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The purpose of this page is to provide further information regarding the provision of Class B cover in accordance with the recommendations of BS 5306-8:2012. Readers are referred to the British Standard itself for full details and reminded that this is a summary only.

Throughout, references to the pertinent parts of the British Standards and/or other legislation have been included as footnotes.

The assessment of cover for a single liquid container in a given room is calculated via the total surface area of the container and the table below¹. This table gives the maximum surface area (in m²) for which a given rated extinguisher(s) is(are) suitable.

Maximum class B surface area	e.g. Circle of diameter	Minimum quantity and rating for each extinguisher
0.14 m²	0.42 m	1 ×	21B
0.23 m²	0.54 m	1 ×	34B
0.37 m²	0.69 m	1 ×	55B
0.47 m²	0.77 m	1 ×	70B
0.59 m²	0.87 m	1 ×	89B
0.75 m²	0.98 m	1 ×	113B
0.96 m²	1.11 m	1 ×	144B
1.22 m²	1.25 m	1 ×	183B
1.41 m²	1.34 m	2 ×	113B
1.80 m²	1.51 m	2 ×	144B
2.29 m²	1.71 m	2 ×	183B
2.88 m²	1.91 m	3 ×	144B
3.66 m²	2.16 m	3 ×	183B
4.66 m²	2.44 m	3 ×	233B

For example, a room containing a flammable liquid container measuring 1m×0.5m has an area of 1×0.5 = 0.5m² which requires 1×89B. Note, this cover does not include the spillage cover given below.

• In cases where there are multiple containers less than 2m apart, they should be considered as an undivided container equivalent to a single container with a surface area equal to the total surface area of the group².
• In cases where there are multiple containers that are more than 2m and less than 20m apart, they should be considered as forming a divided group. The surface area of the largest single container, the total surface area of the largest undivided group (as above), or one third of the total surface area of all of the containers, whichever is the greater, is used to calculate the minimum rating³.
• In cases where the containers are more than 20m apart, the containers should be considered on an individual basis⁴.
• Additional extinguishers should also be supplied to cover the hazard from spillage. The recommended minimum rating = 10×Anticipated Spillage Volume (in litres)⁵. Refer to the British Standard for full details.
• The table above gives minimum cover levels. Additional extinguishers should be provided where the liquids have a low fire point, or are particularly difficult to extinguish⁶.
• Where high rated extinguishers are installed, additional low rated extinguishers should be provided and sited near any anticipated points of fire occurrence⁶.

• Where such fire hazards also require a fixed system, a class B extinguisher(s) is nonetheless also required to cover the risk of spillage or fires outside of the fixed system’s coverage⁶.

• Where such fire hazards are in areas prone to still air (where Class B fires are harder to extinguish) additional class B extinguisher(s) should be provided⁶.
• Class B extinguishers should be sited such that there is not necessary to travel further than 10m to the site of any potential class B fire⁷. These travel distances should be reduced if there are doorways on this route⁷.

1. As per BS 5306-8:2012, clause 8.3.2.2, Table 1.
2. As per BS 5306-8:2012, clause 8.3.2.3.
3. As per BS 5306-8:2012, clause 8.3.2.4.
4. As per BS 5306-8:2012, clause 8.3.2.1 and 8.3.2.2.
5. As per BS 5306-8:2012, clause 8.3.2.5.
6. As per BS 5306-8:2012, clause 8.3.3.
7. As per BS 5306-8:2012, clause 6.4.