Gas detection instruments must often be used or installed in areas considered to be “hazardous locations.” Generically, all sorts of hazards might be recognized in a particular environment, but the term “hazardous locations,” as it applies to electrically-operated equipment refers to provisions established by the National Fire Protection Association (NFPA), and various other organizations.
NFPA publishes the National Electrical Code®, the stated purpose of which is “[T]he practical safeguarding of persons and property from hazards arising from the use of electricity.” (Article 90 of the Code)
Article 500.5 (A) of the Code discusses how locations will be classified, and because of this the terms “hazardous location” and “classified location” are used interchangeably:
Locations shall be classified depending on the properties of the flammable gas, flammable liquid-produced vapor, combustible-liquid produced vapors, combustible dusts, or fibers/flyings that may be present, and the likelihood that a flammable or combustible concentration or quantity is present.
Three classes of hazardous locations are defined…
|Class I||Flammable gases or vapors|
|Class II||Combustible dusts|
|Class III||Combustible fibers or flyings|
The division of a location defines the frequency that the hazard exists or may exist in a location…
Class I, Division 1 location:
Ignitable concentrations of flammable gases, flammable liquid-produced vapors, combustible liquid-produced vapors, or combustible liquids above their flash points…
- Which can exist under normal operating conditions
- Which may exist frequently because of repair, maintenance operations, or leakage
- Which might be released due to a breakdown or faulty operation of equipment or processes, and might also cause simultaneous failure of electrical equipment in such a way as to directly cause the electrical equipment to become a source of ignition.
Class I, Division 2 location:
Volatile flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors…
- Are handled, processed, or used, but they will normally be confined within closed containers or closed systems from which they can escape only in case of accidental rupture or breakdown
- Are normally prevented from becoming ignitable by positive mechanical ventilation and which might become hazardous through failure or abnormal operation of the ventilating equipment
- (or combustible liquid-produced vapors above their flash points) Might occasionally be communicated by virtue of being adjacent to a Class I, Division 1 location, 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 II, Division 1 location:
Airborne combustible dust in quantities sufficient to produce explosive or ignitable mixtures…
- Under normal operating conditions
- Due to mechanical failure or abnormal operation of machinery or equipment—also providing a source of ignition through simultaneous failure of electrical equipment, through operation of protection devices, or from other causes
- In which Group E combustible dusts (defined below) may be present in quantities sufficient to be hazardous
Class II, Division 2 location:
Airborne combustible dust in quantities sufficient to produce explosive or ignitable mixtures…
- Due to abnormal operations
- As a result of infrequent malfunctioning of handling or processing equipment
- Due to abnormal operation or failure of electrical equipment. A related hazard would have the dust accumulate such that it could interfere with the safe dissipation of heat from electrical equipment.
A Class III, Division 1 location is a location in which easily ignitable fibers/flyings are handled, manufactured, or used.
A Class III, Division 2 location is a location in which easily ignitable fibers/flyings are stored or handled other than in the process of manufacture.
A further designation is the so-called “material groups.”
Within Class I, four groups are defined, in accordance with the maximum experimental safe gap and minimum igniting current ratio…
MESG (Maximum Experimental Safe Gap) = The maximum clearance between two parallel metal surfaces that has been found, under specified test conditions, to prevent an explosion in a test chamber from being propagated to a secondary chamber containing the same gas or vapor at the same concentration.
MIC (Minimum Igniting Current) Ratio = The ratio of the minimum current required from an inductive spark discharge to ignite the most easily ignitable mixture of a gas or vapor, divided by the minimum current required from an inductive spark discharge to ignite methane under the same test conditions.
Group A Acetylene
Group B Flammable gas, flammable liquid-produced vapor, or combustible liquid-produced vapor mixed with air that may burn or explode, having either an MESG value less than or equal to 0.45 mm or an MIC ratio less than or equal to 0.40. A typical Class I, Group B material is hydrogen.
Group C Flammable gas, flammable liquid-produced vapor, or combustible liquid-produced vapor mixed with air that may burn or explode, having either an MESG value greater than 0.45 mm and less than or equal to 0.75 mm, or an MIC ratio greater than 0.40 and less than or equal to 0.80. A typical Class I, Group C material is ethylene.
Group D Flammable gas, flammable liquid-produced vapor, or combustible liquid-produced vapor mixed with air that may burn or explode, having either an MESG value greater than 0.75 mm or an MIC ratio greater than 0.80. A typical Class I, Group D material is propane.
Further guidance on Class I areas is available in NFPA document 497, entitled Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas.
Within Class II, three groups are defined…
Group E Atmospheres containing combustible metal dusts, including aluminum, magnesium, and their commercial alloys, or other combustible dusts whose particle size, abrasiveness, and conductivity present similar hazards in the use of electrical equipment. It is noted that certain metal dusts—including zirconium, thorium, and uranium—have extremely low ignition temperatures [as low as 68°F (20°C)] and minimum ignition energies lower than any material classified in any of the Class I or Class II groups.
Group F Atmospheres containing combustible carbonaceous dusts that have more than 8 percent total entrapped volatiles or that have been sensitized by other materials so that they present an explosion hazard. Coal, carbon black, charcoal, and coke dusts are examples of carbonaceous dusts.
Group G Atmospheres containing combustible dusts not included in Group E or F, including flour, grain, wood, plastic, and chemicals.
Further guidance on Class II areas is available in NFPA document 497, entitled Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas.
