It is likely the coal mining industry spawned the first acknowledgement of “hazardous locations” and made attempts to mitigate ignition of explosive and flammable atmospheres. Not only is coal dust very flammable, mined coal begins to evolve gas when released from its tight confines. Methane gas can take 1000 hours to fully escape from mined coal. The obvious difficulty was that when coal was released from its formation it would begin to evolve methane gas which would concentrate as long as the mined coal was allowed to stay in the confined space within the mine. Particularly bad was the situation where coal would be left in a pile for the next shift on the next day.
Of course in early times, electricity was not used in mines and oil or wax fueled headlamps and lanterns were the norm. To deal with the evolving gas problem, it was common procedure for the “fireman” to be the first to enter a mine in the morning, crawling low to the floor with a lantern to intentionally “burn off” the accumulating methane gases in a “safe” manner. Naturally, this worked a little too well, and the position of “fireman” became very undesirable. In fact, convicted felons could reduce their sentences by “volunteering” to be firemen at mines.
Although the methane gas was troublesome the greater hazard was coal dust. An ignition of coal dust was much more serious than the methane burn because the coal dust created a much higher energy explosion. This was the reasoning for sending the “fireman” in at the beginning of the shifts, when the methane was present and the coal dust had typically (hopefully) settled from the air. It didn’t always turn out very well.
Forced air ventilation of mines was a significant improvement and lessened the accumulation of evolved methane. Dilution of the methane with outside air would usually keep the concentrations below the lower flammable limits.
In the late 1800’s electricity was the new workhorse in the coal mines. Although very useful to reduce labour and provide light, sparks were quickly discovered to be able to ignite coal dust and methane.
It didn’t take long before electrical motors of the totally enclosed type were found to initiate fire and explosion far less frequently and the concept of protection using tight fitting enclosed equipment to keep ignition events inside the equipment was born. This is now a staple of classified electrical equipment.
For a while conductors were strung through the mines connected to low voltage signaling bells. Miners could short the wires with any metallic tool to signal coal pickup. It was initially thought that using low voltage would prevent sparks big enough to ignite coal dust and provide a safe signaling method, but the inductive coils on the bells would prove them wrong. Two disastrous incidents in England in 1912 and 1913 taught this lesson is horrific fashion.
The Hillcrest Mine Disaster in the Crowsnest pass of Alberta is another example of the devastating consequences of coal mine explosions. Friday morning at 9:30 AM June 19, 1914 189 men died, 130 women were widowed and 400 children were left fatherless.
It wasn’t long before the current and voltage levels of these systems was successfully reduced to a safe level, and so began the principles of intrinsic safety.
Regrettably thousands of men gave their lives before the coal and mining industries understood and vigorously adopted sufficient area classification and suitable installation practices and equipment to protect these hard working men.
To this day, knowing the precise properties of the gas, vapor, liquid or dust that is present in the area of concern is paramount to selecting suitable electrical equipment.
Starting with coal mines, other substances have been individually tested and a huge body of knowledge has been collected which can now be referenced to find each materials critical properties. Information that is of interest to the Area Classification Engineer includes the autoignition temperature, lower flammability limit, upper flammability limit, density, flash point, minimum ignition energy, and minimum safe experimental gap to name a few. All of these factors are taken into careful consideration to specify the types of equipment to be used in an installation and the equipment installation methods to be observed.
Materials that are unique, new or combinations of many materials can be tested in accordance with well defined testing procedures to determine their critical properties for purposes of Area Classification.