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Hazardous locations and the requirements for electrical systems installed within them are very complex. In this module we are going to introduce you to the concept of hazardous locations, how they are divided up or classified, and briefly discuss some of the protective measures that are required for the locations.

Before making any installations in a hazardous location, we recommend three things:
1.   Study all codes and regulations carefully before installation
2.   Seek advice form experienced personnel
3.   Consult with the AHJ before installation

History of Hazardous Locations

The topic of hazardous locations first appeared in the NEC in 1923, when a new article entitled “Extra-Hazardous Locations” was accepted. This article addressed rooms or compartments in which highly flammable gasses, liquids, mixtures, or other substances were manufactured, used, or stored.

In 1931, “classifications” consisting of Class Class II and I were developed and added to the code. In 1935 “groups” were added to classifications to better define the hazards presented by different chemicals. These categories were developed based solely on the recommendations of chemists without any testing.  Divisions were not recognized until the 1947 edition of the code.

It was not until the late 1950s when the Westerberg Chamber was invented that chemicals were actually tested for combustibility and their reaction to electrical currents. Since that time we have learned a great deal about the reaction of chemicals to electrical currents. This knowledge has lead to advancements in safety and also increases in the installation requirements of electrical systems in hazardous locations.


As with many of our modules we have decided to start with some definitions of the common terms you will be exposed to when discussing hazardous locations.

Approved: Acceptable to the authority having jurisdiction.

Classified Location:
Used interchangeably by many groups with term “Hazardous Location”– it defines an area that is divided into a Class, Division and Group (or into Class, Zone and Group) that may exist due to the presence of flammable gases, vapors, combustible dusts or ignitable fibers or flyings.

Dust-ignition proof
Enclosed in a manner to exclude dusts, and will not permit arcs, sparks or heat generated inside the enclosure to cause ignition of dust accumulation or dust suspension outside the enclosure.

Apparatus or devices constructed so that dust will not enter the case under specified test conditions.

Explosion-proof Apparatus
Enclosed in a case capable of withstanding an explosion that might occur within it; and by containing and controlling release of hot gases and flames, prevents ignition of vapors outside the case.
Factory Sealed
Apparatus constructed in such a way as to contain any arcing within the apparatus itself. Listed factory sealed equipment does not require use of additional seal fittings. Factory sealing is normally accomplished by a combination of interior “potting”, close tolerance shafts and/or use of “threaded” flame paths built into the device.

Flash Point: The minimum temperature at which a liquid gives off vapor in sufficient concentration to form an ignitable mixture with air near the surface of the liquid, as specified by test.

Equipment with a label, symbol, or mark of an organization (UL, FM, ETL etc.) Label my indicate a specific use of the equipment.

Equipment included in a list published by an organization (UL, FM, ETL etc.) acceptable to the authority having jurisdiction.

Liquid, Combustible: A liquid having a flash point at or above 100 degrees F.

Liquid, Flammable:  A liquid having a flash point below 100 degrees F that is classified a Class I liquid.

MESG (Maximum Experimental Safe Gap) The maximum clearance between two parallel metal surfaces that has been found, under specific test conditions, to prevent an explosion in a test chamber from being propagated to a secondary chamber containing the same vapor at the same concentration. Here is how it is developed. A device known as a Westerberg Chamber is used. The Westerberg Chamber has two round containers that are attached with a tunnel between them. The two chambers are filled with the same amount of combustible gas. The tunnel between the two chambers has two four inch pieces of metal. The pieces of metal can be adjusted to form a gap that allows the gas to pass between the two chambers. The gap is set at a minimum amount and the gas in one chamber is ignited. If the gas in the next chamber does not explode the chambers are refilled and the gap increased. This process is repeated until the gas in the second chamber ignites when the gas in the first chamber ignites. The last measurement of the gap before the gas in the second chamber ignited is known as the MESG.

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.

Pressure Piling
Pre-pressurization of an unburned mixture ahead of a moving flame front (typically in rigid conduit).

Sealing Fitting
A fitting for conduit or cable, that when properly installed with approved sealing compound, will prevent the passing of flames/fire from one portion of an electrical installation to another. They also prevent transmission of gases/vapors within a conduit system.

“T” Number
Equipment that is heat producing (fixtures, motors, etc.) must be tested and marked with an Identification Number (T Number). Nameplate on such equipment must show class, group and operating temperature, based on operation in a 40°C ambient (see NEC® 500-3 for exceptions). Non-heat producing equipment does not have this requirement.