This standard identifies parameters that affect the fire resistance of ducts. It also identifies the factors that need to be considered when deciding whether, or by how much, the variation of the parameter can be extended both ways, even in plus or in minus, when contemplating the fire resistance performance of an untested, or unstable variation in the construction. This standard, where applicable, gives guidance on additional tests that are needed to extend the field of application. The standard gives the principles behind how a conclusion on the influence of specific parameters/constructional details relating to the relevant criteria (E, I, S) can be achieved. It should be noted that fire-resisting ducts are exposed to different conditions to other elements of construction; in particular they are subjected to significantly different pressure regimes. Also integrity is evaluated in part by leakage measurements. Consequently, this European standard may adopt a different approach to other extended field of application standards. This standard only applies to ducts already tested to EN 1366-1. It does not cover ducts used for smoke control.