Ask Ovia: What is the difference between ratings of TP (a) and TP (b)?


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The column where you can ask your lighting questions to Ovia’s Head of Lighting Mike Collins, and his team of technical experts. These classifications relate to the fire rating of thermoplastic diffuser materials. Thermoplastics undergo specific tests to certify their ability to withstand heat and fire. The requirements are determined by Document B approved by the Building Regulations, which specifies the fire safety of buildings. Section B2 covers the spread of indoor fire. The approved document of the Building Regulations B2, Annex A, Performance of Materials, Products and Fittings, Sections 17-20 sets out the specific material standards and tests that thermoplastics must comply with to meet either of the two classifications: TP (a) – The test procedure requires that the material self-extinguishes , and any subsequent flame and glow shall not exceed five seconds once the source of the flame has been removed. TP (b) – The test procedure requires the introduction of a flame to the material. If the material burns, the flame spread should be no more than 50 mm per minute. A 600 x 600 mm plate takes about 12 minutes to completely burn out. TP(a) rated thermoplastic diffusers are generally made of polycarbonate, with no restrictions on their use in fire rated or fire rated roofs. TP (b) Classified diffusers are restricted in their use due to the material type or thickness of the diffuser. These materials are generally made of acrylic or polystyrene, and are more problematic due to the fact that they are more flammable. Limitations of TP(b) rated distributors in principle: Turnover areas: The total area of ​​the distributor panels must not exceed 15% of the total floor area. The maximum area of ​​a single diffuser or group of diffusers should not exceed 5 square metres. There should be at least 3 m distance between each 5 m2 diffuser assembly. Offices and other rooms: The total area of ​​the diffuser panels should not be more than 50% of the total floor area. The allowable space for spreader panel assembly should be no more than 5m2 with at least 3m spacing between each 5m2 spreader assembly. Have a question for Mike and the Ask Ovia team? You can submit your inquiry by clicking here. Visit the Ask Ovia microsite by clicking here


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