is protected against dust and powerful jets of water. It also stands for protection against corrosion.
NEMA: This organization has a standard for enclosures (standard 250—latest edition 2014). Similar to IEC the individual rating is identified against various conditions, as shown in Table X/3.6.8-1. One important point is that NEMA specifies indoor and outdoor separately, and for outdoor a corrosion test is done.
A chart showing conversion of NEMA to IP and vice versa is presented in Fig. X/3.6.8-2 (based on NEMA 250:2003).
3.6.9. Markings
Typical markings in line with IEC 60079 are depicted in Fig. X/3.6.9-1. The typical meaning of each of the markings is indicated in the figure.
Normally, the following information is marked on the plate:
• Manufacturer’s name details and model and serial number, etc.
• Conformity mark and ID number
• Designation for identification
• Application zone including:
• Group, vapor/dust/mine
• Categories of approval for specific zones
• Type(s) of protection the equipment fulfills
• Explosion group and subgroup
• Temperature class
• Ambient specification
• Test laboratory where the test certificate was issued
• Standard with versions for certification
Some examples:
• Equipment group II other areas
• Category 2 suitable for zone 1, 21
• Gases, vapors marking with prefix G
• Dusts marking with prefix D
• Enclosure protections are important for instrumentation, so further details are given in Appendix II (Fig. APII/2.3-1). IS is very important from an instrumentation point of view, and will now be the focus of the next clause.
Table X/3.6.8-1
NEMA Enclosure Rating With Interpretation
| Type | I | O | Protection Against: Personnel Access | Protection (Ingress): Foreign Solid Object | Protection (Ingress): Water Harmful Effect |
| 1 | X | To hazardous part | Falling dirt | ||
| 2 | X | To hazardous part | Falling dirt | Dripping/light splashing | |
| 3 | X | X | To hazardous part | Falling dirt, windblown dust | Rain, sleet, snow, and external ice formation |
| 3R | X | X | To hazardous part | Falling dirt | Rain, sleet, snow, and external ice formation |
| 3S | X | X | To hazardous part | Falling dirt, windblown dust | Rain, sleet, snow. Operable when ice laden |
| 3X | X | X | To hazardous part | Falling dirt, windblown dust | Rain, sleet, snow, and external ice formation and additional corrosion protection |
| 3RX | X | X | To hazardous part | Falling dirt | Rain, sleet, snow, external ice formation, and additional corrosion protection |
| 3SX | X | X | To hazardous part | Falling dirt, windblown dust | Rain, sleet, snow, external ice formation, and additional level of protection for corrosion. Operable when ice laden |
| 4 | X | X | To hazardous part | Falling dirt, windblown dust | Rain, sleet, snow. Splashing and hose-directed water. Undamaged because of external ice formation on enclosure |
| 4X | X | X | To hazardous part | Windblown dust | Rain, sleet, snow. Splashing and hose-directed water. Additional level of protection for corrosion and undamaged because of external ice formation on enclosure |
| Table Continued | |||||
| Type | I | O | Protection Against: Personnel Access | Protection (Ingress): Foreign Solid Object | Protection (Ingress): Water Harmful Effect |
| 5 | X | - | To hazardous part | Falling dirt and settling airborne dust, lint, fibers and flying | Dripping and light splashing |
| 6 | X | X | To hazardous part | Falling dirt | Hose-directed water, entry of water because of occasional temporary submersion at a limited depth, and undamaged because of external ice formation on enclosure |
| 6P | X | X | To hazardous part | Falling dirt | Hose-directed water, entry of water during prolonged submersion at a limited depth. Additional level of protection for corrosion and undamaged because of external ice formation on enclosure |
| 12/12Ka | X | To hazardous part | Falling dirt, circulating dust, lint, fibers, flyings | Dripping/light splashing | |
| 13 | X | To hazardous part | Falling dirt, circulating dust, lint, fibers, flyings | Dripping/light splashing. Also protection against spraying, splashing, and seepage of oil and noncorrosive coolants |
3.7. Intrinsic Safety
IS circuits: IS in instrumentation and controls for hazardous areas is very important and a common means of explosion protection. Distribution of use of IS circuits among various types of instrumentation items is depicted in Fig. X/3.7-1 based on data from Ref. [34].
