Statistics indicate that residents detect more than 90 percent of all home fires while the blazes are just beginning. Yet, most homes and apartments still do not contain even one portable fire extinguisher.

A good extinguisher, located close at hand, can in many cases be one of the most effective means of preventing disaster.

Years ago reluctance to install home fire extinguishers was partially understandable: the soda-acid units then available were awkward to handle and difficult to maintain.

In recent years, however, greatly imporved and highly efficient models have been introduced. These modern extinguishers come in a variety of sizes and with different fire-fighting chemicals, so choosing one can be confusing. Fortunately there is a clearly defined rating system used by underwriters' Laboratories (UL) to classify all the extinguishers they test.

Selecting a model that bears the UL seal will insure your getting one that meets the requirements for that particular rating. The rating, which is conspicuously marked on the outside of all UL-listed extinguishers, consists of a series of numbers and letters - for example, it might read 5-B:C, or 1-A, or 10-B:C.

Although this seems complicated at first glance, it is really quite simple. The letters refer to the kind of fire the extinguisher can control, and the rating system breaks these down into three catergories:

Class A fire involve wood, paper, fabrics, upholstery, plastic and similar common combustibles where water would ordinarily work well (if readily available in sufficient quantity).

Class B fires involves burning grease, oil, paint, solvents and other flammable liquids. This type of fire is most likely to break out in and around a kitchen stove (frying pans, etc.), as well as in the garage.

Class C fires involve live electrical wires, motors or other equipment where a nonconducting extingishing liquid, powder or other agent is required.

As can be concluded from this, water could be used on Class A fires, but would be dangerous to use on the other two types. In the case of Class B fires, water might actually spread the flames; in the case of a Class C fire, there is serious danger from live currents.

However, once the current is shut off, a Class-C home fire often becomes a Class A fire (burning insulation or other flammable materials may continue to burn).

An extinguisher that carries only the B:C rating would be good for grease all oil fires, as well as electrical fires, but would not be effective on common fires such as burning drapes, carpet or upholstery. Most moderately priced dry chemical extinguishers carry this rating, so remember they won't work well on ordinary combustibles.

If the extinguisher does not carry the UL seal or bear these ratings, you have no way of knowing what kind of fire it will control.

The numbers that appear just before the letter designation give some aproximation of the size fire the extinguisher is capable of controlling. The larger the number, the bigger the fire it can put out.

For e example, an extinguisher rated 10 B:C, and one rated 2-A;40-B:C will handle a Class B fire that is four times as large as one rated 1A:10 B:C, or a Class A fire that is twice as large.There is no size rating for Class C fires becuse this designation only means that the chemical in the extinguisher will not conduct electricity.

When first introduced some years ago, dry chemical fire extinguishers were usually filled with sodium carbonate, and this is still the most popular type. It is a nontoxic, dry powder that only works on B and C fires, however.

To get around this, newer multipurpose models, slightly higher in price, which use dry chemicals that work on Class A fires, as well as on B and C fires, were introduced some years ago. These vary from about $19 to $50, depending on size (weight) and brand, which makes them about 50 percent higher in price than comparable dry chemical units rated for B and C fires only.

The extra protection, plus the peace of mind inknowing that anyone can use it on any fire the house, is well worth the difference in price.

At the lowest end of the price and performance range there are also very small, foam-type kitchen extinguishers rated for Class B fires only. These units cost only $5, and are good for small kitchen fires is promptly caught - for example, a flash fire in a frying pan (but not an electric stove that may have exposed wiring).

In addition to choosing an extinguisher according to size and rating, there are two other factors worth considering. One is whether it is refillable or disposable. Refillable models can be recharged and reused but disposable models must be thrown away after use. Refillable extinguishers cost more initially but will probably cost less if there is more than one fire (recharging costs only about $4 to $6 for most home sizes).

The second feature worth looking for is an indicator or pressure gauge that shows the condition of the charge - the better quality units have an easily read gauge that indicates if pressure is leaking out. The less expensive models (often not refillable) may have only a small pin that is supposed to show if pressure is lost; these are harder to see and not always accurate.