Page 6 - Caswell Firesafe Manual 2018

TECHNICAL MANUAL

FFRDL / TM / BS / 01 / 2018

Fire resistance of ductwork is expressed in minutes duration of exposure for Stability, Integrity and
Insulation. The criteria for failure is stated under BS 476:Pt 24 (1987).

Failure shall be deemed to have occurred as follows:

Stability: If the duct collapses either in the furnace (Type 'A' test) or on either side of the furnace
(Type 'B' test) in such a manner that the duct no longer fulfills its purpose.

An additional criteria added for Smoke Extract systems is that the duct must retain at least 75% of its
cross-sectional area to be functional.

Integrity: The formation on the unexposed side (i.e. outside the furnace) of holes, cracks or openings
that allow flames or hot gases to escape. The method used to determine this is the ignition of a cotton
pad and / or sustained visible flaming of at least 10 seconds.

Failure is also deemed to have occurred if it is possible to insert a 6mm diameter gap gauge and move it
150mm in the opening and / or when a 25mm gap gauge can be inserted through the opening.

Insulation: Failure is deemed to have occurred when the temperature rise (above ambient) on the
unexposed side of the furnace exceeds 140ºC on average or 180ºC as a maximum. Temperature is
measured by attaching thermocouples to the ductwork.

In respect of Kitchen Extract ducts, there is an additional requirement on Type 'A' test that the temperature
rise inside the duct within the furnace shall not exceed the above limits.

The Time vs Temperature curve below demonstrates the severe climb in temperature within the furnace.
In the first 20 minutes of the test the temperature climbs from ambient to approx 750ºC and reaches over
1100ºC after 4 hours.
ISO 834 (E) Fire Resistance Test. Standard Time - Temperature Curve

Plotting temperature rise as a function of time for all BS 476:Pt24 (1987) & ISO 6944 (1985) fire tests

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