Classification of fires
1Key Takeaways
This standard provides a systematic classification of fires based on the type of fuel involved. It outlines distinct categories that help in identifying the appropriate extinguishing methods and equipment. The document defines criteria for categorizing fires into different classes, ensuring a clear and consistent appro…
2Expert Interpretation
This article provides an in-depth analysis of the ISO 3941:2026 Third Edition International Fire Classification Standard, focusing on the newly added L-class lithium-ion battery fire definition, a comparison of the characteristics of the six major fire categories, the background of the standard's technical evolution, and recommendations for the selection and implementation of fire protection equipment.
Background of the Technical Evolution and Revision of ISO 3941:2026
The International Organization for Standardization (ISO) officially released the third edition of ISO 3941:2026 "Classification of Fires" in 2026. This is a major technical revision nearly two decades after the second edition was released in 2007. The core driving force behind this revision comes from the global energy structure transformation and the emergence of new fire risks, especially the new fire protection challenges brought about by the widespread application of lithium-ion battery technology. The standard was led by the SC 2 Subcommittee on Manual Fire Extinguishers under the ISO/TC 21 Technical Committee on Fire Protection Equipment, reflecting the international fire protection field's rapid response to technological developments.
Compared to the 2007 version, the 2026 standard represents a technological breakthrough primarily in two aspects: Firstly, it classifies lithium-ion battery fires separately as Category L, marking the international community's formal recognition of the dangers of electrochemical fires. Secondly, the addition of Chapter 4, a systematic description of the hazards associated with fire categories, provides a more comprehensive theoretical basis for the design, testing, and selection of fire-fighting equipment. This revision not only reflects technological progress but also embodies a paradigm shift in international standardization work from simple classification to comprehensive risk assessment.In-depth Analysis of the Definitions and Characteristics of the Six Fire Categories
ISO 3941:2026 systematically divides fires into six categories, each precisely defined based on the physicochemical properties of the fuel. This classification system is not only a result of academic research but also an important guiding tool for fire-fighting practice.
| Fire Category | Fuel Type | Typical Material | Combustion Characteristics | Key Considerations for Extinguishing Fire |
|---|---|---|---|---|
| Class A | Solid Materials | Wood, Paper, Fabric, Plastic | Surface or Deep Burning, Producing Embers | Distinguish Between Surface Fire and Deep Fire, Requires Complete Saturation |
| Class B | Liquid or Liquefiable Solids | Gas, Alcohol, Grease, Paraffin | Rapid Spread, Prone to Reignition | Fuel Polarity and Container Type Affect Extinguishing Methods |
| Class C | Gas | Natural gas, propane, hydrogen | Explosive, fast-burning | Prioritize cutting off the gas supply; blindly extinguishing the fire may cause an explosion |
| Class D | Metals | Magnesium, sodium, titanium, potassium | High temperature, violent reaction | No standard test fire; special extinguishing agent required |
| Class F | Cooking medium | Animal and vegetable oils | High temperature, prone to splashing | Do not inject water directly; special wet chemical agent required |
| Class L (New) | Lithium-ion batteries | Lithium-ion cells/packs without metallic lithium | Electrochemical fires, high energy density | Thermal runaway chain reaction, toxic gases, and reignition risk |
The standard specifically emphasizes that all categories of fires need to consider the common hazard of toxicity of combustion products. This requirement reflects the comprehensive expansion of modern fire protection concepts from simple fire extinguishing to personnel safety and environmental impact. In practical applications, firefighters need to select appropriate fire extinguishing strategies and equipment based on accurate assessment of the materials burning on site.
Class L lithium-ion battery fires: Professional response to new risks
The introduction of Class L fires is the biggest highlight of this standard revision. The standard clearly defines Class L fires as "fires involving lithium-ion batteries and battery packs that do not contain metallic lithium." This definition clearly distinguishes lithium-ion battery fires from traditional metallic lithium fires (belonging to Class D), reflecting a profound understanding of the mechanism of electrochemical fires. Section 5.7 of the standard details seven specific hazards of lithium-ion battery fires: high-temperature explosive gas emissions, gas toxicity, physical obstructions affecting the reach of extinguishing agents, thermal runaway chain reactions, ejection of burning cells, risk of electrolyte leakage, reignition due to residual energy, and the risk of electric shock. The identification of these hazards provides clear technical requirements for fire equipment manufacturers to develop dedicated fire suppression systems. A typical application case is electric vehicle fire suppression. When an electric vehicle battery pack experiences thermal runaway, firefighters face not only open flames but also: thermal runaway propagation between battery modules can last for hours; residual energy within damaged cells can reach 30% of the total capacity, easily leading to reignition; extinguishing agents have difficulty penetrating the battery pack casing to reach the internal heat source; and highly toxic gases such as hydrogen fluoride produced by electrolyte decomposition require specialized protection. These complex situations necessitate the use of fire suppression technologies and equipment specifically designed for Class L fires.
