Electrical Equipment in Hazardous Areas

Electrical engineering

Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical...

, a hazardous location is defined as a place where concentrations of flammable gases, vapors, or dusts occur. Electrical equipment that must be installed in such locations is especially designed and tested to ensure it does not initiate an explosion, due to arcing contacts or high surface temperature of equipment.

For example a household light switch

Light switch

A light switch is a switch, most commonly used to operate electric lights, permanently connected equipment, or electrical outlets. In torches the switch is often near the bulb, but may be in the tail, or even the entire head itself may constitute the switch .-Wall-mounted switches:Switches for...

may emit a small, harmless visible spark when switching; in an ordinary atmosphere this arc is of no concern, but if a flammable vapor is present, the arc might start an explosion. Electrical equipment intended for use in a chemical factory or refinery

Oil refinery

An oil refinery or petroleum refinery is an industrial process plant where crude oil is processed and refined into more useful petroleum products, such as gasoline, diesel fuel, asphalt base, heating oil, kerosene, and liquefied petroleum gas...

is designed either to contain any explosion within the device, or is designed not to produce sparks with sufficient energy to trigger an explosion.

Many strategies exist for safety in electrical installations. The simplest strategy is to minimize the amount of electrical equipment installed in a hazardous area, either by keeping the equipment out of the area altogether or by making the area less hazardous by process improvements or ventilation with clean air. Intrinsic safety

Intrinsic safety

Intrinsic safety is a protection technique for safe operation of electronic equipment in explosive atmospheres and under irregular operating conditions. The concept was developed for safe operation of process control instrumentation in hazardous areas, particularly North Sea gas platforms...

and non-incendive equipment and wiring methods are practices where apparatus is designed with low power levels and low stored energy, so that an arc produced during normal functioning of the equipment or as the result of equipment failure has insufficient energy to initiate ignition of the explosive mixture. Equipment enclosures can be pressurized with clean air or inert gas and designed with various controls to remove power or provide notification in case of supply or pressure loss of such gases. Arc-producing elements of the equipment can also be isolated from the surrounding atmosphere by encapsulation, immersion in oil, sand, etc. Heat producing elements such as motor winding, electrical heaters, including heat tracing and lighting fixtures are often designed to limit their maximum temperature below the autoignition temperature of the material involved. Both external and internal temperatures are taken into consideration.

As in most fields of electrical installation, different countries have approached the standardization and testing of equipment for hazardous areas in different ways. As world trade becomes more important in distribution of electrical products, international standards are slowly converging so that a wider range of acceptable techniques can be approved by national regulatory agencies.

Area classification is required by governmental bodies, for example the U.S. Occupational Safety and Health Administration

Occupational Safety and Health Administration

The United States Occupational Safety and Health Administration is an agency of the United States Department of Labor. It was created by Congress of the United States under the Occupational Safety and Health Act, signed by President Richard M. Nixon, on December 29, 1970...

and compliance is enforced.

Documentation requirements are varied. Often an area classification plan-view is provided to identify equipment ratings and installation techniques to be used for each classified plant area. The plan may contain the list of chemicals with their group and temperature rating, and elevation details shaded to indicate Class, Division(Zone) and group combination. The area classification process would require the participation of operations, maintenance, safety, electrical and instrumentation professionals, the use of process diagrams and material flows, MSDS and any pertinent documents, information and knowledge to determine the hazards and their extent and the countermeasures to be employed. Area classification documentations are reviewed and updated to reflect process changes.

History

Soon after the introduction of electric power into coal mine

Coal mining

The goal of coal mining is to obtain coal from the ground. Coal is valued for its energy content, and since the 1880s has been widely used to generate electricity. Steel and cement industries use coal as a fuel for extraction of iron from iron ore and for cement production. In the United States,...

s, it was discovered that lethal explosions could be initiated by electrical equipment such as lighting, signals, or motors. The hazard of fire damp or methane accumulation in mines was well known by the time electricity was introduced, along with the danger of suspended coal dust. At least two British mine explosions were attributed to an electric bell signal system. In this system, two bare wires were run along the length of a drift, and any miner desiring to signal the surface would momentarily touch the wires to each other or bridge the wires with a metal tool. The inductance

Inductance

In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...

of the signal bell coils, combined with breaking of contacts by exposed metal surfaces, resulted in sparks which could ignite methane, causing an explosion.