From the foregoing it is clear that the IS technique is used in cases of low-energy signaling. For instrumentation and control and communication (fieldbus), low-energy level signals are utilized in an effective way. IS circuits utilize the low-energy signaling technique, which prevents an explosion occurring from that energy, that is, to ensure that the available energy level is below the energy level required to cause ignition. This prompts the question: if instrumentation uses low-energy levels then why IS? There are a few reasons for this but one simple answer is that if there is a fault in the system, then a high-level signal may flow! IS prevents this from happening. This is well understood from Fig. X/3.7.2-1. Spark, arc, and hot surface are the major reasons for such chances of ignition. This is also clear from IS definition.
• IS equipment definitions:
• IEC definition of IS: As per IEC 60079-11, IS can be conceived as a “type of protection based on the restriction of electrical energy within apparatus and of interconnecting wiring exposed to the potentially explosive atmosphere to a level below that which can cause ignition by either sparking or heating effects.”
Figure X/3.7-1 Intrinsic safety circuit use (in %). IP, ingress protection; RTD, resistance temperature detector; T/C, thermocouple.
• ISA R12-6 defines IS as “A type of protection in which a portion of the electrical system contains only intrinsically safe equipment (apparatus, circuits, and wiring) that is incapable of causing ignition in the surrounding atmosphere” (ref: Clause 3.12 of ANSI/ISA-RP12.06.01-2003)
• Categories of IS: There are three categories of IS circuit, namely, ia, ib, and ic. The category distinctions are as follows:
• “ia”: “ia” tops the category offering the highest level of protection. It is generally considered as being adequately safe for use in the most hazardous locations (zone 0), can offer fault tolerance up to two “faults,” and has a factor of safety of 1.5.
• “ib”: “ib” is in the middle of the category. “ib” apparatus is adequately safe with a single fault with a factor of safety of 1.5. It is considered for use in hazardous areas—zone 1.
• “ic”: “ic” apparatus has a factor of safety of unity and is considered for use in hazardous zone 2. The “ic” concept (2005) is relatively new and replaces the “energy-limited” (nL) of the type “n” standard IEC 60079-15 and possibly the “nonincendive” concept of North American standards [33]. “ic” is considered equivalent to Cat3 of ATEX, and self-certification by manufacturers with a suitable testing facility is accepted for “ic.”
• Comparison: Comparison and usage of three categories are shown Table X/3.7-1.
• Advantages of IS: IS enjoys a number of advantages over other types of explosion protection. The most prominent ones are as follows:
• The IS technique is a well-accepted technique internationally for both gas and dust. Relevant standards provide detailed guidance, which is not available in other techniques.
• IS, as a technique, is applicable for all the zones based on its level, as shown in Table X/3.7-1. Incidentally, “ia” is the only technique that gives the most satisfactory results in zone 0.
• Intrinsically safe apparatus and systems are usually allocated a group IIC gas classification, which ensures that the equipment is compatible with all gas/air mixtures. Occasionally, IIB systems are used, as this permits a higher power level to be used [33].
• A temperature classification of T4 (135°C) is achievable to meet the requirements of most of the industrial gases.
Table X/3.7-1
Comparison and Usage of Intrinsically Safe (IS) Categories
| IS Level | Fault Tolerance | Factor of Safety | ATEX Category | IEC EPL | Zone |
| ia | 2 | 1.5 | 1 | 0 | 0 |
| ib | 1 | 1.5 | 2 | 1 | 1 |
| ic | 0 | 1 | 3 | 2 | 2 |
• Live maintenance is possible in IS circuits without a gas clearance certificate. However, suitable care should be taken. In all installations, IS circuits are properly marked. IS apparatus is well documented to avoid any mistake.
• Normal cables can be used in IS circuit applications only with special care to be taken for long cable for inductance and capacitance value especially in zone 0, 1 and group C gas. Normally, blue-colored cables are used for IS circuits to distinguish them from others. IS cable installations need special attention; in this connection associated standards (e.g., NORSOK) may be referred to.
Generally, each apparatus in a system is individually certified. From standard IEC 60079-25, for certification of the entire system necessary documentation is to be developed utilizing the information from each device/apparatus including cable details. Starting with general discussions, let us look a little deeper into the system with special reference to the design aspects.
