Hazardous Areas Instrumentation System

Instrumentation System. Hazardous Areas. Class I Flammable Gases or Vapors. Class II Combustible Dusts. Class III Ignitable Fibers or Flyings [Group D]

Hazardous Areas

An area classified as “hazardous” is an area in which the atmosphere contains, or may contain, flammable or explosive gases, dusts, or fibers. In this type of area, fire or explosion could occur when all three basic conditions are fulfilled:

1. Flammable gas, vapor, dust or fibers must be present.
2. The combustible material must be mixed with air in the proportions required to produce a flammable mixture.
3. A source of ignition must act to ignite the mixture. The source under consideration is any portion of the instrumentation system that could release sufficient energy to cause ignition. The incendiary energy may be released by a spark or hot surface.

In the United States, the National Electrical Code, Article 500 for both Zones and Divisions, Canada, Canadian Electrical Code, Section 18 for Zones, defines a hazardous area by applying a three-part classification. In Europe the classification is in type of industry and degree of hazard. See the following tables, for relationship of North America, European definitions and comparisons.

Applying safety barriers requires some knowledge of intrinsic safety principles and terminology. The two main systems in international use are summarized below.

IEC COUNTRIES

(Countries accepting International Electrotechnical Commission Standards, e.g. Australia, Europe,
S.Africa)

Intrinsic safety

Technique that achieves safety by limiting the ignition energy and surface temperature that can arise in normal operation, or under certain foreseeable fault conditions, to levels that are insufficient to ignite an explosive atmosphere.

Intrinsic safety standards

Exia: safety maintained with up to two component faults: hazardous area equipment may be used in Zones 0, 1 and 2.

Exib: safety maintained with up to one component fault: hazardous area equipment may be used in Zones 1 and 2.

Type N equipment (safe in normal operation) may be used in Zone 2.

Type of industry

Explosive mixtures of air with flammable gases or vapors (dusts under consideration).

Degree of hazard

Zone 0: explosive gas-air mixture continuously present, or present for long periods.

Zone 1: explosive gas-air mixture is likely to occur in normal operation.

Zone 2: explosive gas-air mixture not likely to occur and, if it occurs, it will exist only a short time.

Ignition by spark

Apparatus is grouped according to the ignition energy produced under fault conditions, in terms of the gas-air mixture that it will not ignite

Representative gases are:
Group IIC: hydrogen, acetylene
Group IIB: ethylene
Group IIA: propane

Group I: methane

Ignition by hot surface

Hazardous area apparatus is classified according to the maximum surface temperature produced under fault conditions at an ambient temperature of 40°C (or as otherwise specified).

T1: 450°C	T2:300°C	T3: 200°C
T4: 135°C	T5: 100°C	T6: 85°C

Gas characteristics

Details of gas grouping by ignition energy and of gas ignition temperatures are contained in: British Standard Code of Practice for Electrical Apparatus and Associated Equipment for use in Explosive Atmospheres (other than Mining Applications), BS5345: Part 1 – Basic requirements for all parts of the code.

Approval

National certifying authorities issue Apparatus and/or System Certificates for approved equipment, or Listings of approved equipment, defining all or most of the following:

IS standard Apparatus (gas) group

Temperature Class Installation conditions

Cable parameter limits Zone division classification

In Europe, common CENELEC standards will be adopted by all national authorities in due course.

USA and CANADA
(Are adopting IEC terminology )

Intrinsic safety

Technique that achieves safety by limiting the ignition energy and surface temperature that can arise in normal operation, or under certain foreseeable fault conditions, to levels that are insufficient to ignite an explosive atmosphere.

Intrinsic safety standards

Exia: safety maintained with up to two component faults: hazardous area equipment may be used in Zones 0, 1 and 2.

Exib: safety maintained with up to one component fault: hazardous area equipment may be used in Zones 1 & 2.
Or
One standard only: safety maintained with up to two component faults: hazardous area equipment may be used in Divisions 1 and 2.

Type N equipment (safe in normal operation) may be used in Zone 2.
Or Non-incendive equipment (safe in normal operation) may be used in Division 2.

Type of industry

No classification, but mining and surface industries dealt with by different authorities.

Degree of hazard

Zone0/1 or Class I, Division 1: hazardous concentrations of flammable gases or vapors continuously, intermittently or periodically present under normal operating conditions.

Zone 2 or Class I, Division 2: volatile flammable liquids or flammable gases present, but normally confined within closed containers or systems, from which they can escape only under abnormal operating or fault conditions.

Ignition by spark

Explosive atmospheres are grouped according to their ignition energy.
Representative atmospheres are:

Standard version	IEC
Group A: acetylene	IIC
Group B: hydrogen	IIC	Class I
Group C: ethylene	IIB
Group D: propane	IIA
Group E: metal dust
Group F: carbon dust		Class II
Group G: flour, starch, grain

Ignition by hot surface

Hazardous area apparatus is classified according to the maximum surface temperature produced under fault conditions at an ambient temperature of 40°C (or as otherwise specified).

