British Standard 476 part 24 (1987) ISO 6944 (1985)

The purpose of BS476 Part 24 (1987) and ISO6944 (1985) is to measure the ability of a ductwork system to resist the spread of fire, from one fire compartment to another, without the aid of fire dampers. It should be noted that the test relates to a complete ductwork installation and therefore joints, supports and the fire stopping through the furnace wall all form an integral part of the test.

  • BS 476 Fire Tests on building materials and structures: Part 24 (1987) Method for determination of the fire resistance of ventilation ducts.
  • ISO 6944 (1985) Fire resistance tests – Ventilation ducts


Giving temperature rise as a function of time for all British Standard 476 Part 24 (1987) & ISO 6944 (1985) Fire tests.

Requirements of British Standard 476 Part 24 (1987) and ISO 6944 (1985)

The fire resistance of ventilation ductwork, shall, according to the Standard, be expressed in minutes of duration of heating until failure occurs according to one of more of the following criteria:

  1. Stability
  2. Insulation
  3. Integrity

Criteria of failure


Stability failure shall be deemed to have occurred in duct type A, within the furnace, and in duct types A and B, outside the furnace, when the duct collapses in such a manner that the duct no longer fulfils its intended function. In addition, stability is measured by the ability of a smoke extract duct to retain at least 75% of its cross sectional area along its whole length, both inside and outside of the furnace.


Insulation failure shall be deemed to have occurred when the temperature rise above initial ambient temperature, on the unexposed surface of the test specimen outside the furnace, exceeds either:

  1. 140° C as an average value, or
  2. 180° C as a maximum value

For kitchen extract duct type A, the above temperatures rise limits also apply to the inside surface of the duct within the furnace.


The presence and the formation of cracks, holes or other openings outside the furnace through which flames or hot gases can pass shall constitute integrity failure.

Definitions of Types A & B Fire Exposure

The testing standard differentiates between types of fire exposure; two classifications apply as detailed below.