What percentage of incombustible dust must be maintained in underground roadways adjacent to hot work outside a permanent underground workshop?

• A. 90% incombustible content
• B. 85% incombustible content
• C. 75% incombustible content
• D. 100% incombustible content

Answer: (B)

Incombustible dust content is a safety measure to limit the fuel available for a potential ignition. In underground roadways where hot work is performed outside a permanent underground workshop, a high proportion of the dust present should be incombustible so that any dust that becomes airborne or settles does not readily support combustion.

Maintaining 85% incombustible content strikes a practical safety balance: it reduces the combustible portion of dust to a level that lowers the risk of a coal-dust ignition in the event of sparks or heat from hot work, without demanding an impractical, perfect all-incombustible environment. The other options either allow too much combustible dust (75%) or set an even stricter standard that is more difficult to achieve consistently (90% or 100%), without additional safety benefit in this specific context.