What shall be the minimum percentage of incombustible material after rock dust has been applied to all entries?

• A. 80%
• B. 70%
• C. 90%
• D. 100%

Answer: (A)

The key idea is that rock dusting raises the inert, incombustible content of the dust in the mine to reduce explosion risk. After rock dust is applied to all entries, the material on surfaces must be at least eighty percent incombustible by weight. That means the mixture should contain not less than 80% rock dust (incombustible) and up to 20% combustible material such as coal dust.

This minimum ensures there’s enough inert material to absorb heat and disrupt flame propagation if a ignition occurs. Higher percentages (like ninety or one hundred) can occur but are not the required standard; eighty percent is the mandated minimum.