AL Tunnel Practice Questions - Alabama Tunnel Contractor Examination Exam

When 10 percent or more of the LEL for methane or other flammable gas is measured 12 inches plus or minus 0.25 inch from the roof, face, floor, or walls for more than a 24-hour period, or when the geology indicates that condition is likely When oxygen falls to 20.9 percent, the normal outdoor value Only after an ignition has already occurred Whenever any trace of carbon dioxide is present