What are the five fire classes and which extinguishing agent is appropriate for each?

NFPA 10 and OSHA 1910.157 define five fire classes, each requiring a specific extinguishing agent type. Class A fires involve ordinary combustibles — wood, paper, cloth, rubber, plastics. Appropriate agents: water (APW), multipurpose dry chemical (ABC), foam. Water cools the fuel below its ignition temperature and prevents reignition. Class B fires involve flammable and combustible liquids and gases — gasoline, diesel, acetylene, propane, alcohol. Appropriate agents: CO2 (smothers by oxygen displacement), dry chemical (ABC, BC), aqueous film-forming foam (AFFF). Water must NEVER be used on Class B fires — water droplets cause burning liquid to splash and expand the fire; steam production can cause explosive steam events with deep-fat fryers. Class C fires involve energized electrical equipment — switchgear, motors, control panels. Appropriate agents: CO2, dry chemical. Water and foam conduct electricity — using them creates electrocution risk for the operator. The C designation indicates the agent is non-conductive; once equipment is de-energized, a Class A or B agent may become appropriate depending on the burning material. Class D fires involve combustible metals — magnesium (Mg), titanium (Ti), sodium (Na), potassium (K), lithium (Li). Appropriate agent: specific dry powder matched to the metal type. This is a critical routing distinction: CO2 reacts violently with burning sodium and potassium (oxidizes the metal further, accelerating combustion); water reacts with burning Mg and Na to produce hydrogen gas (explosion risk). Only metal-specific agents (Lith-X for Li, Met-L-X for Mg, Na-X for Na/K) provide suppression. Class K fires involve cooking oils and fats in commercial cooking equipment — vegetable oils, animal fats at temperatures above 350°F. Appropriate agent: wet chemical (potassium acetate or potassium citrate solution). Wet chemical saponifies (converts) the burning fat into a soap-like foam that blocks reignition. Regular dry chemical (ABC) is prohibited for Class K — it does not saponify the fat and high-velocity discharge can splatter burning oil, spreading the fire. CO2 cools the oil below flash point momentarily but reignition occurs as the CO2 disperses.

What does the alphanumeric rating on a fire extinguisher label mean?

Fire extinguisher ratings under UL 711 consist of a number followed by a letter (for Class A and B), or just a letter (for Class C, D, K). The number quantifies extinguishing capability; the letter identifies the fire class. Class A rating (e.g., '2-A'): the number represents multiples of 1.25 gallons of water in suppression equivalent. A 1-A rating is equivalent to 1.25 gallons; 2-A equals 2.5 gallons; 4-A equals 5.0 gallons. Practical meaning: a 2-A extinguisher can suppress a larger Class A fire than a 1-A. Class B rating (e.g., '10-B'): the number represents the square footage of a class B fire that a trained operator can extinguish. A 10-B extinguisher can suppress a 10 square foot Class B fire; a 40-B can handle a 40 square foot fire. OSHA and NFPA note that the B rating assumes a trained operator — untrained operators may achieve only 50–60% of the rated coverage area. Combined rating example: '2-A:10-B:C' means the extinguisher is rated 2-A for Class A fires, 10-B for Class B fires, and is safe (non-conductive) for use on Class C (energized electrical) fires. The C designation carries no numeric rating because there is no UL test for electrical fire suppression capacity — C means only that the agent is non-conductive at the tested voltage. Class D and K ratings are not numeric under the current standard — extinguishers for these classes must be specifically listed and labeled for the metal type or cooking medium involved.

What are the OSHA 1910.157 travel distance rules for fire extinguisher placement?

OSHA 1910.157(d) requires that portable fire extinguishers be distributed so that the maximum travel distance from any employee to an appropriate extinguisher does not exceed regulatory limits, which differ by fire class hazard. Class A hazards: maximum travel distance of 75 feet (22.9 m) from any point in the protected area to the nearest Class A extinguisher. This applies to most general occupancy areas with ordinary combustibles (offices, warehouses with paper/wood). Class B hazards: maximum travel distance of 50 feet (15.2 m). The shorter travel distance for Class B reflects the greater intensity and faster spread of flammable liquid fires. Class C hazards: no separate travel distance rule — Class C requirements are met by placing appropriately rated extinguishers (CO2 or dry chem) that are also rated for the Class A or B fuel source present near the electrical hazard. Class D hazards: extinguisher placed within 75 feet of the combustible metal operations, but the specific metal-type agent must be available. Class K hazards: extinguisher placed within 30 feet of cooking equipment per NFPA 10. This is significantly shorter than other classes due to the rapid escalation of cooking oil fires. NFPA 10 additionally requires that Class K extinguishers be the last line of defense after a fixed automatic suppression system (hood suppression system) — a Class K portable extinguisher alone does not satisfy OSHA 1910.157 requirements for commercial cooking operations that also require a fixed suppression system under NFPA 96.

