BS 476-12:1991 download.Fire tests on building materials and structures Part 12: Method of test for ignitability of products by direct flame impingement.
1 Scope
This Part of BS 476 describes a method of test for the determination of the ignitability of materials, composites and assemblies subjected to direct impingement of flames of different size and intensity but without impressed irradiance.
NOTE The titles of the publications referred to in this standard are listed on the inside back cover.
2 Definitions
For the purposes of this Part of BS 476, the following definitions apply.
sustained ignition
after withdrawal of the ignition source, the presence of a flame on the surface of the specimen that persists for at least 10 s
2.2
transient ignition
after withdrawal of the ignition source, the appearance of flashes, or flames, which are not sustained for a continuous 10 s
2.3
progressive smouldering
an exothermic oxidation not accompanied by flaming which is self-propagating, i.e. independent of the ignition source, and may be accompanied by incandescence
2.4
flaming debris
matter flowing or separating from the specimen during the test procedure and falling below the initial lower edge of the specimen and continuing to flame as it falls
2.5
after flame
persistence of flaming ofa specimen, under specified test conditions, after the ignition source has been removed
2.6
afterglow
persistence of glowing of a specimen, under specified test conditions, after cessation of flaming or, if no flaming occurs, after removal of the ignition source
2.7
flame application time
the time (in seconds) for which the burner is in its final testing position
These limits provide adequate oxygen without disturbing the burning behaviour.
NOTE 3 Testing with ignition sources A. B and C may be carried out in either of these environments, or in a suitably ventilated fume cupboard.
5.2 Propane supply and regulation. A supply of commercial propane complying with BS 4250-1 connected to the ignition sources by the following.
a) A cylinder regulator complying with BS 3016 with an outlet pressure of I bar1 absolute.
b) An appropriate length of 5 mm internal diameter high pressure hose to BS 3212.
c) Two flowmeters with integral needle valves and accuracies of± 0.25 %, with flow ranges covering 25 mL/min to 160 mL/min and 1 h/mm to 10 L/min. connected to a common manifold top and bottom to allow measurement with either meter.
d) A length of 5 mm internal diameter hose not greater than 3.0 m in length to connect the flowmeters to the burner carriage via a gas cock mounted on the carriage.
e) A burner carriage consisting of a movable column which incorporates a 10 mm OD x 1 mm wall stainless steel tube terminated by a threaded boss to which the various burners can be fixed (see Figure 1).
NOTE Some difficulties have been reported with the supply and measurement of the propane particularly where the propane cylinder has, of necessity, to be stored in an environment cooler than the defined test conditions and!or some distance from the test rig. In these cases, and other situations where difficulties occur, it is important that there is a sufficient length of tubing inside the controlled environment (10 °C to 30 °C) to ensure that the propane equilibrates to the required temperature before flow measurement. One way to assist this is to pass the propane (before flow measurement) through a metal tube immersed in water maintained at 25 °C.
Great care also needs to be exercised with the measurement and setting of the flow rate of the propane. Direct reading flow meters. even those obtained with a direct propane calibration, need to be checked when initially installed, and also at regular intervals during testing, with a method capable of measuring accurately the absolute propane flow at the burner tube. One way of doing this is to connect the burner tube with a short length of tubing (about 7 mm internal diameter) to a soap bubble flowmeter, such that the upward passage of a soap film meniscus in a glass tube of calibrated volume (e.g. a burette) over a known period of time gives an absolute measurement of the flow. Fine control valves which can each be preset to one of the desired propane flow rates, with simple switching means from one to the other, have also proved helpful.
5.3 Ignition sources
5.3.1 Ignition source A. A burner consisting of a stainless steel tube with a bore of 0.5 ± 0.1 mm and with a minimum length of 35 mm.
NOTE I A hypodermic syringe, with the tapered end cut off square and cleanly, has been found suitable.
8.2 Number and cutting
Take a minimum of six full thickness representative specimens for each ignition source to be applied and each flame application time (three for surface ignition and three for bottom edge ignition). If the material under test is asymmetric through its thickness, and in practice either face may be exposed to a source of ignition, test a separate series of specimens on each face. Similarly, for materials which are chemically or physically not uniform in any other plane, test by as many series of specimens as needed to represent the actual behaviour of the material when exposed to the various ignition sources.
Most composite materials and many homogeneous ones are installed with covered edges. and edge ignition tests shall be carried out on specimens of this form. If such materials may be used with unprotected edges they shall be tested in this form.
NOTE 1 To take an extreme sample, a composite material with a grain/material direction and asymmetrical through its thickness, which could be installed with edges protected or otherwise. wot1d require 8 sets of 3 specimens for bottom edge ignition and 4 sets of 3 specimens for face ignition, for each chosen ignition exposure in order to characterize it.s ignition behaviour.
Where materials will be fixed to substrates in practice, analogous procedures shall be used to prepare test specimens.
NOTE 2 Care is needed when preparing specimens for bottom edge exposure of materials applied to substrates since in practice the substrate may extend beyond the bottom of the material to be tested and not itself be subject to edge exposure. The configuration of the test specimen should reflect the practical aspects of such factors as type of substrate, fixing to substrate and presence and form of joints.
8.3 Suitability of a product for testing
8.3.1 An essentially flat product having one of the following characteristics is suitable for evaluation by this method:
a) an essentially flat exposed surface; b) a surface irregularity that is evenly distributed over the exposed surface provided that:
1) at least 50 % of the surface of a representative square area
of 250 mm x 250mm lies within a depth of 6 mm from a plane taken across the highest points on the exposed surface; or
2) for a surface containing cracks, fissures or holes which do not exceed 6.5 mm in width nor 10 mm in depth, the total area of such cracks, fissures or holes at the surface does not exceed 30 % of a representative square area of 250 mm x 250 mm of the exposed surface.