The Historic Path to Hazardous Location Standards & the Temp. Class. System

In a world where anyone can hop on a plane and traverse through clouds and open sky, where we simply flip a switch to keep the darkness at bay, and where most questions can be answered in mere seconds with the touch of a fingertip to a screen, it’s all too easy to lose our sense of wonder and awe about how truly remarkable the technologies populating our world are.  We rarely stop to think about the airplane as a huge tank of fuel being burned in massive engines with electricity zigzagging around an interior pocket packed full of vulnerable humans, a home as a casing for wires drawing in what amounts to tiny bits of controlled lightning producing on demand heat and light for the family inside, and a phone battery as a handheld factory for chemical reactions that make lightning fast transmissions of information possible.

The ease with which we move amongst these engineering marvels makes it equally easy to become disconnected from the inner workings of these products, especially the safeguards put in place to make these services and devices safe for everyday use.  The story of how things become safe for the consumer naturally starts with the story of how goods are produced.  Fundamentally, the development and testing of safety standards begins with the people who spend their lives unearthing and manufacturing the raw materials that are harnessed and rebirthed as the fuel and sustenance for modern technological life.  We can sit in an airplane, live in a home, and carry a cellphone safely because of the explosion proof standards and technology developed for industrial hazardous locations.

As with any complex system, the origins of safety standards and protocol require a winding path into the past.  In the case of the electrically fueled Digital Age, this trip leads to the coal powered Industrial Revolution.  Often called the most important societal development since the first Agrarian Revolution over 10,000 years ago, the Industrial Revolution left no industry or human population in Western Civilization untouched.  And, of course, the foundation of the Industrial Revolution was the steam engine. The mechanization made possible by the advent and employment of the coal powered steam engine changed the very fabric and focus of society.  While the first steam engine was used to drain water from mines, it eventually evolved into an engine for societal and technological progress.  From agricultural equipment to textile manufacturing to chemical and metal production, the steam engine allowed for faster and more standardized production as well as for the rapid transport of goods on steam powered ships and locomotives.  This, in turn, led to urban expansion as the production of goods shifted from individual craftsmen in rural locals to more centralized factory sites in cities.  And as the population moved farther from the forests and rivers that had previously supplied the raw materials for power, the demand for coal surged. So the story of modern life as we know it – the world of portable electricity and uniform safety regulations – begins with an unassuming small, black lump nestled deep in the earth for eons, waiting for its full potential to be realized.

Characterized by complete darkness, deep tunnels, long winding passages, confined spaces, lethal methane gas with the potential to asphyxiate or ignite, and thick coal dust floating through the limited air and ripe for combustion, coal mines are a veritable cornucopia of hazards.  Crawling into one of these gas and dust filled spaces with an open flame strapped to your forehead seems not only counterintuitive but more like an act of insanity to our modern minds, but that’s exactly what miners did until the use of electric headlamps became common in the 1920s.

The first oil burning safety lamps – known as the Davy, Geordie, and Clanny lamps –  were designed to cool the air surrounding the flame to a temperature below the firedamp ignition point while still allowing the air to pass freely using a wire mesh enclosure or the restriction of the amount and rate of intake air flow and exhaust through a system of fine tubes and a glass cap shield with a gap at the bottom or a combination of these methods.  Though these lamps were an improvement over simple candles and open oil wick lamps, they were far from foolproof and actually ended up causing explosions when drafts blew the flame outside the wire mesh or cracked glass allowed unrestricted access of firedamp to the enclosed flame. Later versions of oil burning safety lamps brought substantial improvements throughout the late 1800s, but getting miners to use these lamps consistently proved difficult.  Miners found these handheld safety lamps to be cumbersome and clumsy and complained that the illumination generated was dull and made working in the deep darkness difficult.  What often happened was that a miner would use the safety lamp to test for dangerous conditions (lack of oxygen and methane pockets) and continue to use open flame lamp as the actual light source.

