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These ratings relate to the fire-rating of thermoplastic diffuser materials. Thermoplastic materials undergo specific tests to certify their ability to withstand the application of heat and fire.
The requirements are established by Building Regulations Approved Document B, which sets out the fire safety of buildings. Section B2 covers internal fire spread.
The Building Regulations Approved Document B2, Appendix A, Performance of Materials, Products and Structures, Sections 17 to 20 stipulates the material criteria and specific tests with which the thermoplastics must comply to meet either rating:
TP(a) – The testing procedure requires that the material self-extinguishes, and any flaming and afterglow must not exceed five seconds once the source of flame is removed.
TP(b) – The testing procedure requires a flame to be presented to the material. If the material combusts, the spread of flame must be no more than 50mm per minute. A 600 x 600mm panel would take around 12 minutes to completely combust.
TP(a) rated thermoplastic diffusers are generally made from polycarbonate, with no restrictions on their use in fire-protecting or fire-resisting ceilings.
TP(b) rated diffusers are restricted in their use due to the material type or diffuser thickness. Generally made of acrylic or polystyrene, these materials are more problematic due to being more combustible.
The restrictions of TP(b) rated diffusers in principle:
Circulation spaces: the total area of diffuser panels must not exceed 15% of the total floor area. The maximum area of one diffuser, or group of diffusers, must not exceed 5m2. There must be at least a 3m spacing between each 5m2 diffuser grouping.
Offices and other rooms: the total area of diffuser panels must not exceed 50% of the total floor space. The permissible area for grouping diffuser panels must not exceed 5m2 with at least a 3m spacing between each 5m2 diffuser grouping.
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