Can I connect a Rf-t fire damper to flexible ducts and when is it recommended?

Rf-t fire dampers are tested in accordance with the European test standard EN 1366-2. This standard prescribes the test program and conditions in order to determine the fire resistance performance of a fire damper for a given lapse of time (30, 60, 90, 120’, …) at a specific negative pressure level (300, 500, 1500 Pa).
The same goes for every component of a HVAC-system. The aim of this approach is to verify and guarantee the performance of each component in the system according to relevant parameters. These parameters vary for each component: ducts for example are tested in accordance with EN 1366-1.
The interaction between the different components is not included and specified in the European standards. This is governed by local best practices or guidelines. Installing a flexible duct section between a damper and a duct is one such element of interaction which is not included in the test setup for a fire damper. It might reveal useful for dampers installed in flexible walls that tend to deflect in the event of fire. A damper installed in a concrete wall with mortar sealing on the contrary doesn’t require such flexible duct sections since the wall is unlikely to deform in the event of fire.
Fire damper manufacturers are free to test their dampers with flexible duct sections. Should a manufacturer opt for flexible duct sections during the test, then the tested damper must always be installed with flexible duct sections, even if this is not necessary for a given configuration. Otherwise the installation is not in conformity with the classifications and Declaration of Performance.
If a fire damper has been tested without flexible duct sections on the contrary, the design engineer or the installation contractor is free to prescribe or connect flexible duct sections for specific situations where it is deemed safer (namely metal stud walls). This is the case for Rf-t fire dampers.