Classification of Fires and Fire Extinguishers for the chemistry laboratory

Different types of fires and the extinguishers approved to fight them. Rating, classification, and uses of fire extinguishers.

Fires and Fire Extinguishers

Fires
Fires are classified according to the materials they burn. They are coded with the capital letter of the alphabet A, B, C, D, F, etc.. with the word 'Class' placed in front of each letter. Example – Class A fires, Class B fires, Class C fires etc..

Class A Fires – Fires involving combustibles such as : wood, paper, boxes, plastic, packing material etc..
Class B Fires – Ignition of flammable liquids such as : solvents, kerosene, gas, grease etc..
Class C Fires – Fires arising from electrical equipment such as : AC outlets, wiring, appliances, flammable gases etc..
Class D Fires – Combustible metal fires such as : Mg, K, Na, Al, Titanium, Lithium (includes powders and swarfs).
Class E Fires – Electrical fires : Fires involving electrical apparatus.
Class F Fires – Fires involving coking oils and Fats : burning hot oil, cooking oil, lard.
Class K Fires – Fires in cooking utensils and appliances caused by oils and fats.
Note
• There is no class E or class F fires in the US system of classification. Class E is equivalent to class C and class F is equivalent to class K.
• Class E is no longer applicable because when power is turned off, electrical fires can fall into any category.

Fire Extinguishers
Fire extinguishers are classified according to 'class' of fires they are made to put out. An ABC extinguisher can be used to put out a Class A, Class B, or Class C fire.
An AB extinguisher is used to out a Class A or Class B fire.
And a D extinguisher can be used to extinguish a Class D fire only.

Fire Extinguisher colors
There is no official standard for color in the US classification. Red is the preferable color except
Class D – yellow
Water – silver
Water mist – white

Fire Extinguisher Ratings
The Numerical Rating of Fire extinguishers is a measure of the Fire Power of the extinguisher.
For example, a class ABC rating is given as 5-A :10-B:C
The No. in front A gives the number of gallons of water the extinguisher is equivalent to.
One (1) unit is equal to 1.25 gallons of water. Hence, 5A = 5 × 1.25 which is equal to 6.25 gallons of water.
The No. in front B gives the number of square feet of fire it can extinguish. In the example, this will be 10 sq. ft.
The No. in front C. There is no number in front of C, which means that the extinguisher does not have a rating and the extinguishing agent is non conductive.
Class D extinguishers have no numerical rating and is non conductive.

Summary of Fires and Fire extinguishers

Extinguisher type	Type of Fire	Examples of Fire type	Commentary
Water	Class A only	Ordinary materials, paper, wood, boxes, plastics, packings etc...	Not recommended for lab or electrical fires; leaves area water-logged
Water mist	Class A Where possible class C hazard exists.	Hospital environments, books, clean-rooms, MRI and NMR rooms	Misting nozzle provide safety from electrical shock and reduce scattering of burning material
Dry chemical (powder) • BC - Na or K carbonate • ABC - ammonium phosphate	Class A Class B Class C	Ordinary materials Combustible liquids, solvents and gases. Electrical fires, appliances	Overlaying powder reduces re-ignition Leaves sticky or corrosive residue that can damage electrical equipment Isolate gas supply first for gas fires Disconnect power for electrical fires
Dry metal powder • Copper agent • NaCl agent	Class D only	Combustible metal fires : (Cu) Metal and lithium alloys (Nacl) Mg, Na, K, Uranium and Al	(Cu) Powder adhere to vertical surfaces (NaCl) Cakes over surface, excludes air, dissipates heat
Carbon dioxide CO2	Class A Class B Class C	Every day combustible materials Flammable solvents and electrically charged equipment and appliances Flammable gases	Leaves no harmful residue, but may re-ignite with class A fires Disable gas and electricity supply if necessary Caution! Reacts with class D fires
Foam (Film forming)	Class A Class B	Combustible solids Flammable liquids	Not recommended but safer than water if inadvertantly used on electrical fires
Wet chemical (Sprayer)	Class A Class F Class K	Wood, paper, fabrics etc.. Fats, Lard, vegetable oils Animal oil, cooking oil, butter	Flash prevention spray Soapy foam prevents re-ignition

Emergency numbers :

• Fire Department Emergency – Dial 990 (T&T Fire Services)
• UWI Fire Safety – 662 4707