A duct, irrespective of its cross section, serving more
than one tweendeck should be fitted, near the penetration of each
deck served, with a fire or smoke damper.
Such dampers should close automatically by means of a fusible
link or other suitable device, and manually from the deck in which
the passage of smoke, due to a fire in the deck immediately below
which is served by the same duct, will be avoided.
Where, within a main vertical zone, a fan serves more than
one tweendeck through separate ducts, each of these dedicated to a
single tweendeck, each duct should be provided with a manually operated
smoke damper fitted close to the fan.