Thus, the use of carbide lamps became popular during the first decade of the 1900s.  Carbide lamps generate light by creating a chemical reaction between water and calcium carbide that results in the production of acetylene gas, which was then lit by hand and burned as an open flame backed by a metal reflector to focus the light.  Though intended for use in non-gaseous mines, these lamps were used by coal miners who preferred their bright illumination to the relative dimness of the safety lamps. Lighter in weight and available in headlamp models, carbide lamps were extremely popular with miners because of their powerful illumination, hands free operation, and ability to burn up to 8 hours on a single charge.  However, the obvious trade off was the inherent explosion danger in using an open flame lamp in the presence of methane.

Luckily, carbide lamps proved to be only a stopgap measure within the coal mining industry as the early 20th century ushered in the era electric headlamps for miners.  Electricity had been available in urban homes for years at this point, but the gradual expansion into rural areas would last into the mid 1900s as the building of transmission systems and installation of lines outside of city boundaries was considered cost prohibitive by the utility companies.  Similarly, running electric lines into mines was too expensive and complicated to be feasible solution.  And while scientific trials of portable electric lamps were ongoing on throughout the late 19th century, development and progress was slow going. Scientists and inventors kept running up against the same obstacle: creating a battery that was safe, reliable, and lightweight enough to be carried by miners for the duration of a shift.

Two major mining explosions in 1907 and 1909 resulted in such a catastrophic loss of life, a combined total of 621 miners killed, that the United States Bureau of Mines (USBM) was formed in 1910 to address mine safety concerns.  In collaboration with engineers from the USBM, Thomas Edison developed the first fully portable electric mining headlamp that utilized newer, more efficient tungsten filament bulbs and an alkaline battery pack.  The nickel and iron elements in the battery allowed for a lighter, more compact design and could power a headlamp for up to 12 hours on a single charge.

Though electric illumination was far safer than open flame methods, there still lingered very valid concerns about ignition potential when broken bulbs allowed hot filaments exposure to the open air in a mine.  So, in 1912, USBM conducted ignition tests with bulbs from 8 different electric lamp manufacturers.  Each bulb was punctured and exposed to a natural gas and air mixture while connected to a power source.  It was found that the exposed and electrified filament caused ignition in atmospheres with as little as 5% natural gas. This testing led to redesigning the bulb clip on mining lamps in such a way that if the bulb broke, it would be instantly disconnected from the power source and the filament allowed to quickly cool, thus preventing ignition of the surrounding atmosphere.

It was at this point that the USBM began to collaborate with electric mine lamp makers to create minimum quality specifications and a conformity code for lamp parts as well as instituting a voluntary approval process with numbered plates attached to lamps verified as safe by the agency.  Although other agencies – such as the Underwriters’ Fire Bureau and the National Fire Protection Agency – already existed to address the hazards posed by electricity, at that time these groups mainly dealt with electrical connections and sprinkler safety within buildings and homes.  Because of its focus on mines, the USBM’s mission inevitably progressed beyond electricity into the dangers of electricity mixed with environmental hazards, namely gases and vapors.  Further testing and research into the prevention of gas ignition as well as the functionality and other safety concerns regarding electric lamps (battery leakage, weight, light production capabilities, etc.) led to conformity suggestions and eventually a formal scheme for approval requirements.  The Edison cap lamp was the first to be approved under the USBM’s newly created regulations with seven other models quickly following, and by 1917 over 70,000 electric lamps were being used in mines. As a result, another revolution was born – the adoption of uniform safety standards for electric equipment within hazardous work areas, in particular, the gas and vapor temperature classification system.

Over the years, as technology and understanding progressed, more sophisticated testing yielded more exact information about flammability limits and minimum ignition temperature of gases and vapors present within hazardous locations.  Eventually, the National Fire Protection Agency became more involved with hazardous location electrical safety created the North American Class/Division system to classify the nature of the hazardous substance present (Class), the probability of its presence (Division), and to define and group the substance based on its inherent properties (Group).

Class describes which type of material may be present at a location:

  • Class I – flammable gases and vapors
  • Class II – combustible dusts
  • Class III – ignitable fibers and flyings

Division describes the likelihood of the material being present in a flammable concentration

  • Division 1 – ignitable concentrations exist under normal operating conditions
  • Division 2 – ignitable concentrations may be present (through handling, processing, or use) but are normally within containers or a closed system

Group describes the groupings of materials according their properties and lists the minimum ignition temperature for each individual material

  • A, B, C, D – gases and vapors
  • E, F, G – dusts, fibers, and flyings


Modern explosion prevention protocols and strategies are built upon the same two principles seen throughout the evolution of coal mine safety:

  • The primary explosion protection method is to prevent the formation of an explosive atmosphere.
  • The secondary explosion protection method is to prevent ignition of an explosive atmosphere from a heat source.

