Class I Div 2 – Conditions and Requirements

David
Etukudo

Class I, Div 2 is a hazardous area classification according to the National Electrical Code (NEC). Also, the National Fire Protection Association and Underwriters Laboratories utilize the same classification. Class I, Div 2 refers to an area where explosive or flammable liquids, gases or vapor is not likely to exist during normal operation. This article reviews the conditions that qualify a location as Class I Div 2, Class I Groups, and the requirements of a Class I Div 2 area.

Fire triangle showing elements of a hazardous location such as Class I Div 2
Courtesy: AssuredSystems

Class I Div 2 Conditions

One major safety concern in workplaces is the occurrence of fire and explosions. Generally, equipment and human activity within any area should operate safely in line with the hazards present per time. Therefore, codes such as the NEC categorize locations according to the likely dangers. The conditions that make a location Class I Div 2 are as follows:

  • A location where there is use, processing, or handling of flammable liquids or gases. But, these hazardous fluids are secure within containers or systems where their release would be from an accident or abnormal operation.
  • Also, an area where positive mechanical ventilation prevents hazardous concentrations of gases, except through failure of ventilating equipment.
  • Any location that is adjacent to a Class I Div 1 location, and to which flammable gas/liquids might transfer.

Some common Class I areas include petroleum refineries, gasoline storage areas, spray finishing areas, and dry-cleaning plants.

Class I Groups

A further level of classifying hazardous areas beyond Divisions is in Groups. Moreover, these Groups are according to the type of chemicals present within the location. This classification is important because the ignition temperature depends on the type of gas as the following table shows.

GroupChemicalsIgnition Temperature (°C)
AAcetylene305
BHydrogen
Butadiene
Ethylene Oxide
Propylene Oxide
520
420
429
449
CEthylene
Cyclopropane
Diethyl Ether
Acetaldehyde
Hydrazine
450
503
160
175
249
DGasoline
Ethane
Butane
Ethanol
Methanol
Hexane
Propane
Acetone
Ammonia
Benzene
Petroleum Naphtha
280, 536
472
288
363
385
225
450
465
498
498
288
Class I Chemical Groups and Ignition Temperature

Note, these groups cover only chemicals for Class I, Div 1 and Div 2.

Class I Div 2 Requirements

To maintain safety, there are several equipment requirements in accordance with articles 500 and 501 of the NEC. These requirements cover areas including wiring and a host of electrical and electronic equipment. The following sections highlight some of these requirements.

Class I Div 2 Distance Requirements

In line with the NEC, there should be a minimum clearance between any hazardous source and equipment that could ignite it. However, this clearance depends on the nature of the hazardous source and the presence of features such as a vent, fan, or door.

Distance Requirements for Class I Div 2 Building

For a building containing flammable gas, certain areas around openings such as windows, doors, and vents are Class I Div 2 locations. Industry codes determine that a 3 ft radius around vents, windows, and shut doors are Div 2 hazardous areas. Moreover, an extent of 10 ft around open doors also ranks under a Div 2 location.

Distance Requirements for Above Ground Tank

The immediate area around an above-ground tank containing flammable liquid ranks as a Class I Div 2 location. But the area surrounding the tank’s vents, up to a radius of 5 ft, are Class I Div 1 locations, which have more stringent requirements. Beyond this 5 ft radius and up to 10 ft, qualifies as a Class I Div 2 location.

Distance Requirements for Skid-Mounted Equipment

Generally, distance requirements for skid-mounted equipment categorize according to the distance from the equipment and skid, respectively. The figure and table below show this categorization of the Class I Div 2 location.

Categorization of Class I Div 2 distance requirements for a skid-mounted hazardous equipment
Courtesy: Newterra
LocationABC
Indoor5 ft25 ft3 ft
Outdoor3 in10 ft18 in
Distance requirements for a skid-mounted hazardous equipment

Seal Off Requirements

Also, there are industry requirements for sealing off gases in Class I, Div 2 locations. For each conduit leaving a Class I, Division 2 location, there shall be a conduit seal at the boundary. Moreover, the installation of the seal fitting could be on either side of the boundary and be within 10 ft of the boundary. In addition, to minimize gas transfer, the sealing connection should be threaded, and there should be no union or coupling between the seal fitting and the point where the conduit leaves the hazardous area.

Seal off requirements for a Class I, Div 2 location
Courtesy: EMERSON

Other Sealing Requirements

There are several other sealing requirements with a few of them as follows:

  • Fittings: Any enclosure containing connections or equipment shall have an integral seal fitting. Moreover, this fitting should serve alongside specific sealing compounds. In a case where a fitting can work with multiple compounds, then, refer to seal fitting installation instructions for proper preparation of the compound.
  • Compound: The sealing compound shall be compatible with the process fluid and atmosphere. Also, its melting point shall not be less than 93°C (200°F). In seals that are complete, the compound’s thickness shall not be less than the trade size of the fitting. However, in no case shall the thickness be less than 16 mm (5/8 in).
  • Splices and Taps: Any fitting that uses a sealing compound shall have no splices and taps. Conversely, fittings that use splices or taps shall not have any sealing compound.
  • Conductor or Optical Fiber: The cross-sectional area of the conductors or optical fiber shall not exceed 25% of the area of the rigid metal conduit of the same trade. Except, the seal is specifically set for a higher percentage fill. Such fittings that are available for a higher percentage fill can go up to 40% fill. This can help reduce the total number of conduit-runs in an installation.

Enclosure Requirements

Generally, NEMA 7 enclosures meet the requirements for Class I, Div 2 indoor locations because of the following features:

  • Explosion-Proof Enclosure: These are enclosures that can withstand the pressures from an internal explosion of specified gases. Moreover, they contain such an explosion sufficiently to prevent ignition of the explosive atmosphere surrounding the enclosure.
  • Purged and Pressurized Enclosures: A sufficient flow of clean air or inert gas prevents the accumulation of ignitable gases within the enclosure.
  • Oil Immersion: The arrangements of arcing contacts or connections remain in an oil immersion. Thus, any arcing is always confined under the oil and poses no risk of ignition.
  • Intrinsically Safe: This term describes wiring that is incapable of releasing sufficient electrical or thermal energy to ignite flammable gases under normal conditions.