NEBOSH Practice Test

What does the Upper Flammable Limit (UFL) indicate?

1. The lowest concentration necessary for ignition

2. The ideal temperature for combustion

3. The maximum concentration of fuel in air for combustion

4. The safest concentration of a chemical substance

The correct answer is: The maximum concentration of fuel in air for combustion

The Upper Flammable Limit (UFL) refers to the maximum concentration of a combustible substance in air that can lead to combustion. When the concentration exceeds this limit, there is not enough oxygen available in the mixture to sustain a fire, which means ignition cannot occur, even if an ignition source is present. Understanding the UFL is crucial for safety in environments where flammable gases or vapors are handled, as it helps in preventing the formation of dangerously rich mixtures that won't ignite but can still present other risks such as pressure buildup or toxic exposure. The other options do not accurately describe the UFL: the lowest concentration necessary for ignition relates to the Lower Flammable Limit (LFL), while the ideal temperature for combustion connects more to the concept of ignition temperature rather than UFL. The safest concentration of a chemical substance could pertain to exposure limits for health and safety rather than flammability specifically.