Divisions or zones (gases, vapors and dust)

In an industrial plant such as a refinery

Refinery

A refinery is a production facility composed of a group of chemical engineering unit processes and unit operations refining certain materials or converting raw material into products of value.-Types of refineries:Different types of refineries are as follows:...

or chemical process plant, handling of large quantities of flammable liquids and gases creates a risk of leaks. In some cases the gas, ignitable vapor or dust is present all the time or for long periods. Other areas would have a dangerous concentration of flammable substances only during process upsets, equipment deterioration between maintenance periods, or during an incident. Refineries and chemical plants are then divided into areas of risk of release of gas, vapor or dust known as divisions or zones. The process of determining the type and size of these hazardous areas is called area classification. Guidance on assessing the extent of the hazard is given in the NFPA 497 Standard, or API 500 and according to their adaptation by other areas gas zones is given in the current edition of IEC 60079.10. For hazardous dusts, the guiding standard is IEC 61421.10.

Typical gas hazards are from hydrocarbon compounds.

Safe area: An area such as a residence or office would be classed as safe area, where the only risk of a release of explosive or flammable gas would be such things as the propellant in an aerosol spray

Aerosol spray

Aerosol spray is a type of dispensing system which creates an aerosol mist of liquid particles. This is used with a can or bottle that contains a liquid under pressure. When the container's valve is opened, the liquid is forced out of a small hole and emerges as an aerosol or mist...

. The only explosive or flammable liquid would be paint and brush cleaner. These are classed as very low risk of causing an explosion and are more of a fire risk (although gas explosions in residential buildings do occur). Safe area on chemical and other plant are present where the hazardous gas is diluted to a concentration below 25% of its lower flammability limit

Flammability limit

Flammability limits, also called flammable limits, give the proportion of combustible gases in a mixture, between which limits this mixture is flammable. Gas mixtures consisting of combustible, oxidizing, and inert gases are only flammable under certain conditions. The lower flammable limit ...

(or lower explosive limit (LEL)).

Division 2 or Zone 2 area: This is a step up from the safe area. In this zone the gas, vapor or mist would only be present under abnormal conditions (most often leaks under abnormal conditions). As a general guide for Zone 2, unwanted substances should only be present under 10 hours/year or 0–0.1% of the time.

Division 1 or Zone 1 area: Gas, vapor or mist will be present or expected to be present for long periods of time under normal running. As a guide for Zone 1, this can be defined as 10–1000 hours/year or 0.1–10% of the time.

Zone 0 area: Gas or vapor is present all of the time. An example of this would be the vapor space above the liquid in the top of a tank or drum. The ANSI/NEC classification method consider this environment a Division 1 area. As a guide for Zone 0, this can be defined as over 1000 hours/year or >10% of the time.

Zones (dusts)

In the case of dust

Dust

Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...

s there is still a chance of explosion. An old system of area classification to a British standard used a system of letters to designate the zones. This has been replaced by a European numerical system, as set out in directive 1999/92/EU implemented in the UK as the Dangerous Substances and Explosives Atmospheres Regulations 2002

The boundaries and extent of these three dimensional zones should be decided by a competent person. There must be a site plan drawn up of the factory with the zones marked on.

The zone definitions are:

Zone 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.

Zone 21: A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur, occasionally, in normal operation.

Zone 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

Gas groups

Explosive gases, vapors and dusts have different chemical properties that affect the likelihood and severity of an explosion. Such properties include flame temperature, minimum ignition energy, upper and lower explosive limits, and molecular weight. Empirical testing is done to determine parameters such as the maximum experimental safe gap, minimum ignition current, explosion pressure and time to peak pressure, spontaneous ignition temperature, and maximum rate of pressure rise. Every substance has a differing combination of properties but it is found that they can be ranked into similar ranges, simplifying the selection of equipment for hazardous areas.

Flammability of combustible liquids are defined by their flash-point. The flash-point is the temperature at which the material will generate sufficient quantity of vapor to form an ignitable mixture. It is a critical data in determining whether the area needs to be classified or not. A material may have a relatively low autoignition temperature yet its flash-point is above the ambient temperature then the area may not need to be classified. Conversely if the same material is processed(heated) above ambient temperature and the handling temperature is above its flash-point, the area MUST be classified.

