Hazardous area classification for electrical systems

Production facilities contain, or may contain, flammable gases and vapors in normal operations. In the right concentration with air, these can form an explosive environment that is ignitable by hot surfaces, electrical arcs, and sparks. To prevent this from happening, facilities must be classified properly, so that all electrical equipment and systems are properly selected and installed.

North American standards

In the U.S., facilities are classified according to NEC, and a nationally recognized testing laboratory must approve all arcing electrical equipment installed in the classified areas.

The four steps involved in hazardous area classification are:

1. Determine the type of hazard or "class" that might be present—combustible gas (Class I), combustible dust (Class II), or fibers (Class III).
2. Identify the specific "group" for the hazardous substance (Group A through Group G).
3. Determine the degree of the classification (Division 1 or Division 2).
4. Determine the extent of the classified locations.

Groups A through G are acetylene, hydrogen, ethylene, propane/methane, metal dust, coat dust, and grains/fibers, respectively. Almost all classifications in oil and gas facilities are Class I, Group D. Class I locations are those in which flammable gases or vapors are or might be present in the air in the quantities sufficient to produce explosive or ignitable mixtures.

Class I locations are either Division 1 or Division 2, or are unclassified:

Class I, division 1

Locations (1) in which ignitable concentrations of flammable gases or vapors can exist under normal operating conditions, (2) in which ignitable concentrations of such gases or vapors may exist frequently because of repair or maintenance operations or because of leakages, or (3) in which breakdown or faulty operation of equipment or processes might release ignitable concentrations of flammable gases or vapors and might also cause simultaneous failure of electrical equipment in such a way as to directly cause electrical equipment to become a source of ignition.

Class I, division 2

Locations (1) in which volatile, flammable liquids or flammable gases are handled, processed, or used, but in which the liquids, vapors, or gases normally will be confined within the closed containers or closed system from which they can escape only in case of accidental rupture or breakdown of such containers or systems, or in case of abnormal operation of equipment; (2) in which ignitable concentrations of gases or vapors normally are prevented by positive ventilation, and which might become hazardous through failure or abnormal operation of the ventilation equipment; or (3) which are adjacent to a Class I, Division 1 location, and to which ignitable concentrations of gases or vapors might occasionally be communicated unless such communication is prevented by adequate positive-pressure ventilation from a source of clean air, and effective safeguards against ventilation failure are provided.

Unclassified

All areas in a facility that are not Division 1 or Division 2 are considered unclassified. Arcing electrical equipment in the unclassified areas need not be explosion-proof. General-purpose enclosures are accepted in these areas.

API RP 500 shows typical examples of classifications of equipment in oil and gas production facilities, including the extent of the classified areas around such equipment. Figs. 1 through 5 provide examples of classified locations in a typical oil-and-gas-production facility.

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  Fig. 1—Nonenclosed, adequately ventilated well on which work is being performed.

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  Fig. 2—Nonenclosed beam-pumping well in an adequately ventilated area with an inadequately ventilated cellar.

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  Fig. 3—Nonenclosed beam-pumping well in an adequately ventilated area without a cellar.

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  Fig. 4—Electrical submersible pumping well in a nonenclosed, adequately ventilated area without a cellar.

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  Fig. 5—Electrical submersible pumping well in a nonenclosed, adequately ventilated area with an inadequately ventilated cellar.

Refer to API RP 500 and the NEC for further details regarding hazardous-area classification.

International Electrotechnical Commission (IEC) standards

Whereas the classification based on the NEC and API standards is used in the U.S. and a few other countries in the world, an IEC-created zone classification system is widely accepted elsewhere.

The IEC zone classifications basically are:

Class I, zone 0

Locations (1) in which ignitable concentrations of flammable gases or vapors are present continuously or (2) in which ignitable concentrations of flammable gases or vapors are present for long periods of time.

Class I, zone 1

Locations (1) in which explosive or ignitable concentrations of flammable gases or vapors are likely to exist under normal operating conditions; (2) in which ignitable concentrations of flammable gases or vapors may exist frequently because of repair or maintenance operations or because of leakages; (3) in which equipment is operated or processes are carried on, of such a nature that equipment breakdown or faulty operations could result in the release of ignitable concentrations of flammable gases or vapors and cause simultaneous failure of electrical equipment in a mode to cause the electrical equipment to become a source of ignition; or (4) that is adjacent to a Class I, Zone 0 location, from which ignitable concentrations of gases or vapors could be communicated, unless such communication is prevented by adequate positive-pressure ventilation from a source of clean air, and effective safeguards against ventilation failure are provided.

Class I, zone 2

Locations (1) in which ignitable concentrations of flammable gases or vapors are not likely to occur in normal operation and if they do occur will exist only for a short period; (2) in which volatile, flammable liquids, flammable gases, or flammable vapors are handled, processed, or used, but in which the liquids, gases, or vapors are confined within closed containers or a closed system from which they can escape only in case of accidental rupture or breakdown of such containers or system, or as a result of the abnormal operation of equipment with which the liquids or gases are handled, processed, or used; (3) in which ignitable concentrations of flammable gases or vapors normally are prevented by positive mechanical ventilation, but which may become hazardous because of failure or abnormal operation of the ventilation equipment; or (4) which are adjacent to a Class I, Zone 1 location from which ignitable concentrations of flammable gases or vapors could be communicated unless such communication is prevented by adequate positive-pressure ventilation from a source of clean air, and effective safeguards against ventilation failure are provided.

Unclassified

All areas in the facility that are not Zone 0, Zone 1, or Zone 2 are considered unclassified. Arcing electrical equipment in unclassified areas need not be explosion-proof. General-purpose enclosures are acceptable in these areas.

The IEC zone classification also differs from North American standards in its grouping of the hazardous gases or vapors as either Group I or Group II. Group I is for use in describing atmospheres that contain firedamp (a mixture of gases, composed mostly of methane, found underground, usually in mines). Group II covers all other flammable gases. Group II is subdivided into IIC, IIB, and IIA, according to the nature of the gas or vapor:

• Group IIC is equivalent to a combination of Class I, Group A and Class I, Group B in the NEC system.
• Group IIB is equivalent to Class I, Group C in the NEC system.
• Group IIA is equivalent to Class I, Group D in the NEC system.

See NEC, Chap. 5, Article 505 for further details of the IEC zone classifications.

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See also

Electrical grounding

Electrical distribution systems

Power factor and capacitors

Electrical systems

Alternating current motors

Induction motors

Synchronous motor

Motor specifications

NEMA motor characteristics

Alternating current motor drives

Motor enclosures

PEH:Electrical_Systems