3.7.1. General Discussions
A circuit in which any spark or thermal effect is incapable of causing ignition of a mixture of flammable or combustible material in air, under prescribed test conditions, is an IS circuit. Intrinsically safe circuits have three components: field device, referred to as the intrinsically safe apparatus; barrier (associated apparatus), the energy-limiting device, and the associated wiring. From this it is clear that intrinsically safe apparatus is interconnected by wiring and the safety of each device is highly dependent on the performance of the other pieces of apparatus in the circuit. So, this safety technique is a system concept and relies on accurate design of the entire system. In an intrinsically safe circuit, design and analysis starts with the field device, which will determine the type of barrier to be used for proper and safe functioning of the system under normal operating conditions as well as under fault conditions. The numerous products available in the market and various classes of calculation make things increasingly confusing.
Table X/3.7.1-1
Parameter Limits for Simple Apparatus (ISA)
| Voltage | Current | Power/Energy |
| 1.2–1.5 V | 0.1 A | 25 mW/20 μJ |
• Circuit parameters: Intrinsically safe apparatus is classified as simple apparatus or nonsimple apparatus.
There are a number of publications on simple apparatus worldwide, defined in similar terms. A few of them are EN 50020:1995 (P: 3.11, 5.4), EN 60079-14:1997 (P: 3.21), IEC 60079-11—appendix, and ANSI/ISA RP12.6-1987 (P: 3.12). As per the ATEX directive (original), “simple apparatus is not capable of causing an explosion through its own potential source of ignition.” Also “Simple apparatus is considered not to require certification from a notified body.” This means that self-certification is acceptable. Electrical junction boxes, RTDs, thermocouples, contact passive devices, and semiconductors are examples of simple apparatus. As per ISA a simple apparatus cannot generate or store energy limited by a few parameters, as given in Table X/3.7.1-1.
Naturally, those devices that cross the limit given in Table X/3.7.1-1 are considered as nonsimple devices. Transmitters, relays, solenoid valves, etc. are examples of nonsimple devices and require certification by a notified body. One thing is clear, however; all these are mainly connected with energy and/or storage of energy. In an entity approval for nonsimple apparatus, each apparatus is examined separately and assigned a set of entity parameters. For nonsimple devices these parameters are: maximum voltage (Vmax) allowed, maximum current allowed (Imax), internal capacitance (Ci), and internal inductance (Li). The first two parameters indicate that when either of these are exceeded, then intrinsically safe apparatus (a field device) can heat up or spark in the hazardous area. Li and Ci indicate the ability of the device to store energy. For designing the system it is necessary to obtain matching parameter data for associated apparatus. These are:
Voc: Maximum open-circuit voltage that is allowed to appear across the intrinsically safe connections of the associated apparatus under fault conditions.
Isc: Maximum allowable short-circuit current that can be drawn from the intrinsically safe connections of the associated apparatus under fault conditions.
Ca: Allowed capacitance that can safely be connected to the associated apparatus.
La: Allowed inductance that can safely be connected to the associated apparatus. The relationship between the two sets of parameters is as indicated in Table X/3.7.1-2.
• Zener diode: A diode is a solid-state device; when it is forward biased it allows current flow in one direction only. A Zener diode is a special diode that blocks current flow in reverse bias until a critical voltage is reached. At critical voltage on account of avalanche it is able to conduct current in this reverse bias mode without damaging itself. (This current cannot be controlled until the source is controlled.) Any standard book on solid-state devices may be referred to recapitulate Zener diode.
Table X/3.7.1-2
Relation Between Parameters of Barrier and Field Devices
| Barrier Parameter | Relations | Field Device Parameter |
| Open-circuit voltage: Voc | ≤ | Maximum voltage: Vmax |
| Short-circuit current: Isc | ≤ | Maximum current: Imax |
| Allowed capacitance: Ca | ≥ | Internal capacitance: Cia |
| Allowed inductance: La | ≥ | Internal inductance: Lia |
• Generalized circuit: A typical barrier circuit (DC) is shown in Fig. X/3.7.1-1.
