By Jack Adams, Laboratory Manager at the LIA

When a disaster such as the one at Grenfell Tower occurs, it is always tempting to concentrate on what went wrong in that specific event, and not take a wider view. With Grenfell there was a clear breakdown of good and accountable design, in the terms of the Grenfell report a lack of a golden thread throughout the process. This is being addressed and the golden thread is now a key requirement in the design and construction of the built environment. But the perfect application of imperfect rules is still a possibility. Using processes and materials that comply with building requirements but do not use new best practice methods remains possible.

As an example of this we can consider the Part B approved guidance document of the Building Regulations concerning lighting and the spread of flame, which uses the system of plastic diffuser classification for TP(a) and TP(b). From the building regulations it is obvious that if a TP(a) product is used in a lighting design it will comply, subject to a few restrictions, and that if a TP(b) product is used it can still comply, albeit with more restrictions for the spacing of these diffusers. However, it can be argued that the test methods used to determine the TP() classification have not kept up with current luminaire design practices, especially since the widespread use of LED technology.

To classify materials as either TP(a) or TP(b) BS 2782-0:2004 Method 508A: Rate of burning, laboratory method is used. In the 2011 version of the standard Method 508A is detailed in Annex B, and clearly states that “The method has been declared obsolescent but is made available here because it is referred to in the Approved Document B Building Regulations”. Method 508A was originally defined within BS 2782-5:1970 and was withdrawn by BSI in 1992. Therefore the building regulations are using a test method no longer considered as best practice to classify thermoplastic materials.

When Method 508A was developed light sources were generally incandescent or discharge lamps and optics were single piece prismatic controllers or opal diffusers (see figure 1). The test method reflects this in that it is valid for testing only a single material. In addition regardless of the thickness of the material used for the actual optic, a strip of the material under test with dimensions 150mm x 13mm x 1.5mm is used. However current methods of optical control are generally via a sandwich of materials with dissimilar burning properties and varying thicknesses (see figure 2).

Method 508A as written and generally interpreted is not suitable for testing this type of composite construction. As currently presented the Building Regulations, as guided by Part B, can be interpreted as only considering the lower face of the luminaire, that is the part of the thermoplastic that faces directly into a room or space. When this face was a part of a single piece optic, or at least of a sturdy and relatively thick construction, this was not particularly unreasonable. In modern optics the lower thermoplastic layer may be quite thin in cross-section, and therefore significantly different from the sample used in testing, and the other materials used in the optical sandwich may have little resistance to the spread of flame. This means that the safety of the space relies upon the full diffuser behaving the same as the lower thermoplastic material under conditions of fire, being undamaged, and remaining so through life.

As mentioned it can be interpreted to test the lowest layer of this optical sandwich and specify the TP() classification for the luminaire based upon the characteristics of this material only. However, the legal requirements of the building regulations are to initially resist the spread of flame and once a flame is established limit the growth of the fire. Whilst testing only the bottom material will give some indication of the initial resistance to ignition, once ignition has occurred and a flame is established it will no longer give any reliable indication on how the flame will grow and spread on all of the materials being used in the diffuser construction.

So, the question we need to consider is why do we use a test method that has been declared obsolete and only still exists as it is called up within building regulations, and why do we use this method to test a sample that is completely unrepresentative of the optical construction in use? Because if we do not solve this problem the golden thread is potentially meaningless, at least for lighting products. For the first part of the question a relatively easy solution can be proposed.

BS EN 60695-11-10, Fire hazard testing – Test flames. 50 W horizontal and vertical flame test methods, has similarities to the Method 508A and could be reasonably applied. This would move from an obsolete to a current test method. It would also allow expected mounting orientation to be considered as it may test a material in both horizontal (e.g. ceiling mounted) and vertical (e.g. wall mounted) orientation.

However, this would not solve the issue of the test sample configuration as the test method is for testing of single materials, not for optical sandwiches consisting of a number of layers of differing materials. Therefore the test method would need an element of development to overcome this, however this could be defined within building regulations if necessary. (Note that testing each component of the optical construction individually is not a solution as the properties when exposed to flame differ between a material in isolation and as part of an optical sandwich).

So, when a product is certified to be TP(a) or TP(b), the question a customer should ask is how this test was performed and is it representative of the actual product being purchased. Declaring a TP() rating based purely on testing an unrepresentative sample of the bottom layer of the optics should not be considered acceptable.

The LIA is strongly of the opinion that to meet the legal requirement of the Building Regulations, modern composite diffusers need to have their TP() rating based upon testing of the complete sandwich of materials as used within the recessed luminaire. This to demonstrate a complete level of legal compliance to the national requirements.

For more information on testing a multi-layer optical controller for TP() classification contact the LIA Laboratory. Also see the LIA Laboratory Decision Sheet DS-1 LIA_07_2020 (

This topic is also discussed in annex A of the LIA Technical Statement 39, UK Building Regulations – Fire Safety – Spread of flame Use of Thermoplastic Materials & TP(a)/TP(b) rating and has also been communicated to the UK Governments. (

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May 2024 issue

2024 A1 Buyers Guide