Class III
Little Things Can Become
A close look at Class III hazardous locations.
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 Several years ago three firefighters were injured in an explosion at an Oregon lumber mill - two hours after they discovered a fire in a dust collection bin. The first respondents thought they had it "under control" when the explosion occurred. Even though the initial fire was under control, the hazardous location conditions still existed in the collection bin contributing to a ‘delayed’ explosion.
When a dust explosion occurs, a company is usually caught
off‐guard due to the ‘invisible’ risk factors present at the time. Materials that can be completely safe can become extremely dangerous when present in dust form.

This story serves as a lead-in to an often ignored category – Class III Hazardous Location rating. Let’s start with a very short review of the Classes.

Class I
Hazardous Location rating covers the gases that can cause explosions and fires. In terms of size, gases are composed of very small particles.

Class II
Hazardous Location ratings cover the explosive particles referred to as ‘Dust’, which are still very small, but larger particles than gas molecules.

Class III
Hazardous Location ratings cover even larger particles like carpet fibers. But what is the difference between Class II and Class III particles?

How does one measure dust and “bigger” dust?
The size of contaminants and particles are usually described in microns, a metric unit of measure that is one-millionth of a meter. The word micron comes from the Greek word mikros meaning “small”, and is abbreviated using the Greek letter ‘ mu ’ shown as ‘µ’.
For comparison, one US ‘yard’ is 36 inches, and one meter is 39.37 inches.
  • one micron is one-millionth (µ) of a meter (m), or 1 µm (1 micron)
  • 1 inch = 25,400 µm (25,400 microns)
  • 1 µm (1 micron) = 1 / 25400 inch
Something this small is tough to even think with, but this is the way small particles like dust are measured for Classes II & III. The human eye can in general see particles larger than 40 µm (40 microns). To get some idea as to size, look at the following picture.
Let’s take a look at some different sizes of particles:

Particles larger than  100 μm (100 microns)
  • These particles will fall out of the air quickly
  • They include snow, hair, room dust, carpet fibers, sawdust, soot, coarse beach sand and gravel
Particles in the range  1 µm (1 micron) to 100 μm (100 microns)
  • These particles will settle out of the air slowly
  • They include fine ice crystals, pollen, windblown dust, grain dust, fine wood dust, coal dust, silt, and fine sand
Small particles less than  1 μm (1 micron)
  • These particles will fall out of the air slowly, and can take days to years to settle out of a quiet atmosphere. In an atmosphere with constantly moving air, they may never settle out
  • These particles can include sugars, very fine grain powder, soot, oil smoke, clay, and fumes
The National Electrical Code (NEC) describes Class II locations as those that are hazardous because of the presence of combustible ‘dust.’ To be categorized as a ‘dust’ covered in Class II Hazardous Location ratings, the NEC considers combustible material must exist as a finely divided solid of 420 µm (420 microns) or less. So – what about Class III?

Next Journal: Class III – part 2
Bill Nagengast, Lighting Engineer
Solas Ray Lighting
Holds over 20 patents in the lighting industry.
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