ic layout of the tunnel test apparatus. A detailed description
can be found in NFPA 255. In summary, a 20- to 24-in. (508- to
610-mm) wide by 24-ft (7.3-m) long test specimen (normally
up to 0.15 m thick) is placed face down on a ledge at the top of
the furnace to form the ceiling of the test chamber. A gas burner
projects a 4½-ft (1.4-m) long flame (89 kW intensity) onto the
underside of the test specimen while air flows through the tunnel
at a controlled linear air velocity of 240 ft/min (approximately
73 m/min). Flame spread over the face of the test specimen is
observed through windows in the side of the “tunnel.” Flame
propagation is recorded as a function of time over the 10 minute
test duration. The record of flame propagation versus time
is used to calculate a flame spread index, by comparison with
two standard reference materials. Red oak flooring is arbitrarily
assigned a flame spread index of 100 and a cementitious board
material is assigned a flame spread index of 0. According to the
test standard, red oak flooring spreads flame 24 ft (7.3 m) to
the end of the test specimen in 5½ minutes ± 15 seconds. The
flame spread indices of products are calculated on the basis of a
graph of flame spread versus time. The lower the flame spread
index, the better the presumed performance. However, as stated
above, there is not necessarily a correlation between the measured
flame spread index and product behavior in actual fires.
NFPA 255 simply provides a basis for comparing one product
with another under a standardized set of test conditions.