Fire Extinguishers

First recognize that there are four different kinds of fires:

• Class A fires are ordinary materials like burning paper, lumber, cardboard, plastics etc.
• Class B fires involve flammable or combustible liquids such as gasoline, kerosene, and common organic solvents used in the laboratory.
• Class C fires involve energized electrical equipment, such as appliances, switches, panel boxes, power tools, hot plates and stirrers. Water is a particularly dangerous extinguishing medium for class C fires because of the risk of electrical shock.
• Class D fires involve combustible metals, such as magnesium, titanium, potassium and sodium as well as pyrophoric organometallic reagents such as alkyllithiums, Grignards and diethylzinc. These materials burn at high temperatures and will react violently with water, air, and/or other chemicals. Handle with care!!

Here are typical uses for common extinguishers:

• Water extinguishers are suitable for class A (paper etc.) fires, but not for class B, C and D such as burning liquids, electrical fires or reactive metal fires. In these cases, the flames will be spread or the hazard made greater!
• Dry chemical extinguishers are useful for class ABC fires and are your best all around choice. They have an advantage over CO₂ extinguishers in that they leave a blanket of non-flammable material on the extinguished material which reduces the likelihood of reignition. They also make a terrible mess -- but if the choice is a fire or a mess, take the mess! Note that there are two kinds of dry chemical extinguishers! BC and ABC
• CO₂ (carbon dioxide) extinguishers are for class B and C fires. They don't work very well on class A fires because the material usually reignites. CO₂ extinguishers have an advantage over dry chemical in that they leave behind no harmful residue -- a good choice for an electrical fire on a computer or other delicate instrument. Note that CO₂ is a bad choice for a flammable metal fire. CO₂ extinguishers are not approved for class D fires!
• Metal/Sand Extinguishers are for flammable metals (class D fires) and work by simply smothering the fire. You should have an approved class D unit if you are working with flammable metals.

Using fire extinguishers

1. Pull the pin on the fire extinguisher.
2. Stand several feet from the fire, depress the handle and sweep back and forth towards the fire.
3. Direct the extinguisher at the base of the flames until the fire is completely out.
4. Recharge any discharged extinguisher immediately after use. If you discharge an extinguisher (even just a tiny bit) or pull the pin for any reason, call your fire extinguisher service person as soon as possible.