Under normal conditions, the device really has no function except to allow a field device to function properly. Under fault conditions, the barrier has to protect the field circuit by preventing excess voltage and current from reaching the hazardous area. Three components in the barrier that limit current and voltage are: a resistor, a set of Zener diodes (three Zener diodes for “ia” and two for “ib”), and a fuse. The resistor limits the current to Isc. The set of Zener diodes limits the voltage to Voc. The fuse will blow when the set of Zener diodes conducts. This interrupts the circuit, which prevents the diode from burning and allowing excess voltage to reach the hazardous area. There are always at least two Zener diodes in parallel so that each can fall back in case of failure of the other. Because the set of Zener diodes is in single configuration, operation is in one direction only, hence it can be considered as DC. Cables have distributed inductance and capacitance, and hence possess energy storage capabilities, so they can affect system safety. As a result the system design imposes restrictions on the amount of each of these parameters. These are shown as a bulk separately in Fig. X/3.7.1-1. In this connection, Table A.2 in IEC 60079-11 for cable parameters may be referred to. Special calculations and considerations are applied for system analysis because of cable faults—especially when multicore cables (see IEC 60079-14 for type A and B cables in connection with multicore cables) are used in the installations [33]. Other important issues here are that: (1) voltage drop across the resistor under normal operating conditions has to be taken into account in the design so that there is sufficient voltage at the field device and (2) Zener barriers are properly earthed as shown because without an IS earth they are not safe. To obtain independence from earthing, galvanic isolators could be deployed. In the case of a galvanic isolator (another type of IS module) grounding is not done and hazardous side connection is isolated from the safe side connection and ground loop. In the case of galvanic isolators (Fig. X/3.7.1-4) longer cables and higher voltages can be used.
• Safety energy level: As already discussed, there are different levels of energy needed to cause ignitions in different groups of gases. Ignition curves for various groups of gases are shown in Fig. X/3.7.1-2. It is worth noting that the curves shown here are not to true scale and are replicas of actual curves. They are used for illustration purposes only. Actual curves are available in any standard handbook on IS systems. In this log–log graph with current on the vertical axis and voltage on the horizontal axis, ignition energy required for various gas groups has been depicted. The portion to the left and below the group curve is the safe zone and shown by shading. Also for a better understanding of the gas group—shown by a dotted line—safe and unsafe operating points are also marked in the figure. A safety factor of 1.5 is applied for calculating the allowable energy.
These curves reflect the worst-case scenario. Because of the low power required by today’s electronics, most manufacturers of intrinsically safe equipment find it safer (and easier) to start by designing the equipment for the worst-case specifications [24].
• Safety barrier types: There are two types of safety barrier: DC and AC type. The DC type is shown in Figs. X/3.7.1-1 and X/3.7.3-1A where it is seen that Zener diodes are in one configuration only. In this type, polarity is an important factor and if it is connected in reverse it will not function. On the other hand, to overcome the polarity problem, the AC type (double) is used, especially for thermocouples and RTDs. An AC double-type barrier is shown in Fig. X/3.7.1-3. Here it is seen that two sets of Zener diodes are in back-to-back connection to accept any polarity of connections. Because in this connection both fault current directions are accepted, it is an AC-type barrier.
• Galvanic isolator: A typical galvanic isolator IS module is shown in Fig. X/3.7.1-4.
As seen in the figure the module consists of the following components:
• Energy limiting circuit (in hazard side connection)
• Hazard area circuit
• Safe area circuit
• Power supply is also galvanically isolated through the aforesaid transformer primary coil.
• Optocoupler/approved relay
A few relevant issues shall include but are not limited to the following:
• Transformer-isolated barriers do not require a high integrity IS ground on account of isolation of two areas by the transformer.
• The system will be floating unlike grounding need for IS barrier.
• Transformer-isolated barriers are repairable IS barriers, which are encapsulated.
• Transformer-isolated barriers are mainly used when grounding is impractical and when regulated power supply is too costly. Some application examples are transmitters/thermocouples/switching inputs.
• These are used where noise caused by the ground loop is undesirable and when total resistance of the loop exceeds the specification for the desired instrument application.
There is approved safety segregation; only light from the optocoupler can cross approved safety segregation. There is no grounding, so the hazard side is isolated from the safe side and ground loop. Highway Addressable Remote Transducer communication in a transmitter is an example of its application.
• IS approaches: Fundamentally there are two approaches for IS circuits:
• Grounded IS barrier
• Grounded/ungrounded repeaters (galvanic isolator)
Each of these systems has some advantages and disadvantages, which are listed in Table X/3.7.1-3. Repeaters require certain components for which approval from notified bodies is required, such as a transformer in ungrounded repeaters. Usually in case of repeaters, a single product can be used for different applications. However, these are costly and require more space. In the case of a grounded safety barrier, precise engineering and selection of IS products are necessary.
