Maintenance
Diligence is required to make sure that a non-plenum airspace stays that way. A non-plenum airspace can become a plenum airspace by accident if the ductwork is disconnected and not properly repaired and resealed. Ductwork disconnection can occur due to building damage such as earthquakes, aging, or adverse environment causing the metal to corrode and fall apart, or simply negligence on the part of building contractors that leave work unfinished. In all such cases, discovery, and repair of such problems to eliminate unintended plenums is difficult due to the hidden nature, limited space, and difficult access of most installed drop ceilings. For highest fire safety, it is best to assume all drop-ceiling and raised-floor airspaces are plenums, whether or not they are officially designated as such, and to always use plenum cable in these spaces.References
{{HVAC, state=collapsed Heating, ventilation, and air conditioning