T1: 450°C	T2:300°C	T3: 200°C
T4: 135°C	T5: 100°C	T6: 85°C

Gas characteristics

Details of gas grouping by ignition energy and of gas ignition temperatures are contained in:NFPA, Vol. 5 Electrical, Article 500, Table 500-2(c) (ignition energy)

CSA No. C22-1 (ignition energy) Section 18

Approval

National certifying authorities issue Apparatus and/or System Certificates for approved equipment, or Listings of approved equipment, defining all or most of the following:

IS standard Apparatus (gas) group

Temperature Class Installation conditions

Cable parameter limits Zone division classification

In the USA, OSHA, under NRTL, requires either CSA, FM, UL or other NRTL approved agencies approval. In Canada, CSA is required.

Class I Flammable Gases or Vapors
Zone 0,1 or Division 1(Exia/b) Exists under normal conditions Exists for long periods of time May exist because of: repair operations maintenance operations leakage Released concentration because of: breakdown of equipment breakdown of process faulty operation of equipment faulty operation of process which causes simultaneous failure of electrical equipment	Zone 2 or Division 2(Exn) Liquids and gases are in closed containers or the systems are: handled processed used Concentrations are normally prevented by positive mechanical ventilation. Adjacent to a Class I, Division 1 location.

Group A: Atmospheres containing Acetylene [IIC]
Group B: [IIC]	Atmospheres such as Butadiene, Ethylene Oxide, Propylene Oxide, Acrolein or Hydrogen (or gasses or vapors equivalent in hazard to hydrogen such as manufactured gas.)
Group C: [IIB]	Atmospheres such as Cyclopropane, Ethyl Ether, Ethylene or gases or vapor equivalent in hazard.
Group D: IIA	Atmosphere such as Acetone, Alcohol, Ammonia, Benzine, Benzol, Butane, Gasoline, Hexane, Laquer Solvent vapors, Naptha, Natural Gas, Propane or gases or vapors equivalent in hazard.

Class II Combustible Dusts
Division 1 Exists under normal conditions Combustible mixture produced by: mechanical failure of equipment or machinery abnormal opertion of equipment and provide source of ignition flame simultaneous failure of electrical equipment simultaneous failure of operation of protection devices other causes Electrically conductive dusts may be present	Division 2 Not normally in the air Accumulation normally sufficient ti interfere with normal operation of electrical equipment or other apparatus In the air as a result of infrequent malfunctioning of: handling equipment process equipment Accumulations are sufficient to interfere with the safe dissipation of heat from electrical equipment Accumulations may be ignitable by abnormal failure of electical equipment
Group E: Atmospheres containing combustible metal dusts (regardless of resistivity), dusts of similarly hazardous characteristics (<100 k ohms/cm) or electrically conductive dusts.
Group F: Atmospheres contains combustible Carbon Black, Charcoal or Coke Dusts which have >8% total volitile material or if these dusts are sensitized so that they present and explosion hazard and having a resistivity >100 k ohms/cm less than or equal to 100 M ohms/cm.
Group G: Atmospheres containing dusts having a resistivity >100 K ohms/cm or electrically nonconductive dusts.

 

Class III Ignitable Fibers or Flyings [Group D]
Division 1 Fibers or materials producing combustible Flyings are manufactured, stored or handled.		Division 2 Fibers are handled except during the manufacturing process or are stored, except during the process of manufacture.
	Not Grouped Manufacturers such as textile mills, cotton related mills or clothing plants. Fibers and Flyings include Rayon, Cotton, Sisal, Hemp, Jute and Spanish Moss.

Non-hazardous (Safe) Area
An area classified as non-hazardous has a small probability of a flammable mixture being present. It is also called a “safe area” and includes most control rooms.

The Instrumentation System

An electrical system generally contains potential sources of ignition that are of concern in a hazardous area installation. Therefore, the types of ignition sources and applicable methods of preventing ignition must be considered by an instrument manufacturer and user. Systems that are designed to meet certain safety criteria may receive certification from a safety standards approval agency such as Canadian Standards Association (CSA), BASEEFA, SIRA or LCIE.

Sources of Ignition
Generally, a potential source of ignition from an electrical system is any spark or hot component that releases energy sufficient to ignite a combustible mixture surrounding it. The ignition source may occur in any of four mechanisms: a) discharge of capacitive circuits, b) interrupting (opening) of inductive circuits, c) opening or closing of resistive circuits with slow intermittent interruption increasing the ignition capability (hazard), and d) high temperature sources. The ignition mechanisms may occur in relay contacts, switch contacts, fuses, short circuits (from damage or component failure), and arc-over between components or conductors. The components or circuits that present a potential ignition source may be designed in a variety of ways in order to prevent ignition of a hazardous atmosphere.