Table X/3.7.1-3
Pros and Cons of Different Approaches to an Intrinsic Safety (IS) Circuit
| Pros and Cons | Grounded IS Barrier | Grounded Repeater | Galvanic Isolator |
| Advantage | Less costly and smaller size Precise signal response | Single product use Usable in transmitters with higher power supply | Single product use No need for ground Usable in transmitters with higher power supply |
| Disadvantage | Precise engineering and grounding are necessary | More expensive, and more power and space needed | More expensive, and more space needed Radiofrequency interference |
3.7.2. Thermocouple/Resistance Temperature Detector (RTD) Input
Discussions begin with thermocouple/RTD inputs. Since both belong to the simple apparatus category they do not need certification by a notified body. Polarity, rated nominal voltage, and internal resistance are major design parameters to be considered. Both have similar requirements for these parameters.
• Thermocouple input: A thermocouple is a simple device, so it is incapable of creating or storing enough energy to ignite any mixture of volatile gases. When the energy level of a typical thermocouple circuit is seen against the ignition curve (like one shown in Fig. X/3.7.1-2) of the gas group it is seen that these are very much on the left and lower part of group A.
Now, if a fault occurs on the secondary device, say in the logic solver, then it could cause excess energy to reach the hazardous area, as seen in Fig. X/3.7.2-1. To make sure that the circuit remains intrinsically safe, that is, the fault does not reach a hazardous area, a barrier limiting the energy is required as shown in the bottom part of Fig. X/3.7.2-1.
Design issues: A few design issues are discussed:
Figure X/3.7.2-1 Intrinsic safety (IS) circuit for a thermocouple. LS, logic solver; T/C, thermocouple.
• Rated voltage (Vn): A thermocouple produces a very small voltage. Since the thermocouple produces such a small voltage, it makes sense to choose a double AC barrier with a higher rated nominal voltage (Vn > 1 V) [34].
• Internal resistance (Ri): Because the mV signal has a very small current and is going to a high-impedance voltmeter, the resistance of the barrier will not affect functioning of circuit. However, it is wise to select a barrier with a low resistance (<110 Ω) [34].
• Resistance temperature detector (RTD) input: Like the thermocouple, in the case of an RTD, IS barriers prevent excess energy from possible faults on the safe side, say from the logic solver, from reaching the hazardous area. A typical RTD input IS circuit is depicted in Fig. X/3.7.2-2.
In most cases, three-wire RTDs are used in the industry, so these will be considered here. Mainly, bridge circuits using a modified Wheatstone bridge are used. In these circuits, measured output voltage is a function of the RTD resistance. Requirements for RTDs are similar to thermocouples. Use of a double-channel AC barrier with parameters discussed later will be a cost-effective solution.
• Polarity: The current loop of the RTD has a positive and negative polarity. Hence one each of a standard DC barrier, one standard DC barrier (+ve and −ve), one double AC barrier are to be chosen. The last one is a better choice to avoid polarity problems.
• Rated voltage (Vn): The constant current sent to the RTD is very small at the 10−6 level. RTDs are recommended for use in the range 600–700°C. Even if a very high temperature is considered, the maximum resistance of the RTD Pt100 is 390 Ω at 1560°C [34]. So, the voltage drop across the RTD will be very low in the order of mV. Naturally, the Vn of the RTD loop is similar to the thermocouple, that is, Vn > 1 V.
• Internal resistance (Ri): Any constant current source will have a rated maximum load that it can drive. Considering a minimum standard load of 500 Ω and RTD value at high temperature as 390 Ω, the Ri to be chosen is less than 110 (500–390) Ω.
3.7.3. Transmitter Input/Analog Input
Transmitters are not simple apparatus and there is the possibility for storing energy. So, they need certification by a third party for use in IS applications. Generally, high-level analog (4–20 mADC) inputs also interface in a similar manner. As stated in Clause 3.7.1 and in Table X/3.7.1-3, there are three approaches to IS circuits and all are applicable here. These are detailed in Fig. X/3.7.3-1.
Various approaches shown in Fig. X/3.7.3-1 are not only applicable for transmitters but are also applicable to other applications discussed later. In transmitters, physical parameters are converted to an electrical signal (generally 4–20 mADC) for transmitting to DCS over a long distance. Usually, this mA signal is converted to 1–5 V with the help of conditioning resistance (250 Ω), so that it can be used for analog-to-digital conversion. Here, design issues are type of safety barrier, voltage input (Vn), and internal resistance (Ri).
