Class II dust: A combustible particulate solid that has an Explosion Severity (ES) greater than 0.5 or an Ignition Sensitivity (IS) greater than 0.2. A Class II dust is an appreciable explosion hazard requiring electrical equipment suitable for Class II locations.

Combustible dust: A combustible particulate solid that presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations, regardless of particle size or shape.

Deflagration: Propagation of a combustions zone at a speed that is less than the speed of sound in the unreacted medium.

Dust deflagration index (Kst): The maximum rate of pressure rise during a dust explosion in an equidimensional vessel, multiplied by the cube root of the vessel volume. Mathematically it is defined as:

The dust deflagration index is used to compare the severity of deflagrations caused by different combustible dusts. The value for Kst is used in the St Class system to categorize dusts according to the relative violence of their deflagrations.

 

Kst (bar•m/s)	Class	Example
Kst < 200	St. 1 (weak)	Sugar
200 < Kst < 300	St. 2 (moderate)	Wood flour
Kst > 300	St. 3 (strong)	Aluminum powder

 

Dust-ignitionproof: Electrical equipment enclosed in such a manner that prevents the entry of dusts and does not permit arcs, sparks or heat otherwise generated or liberated inside of the enclosure to cause ignition of exterior accumulations or atmospheric suspensions of a specified dust on or in the vicinity of the enclosure.

Dusttight: Enclosures constructed so that dust will not enter the enclosure under specified test conditions.

Electrostatic Charging Tendency (ECT): The tendency of a material, based on how strongly the material resists the flow of electric current, to accumulate an electric (static) charge.

Explosion: The bursting or rupture of an enclosure or a container due to the development of internal pressure from a deflagration.

Explosion Severity (ES): The product of the maximum explosion pressure (Pmax¬) and the maximum rate of pressure rise (dP/dt)max, normalized to Pittsburgh coal dust. Mathematically, it is defined as:

Hybrid mixture: A mixture of a flammable gas with either a combustible dust or a combustible mist.

Ignition Sensitivity (IS): The product of the minimum ignition temperature (MIT), minimum ignition energy (MIE) and the minimum explosible concentration (MEC) normalized to Pittsburgh coal dust. It is expressed mathematically as:

Limiting Oxygen Concentration (LOC): Maximum oxygen concentration in a mixture of a flammable substance and air and an inert gas, in which an explosion will not occur, determined under specified test conditions.

Maximum explosion pressure (Pmax): The maximum pressure achieved by a deflagration under standard testing conditions.

Maximum rate of pressure rise ((dP/dt)max): The maximum rate of change in pressure during a deflagration.

Minimum Explosible Concentration (MEC): Minimum concentration of dust suspended in air, measured in mass per unit volume, which will support a deflagration.

Minimum Ignition Energy (MIE): Lowest electrical energy stored in a capacitor that, upon discharge, is sufficient to effect ignition of the most ignitable atmosphere under specified test conditions.

Minimum Ignition Temperature (MIT): Lowest temperature of a hot surface on which the most ignitable mixture of the dust with air is ignited under specified test conditions.

NFPA (National Fire Protection Association): The stated mission of the NFPA is to reduce the worldwide burden of fire and other hazards on the quality of life by providing and advocating consensus codes and standards, research, training and education.

OSHA (Occupational Safety and Health Administration): Congress created OSHA with the Occupational Safety and Health Act of 1970 to ensure safe and healthful working conditions for working men and women by setting and enforcing standards and by providing training, outreach, education and assistance.