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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.

Dustproof: 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.

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


Listed: 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.

Electrical Equipment and Safety Standards

Safety is always a consideration when using electrical equipment. Some common concerns are contamination due to unsanitary equipment, protection from and limits on electromagnetic interference, and meeting strict safety requirements in potentially explosive environments. Customers rely on certifying agencies such as 3A, Factory Mutual, Canadian Standards Association, and others to ensure safety in these applications. These agencies examine, test, and certify that each product has been designed to meet specific standards for sanitary applications, hazardous locations, or specific electrical situations. Unlike independent testing laboratories who are unauthorized to issue any label but their own, certifying agencies enable the manufacturer to mark approved products with the corresponding standard committee's label, ensuring the end user that these products have been tested and meet those specific standards.

Certifying Agencies

Sanitary Equipment

3A: Sanitary Standards Administrative Council
The objective of the 3A Sanitary Standards Committee is to formulate standards and accepted practices for equipment and systems used to process milk and milk products. Such standards are developed through the cooperative efforts of local, state, and federal sanitarians, equipment manufacturers, and equipment users so that the standards are acceptable to those involved in the sanitary aspects of dairy and related industries. The 3A Symbol Administrative Council authorizes manufacturers to display the 3A symbol on processing equipment that is in compliance with 3A Sanitary Standards.

USDA: United States Department of Agriculture
The Federal Meat and Poultry Products Inspection Acts authorize the USDA to require that the slaughter of animals and the subsequent processing of meat and poultry products be done in a sanitary manner. The Food Safety Inspection Service (FSIS) inspects to USDA sanitary guidelines for equipment and facilities engaged in these operations. As of November 1997, the USDA no longer approves product to these guidelines but rather requires the facility engaged in these processes to combine equipment in their plant that will pass a USDA Inspection. The manufacturer of the equipment will be solely responsible for ensuring their product will meet the USDA guidelines for inspection.

Hazardous Locations Equipment

FM: Factory Mutual Research Corporation
The Factory Mutual Approvals Division determines the safety and reliability of equipment, materials, or services utilized in hazardous locations in the United States and elsewhere. Factory Mutual certifies to NEC (National Electrical Code) standards for hazardous locations, NEC Standard 500 (Division classification) and also to the new NEC Standard 505 (Zone classification), which attempts to harmonize American and European classifications. For a product to receive approval, it must meet two criteria. First, it must perform satisfactorily, reliably, and repeatedly as applicable for a reasonable life expectancy. Second, it must be produced under high quality control conditions. Factory Mutual also has interlaboratory agreements and can certify to Canadian and European standards.

CSA: Canadian Standards Association
The association includes Canadian consumers, manufacturers, labor, government, and other regulatory agencies among its actively participating influences. These groups work together to generate standard requirements (CSA standards) that demonstrate product quality, enhance market acceptability, and improve quality and safety control procedures in manufacturing and construction for the Canadian marketplace. The standards generated by CSA are the cornerstone for determining a product's eligibility for certification in hazardous locations in Canada. CSA also performs product evaluation, testing, and ongoing inspection to these standards, and also to American and European standards through new interlaboratory agreements.

These are some of the recognized European approval agencies that have certified Viatran transmitters to Cenelec (European Committee for Electrotechnical Standardization) and/or IEC (International Electrotechnical Commission) standards for hazardous locations. Cenelec attempts to harmonize the electrical standards of its member countries. Generally, IEC standards are used. However, in certain instances where IEC standards are considered too vague, Cenelec defines more precise requirements. The member nations of Cenelec, which include and exceed those of the EEC (European Economic Community), are bound to adhere to these international regulations.

