Fire extinguisher efficiency

Factual Questions
1

Are fire extinguishers that are rated for a single type better at putting out fires of that type than a general purpose ABC extinguisher? If so, how much better?

I went to buy a fire extinguisher to carry in my car and I had a choice between one rated ABC and one just BC. I can see how a car related fire is more likely to be a grease/electrical fire, but I’d like to be prepared for any type. I’d have no problem buying two if specialized extinguishers are better at putting out respective fires.

** Yes I realize with more than one extinguisher there would be an added risk of grabbing the wrong one, or incorrectly determining the cause of a fire – but let’s ignore that for now.*

2

paraphrased from from essentials of firefighting second edition

The number/letter rating on a fire extinguisher is the number of square feet of fire you should be able to put out - fire types. So an 10-A rated FE should be able to put out a fire invoving wood/paper with 10 square feet of surface area in the hands of a trained operator.

The ratings speak for themselves…active components selection is not much of an issue, the numerical rating is. So you might find a 10-A 20-B or a 6-A 40-B. The second is obviously far more effective on class B fires (flammable liquids)

Ah but the cause is often a major consideration sir. Ignoring it could lead to a bigger problem. Especially when it comes to energized electrical equipment. A “C” rating on a FE designates its safe to use on energized electrical equipment.

I can go for days here… there are specialized FE’s for many purposed (things like metal fires).

3

Thanks drachillix. I didn’t know about the number part of the rating.

I had thought about your scenario of using an A rated extinguisher on an electrical fire. The plan in my head was to buy one BC rated FE [because I thought it might be better at fighting BC fires] and one that was ABC [to use if there happens to be a paper fire, but also wouldn’t hurt if it were used on a BC fire]. That logic may be irrelevant now.

Is it a hard and fast rule to only use appropriately classed FEs on fires? [I’m guessing the answer is yes.] In the case of using one without a C rating on an electrical fire the answer is obviously yes. But what if there was a paper fire and the only extinguisher available didn’t have an A rating. Would it just not work as well? Could it make the fire worse? What about other combinations?

4

The basic difference between regular dry chemical (BC) and multi-purpose dry chemical (ABC) is that the ABC actually sort of melts on the fire and smothers it, keeping it from re-igniting, while the BC “snuffs” it. In most cases you’ll be better off with an ABC since you can put out more fires with it, but it can be a whole lot more difficult or impossible to clean up.

5

Uh, are you sure? I was under the impression that monammonium phosphate dry chem FE’s and such worked more by chemical interruption of combustion like Halon does. Also Dry chem FE

Also of note, especially when fighting vehicle fires.

Dry chemical, Co2, and Halon FE’s do not provide a “cooling” effect that high flow water has to keep fires from reigniting. A car engine that has been on fire could easily have patches of metal well over 1,000 degrees that can reignite exposed flammables immediately after knocked down.

Car rims can also be nasty, Magnesium and aluminum rims can ignite and when they do…its quite a show, and not one you want to get close to. Specialized FE’s do exist just for this kind of fire but unless you work for example in a machine shop that works with these types of metals its really not worth the expense to have them around, many fire departments don’t even carry this stuff.

A little carb fire or something really small is what car FE’s are for, anything bigger is what insurance is for. Fighting vehicle fires is dangerous business and is best left to the fire department. Your car or especially someone elses is not worth getting seriously injured or killed trying to play superhero. Firefighters get hurt doing this all the time, good samaritans rarely have visored helmets and fire resistant insulated clothing.

IMHO if you get a discount on your insurance, grab a basic 2-A 10-BC or there are some nice 20-BC’s out there. No offense intended but, if you can’t take it down with that you probably don’t have any business messing with it.

6

Close, but not quite.

In the US, there are 5 class ratings for fire extinguishers, A,B,C, D, and K (the classes and their meanings change when you go overseas).

Class A = ordinary combustibles
Class B = Liquids
Class C = Energized electrical equipment
Class D = Metals
Class K = Kitchen fires

Now, when you look at the rating on the side of the extinguisher, it will say something like 10:A,60:B,C. This doesn’t mean that it will put out 10 square feet of ordinary combustibles and 60 square feet of liquids and electrical equipment. The ratings are decided like so:

Class A: The standard unit of “A” is the 2.5 gallon water extinguisher. Each 2.5 gallon water extinguisher is rated as a 2A extinguisher. Since 2.5 gals water is 2.5 gals of water everywhere, it makes a good “standard” unit. The fire that this 2A extinguisher is expected to put out is a certain size and shape wooden crib. If your other-than-water extinguisher puts out this fire, its a 2A. If it puts out 2 of them, its a 4A, and so on.

Class B: Each unit of “B” is the square footage of the surface of the burning liquid. If you have a 60B extinguisher, it will put out 60 square feet of fire on a flammable liquid.

Class C: There is no numerical rating for Class C extinguishers, it only reflects the conductivity of the extinguishing agent. Since your electrical equipment is going to be a Class A or B fire with electricity flowing through/around it, you use the appropriate extinguisher for that hazard plus the C rating so you don’t get shocked.

The number ratings are for a non-trained user of the extinguisher. I know I can put out 400 square feet of burning liquid with a 120BC extinguisher, but I’ve been trained quite a bit in the use of them. The people that use the extinguishers to come up with the ratings are obviously very skilled in their use, so the amount of fire that they can extinguish is down-rated for the average joe.
As for mixing ratings with fires:
In some cases, it can be done without too much harm. In others, you’re going to kill yourself and make the fire larger. Rule of thumb: if you don’t know exactly what the agent is going to do/cause when it hits the material, don’t use it. Can a 2.5 gallon water extinguisher put out a fire in a flammable liquid? Sure can. I don’t suggest anyone try it, but it will work if you know what you’re doing. Stick with the ratings on the extinguisher. You’re also best buying an ABC extinguisher. Can’t really go wrong if you get one.

7

Thanks for the great answers everyone. You’ve got me thinking, and I have another question…

In the real world, would a higher rating imply that one extinguisher was better (I guess by better I mean faster or more completely) at putting out a fire than one with a lower rating? Or just that it could put out more of it?

On a related note are there any training programs out there designed for people who aren’t fire “professionals” that teach things like this. How to best fight different kinds of fires with the common fire extinguishers out there (something beyond “point at base and sweep side to side.”) I’d be interested in something a little more advanced than the basic “check the batteries in your smoke alarm” public service classes.