Article 500.8 of the Code details provisions for maximum allowable surface temperature of equipment, but this would normally not apply to gas detection systems. However, alarm signaling devices should be carefully chosen to meet the requirements of an occupancy. In addition, because of the extreme hazards associated with certain metal dusts, note that portions of portable analyzers might develop temperatures which would be dangerous in these environments. Most portable gas detection equipment is approved for gas/vapor environments (A, B, C, D) only.
National Electrical Code harmonization with certain International Electrotechnical Commission (IEC) standards
Explosive gas atmospheres
IEC standard 60079-10-1 entitled Electrical Apparatus for Explosive Gas Atmospheres—Part 10-1: Classification of Hazardous Areas provides a classification scheme, that has been adopted as an alternative to NEC’s traditional approach outlined above. Zones rather than divisions are defined in this scheme. The following table may be helpful…
Time that hazardous gases are present in ignitable concentrations
Occasionally in normal operation
|Division 1 is split into Zone 0 and 1.|
Zone 0 is a small percentage of locations, usually confined to inside vented tanks.
|Zone 2||Division 2||Not normally present||Zone 2 and Division 2 are essentially the same|
The material group classification under this scheme is somewhat different from the traditional NEC approach, and is detailed in Article 505.6 of the Code. 29 CFR 1910.399 [OSHA] also defines many of these terms.
Combustible dust atmospheres
IEC document 61241-10 entitled Electrical apparatus for use in the presence of combustible dust—Part 10: Classification of areas where combustible dusts are or may be present provides a classification scheme, that has been adopted as an alternative to NEC’s traditional approach outlined above. Zones rather than divisions are defined in this scheme, but combustible metallic dusts (as in NEC Group E) are not included.
Zone 20 Hazardous (Classified) Location An area where combustible dust or ignitable fibers/flyings are present continuously or for long periods of time in quantities sufficient to be hazardous. [Compare to Zone 0]
Zone 21 Hazardous (Classified) Location An area where combustible dust or ignitable fibers/flyings are likely to exist occasionally under normal operation in quantities sufficient to be hazardous. [Compare to Zone 1]
Zone 22 Hazardous (Classified) Location An area where combustible dust or ignitable fibers/flyings are not likely to occur under normal operation in quantities sufficient to be hazardous. [Compare to Zone 2]
A variety of methods are available to safely package electrical equipment for hazardous locations—including encapsulation, oil immersion, powder filling, and hermetic sealing. However, since most instrumentation must be accessible from time to time, the predominant protection methods used for fixed, continuous gas monitoring systems are explosion-proof (including dust-ignition proof) packaging and purging to render the equipment intrinsically safe. Certain portable gas detection instruments (such as Interscan’s UL Classified 4000 Series) are intrinsically safe by design, and do not require purging.
Explosion-proof apparatus (per ANSI/UL 1203): “Apparatus enclosed in a case that is capable of withstanding an explosion of a specified gas or vapor that may occur within it and of preventing the ignition of a specified gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within, and that operates at such an external temperature that a surrounding flammable atmosphere will not be ignited thereby.”
Dust-Ignition proof equipment (per ANSI/UL 1203): “Equipment enclosed in a manner that excludes dusts and does not permit arcs, sparks, or heat otherwise generated or liberated inside of the enclosure to cause ignition of exterior accumulations or atmospheric suspensions of a specified dust on or in the vicinity of the enclosure.”
Note that enclosures and apparatus suited for these environments are specifically marked by class, division (zone), and group.
Intrinsically safe circuit (per ANSI/UL 913): “A circuit in which any spark or thermal effect is incapable of causing ignition of a mixture of flammable material in air under prescribed test conditions.”
Intrinsically safe apparatus (per ANSI/UL 913): “Apparatus in which all the circuits are intrinsically safe.”
Here again, ratings are based on class, division (zone), and group.
NFPA document 496, entitled Standard for Purged and Pressurized Enclosures for Electrical Equipment governs, and defines three types of purging. In general, most purging applications require a minimum enclosure pressure of 0.10 inches (2.5 mm) of water, although some occupancies require more than this, and it is prudent to build in a safety factor in any event. Three types of pressurization are defined…
Type “X” Protects general purpose equipment in Division 1 areas. This type reduces the classification within protected enclosures from Division 1 to non-hazardous. It is required to automatically control electrical power to all protected equipment.
Type “Y” Protects Division 2 rated equipment in Division 1 areas. This type reduces the classification within protected enclosures from Division 1 to Division 2. All protected equipment must be rated for Division 2. Automatic power control disconnects are not required, but visual and/or audible alarms must be initiated when there is loss of pressure.
Type “Z” Protects general purpose equipment in Division 2 areas. This type reduces the classification within protected enclosures from Division 2 to non-classified. Automatic power control disconnects are not required, but visual and/or audible alarms must be initiated when there is loss of pressure.
Type “X” and type “Z” are most often used.
Choosing the protection method
In most cases, purging is preferred as explosion-proof packaging is exceedingly expensive, and may not be available at any price depending on the occupancy. An exception would be small remote gas detection sensing heads, which are traditionally packaged in explosion-proof purpose-designed enclosures.
As you can see, figuring out gas detection installations in hazardous locations can be complex. While this article provides a good introduction, it is by no means comprehensive. Indeed, situations can arise that will go beyond the scope of the standards.
Should you need further assistance in dealing with hazardous locations, please contact us.