Hazardous Locations Classifications
For an area to be classified as hazardous, the following three requirements for a Fire Triangle must be present simultaneously:
      Flammable gas, dust, or fiber
      Ignition source
Hazardous locations are broken into Divisions, Zones, Classes and Groups. These enable the manufacturer to specify exactly the type of hazardous location for which the product has been certified. The first classification describes the presence of flammable material in a hazardous location, either continuously, intermittently, or abnormally. The apparatus grouping states what type of flammable material is present: either gas, dust, or fiber. The temperature codes indicate the maximum temperature the device's external enclosure can reach. This is summarized in Table 1.

Table 1:       Hazardous Locations Classifications

Protective Concepts

For a product to be approved for a hazardous location, it must be designed so that an explosion of the flammable or combustible material surrounding the device does not occur. There are different methods of protection to achieve this. Viatran uses the three most accepted methods in the pressure transmitter market: Intrinsic Safety, Explosion Proof (Flame Proof), and Suitable for Use in Hazardous Locations.

Intrinsic Safety

An Intrinsically Safe piece of equipment is an electrical device that is incapable of causing an ignition of the prescribed flammable gas, vapor, or dust, regardless of any spark or thermal effect that may occur in normal use, or under any conditions of fault likely to occur in practice. This means that the device design is limited in such areas as PC Board layout, surface temperature, protection of electrical components, and power supply to the device. The devices are certified with either specific Intrinsic Safety Barriers (Loop certification) or general Intrinsic Safety Barrier parameters (Entity certification). These barriers are used outside the hazardous location and limit the amount of current, voltage, capacitance, and inductance entering the certified device. Often considered the safest and most technically elegant approach, there are many benefits of an Intrinsically Safe device to the customer. Expensive and cumbersome explosion-proof enclosures and conduit connections are not needed, electric shock is minimized, and controls can be maintained without shutting down the process.

Explosion Proof / Flame Proof
An Explosion Proof (or Flame Proof, as classified in IEC and Cenelec standards) device is an electrical device designed with an enclosure capable of withstanding, without damage, an explosion within it of a specific gas, fiber, or dust. In turn, it prevents ignition of these same materials surrounding the enclosure by a spark or flame from the explosion within. Factory Mutual formerly limited its Explosion Proof standard by requiring that the explosive external material be able to enter the device to cause an explosion. This excluded hermetically sealed devices from approval consideration. FM has recently modified their definition to include these devices. This certification usually requires that devices be designed with sturdy and durable enclosures with conduit connections. The primary benefits of this type of protection are that the device is not limited by low available power nor does it restrict PC Board layout. Viatran has also designed some of the smallest Explosion Proof devices in the industry.

Suitable for Use in Hazardous Locations
Factory Mutual developed this unique approval as a way for products to receive hazardous location approvals that cannot conform to existing protection concepts. There is no documented standard and the definition of this certification is unique to each product. In Viatran's case, this protection concept was utilized for our hermetically sealed products that did not meet FM's former Explosion Proof definition. Products that receive this approval are certified to the same Divisions as a comparable Explosion Proof or Intrinsically Safe device.

CE Marking
"CE" marking is a declaration from the manufacturer that their product conforms to a specific Directive(s) adopted by the EEA (European Economic Area) and is a requirement for the product to be sold into any of the countries in this 18 member group. CE is an abbreviation for the French phrase Conformité Européene, meaning European Conformance. Unlike hazardous location approvals, the manufacturers are solely responsible for ensuring their product's conformance to these Directives which were developed using IEC and Cenelec standards. The Directives that affect transmitters are the EMC (Electromagnetic Compatibility) and LVD (Low Voltage) Directives. These state that the products must meet specific electromagnetic emission and immunity, as well as electrostatic discharge standards. Transmitters that meet EMC standards, as declared by the manufacturer, must be able to withstand interference from the radio frequency spectrum, electrostatic discharge, surges, etc., without the unit's performance being affected. The transmitter must also emit a minimum of the above charges so that it does not affect other nearby electrical devices or systems such as emergency communications or radio and television broadcasts. The Low Voltage Directive addresses basic electrical shock and fire hazard issues. These directives are currently only a requirement for the EEA member nations and are not required for products sold outside this community.