Lower Explosive Limit (LEL) refers to the concentration of a gas or vapor in air below which a flame will not propagate when exposed to an ignition source.

• A. The maximum concentration of gas in air at which a flame will propagate.
• B. The concentration above which a flame will propagate.
• C. The concentration below which an ignition source has no effect on a flame.
• D. The concentration of a gas or vapor in air below which a flame will not propagate when exposed to an ignition source.

Answer: (D)

Lower Explosive Limit is the minimum concentration of a flammable gas or vapor in air that can support flame propagation once an ignition source is present. If the mixture is leaner than this, there isn’t enough fuel to sustain a flame even when ignited. Flammable mixtures exist only between the lower limit and the upper explosive limit, so flame propagation occurs only within that range. The statement describing the concentration below which a flame will not propagate when exposed to an ignition source captures this idea precisely.