Each chemical gas or vapour used in industry is classified into a gas group.

Group	Representative Gases
I	All Underground Coal Mining Coal mining The goal of coal mining is to obtain coal from the ground. Coal is valued for its energy content, and since the 1880s has been widely used to generate electricity. Steel and cement industries use coal as a fuel for extraction of iron from iron ore and for cement production. In the United States,... . Firedamp Firedamp Firedamp is a flammable gas found in coal mines. It is the name given to a number of flammable gases, especially methane. It is particularly commonly found in areas where the coal is bituminous... (methane)
IIA	Industrial methane Methane Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel... , propane Propane Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central... , petrol and the majority of industrial
IIB	Ethylene Ethylene Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone... , coke oven gas and other industrial gases
IIC	Hydrogen Hydrogen Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly... , acetylene Acetylene Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because... , carbon disulphide

Apparatus marked IIB can also be used for IIA gases. IIC marked equipment can be used for both IIA and IIB. If a piece of equipment has just II and no A, B, or C after then it is suitable for any gas group.

A list must be drawn up of every chemical gas or vapor that is on the refinery/chemical complex and included in the site plan of the classified areas. The above groups are formed in order of how volatile the gas or vapor would be if it was ignited, IIC being the most volatile and IIA being the least. The groups also indicate how much energy is required to ignite the gas by spark ignition, Group IIA requiring the most energy and IIC the least.

Equipment Protection Level (EPL)

In recent years also the Equipment Protection Level is specified for several kinds of protection.
The required Protection level is linked to the intended use in the zones described below:

Group	Ex risk	Zone	EPL	Minimum type of protection
I (mines)	energized		Ma
I (mines)	de-energized in presence of Ex atmosphere		Mb
II (gas)	explosive atmosphere > 1000 hrs/yr	0	Ga	ia, ma
II (gas)	explosive atmosphere between 10 and 1000 hrs/yr	1	Gb	ib, mb, px, py, e, o, q, s
II (gas)	explosive atmosphere between 1 and 10 hrs/yr	2	Gc	n, ic, pz
III (dust)	explosive surface > 1000 hrs/yr	20	Da
III (dust)	explosive surface between 10 and 1000 hrs/yr	21	Db
III (dust)	explosive surface between 10 and 10 hrs/yr	22	Dc

Temperature classification

Another important consideration is the temperature classification of the electrical equipment. The surface temperature or any parts of the electrical equipment that may be exposed to the hazardous atmosphere should be tested that it does not exceed 80% of the auto-ignition temperature of the specific gas or vapor in the area where the equipment is intended to be used.

The temperature classification on the electrical equipment label will be one of the following (in degree Celsius):

USA°C		UK °C	Germany °C Continuous - Short Time
T1 - 450	T3A - 180	T1 - 450	G1: 360 - 400
T2 - 300	T3B - 165	T2 - 300	G2: 240 - 270
T2A - 280	T3C - 160	T3 - 200	G3: 160 - 180
T2B - 260	T4 - 135	T4 - 135	G4: 110 - 125
T2C - 230	T4A - 120	T5 - 100	G5: 80 - 90
T2D - 215	T5 - 100	T6 - 85
T3 - 200	T6 - 85

The above table tells us that the surface temperature of a piece of electrical equipment with a temperature classification of T3 will not rise above 200 °C.

Auto-ignition temperatures (vapors & gases)

The auto-ignition temperature of a liquid, gas or vapor is the temperature at which the substance will ignite without any external heat source. The exact temperature value determined depends on the laboratory test conditions and apparatus. Such temperatures for common substances are:

Gas	Temperature
Methane Methane Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...	580 °C
Hydrogen Hydrogen Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...	560 °C
Propane Propane Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...	493 °C
Ethylene Ethylene Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...	425 °C
Acetylene Acetylene Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...	305 °C
Naphtha Naphtha Naphtha normally refers to a number of different flammable liquid mixtures of hydrocarbons, i.e., a component of natural gas condensate or a distillation product from petroleum, coal tar or peat boiling in a certain range and containing certain hydrocarbons. It is a broad term covering among the...	290 °C
Carbon disulfide Carbon disulfide Carbon disulfide is a colorless volatile liquid with the formula CS2. The compound is used frequently as a building block in organic chemistry as well as an industrial and chemical non-polar solvent...	102 °C

The surface of a high pressure steam pipe may be above the autoignition temperature of some fuel/air mixtures.

