Insulation is a factor preventing flashover
Exhausting smoke from a fire creates a potential flashover risk in compartments other than the fire compartment. As part of the EN tests, insulation can help prevent flashover.
Fire-rated insulation ensures that the temperature measured, outside in Type A and inside in Type B, does not exceed 140ºC mean average above ambient during the test, and is deemed automatically failed if any thermocouple rises to 180ºC.
The inclusion of insulation during fire testing means that the system becomes the product. Use of untested combinations of duct and insulation is not permitted without completing a full test programme.