Because the presence of hazardous gases and vapors is unavoidable is most hazardous locations, the principal goal of the primary explosion protection method isn’t so much preventing the presence of the substance as it is controlling the concentration of the gases and vapors present, i.e. maintaining an air mixture either too rich or too lean for explosion to occur.  While hazardous locations typically use a combination of primary and secondary protection to minimize the potential for a fire or explosion, the most predictable aspect of nature – in this case, the atmosphere containing flammable gases and vapors – is its overwhelming unpredictability.  In many hazardous locations, especially areas like coal mines, the air mixture can rapidly and unexpectedly change.  Therefore, the most reliable and controllable facet of explosion safety will always be on the man-made, or equipment, side of the equation.

All fires/explosions need three elements: a fuel source, oxygen or another oxidizing agent, and a heat source.  This tripart equation is referred to as the fire triangle.


The first line of defense within the secondary level of explosion protection is making sure that the electrical current and any potential sparking are safely contained within a sealed enclosure.  But even without an actual flame or spark, equipment can generate enough surface heat to ignite the surrounding explosive atmosphere, so a second layer of protection is needed to ensure that the amount of surface heat generated never rises to the ignition temperature of the gases and vapors present in the work area.  Thus, modern explosion proof equipment carries a temperature classification, or T-code, that represents the maximum surface temperature of the equipment enclosure.

The T Code system was developed as a way reducing the the risk by carefully assessing and controlling heat side of the fire triangle.  Laid out in Article 500, Table 500.8 of the NEC, the T Codes establish a maximum surface temperature threshold for equipment used in hazardous areas.  This temperature is based on 80% of the minimum autoignition temperature for each gas or vapor. (Dust and fiber groups are not addressed in T Code chart because their autoignition temperatures are bit more complicated to calculate.  For some of these materials, the ignition temperature must be reduced and treated as if it were a lower temperature, and the dust layer expected must be figured into the maximum surface heat calculation.  Once the final minimum autoignition temperature of each material is established, it can be matched with the appropriate temperature class in the NEC Table 500.8 chart.) Basically, the T Code is a method of comparison between the maximum amount of surface heat produced by equipment and the ignition temperature of flammable materials. T Codes range from T1 – T6 with incremental values based on gas and vapor groupings for T2, T3, and T4.  Equipment in hazardous locations must always have a T Code lower than the minimum ignition threshold of the substance present in order to effective in explosion prevention. Used in conjunction with the Class/Division system, the T Code classes help us determine the appropriateness of equipment for a given atmosphere.

For example, if we were looking for a light safe for a coal mining operation, it would need to be listed for both Class I and Class II situations due to the presence of gases and vapors as well as dust, at least Division 1 for both classes due the presence of the materials under normal operating conditions, and Groups D and F due to the presence of methane and coal dust.  Similarly, it would need to carry a T Code below the ignition temperature of both methane and coal dust.  So, in this case, we might choose something like this explosion proof LED light tower as it is listed for Class I, Divisions 1 & 2, Groups C and D as well as Class II, Divisions 1 & 2, Groups E, F, G and carries a T5 temperature class rating, which is well below the listed minimum ignition temperature of both methane and coal dust.  Making sure that each the equipment specifications for class, division, groups, and the temperature class are matched precisely to the conditions of the hazardous location is the most effective way to prevent ignition and explosion in hazardous work areas.


These lengthy codes, papers, and charts are the culmination of hundreds of years of accidents, trial and error, and the collaboration of the best scientific minds of several generations.  The same standards and regulations utilized for the mining and processing of raw materials are used in the creation and development safe consumer products and services..  They are the unseen infrastructure that brings order and clarity to the modern technological era, allowing us to safely inhabit a space inside the thrumming electrical highways that make up the world as we know it.  That quick glance at your lightweight, battery powered phone is, in essence, a long look back through the annals of history that ends with the dimly lit face of a miner precariously making his way through the darkness.

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