Which description matches the concept of LEL?

• A. The concentration below which a flame will propagate.
• B. The concentration above which a flame will propagate.
• C. The concentration below which a flame will not propagate when exposed to an ignition source.
• D. The concentration at which ignition occurs regardless of source.

Answer: (C)

Lower Explosive Limit is the minimum concentration of a flammable vapor in air that can support flame propagation. The description that matches this concept is the one stating that below this concentration, a flame will not propagate when exposed to an ignition source. In other words, if the mixture is too lean, there isn’t enough fuel to sustain a flame even if a spark or flame is present. The flammable range sits between the LEL and the upper explosive limit, where a flame can propagate; above the LEL, the mixture becomes too rich for ignition until you reach the upper limit. The other descriptions point to the upper end of the range or to ignition behavior that doesn’t align with how much fuel is required to sustain combustion.