Auto-ignition temperatures (dust)

The auto-ignition temperature of a dust is usually higher than that of vapours & gases. Examples for common materials are:

Substance	Temperature
Sugar Sugar Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...	460 °C
Wood Wood Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...	340 °C
Flour Flour Flour is a powder which is made by grinding cereal grains, other seeds or roots . It is the main ingredient of bread, which is a staple food for many cultures, making the availability of adequate supplies of flour a major economic and political issue at various times throughout history...	340 °C
Grain GRAIN GRAIN is a small international non-profit organisation that works to support small farmers and social movements in their struggles for community-controlled and biodiversity-based food systems. Our support takes the form of independent research and analysis, networking at local, regional and... dust	300 °C
Tea Tea Tea is an aromatic beverage prepared by adding cured leaves of the Camellia sinensis plant to hot water. The term also refers to the plant itself. After water, tea is the most widely consumed beverage in the world...	300 °C

Type of protection

To ensure safety in a given situation, equipment is placed into protection level categories according to manufacture method and suitability for different situations. Category 1 is the highest safety level and Category 3 the lowest. Although there are many types of protection, a few are detailed
!
! Ex Code
! Description
! Standard
! Location
! Use
|-
! Flameproof
| d
| Equipment is robust can stand an explosion from within, without transmitting the flame to the outside

Equipment has flameproof gaps (max 0.006" propane

Propane

Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...

/ethylene

Ethylene

Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...

, 0.004" acetylene

Acetylene

Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...

/hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

Fieldbus

Fieldbus is the name of a family of industrial computer network protocols used for real-time distributed control, now standardized as IEC 61158....

Intrinsically Safe Concept (FISCO) (zone 0, 1 or 2)

| IEC/EN 60079-25
IEC/EN 60079-11
IEC/EN60079-27
| 'ia': Zone 0 &
'ib': Zone 1
'ic: zone 2
| Instrumentation, measurement, control
|-
! Non Incendive
| n
| Equipment is non-incendive or non-sparking.

A special standard for instrumentation is IEC/EN 60079-27, describing requirements for Fieldbus

Fieldbus

Fieldbus is the name of a family of industrial computer network protocols used for real-time distributed control, now standardized as IEC 61158....

Non-Incendive Concept (FNICO) (zone 2)
| IEC/EN 60079-15
IEC/EN 60079-27
| Zone 2
| Motors, lighting, junction boxes, electronic equipment
|-
! Special Protection
| s
| This method, being by definition special, has no specific rules. In effect it is any method which can be shown to have the required degree of safety in use. Much early equipment having Ex s protection was designed with encapsulation and this has now been incorporated into IEC 60079-18 [Ex m]. Ex s is a coding referenced in IEC 60079-0. The use of EPL and ATEX Category directly is an alternative for “s” marking. The IEC standard EN 60079-33 is made public and is expected to become effective soon, so that the normal Ex certification will also be possible for Ex-s
| IEC/EN 60079-33
| Zone depending upon Manufacturers Certification.
| As its certification states>

The types of protection are subdivided into several sub classes, linked to EPL: ma and mb, px, py and pz, ia, ib and ic.
The a subdivisions have the most stringent safety requirements, taking into account more the one independent component faults simultaneously.

Multiple Protection

Many items of EEx rated equipment will employ more than one method of protection in different components of the apparatus. These would be then labeled with each of the individual methods. For example a socket outlet labeled EEx'de' might have a case made to EEx 'e' and switches that are made to EEx 'd'.

Hazardous Locations Classifications Descriptions - ANSI/NFPA(NEC)terms

Class I, Div. 1 - Where ignitable concentrations of flammable gases, vapors or liquids are present within the atmosphere under normal operation conditions.

Class I, Div. 2 - Where ignitable concentrations of flammable gases, vapors, or liquids are present within the atmosphere under abnormal operating conditions.

