Most of the extinguishers you see hanging in an office, a church, or a shop are multi-class dry chemical extinguishers. The label says ABC, and most places start with one because it covers most of what can catch fire in a regular building. Here’s what’s going on inside it.
What a fire needs
A fire needs three things to keep going: fuel, heat, and oxygen. On top of that there’s the chemical reaction between them that keeps feeding itself. Take away any one of those, or break that reaction, and the fire goes out. Every extinguisher does one or more of those jobs. Water cools things down, CO2 pushes the oxygen out, and dry chemical breaks the reaction and smothers the fuel.
What “multi-class” means
Fires get sorted into classes by what’s burning.
- Class A is ordinary stuff like wood, paper, cardboard, cloth, and trash.
- Class B is flammable liquids and gases, like gasoline, oil, grease, paint, solvents, and propane.
- Class C is anything electrical that’s still plugged in or powered.
- Class D is burning metal, like magnesium, and it takes its own extinguisher.
- Class K is cooking oil and grease in a commercial kitchen.
A multi-class extinguisher is rated for more than one of those. An ABC covers A, B, and C, which is why you see so many of them.
How the dry chemical works
An ABC extinguisher is full of a fine yellow powder called monoammonium phosphate. It’s under pressure, so it comes out in a cloud when you squeeze the handle. When the powder hits the fire, it gets in the way of the chemical reaction that keeps the fire burning, and it knocks the flames down fast. On wood and paper, the powder melts and coats the surface, which helps keep it from flaring back up. On an electrical fire, the powder doesn’t carry current back to you.
There are also BC extinguishers, which use sodium bicarbonate or potassium bicarbonate instead. They work well on flammable liquids and electrical fires, but they aren’t rated for wood and paper. You’ll see them in some shops and garages.
The tradeoff with any dry chemical is the mess. The powder gets everywhere, it’s hard on electronics, and the area needs a good cleanup afterward. For a server room or a room full of sensitive equipment, we’d talk with you about a different kind of extinguisher.
Grease fires and kitchens
An ABC will put out a grease fire. If you run a commercial kitchen, the cook line also needs a Class K extinguisher, which sprays a wet chemical made for cooking oil and grease. That’s on top of the hood system over the line.
At home, if a pan catches, put a lid on it and turn off the burner. Never throw water on grease.
After you use one
Once an extinguisher has been used, even for a second, it needs to be recharged before it goes back on the wall. A partly used one can lose the rest of its pressure, and you won’t know until you grab it. Call us and we’ll take care of it. We recharge on site when we can. Our trucks carry loaners. If an extinguisher has to come back to the shop, a loaner goes up in its place until yours is back.
Picking the right one
An ABC is a good place to start in most buildings, but it isn’t the answer everywhere. We match the extinguisher to the hazard. When we walk your building, we look at what’s actually around each one, and we’ll tell you where a Class K, a BC, a CO2, or something else belongs.
If you want somebody to walk your building and make sure the right extinguishers are in the right spots, call 1-800-348-9604. We inspect and service them, we leave a report, and we keep up with the date.