|First Digit||Second Digit|
|0||No protection||0||No protection|
|1||Protection against the ingress of objects larger than 50mm||1||Protection against drops of water falling vertically|
|2||Protection against the ingress of objects larger than 12.5mm||2||Protection against drops of water falling at any angle up to 15° from the vertical|
|3||Protection against the ingress of objects larger than 2.5mm||3||Protection against rain falling at any angle up to 60° from the vertical|
|4||Protection against the ingress of objects larger than 1mm||4||Protection against liquid splashed from all practicable directions|
|5||Protection against the ingress of dust in sufficient quantity to interfere with the satisfactory operation of the equipment||5||Protection against low pressure jets of water from all practicable directions|
|6||Complete protection against the ingress of dust||6||Protection against heavy seas or strong jets of water from all practicable directions|
The Building Regulations Approved Document B2 (buildings other than dwellinghouses) places restrictions on the use of certain thermoplastic materials when they are used as lighting diffusers and form part of a ceiling. TP(a) rated materials may be used almost without restriction, but restrictions exist on the use of TP(b) rated materials. To determine the admissibility of a proposed TP(b) lighting diffuser layout the specifier/installer may use either of the following two diagrams.
This diagram (diagram 27) requires that 3m exclusions zones, where no TP(b) panels are present, are maintained between groups of light fittings where TP(b) panels are present.
This diagram (diagram 27a) requires that a minimum distance equal to the longest dimension of a TP(b) panel is maintained between adjacent panels.
In practice, the existence of diagram 27a means that TP(b) rated diffusers are admissible in almost any commonly used lighting layout.