Figure X/3.7.3-1 Safety barrier and repeater for a transmitter (20 mA). (A) DC safety barrier (+ve), (B) both side safety barrier, (C) safety repeater for transmitter. DCS, distributed control system.
• Barrier type: The type of safety barrier is largely determined by the placement of the conversion resistor. When a conditioning resistor is placed on the supply line, a simple DC positive barrier can be used, as shown in Fig. X/3.7.3-1A. However, in the majority of cases conditioning transmitters are placed in the return path, where a double-channel supply and return barrier are used, as shown in Fig. X/3.7.3-1B. The supply line positive DC barrier prevents a fault on the safe side from transferring excess energy to the transmitter. When looking at the return line, it is seen that there are two diodes in series to ensure that the signal flows only in one direction, that is, return to the DCS, and prevent any excess fault energy from being transferred to the transmitter. Both these approaches are grounded safety barriers. However, there could be situations where there is no proper ground or transmitters have higher operating voltages. In such situations, repeaters—grounded and ungrounded (see Clause 3.7.1 and Table X/3.7.1-3)—could also be deployed, that is, transmitters, with a loop indicator or communicator. Repeaters supply a regulated power supply of 15–17 V to the transmitters to drive a conversion resistor load of 750–1000 Ω [34]. A typical repeater circuit is shown in Fig. X/3.7.3-1C. The repeaters could be grounded or ungrounded galvanic isolators, as shown in Fig. X/3.7.1-4.
• Rated voltage: A regulated supply with optimum tolerance that does not exceed the barrier rating should to be selected. Keeping parity with normal industrial usage, 24 VDC ± 1% supply is a good choice.
• Internal resistance: Normal transmitters at 20 mA require at least a 12 VDC drop across them. When the total loop is considered it is seen that for a 24 VDC power supply major drops are transmitter drop and conditioning resistance drop, which in the highest case of 24 VDC will be 17 (12 + 5) VDC. So, 7 V are left at best for the IS barrier and cable drop. Applying Ohm’s law one gets Rtotal = 7/20 mA = 350 Ω. Taking 10 Ω for the cable it should be <340 Ω.
3.7.4. Digital/Binary Inputs
There are a good number of digital inputs interfaced with the DCS through safety systems. Mostly, these inputs come from mechanical or reed contacts, transistors, limit switches, pushbuttons, etc. All these are simple apparatus and do not need any certification by a third party. There are two ways digital inputs for IS applications are handled, either a switching amplifier (NAMUR sensor) or by grounded IS barrier.
• Switching amplifier (IS): Switching through an intrinsically safe relay or switching amplifier is quite popular in IS applications when power supply and space are available in the associated control panel of the DCS. A switch amplifier is an intrinsically safe relay. A NAMUR-style proximity switch is a two-wire DC sensor that operates at 8.2 V with switch points operating between 1.2 and 2.1 mA [34]. These switches require a remote amplifier for operation. Most of the available switching amplifiers have ratings: an intrinsically safe voltage of 8.2 V 8 mA current (through the contacts in hazardous areas). Switch amplifiers have two outputs: relays for slow switching application and optocouplers for the DCS and faster switching.
• Grounded IS barrier: When power or space is not available, or field switches are powered from other places, then a grounded safety barrier may be chosen. There are two types of grounded IS barriers: current source type and current sink type.
• Current source type: Here this IS type used because it is the same as the supply and return type IS shown in Fig. X/3.7.3-1B. Blue IS cables are used between field sensor and IS barrier.
• Current sink type: For current sink switching, a single-channel DC barrier as seen in Fig. X/3.7.3-1A can be utilized. In this type of binary input, the DCS sees high voltage when the contact is open whereas when the contact is closed the DCS sees lower voltage as is normal with sink type sensing. Suitable care must be taken for proper grounding and to avoid unnecessary group loop formation to avoid noise and influence actual signal measurements.
