1. What is TP(a) / TP(b) in Lighting?

In the UK Building Regulations (Approved Document B: Fire Safety), TP(a) and TP(b) are official classifications that describe the fire performance of thermoplastic lighting diffusers. These diffusers — commonly used in LED panels and downlights — are tested to determine how they react to heat and flames, helping ensure safety in commercial and residential lighting installations.

• TP(a) – Thermoplastic
  TP(a) rated diffusers are considered the safer option in terms of fire performance. To qualify, material must:
   Self-extinguish within 5 seconds after the flame source is removed,
  Or alternatively, they are automatically classified as TP(a) if made from polycarbonate with a minimum thickness of 3mm.
• TP(b) – Thermosetting Plastic
  TP(b) rated materials may continue to burn after the ignition source is removed, but at a rate not exceeding 50mm per minute.

2. Where can TP(a)/TP(b) be used?

• Where can TP(a) be used?
  TP(a) diffusers are considered highly fire-resistant, and TP(a) diffusers have no restriction on their use in ceilings or circulation areas, except in Protected Stairways (staircases leading to a final fire exit), where no thermoplastic material may be used as part of the ceiling structure.

• Where can TP(b) be used?
  TP(b) diffusers have usage limitations due to lower fire performance compared to TP(a)
  In rooms, individual TP(b) diffusers must not exceed 1m², and the combined area of all diffusers must not exceed 50% of the room’s floor area.
  In circulation spaces, diffusers must not exceed 5m² each, and the total coverage must be limited to 15% of the floor area.
  TP(b) diffusers must also be spaced apart by at least their largest dimension.
  TP(b) diffusers are not permitted in Protected Stairways, either.