When air diffusers are built into lower decks, the openings in the deck must be protected against fire by special shut-off units. Closure of the shut-off units is usually triggered by thermal fuses at temperatures above 72 °C. But the Trox series KU-K30 motor-driven shut-off units can reliably compartmentalize deck openings even before the fire bursts into flames (i.e. at temperatures below 72 °C), thereby ensuring that smoke does not spread through the ventilation system. The motors can also be controlled automatically via smoke detectors.