In surface mounted and suspended light fittings, which are by definition not part of the ceiling, the type of material that may be used for the diffuser is regulated by the appropriate product standard, in this case BS EN60598. The diagram here is taken from the UK building regulations and makes it very clear – if a light fitting is surface mounted or suspended then it is not part of the ceiling, so the consideration of TP(a) or TP(b) simply does not apply. When is a lighting diffuser part of a ceiling?Ī lighting diffuser is only judged to be part of the ceiling if it is recessed into it. However, it is nearly 200 pages long, so here’s a quick summary of TP(a) and TP(b) and how these terms relate to lighting in buildings other than dwellings.

(Note that it is described as “for use in England”, but the technical guidance it contains is for the whole of the UK.) You can download a complete copy of Approved Document B2 here. TP(b) diffusers may burn, but not at a speed of more than 50mm per minute. Polycarbonate diffusers over 3mm thick are automatically classified as TP(a), along with any other thermoplastic material that self-extinguishes within 5 seconds when a flame has been removed. No other thermoplastic material may be used. In the UK Building Regulations these are classified as TP(a) or TP(b) and when a lighting diffuser is deemed to be part of a ceiling it must be made of either TP(a) or TP(b) rated material. TP means thermoplastic, and TP materials, such as polycarbonate (PC), acrylic (PMMA) and polystyrene (PS), are commonly used as diffusers in light fittings. TP(a) and TP(b) are terms used in the UK Building Regulations ( UK Building Regulations, Fire Safety, Approved Document B, Volume 2 – Buildings other than dwellings) that classify lighting diffusers according to their flammability.

What are TP(a) and TP(b) in relation to lighting?

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