Final answer:
The question revolves around fire safety standards for ABS or PVC DWV piping in ducts or plenums, which must adhere to a flame-spread index of no more than 25 and a smoke developed index of 50, with certain unspecified exceptions.
Step-by-step explanation:
The question pertains to the building codes regulating the fire safety of plastic piping systems, such as ABS or PVC DWV (Drain, Waste, and Vent), when installed in areas with increased fire risk, like ducts or plenums in buildings. According to standard building codes, these plastic pipes are required to have a limited flame-spread index (not more than 25) and smoke developed index (not exceeding 50).
This standard ensures that, in case of a fire, the spread of flames and the smoke generated by these piping materials does not contribute significantly to the fire danger. The question implies there are exceptions to this rule, but without further context, it is not possible to specify what the exceptions are.
They typically pertain to certain types of applications or locations within a building where the risk of fire spread or smoke generation is differently assessed.