Class 1 and Class 0
This note confirms the requirements for Class 1 as defined in BS 476 : Part 7 and Class "0" as defined in approved document B of the England and Wales Building Regulations 1991.
In order to achieve a Class 1 rating, one has to test the material to BS 476 : Part 7 : 1987 "Method for Classification of the Surface Spread of Flame of Products". The test, which is for wall and ceiling linings involves exposing a flat specimen to a radiation gradient along it's length. A pilot flame is applied at the hot end and flame spread is measured visually along the length. The test thus measures the rate of flame spread over a surface and gives the results in the form of classes : 1,2,3 and 4, with 1 being the best and 4 the worst.
The material to be tested must be mounted onto a substrate as in practice. Typical substrates include plasterboard, calcium silicate boards, chipboard etc.. Air gaps may also need to be included where appropriate.
In order to up rate that Class 1 to a Class "0" rating one has to test the same material to BS 476 : Part 6 : 1989 "Method of test for fire propagation for products". This test involves heating the specimen in a non-combustible box and comparing the temperatures of the flue gasses, with those from a non-combustible board i.e. it is a measure of heat output.
If the board achieves Class 1 when tested against BS 476; Part 7 and then also achieves indices of performance i, < 6 and I < 12 in BS 476 : Part 6, then it can be classified as Class "0".
An alternative way to meet Class "0" is for material to be "non-combustible" or of "limited combustibility". Tests used to evaluate such materials are BS 476 : Part 4 and Part 11 respectively. These tests involve inserting a small specimen (=80cm3) into a furnace at 750°C. Sustained flaming or significant rises of thermocouples inserted in the furnace, specimen, etc are not permitted. Normally non-combustible materials or those of limited combustibility contain only very small percentages of organic materials.