Practical Response Strategies and Technical Points for Various Types of Fires
Class A Fires: Differentiating Between Surface Fires and Deep-Seated Fires
Standard 5.2 clearly states that Class A fires can be divided into surface fires and deep-seated fires. Surface fires, such as burning paper and cloth, are relatively easy to extinguish; deep-seated fires, such as burning wooden beams and foam plastics, require extinguishing agents with strong penetration and sustained cooling capabilities. Traditional water-based fire extinguishers have limited effectiveness against deep-seated fires and require the use of water mist systems or special penetrating agents.
Class B Fires: Focus on Fuel Characteristics
Standard 5.3 emphasizes the need to consider the polarity of the fuel. Polar fuels (such as alcohol) are soluble in water, while non-polar fuels (such as gasoline) are insoluble in water. This difference directly affects the selection of extinguishing agents: protein foam is effective against non-polar fuels, while aqueous film-forming foam (AFFF) is effective against both but environmental issues need to be considered.
Class C Fires: Gas Source Control First Section 5.4 provides a clear safety warning: "Extinguishing a gas fire is generally dangerous if the gas source cannot be cut off." This is because unburned gases can accumulate to form an explosive mixture. In practice, valves should be shut off first, followed by extinguishing with inert gas or dry powder diluted with water. Class D Fires: No Standard Solution Section 5.5 acknowledges the complexity of metal fires, stating that "it is impossible to define a representative standard fire for testing purposes." The combustion characteristics of different metals (magnesium, sodium, titanium) vary greatly, and their forms (powder, flakes, blocks) and fire configurations also vary considerably. This requires on-site commanders to select specialized extinguishing agents (such as Class D dry powder or specialized covering agents) based on the specific metal type. Class F Fires: Professional Considerations for Kitchen Safety Section 5.6 explicitly warns of the explosion risk of directly injecting water into hot oil. Commercial kitchens should be equipped with a **Class K (North American standard) or Class F (international standard) wet chemical fire suppression system**, which forms a covering layer through saponification to isolate oxygen and simultaneously cool oil temperatures below their ignition point.
Standard Implementation Recommendations and Fire Equipment Selection Guidelines
1. Risk Assessment and Classification Labeling
All locations should undergo fire risk classification assessments based on the materials stored and used. It is recommended to install **visual classification labels** in hazardous areas, such as "Class B Hazard Area—Flammable Liquid Storage". For mixed-risk locations (such as data centers containing Class A equipment and lithium-ion UPS), multiple classification labels should be used.
2. Fire Extinguishing Equipment Compatibility Selection
| Fire Category | Recommended Extinguishing Agent Type | Applicable Equipment | Prohibited/Cautionary Methods |
|---|---|---|---|
| Class A | Water, Water Mist, ABC Dry Powder | Water-based Fire Extinguishers, Fire Hydrants | Carbon Dioxide (Ineffective for Deep-level Fires) |
| Class B | Foam, Carbon Dioxide, BC Dry Powder | Foam Extinguishing Systems, Dry Powder Fire Extinguishers | Direct Current Water (May Splash) |
| Class C | Dry powder, carbon dioxide, inert gases. Dry powder fire extinguishers, gas extinguishing systems. Water (conductive hazard). Class D fire extinguishers, dedicated metal extinguishing agents, Class D dry powder fire extinguishers, covering sand. Water, carbon dioxide, conventional dry powder. Class F fire extinguishers, wet chemical extinguishing agents, kitchen-specific extinguishing systems. Water, conventional dry powder. Class L fire extinguishers, dedicated lithium battery extinguishing agents, large amounts of water. Lithium battery-specific extinguishing devices, water mist systems. Small amounts of water (may exacerbate the reaction). 3. Training and Emergency Response Plan Updates Fire safety training courses must include content on Class L fires, including lithium-ion battery identification, initial response procedures, operation of specialized equipment, and long-term monitoring requirements. Emergency response plans should develop tiered response strategies for mixed fire scenarios, especially in emerging high-risk locations such as energy storage power stations, electric vehicle charging stations, and electronic product warehouses. 4. Standardization and Certification Alignment ISO 3941:2026 needs to be harmonized with national fire safety standards. For example, the US NFPA 10 standard classifies fires into five categories: A, B, C, D, and K, with Class K corresponding to ISO Class F. Equipment manufacturers should ensure that their products meet both international classification and local certification requirements, clearly indicating the applicable fire category on product labels.