3.7.5. Analog Output
An example of an analog output is the output from the DCS to converters. When a current to a pneumatic (input) converter in the field in a hazardous area signal is to be sent from the DCS, a grounded barrier may be used. These converters need entity approval. The DC positive polarity grounded IS barrier as shown in Fig. X/3.7.3-1A may be utilized. Voltage rating is determined by type of converter chosen. Since converters have resistance to the tune of 150 Ω, it should be maximum driving load of analog output of the DCS minus converter load. So, if the driving maximum load is 1000 Ω, then 850 Ω will be Ri (including cable). A typical grounded IS barrier is shown in Fig. X/3.7.5-1.
3.7.6. Digital Output
Digital outputs are frequently used for actuation of valves, LEDs, etc. An LED being a simple apparatus does not require entity approval whereas a valve actuator such as a solenoid valve needs entity approval. Positive DC IS barriers are used. In most cases these barriers are rated for 24 VDC. Ri is calculated from the selected device. Typical values for LEDs and solenoid valves can be 480 and 350 Ω [34]. System configuration is similar to what has been shown in Fig. X/3.7.5-1, only +4–20 mA will be replaced by +24 VDC via the contact. The return line will be grounded at the DCS as shown. The IS barrier will be grounded. With discussions on IS barrier for various I/Os concluded we will now move on to wiring practices.
3.7.7. Wiring and Installation
Short discussions on installation practices are as follows:
• IS cable: There is not much difference between the specification of “IS” cables and other instrumentation cables. IS cables in the field are recognized by their light blue-colored (similar to RAL 5015) outer sheathe and proper labeling. A short description of IS cables in their general form is presented in Table X/3.7.7-1.
Table X/3.7.7-1
Short Intrinsic Safety (IS) Cable Specification (General)
| Item | Specification | Item | Specification |
| Conductor | Finely stranded tinned Cu | Sheath color | Light blue (RAL 5015) |
| Insulation | Fluoropolymer/PVC | Standards | IEC 62228/VDE, etc. |
| Cores/pairs | 1, 2, 6, etc. as required | Flame resistance | Flame retardant |
| Screening | Individual/overall | Capacitancea | 100–120 pF/m |
| Screen | Braided wire tinned Cu | Inductance | 0.5 μH/m |
| Outer sheathe | PVC chemical resistant | Voltage rating | 500 V |
• Wiring: Two major differences between IS wiring and other unclassified wiring are separation and identification. The intrinsically safe conductors are separated from non-IS wiring by using separate conduits or by a separation of 2 inches of air space. In an enclosure the conductors can be separated by a grounded metal or insulated partition. Intrinsically safe wiring is light blue, and other conductors must not have the same color. The raceways, trays, open wiring, and terminal boxes must be labeled for intrinsically safe wiring to prevent unintentional interference with the circuits. Usually, 25 × 40 mm size labels are used, and these are placed at a distance of not more than 7.5 m apart. Fig. X/3.7.7-1 depicts the safe distance between wiring in a cabinet.
• Barrier enclosure and placement: Normally, barriers are located in a safe area in a NEMA 4/12 enclosure depending on indoor or outdoor installation because dust and moisture can reduce the air segregation distance. Another criterion is that they should be as close as possible to the hazardous area to minimize cable runs so that there is not much of an increase in capacitance of the circuit because of the cable. In such cases, the enclosure to be chosen is the one that is most suitable for the hazardous area. In the case of an explosion-proof enclosure, suitable sealing shall be used, as shown in Fig. X/3.7.7-2.
Figure X/3.7.7-2 Intrinsic safety (IS) barrier in a hazardous area. Developed based on an idea from Intrinsic safety circuit design; P.S. Babiarz; Omega US; http://www.omega.com/temperature/Z/pdf/z131-148.pdf. Courtesy: OMEGA USA.
• Barrier grounding: As discussed earlier, IS can be grounded or ungrounded. In the case of a grounded IS barrier the ground is important as it is the path for releasing excess energy. Major grounding rules are:
• Permanent, secured, separate isolated grounding path to be used.
• Grounding path must be visible and subject to regular inspection.
• Grounding path resistance is normally less than 1 Ω and a minimum 12 AWG must be used.
• Replacement and inspection: When fuses blow it is necessary to replace the module. This is done by first disconnecting the module in the following sequence: nonhazardous terminal (isolation of source), then hazardous terminal, and finally ground terminal. Another important aspect of inspection is that, as per IEC 60079-17, “a technical person with executive function” should be identified and should be responsible for inspection.
With this, the discussion on electrical classification is concluded. However, some supplementary information is also presented in Appendix II.
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