Area Classification Drawing

Written by Matt Milbury on . Posted in Electrical Engineering

A hazardous area classification drawing (also known as an area classification drawing) outlines the classifications of areas where flammable liquids, gasses or vapors are handed, processed or stored. It is created based on input from the Process Flow Diagrams, Piping & Instrumentation Diagrams and the Equipment Location Plan. The intent of the drawing is to communicate to engineers, operators and contractors information on the hazardous material that may be present and the probability that it is in the atmosphere. This knowledge allows for engineers and designers to select the right equipment and for contractors to know how to properly install the equipment.

PSV Area Class ExampleExample of a classified area on a pressure relief valve, PRVIn the United States, the most common way of classifying areas is specified in NFPA 70 - National Electric Code. In the oil and gas industry, API RP500 (Classification of Locations for Electrical Installation at Petroleum Facilities Classified as Class I, Division 1 and Division 2) is commonly followed. However, it should be noted that in the scoping section of API RP 500, it very clearly states that they follow the methods described NFPA 79 (NEC).

It should be noted that an area classification plan does not take into account catastrophic failures of a piece equipment, such as a tank. While these are not considered in the initial development of a hazardous location plan, they should be considered during the Process Hazard Analysis (PHA) or the Hazard and/or Operability Study (HAZOP).

Area Classifications, Divisions, Groups

Area classifications are broken into three categories, Class I, II and III which apply to flammable gasses, flammable dusts and fibers, respectively. Each of these classifications are split into two Divisions:

Division 1 (Div 1) - Locations where flammable or ignitable concentrations of gas, dust or fibers are expected to be present during normal operation. Examples of this might be an open sump or cellar where heavy gasses can accumulate.

Division 2 (Div 2) - Locations are areas where flammable or ignitable concentrations of gas, dust or fibers can accumulate during abnormal conditions. An example of this would be the area around a tank where the pressure safety device vents to atmosphere. While gas is usually not present, when the valve is venting, gas could be present.

Groups are a way of further identifying hazardous materials. Group information is displayed in the table below:

NEC Division System Gas & Dust Groups 
 AreaGroupRepresentative Materials
Class I, Division 1 & 2  A  Acetylene
 B  Hydrogen
 C  Ethlene
 D  Propane
Class II, Division 1 & 2  E  Metal Dusts (e.g. magnesium), Division 1 only
 F Carbonaceous dusts (e.g. carbon & charcoal)
G  Non-conductive dusts (e.g. flour, grain, wood & plastic)
Class III, Division 1 & 2 None  Ignitible fibers/ flyings (e.g. cotton, lint, flax, rayon)

Information on a Hazardous Area Classification

Area classification drawings should be prepared in the following way:

  • The classification and extent of each area shall be shown with major structures and equipment indicated.
  • Drawings shall provide plan views and sectional views, as required, to clarify area classification.
  • All equipment that is considered a source of flammable liquid, gas or vapor must be shown on the drawings. Each source shall be identified by equipment number, the flammable materials handled, flash points and ignition temperature, and atmosphere groups according to the standards to which electrical equipment is specified. It might make sense to include this information as a table on the drawing.
  • Drawings should specify the edition date of the standard used for the classification and note exceptions which have been taken. This is important historical information that may not serve an immediate purpose but will be useful in the future.

Applicable Codes

It is important to be familiar with the design codes in the applicable industry and apply them as instructed:

ANSI / NFPA Standards

  • NFPA 70 National Electrical Code (NEC)
  • NFPA 496 Standard for Purged and Pressurized Enclosures for Electrical Equipment (National Fire Codes, vol. 7)
  • NFPA 497 Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas (Revised and Redesigned from NFPA 497A - 1992 and part of NFPA NFPA 497M - 1991 (National Fire Codes, vol. 11)).

API Recommended Practices (RP)

  • API RP 500 Classification of Locations for Electrical Installations at Petroleum Facilities as Class I, Division 1 and Division 2
  • API RP 505 Classification of Locations for Electrical Installations at Petroleum Facilities as Class I, Zone 0, Zone 1, and Zone 2

International Electrotechnical Commission (IEC) Standard

  • 60079-10 Electrical Apparatus for Explosive Gas Atmospheres - Part 10: Classification of Hazardous areas
  • 60079-14 Electrical Apparatus for Explosive Gas Atmospheres - Part 14: Electrical Installations in Hazardous Areas (Other Than Mines)

Area Classification Drawing Example

Using the Equipment Location Plan on another page, classified areas are added.  In this example, new equipment is not being installed in the classified area so no further detail is necessary.  Most hazardous area location plans will show the size of the hatched area relative to the equipment.

Area Classification Plan Example