Which percentage does 'Power' withdrawal refer to in the gas concentration table?

• A. 1.25%
• B. 0.25%
• C. 2.0%
• D. 5%

Answer: (A)

Gas concentration tables in mining safety use fractions of the Lower Explosive Limit (LEL) for the gas. For methane, the LEL is about 5% in air. The row labeled Power withdrawal shows the threshold at which power should be isolated to avoid ignition, and that threshold is set at 25% of the LEL. calculating 25% of 5% gives 1.25%. So the withdrawal applies when methane concentration reaches 1.25%.

The other options don’t fit because they correspond to different fractions of the LEL (0.25% would be 5% of LEL, 2.0% would be 40% of LEL, and 5% is the full LEL), not the 25% LEL level used for power withdrawal.