- Classes of Fires
- How are Extinguishers Rated?
- Definitions of Hazards
- How Many Extinguishers are Needed?
- Extinguisher Size & Placement
We are fully conversant with the 1991 NZ Building Code and can therefore assist you in the design of such things as:
- Means of Escape
- Fire Ratings
- Distribution of Fire Equipment
- Purpose Group Classification
- Evacuation Plans
- Alarm Types
We work closely with the Fire Service, constantly bearing in mind the needs and expectations of our clients.
Fire Extinguishers - Before You Choose.....
Fire can be categorised by class. The letter designation that accompany the following descriptions relate to the standard for portable fire extinguishers
Class A - Trash, Wood, Paper
Class A fires involve ordinary combustible materials - paper, wood, fabrics, rubber, and many plastics. Quenching by water or insulating by a multipurpose (ABC) dry chemical agent is effective.
Class B - Liquids, Grease
Class B fires occur in flammable liquids - gasoline, oils, greases, tars, paints, lacquers, and flammable gases. Dry chemicals and carbon dioxide agents extinguish these fires.
Class C (E for Australian Standards) - Electrical Equipment
Class C fires take place in live electrical equipment - motors, generators, switches, and appliances. Non-conducting extinguishing agents such as dry chemicals, or carbon dioxide are required to extinguish them.
Class D - Combustible Metals
Class D fires occur in combustible metals such as magnesium, titanium, zirconium, sodium, lithium, and potassium. Sodium carbonate, graphite, bicarbonate, sodium chloride, and salt-based chemicals extinguish these fires.
The fire rating of an extinguisher provides a guide to its extinguishing ability. As a result of fire testing by Underwriters' Laboratories, Inc. or other Certifying Bodies, fire extinguishers carry a classification on their nameplates that consists of a numeral followed by a letter. The numeral indicates the approximate relative fire extinguishing capacity of the extinguisher on the class of fire, which is identified by a letter. For example, a 4-A extinguisher can put out approximately twice as much fire as a 2-A extinguisher, and a 20-B:C extinguisher can put out approximately twice as much flammable liquid fire as a 10-B:C extinguisher. For Class B extinguishers, the numeric rating also indicates the fire suppression capacity of the extinguisher when used by an inexperienced operator. That is, a novice can put out a fire encompassing 10 sq. ft. (.9 m2) with a 10-B:C extinguisher and a 20 sq. ft. (1.8 m2) fire with a 20-B:C extinguisher. The fire suppression capacity is related to the experience of the operator. For example, an experienced operator can put out a fire encompassing 25 sq. ft. (2.3 m2) with a 10-B:C extinguisher and 50 sq. ft. (4.6 m2) with a 20-B:C extinguisher. Class C extinguishers carry only the symbol and have no numerical rating because such fires are essentially Class A or Class B fires involving energised electrical equipment. For Class D fire extinguishers, there is no numerical rating. Fire extinguishers and extinguishing agents for use with Class D hazards shall be of types approved for use on the specific combustible metal hazard.
Note: Fire extinguishers labelled for Class D fires are not equally effective on all combustible metal fires. Care must be taken that the extinguishing agent chosen will be effective on the combustible metal present (this should be detailed on the fire extinguisher nameplate), since use of the wrong extinguishing agent can increase or spread the fire.
Light (Low) Hazard
Locations where the total amount of Class A combustible materials, including furnishings, decorations and contents, is minor. These may include offices, classrooms, churches, assembly halls, etc.
Ordinary (Moderate) Hazard
Locations where the total quantity of Class A combustibles and Class B flammables is greater than expected for Light (Low) Hazards. These locations could be offices, classrooms, commercial shops, light manufacturing or research operations, auto showrooms, parking garages, workshops, or support service areas.
Extra (High) Hazard
Locations where the total amount of Class A combustibles and Class B flammables in storage, production use, and/or finished product, is over that expected for Ordinary (Moderate) Hazards. These locations could be sites for woodworking, vehicle repair, aircraft and boat servicing, painting, dipping and coating (including flammable liquid handling) or be product showrooms, convention centres or storage areas.
The number of extinguishers to be installed is determined by the authority having jurisdiction (In New Zealand The Rule of Thumb is One 2.25 kg Multi-Purpose Dry Chemical Fire Extinguisher per 210 m2 starting with a minimum of two. However level of risk needs to be taken into account). This determination is based on the rapidity with which a fire may spread, the intensity of the heat that may develop, the travel distance (actual walking distance) from any point to the nearest fire extinguisher, and the accessibility of the fire. Where there are also special hazards, install additional units.
Class A Extinguisher Distribution
The table below is a guideline for determining the minimum number and rating of extinguishers for Class A and Class B fire protection. An analysis of specific areas, process hazards, or building configurations may indicate the need for more extinguishers or for extinguishers with higher ratings. The first step in calculating Class A fire extinguisher needs is to determine the proper class of occupancy (light, ordinary, or extra hazard). Depending on the rating of the extinguisher (1-A to 40-A), the maximum area that it will protect can be determined.
For example, a 2-A fire extinguisher will protect 210 of ordinary hazard area. Where the floor area of a building is less than 210 at least two extinguishers should be provided. As fire extinguishers once activated will loose pressure after a time and therefore not work a backup is always required. The requirements also specify that the travel distance (actual walking distance) from any point to the nearest extinguisher shall not exceed 20m. Extinguisher placement must fulfil both distribution and travel distance requirements.
Class B Extinguisher Distribution
Fire extinguishers for the listed grades of hazard should be provided on the basis of the table below. The table cites minimum sizes acceptable. Extinguishers must be placed so that the maximum travel distance does not exceed those specified in the table. Extinguishers with a lower rating, suitable for small specific hazards within the general hazard area, may be desirable in addition to those required.
Class C Extinguisher Distribution
Extinguishers with Class C ratings are required where putting out an energised electrical equipment fire requires a non-conducting extinguishing agent. This includes fire either directly involving or surrounding electrical equipment. Since the fire itself is a Class A or Class B hazard, the extinguishers are sized and placed on the basis of the anticipated Class A or Class B hazard. Whenever possible, electrical equipment should be de-energised before an extinguisher is used on a Class C fire.
Class D Extinguisher Distribution
Extinguishing equipment for combustible metal fires should be placed no more than 20 m from the hazard. The amount of extinguishing agent needed is measured by the surface area of combustible metals present, plus the potential severity of the fire as influenced by the shape and form of the metal.
For Class A Hazards
Maximum Square Footage1 Protected per Extinguisher
|Class A Rating Shown on Extinguisher||Light (Low) Hazard Occupancy||Ordinary (Moderate) hazard Occupancy||Extra (High) Hazard Occupancy|
|2-A||6000 SQ. FT||3000 SQ. FT||--|
|4-A||11,250||6000||4000 SQ. FT|
For Class B Hazards2
|Type of Hazard||Basic Minimum Extinguisher Rating||Maximum Travel Distance to Extinguishers|
|Light (low)||5B||30 ft. (9m)|
|Ordinary (moderate)||10B||30 (9)|
|Extra (high)||40B||30 (9)|
2 Other than for fires in flammable liquids of appreciable depth; spill fires or running fires where the depth of liquid does not accumulate appreciably; pressurized flammable liquid or gas fires from damaged vessels or product lines; cooking grease fires of appreciable depth. Variations between indoor and outdoor conditions can further complicate these conditions.
Note: The charts above are intended only as a guide to proper extinguisher size and placement. Actual conditions may call for more or fewer extinguishers than the charts indicate. For more information, refer to NFPA Standard No. 10 or contact the Fisher Safety Specialists at 1-800-926-8999.