To limit the probability that fire will spread from one fire compartment to another fire compartment by means of the combustible coverings and linings of ducts, which could lead to harm to persons in the other fire compartment.
To limit the probability that fire will spread from one fire compartment to another fire compartment through gaps between a duct and the fire separation through which it passes if the combustible material were to be burnt in a fire occurring on one side of the fire separation, which could lead to harm to persons in the other fire compartment.
To limit the probability that combustible coverings and linings of ducts will be ignited from nearby heat sources, which could lead to the spread of fire along the ducts to other parts of the building, which could lead to harm to persons.