Hazardous Area Classified

What Constitutes A Hazardous Area?

I.     Hazardous Area Defined
II.    Class Defined
III.   Division Defined
IV.  Class and Division Locations
V.   Groups Defined
VI.  Temperature Identification
VII. Creating an Intrinsically Safe Environment

Hazardous Area Defined

Hazardous areas are those which constitutes a potential fire/explosion. The National Electric Code, (Article 500), as set forth by the NFPA, classifies these hazards by Class, Division, and Group.

The CLASS designation represents the broad category of fuel type. It is listed as seen below.

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Class

In the United States, we use the term CLASS to divide the types of fuel into families. Unfortunately the terms Class I,II, and III have taken on a street meaning that they should not. An explosion that might occur in a Class I area should not be thought of as being more dangerous, more damaging, or more probable than an explosion in a Class II area. The mangled and melted steel after a gas explosion isn’t much different from the fractured and pulverized reinforced concrete after a grain dust explosion.

The original concept of CLASS was more meaningful than it is today.

Class I:

gas, was defined as a molecule or a compound (multiple different molecules) or a fuel mixed with oxygen molecules.

Class II:

dust, was defined as a mixture of compounds and oxygen molecules.

Class III:

fibers and lint, was defined as particles bigger than dust.

Class I, II, and Class III are handled in a similar manner by today’s technology; Intrinsically safe, purged and pressurized, and immersion methods. All these methods are designed to prevent even the smallest explosion.

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Division

Here in the United States, we often physically divide a hazardous area into two parts. The line that divides these two parts is based upon the probability that a dangerous fuel to air mixture will occur. Here’s an example:

Division 1:

The inside of a gasoline storage tank.

Division 2:

A storage room for oil based paints (a few times a year maybe a can is opened to verify the color, then covered and sealed again).

For my examples, I picked two opposite and obvious extremes.

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Class and Division Locations

The National Electrical Code gives the following definitions for hazardous area locations:

Class I, Division 1 Locations are Areas:

1. in which ignitable concentrations of such gases or vapors can exist under normal operation conditions;
   
   
2. in which ignitable concentrations of such gases or vapors may exist frequently because of repair or maintenance operations or because of leakage;
   
   
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 electric equipment.
   
   

Class I, Division 2 Locations are Areas:

1. in which volatile flammable liquids or flammable gases are handled, processed, or used, but in which the liquids, vapors or gases will normally be confined within closed systems 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 are normally prevented by positive mechanical ventilation, and which might become hazardous through failure or abnormal operation of the ventilating equipment;
   
   
3. that are adjacent to a Class I, Division I location, and to which ignitable concentrations of gases or vapors might occasionally be communicated unless such communication is prevented by positive-pressure ventilation from a source of clean air, and effective safeguards against ventilation failure are provided.
   
   

Point 2 and 3 of Class I, Division 2 above both deal with positive ventilation. The difference is the last part of point 3, "effective safeguards against ventilation failure are provided". If effective safeguards are incorporated, then the area is not classified. It in non-hazardous. If there is positive ventilation, but without an effective safeguard against ventilation failure, then the location is a Division 2 area.

Class II, Division 1 Locations are Areas:

1. in which combustible dust is in the air under normal operation conditions in quantities sufficient to produce explosive or ignitable mixtures.
   
   
2. where mechanical failure or abnormal operation of machinery or equipment might cause such explosive or ignitable mixtures to be produced, and might also provide a source of ignition through simultaneous failure of electric equipment, operation of protective devices, or from other causes.
   
   
3. in which combustible dusts of an electrically conductive nature may be present in hazardous quantities.
   
   

Class II, Division 2 Location

Is a location where combustible dust is not normally in the air in quantities sufficient to produce explosive or ignitable mixtures, and dust accumulation are normally insufficient to interfere with the safe dissipation of heat from electrical equipment, or may be ignitable by abnormal operation or failure of electrical equipment.

Class III, Division 1 Location

Is a location in which easily ignitable fibers or materials producing combustible flyings are handled, manufactured, or used.

Class III, Division 2 Location

Is a location in which easily ignitable fibers are stored or handled.

I have included an exact description of Class-Division text from the National Electric Code. I’ve done this because this is the most subjective topic concerning hazardous areas. The determination of the Division requires understanding, common sense, and experience. If in doubt, Division 1 equipment can be used in Division 2 areas if the Class and Group ratings are the same. This is per the 1990 National Electric Code, Article 500-3; a. paragraph 2.

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Groups

The GROUP may be the most meaningful nomenclature of the hazardous area terms. A group identifies materials with similar explosion properties.

For Class I materials, the determination of the group is based heavily upon the requirements of explosion proof enclosure technology, i.e. maximum explosion pressure, maximum safe clearance between parts of a clamped joint in an enclosure, and ignition energy.

For Class II materials, the determination is based upon the tightness of the joints of assembly and shaft openings, to prevent entrance of dust in the dust-ignition proof enclosure; the blanketing effect of layers of dust on the equipment that may cause overheating; electrical conductivity of the dust; and the ignition temperature of the dust.

Class III materials, lint shavings and fibers, are not broken down into groups.

The following tables list the materials that are considered representative of the Groups and Classes.

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Temperature Identification

Unfortunately, materials refuse to neatly arrange themselves by explosion pressure, maximum safe gap, ignition energy and ignition temperature. There are many materials with a high ignition temperature and a low ignition energy or vice versa. As materials are grouped largely by ignition energy, ignition temperature must be considered independently.

Prior to the 1971 National Electrical Code, the minimum ignition temperature of a Class I Group was considered the limit for all materials in the group (see table below). However, this was overly restrictive and new gases continually caused a need for revision of the limits.

In 1971, temperature identification numbers were introduced (see table below). Today, unless the maximum operating temperature of the equipment at its maximum marked ambient temperature is less than the most critical temperature rating T6 85?C, the equipment must be marked with the appropriate temperature identification code. This enables the user to ensure that specific equipment will not ignite a specific material due to hot surfaces as well as to sparks. Class II Group temperatures did not change like the Class I Group temperatures.

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Creating an Intrinsically Safe Environment

MTI carries a full line of Intrinsically Safe and Explosion Proof Indicators, Junction Boxes, and Load Cells. All meet full Factory Mutual Approval for Intrinsically Safe applications. Determination of systems classification to any hazardous environment must be ultimately made by the customer and local authorities.

If you are unsure of the design criteria involved in determining how your application applies to intrinsic safety guidelines, ask our engineers. We maintain up-to-date National Standards on all relative information and guidelines to intrinsic safety. MTI has the experience necessary to guide you through the process of meeting the stringent specifications which will satisfy your insurance underwriter. As a provider of such equipment, we leave no details unchecked. All systems go through a complete verification process prior to shipment. You're assured that the equipment you receive will meet the exact requirements that you have specified. Call one of our hazardous area application specialists for immediate answers to any of your hazardous area weighing questions.

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