Class II, Div. 1 - Where ignitable concentrations of combustible dusts are present within the atmosphere under normal operation conditions.

Class II, Div. 2 - Where ignitable concentrations of combustible dust are present within the atmosphere under abnormal operating conditions.

Class III, Div. 1 - Where easily ignitable fibers or materials producing combustible flyings are present within the atmosphere under normal operation conditions.

Class III, Div. 2 - Where easily ignitable fibers or materials producing combustible flyings are present within the atmosphere under abnormal operating conditions.

Common Materials within Associated Class & Group Ratings, such as "Class I, Division 1, Group A":

Class I Areas: Group A: Acetylene / Group B: Hydrogen / Group C: Propane and Ethylene / Group D: Benzene, Butane, Methane & Propane

Class II Areas: Group E: Metal Dust / Group F: Carbon & Charcoal / Group G: Flour, Starch, Wood & Plastic

Class III Areas: NO GROUP: Cotton & Sawdust

For more information see Article 500 of NFPA 70 - The National Electric Code, as published by the National Fire Protection Association.

Equipment category

The equipment category indicates the level of protection offered by the equipment.

Category 1 equipment may be used in zone 0, zone 1 or zone 2 areas.

Category 2 equipment may be used in zone 1 or zone 2 areas.

Category 3 equipment may only be used in zone 2 areas.

Labeling

All equipment certified for use in hazardous areas must be labelled to show the type and level of protection applied.

1. In Europe the label must show: -

a) The CE mark.

b) The code number of the certifying body for the ‘quality certificate’.

c) CE marking is complemented with the Ex mark, followed by the indication of the Group,
Category and, if group II equipment, the indication relating to gases (G) or dust (D). For
example: -
Ex II 1 G (Explosion protected, Group 2, Category 1, Gas)

2. In addition, the normative marking will be able to establish the specific type or types of protection being used, for example: -

EEx ia IIC T4. (Type ia, Group 2C gases, Temperature category 4).

EEx nA II T3 X (Type n, non-sparking, Group 2 gases, Temperature category 3, special conditions apply).

3. The suitability of equipment for the specific hazardous area in the ANSI/NFPA(NEC) regulated market is required to be tested by a NRTL(Nationally Recognized Testing Laboratory). Such institutes are UL, FM or ETL, just to name a few.

a) Dependent on the specific equipment and the applicable division the label will always list the Class(es), Division(s) and may list the Group(s) and temperature Code.

b) Directly adjacent on the label one will find the mark of the listing agency.

c) Some manufacturers claim "suitability" or "built-to" hazardous areas in their technical literature, but in effect lack the testing agency's certification and thus unacceptable for the AHJ (Authority Having Jurisdiction) to permit operation of the electrical installation/system.

4. Also included in the marking are: -

a) The manufacturers name or trademark and address.

b) The apparatus type, name and serial number.

c) Year of manufacture.

d) Any special conditions of use.

e) The NEMA

Néma

Néma is a town in southeastern Mauritania, close to the border with Mali. It is located at around . It is the capital of Hodh Ech Chargui Region and of the Néma Department....

enclosure rating or IP code may also be indicated, but it is usually independent of the Classified Area suitability.

5. Manufacturers will provide specific installation instructions that must be followed for:

a) the equipment to function properly, and

b) to provide the designed protection in the Classified Area according to the listing.

British standards

Industrial electrical equipment for hazardous area has to conform to standard BS 60079 and in some cases, certified as meeting that standard. Independent test houses (known as Notified Bodies)are established in most European countries, and a certificate from any of these will be accepted across the EU. The DTI appoint and maintain a list of Notified Bodies within the UK, of which Sira

Sira (group of British companies)

Sira is a UK-based Notified Body, specialising in ATEX, IECEX and North American product approvals. It began life as the British Scientific Instrument Research Association in 1918 and was once part of a larger group of British engineering companies, based in south London, that designed test...

and Baseefa

Baseefa

Baseefa is a British certification body for equipment intended for use in potentially explosive atmospheres. It is based in Buxton, Derbyshire, England...

are the most well known. All equipment certified for use in hazardous areas must be labelled to show the level of protection offered.