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

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 diffuser according to their flammability. TP means thermoplastic, and TP materials, such as polycarbonate (PC), acrylic (PMMA) and polystyrene (PS), are commonly used as diffuser in light fittings. 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. No other thermoplastic material may be used.

Polycarbonate diffuser 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. TP(b) diffuser may burn, but not at a speed of more than 50mm per minute.

here’s a quick summary of TP(a) and TP(b) and how these terms relate to lighting in buildings other than dwellings.

When is a lighting diffuser part of a

A lighting diffuser is only judged to be part of the ceiling if it is recessed into it. 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.

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. Note that this is a product standard, not a building standard, and 60598 makes no mention of TP(a) or TP(b) at all.

When a lighting diffuser, such as in a 600 x 600 LED recessed panel, is part of a ceiling it is mandatory that it must be made of TP(a) or TP(b) rated materials.e difference between TP(a) and TP(b) rated materials?

The difference between TP(a) and TP(b) rated materials is in their composition and thickness or how they react under test conditions when a flame is applied to them.  

To be classed as TP(a) a lighting diffuser material must be either:

Polycarbonate at least 3mm thick, or

Polycarbonate with a class 1 rating when tested to BS 476-7, or

Any other material that when tested to BS 2782-0 Method 508A performs so that the test flame extinguishes before a certain point on the test sample and any flame or afterglow self-extinguishes within 5 seconds of the source of ignition being removed.

To be classed as TP(b) a lighting diffuser material must be either:

Polycarbonate up to 3mm thick, or

Any other material from 1.5mm – 3.0mm thick that, when tested to BS 2782-0 Method 508A has a maximum rate of burning of 50mm/minute.

Can light fittings with TP(a) rated diffuser be used anywhere?

Yes, almost.

Diffuser made of TP(a) rated material can be used to an unlimited extent in the ceilings of rooms (offices, classrooms etc) and circulation areas (corridors, lift lobbies etc). The UK Building Regulations do not require minimum spacing between TP(a) rated diffuser nor do they limit the % of the ceiling area that can be made of TP(a) rated diffuser.

The advantage therefore in using light fittings with TP(a) rated diffuser is that no checks or calculations are required to ensure that their usage is within the stipulations of the building regulations.

The exception is in respect to “protected stairways”.

A Protected Stairway is, in the words of Approved Document B, “A stair that leads to a final exit to a place of safety and that is adequately enclosed with fire resisting construction. Included in the definition is any exit passageway between the foot of the stair and the final exit.”

In a Protected Stairway no thermoplastic materials at all (neither TP(a) nor TP(b) rated) can be used as part of the ceiling.

What restrictions apply to the use of TP(b) rated diffuser?

Limited restrictions apply to the use of TP(b) rated diffuser in both rooms and circulation areas, and, as with TP(a) rated diffuser, TP(b) rated diffuser cannot be used at all as part of the ceiling in a protected stairway.

In rooms

The maximum area of any one TP(b) diffuser must be not more than 1m². So, 600x600 and 600x1200 fittings with TP(b) rated diffuser are permitted.

The maximum total area must be not more than 50% of the room in question. This means that any commonly used lighting layout would be permissible with TP(b) rated diffuser.

The distances between fittings must comply with the distances stipulated in the UK Building Regulations, Fire Safety, Approved Document B, Volume 2, diagrams 6.2 or 6.3 – as shown below. Again, any commonly used lighting layout would permit the use of TP(b) rated diffuser.

A minimum space of 3m must be kept between any two 5m x 5m groups of light fittings with TP(b) rated diffuser. If this presents a problem in, for example, an open-plan office lit mostly with 600 x 600 panels the solution could be to selectively introduce a TP(a) rated panel or use a different fitting altogether in the 3m space, such as a down light.

In circulation areas

The maximum area of any one TP(b) diffuser must be not more than 5m². This is enormous, so any commonly used size of fitting would be permitted.

The maximum total area of TP(b) rated material must be not more than 15% of the circulation area in question. This means that most commonly used lighting layouts would be permissible with TP(b) rated diffuser.

The distances between fittings with TP(b) rated diffuser must be at least 3m.

Finally, TP(b) rated diffuser may not be used at all as part of the ceiling in a Protected Stairway.

What are the Pros and Cons of TP(a) and TP(b) rated diffuser?

Ease of Application:

If you select fittings with TP(a) rated diffuser you know that you can install them anywhere (except for in Protected Stairways) without having to check spacing or the total area they will cover

If you select TP(b) rated diffuser you should check that the spacing and total area are within the stipulations of the UK Building Regulations.

Efficiency: To achieve a TP(a) rating a diffuser will usually be made of polycarbonate and will be relatively thick, often 3mm or more. This has negative consequences for the efficiency (lumen per watt) of the light fitting for two reasons.

First, polycarbonate has a lower light transmission rate than some other materials, including acrylic (PMMA) and polystyrene (PS).

Secondly, the amount of light lost as it transits through a material is directly proportional to the thickness of the material, so, if other factors are all equal, thinner diffuser materials result in greater efficiency.

TP(a) v TP(b) – a summary

On the great majority of lighting projects recessed fittings with TP(b) rated diffuser are a legitimate and responsible choice.

Fittings with TP(a) rated diffuser can be used in a few additional circumstances, but in the great majority of situations their use is not required and the additional expenditure and potential loss of efficiency has no cost justification.

Some lighting manufacturers, including NVC , offer a range of LED panels with a choice of TP(a) and TP(b) ratings. By using such manufacturers and following their advice in product selection contractors can be confident they are using the most cost-effective solution while maintaining strict compliance with the UK building regulations.

What is the difference between TP(a)/TP(b) and Fire Rated?

TP(a)/TP(b) relates to the rating of the material from which a lighting diffuser, when it is part of the ceiling, must be made. It is important in “buildings other than dwellings” and its purpose is to restrict the spread of fire horizontally across a ceiling.

“Fire Rated” is completely different. This refers to light fittings that are recessed into a ceiling that is itself a barrier to the spread of fire vertically from one floor in a building to the one above and which may also endanger fire and rescue personnel if it collapses quickly in the event of a fire.