What are the NFPA 10 inspection intervals for portable fire extinguishers?

NFPA 10 (Standard for Portable Fire Extinguishers) establishes four distinct inspection and maintenance intervals for portable fire extinguishers, each serving a different purpose. Monthly visual inspection: performed by the owner/occupant, not a professional. Checks that the extinguisher is visible and accessible (not blocked), the pressure gauge is in the green zone (or weight is verified for CO2), the tamper seal on the pull pin is intact, the label and operating instructions face outward and are legible, and there is no obvious physical damage or corrosion. Monthly inspections must be documented in a log. Annual maintenance inspection: performed by a licensed fire extinguisher service company. Involves a complete hands-on examination — verifying operating mechanism, hose and nozzle condition, safety valve, discharge test (for CO2 and other types), label accuracy, and for dry chemical extinguishers, verifying that the powder is not caked. Weight is verified for CO2 cylinders. Results tagged on the extinguisher. 6-year internal inspection: applicable to stored-pressure dry chemical, stored-pressure water, and stored-pressure loaded-stream extinguishers. Requires emptying the agent, inspecting the interior cylinder for corrosion and damage, inspecting the valves, reassembling, refilling with new agent, and recharging. Identifies internal corrosion invisible from outside. Hydrostatic pressure test: required at 12-year intervals for most stored-pressure extinguishers (water, dry chem, foam). CO2 cylinders require hydrostatic testing every 5 years due to the higher pressure. Tests structural integrity of the pressure vessel by filling with water and applying pressure at 125–167% of service pressure. Cylinders that fail are destroyed; date of last hydrostatic test must be stamped on cylinder neck.

Why can't a standard ABC dry chemical extinguisher be used on a Class K commercial kitchen fire?

ABC multipurpose dry chemical extinguishers are prohibited for use on Class K commercial cooking equipment fires under NFPA 96 and NFPA 10 for two reasons. First, ABC dry chemical (monoammonium phosphate) does not saponify cooking oils. Saponification is the chemical reaction where an alkali (potassium acetate or potassium citrate in wet chemical agents) reacts with animal fats or vegetable oils to form a soapy foam blanket that seals the oil surface and prevents reignition as the oil cools through its flash point. Without saponification, the oil remains at extremely high temperature (well above 700°F in a commercial deep fryer fire) after the dry chemical flame knockdown, and reignition occurs as soon as the powder dissipates. Second, ABC dry chemical is stored under pressure and discharges at high velocity. When discharged into a burning fryer or open cooking vessel, the high-velocity gas stream scatters burning oil outside the fryer onto the operator, cooking equipment, and kitchen surfaces, dramatically expanding the fire. Wet chemical agents are formulated for low-pressure discharge specifically to avoid this splatter effect. NFPA 96 Section 10.5.1 requires that commercial cooking equipment with a grease collection system have a UL 300 listed fixed hood suppression system as the primary suppression. The Class K portable extinguisher is a backup for fires outside the hood or for reignition after automatic discharge. A Shopify merchant selling commercial kitchen extinguishers must encode fire_extinguisher.fire_class_rating = 'K' on wet chemical extinguishers and explicitly exclude the ABC rating to prevent AI agents from routing dry chemical extinguishers into commercial kitchen applications.

AI Agent Product Routing — Fire Extinguishers (OSHA 1910.157, NFPA 10, Class A/B/C/D/K Agent Selection)

Fire Extinguisher Schema for AI Agents — Class A/B/C/D/K Agent Selection, OSHA 1910.157 Travel Distance Rules, and NFPA 10 Monthly/Annual/6-Year/Hydrostatic Inspection Intervals

Fire extinguisher routing errors are life-safety failures. Using water on a Class B (flammable liquid) fire spreads burning fuel; CO2 on a Class D (combustible metal) fire can cause violent reactions with burning sodium or potassium; ABC dry chemical on a Class K (cooking oil) fire fails to prevent reignition and scatters burning oil. Correct AI-agent routing requires encoding fire class, agent type, travel distance compliance, and maintenance schedule status as machine-readable metafields.

TL;DR — Key Encoding Rules Encode fire_extinguisher.fire_class_rating = 'A' | 'B' | 'C' | 'D' | 'K' | 'A,B,C' to prevent cross-class routing errors. Encode fire_extinguisher.agent_type explicitly — agents within the same class vary (wet-chemical vs dry-chem for K). Encode fire_extinguisher.travel_distance_class_a_max_ft = 75 and travel_distance_class_b_max_ft = 50 for facility compliance gap detection. Encode fire_extinguisher.class_k_prohibited = true on ABC dry-chem to prevent commercial kitchen routing.

Fire Class Reference — Agent Selection Constraints

Five fire classes each have specific agent requirements. Mismatching agent to fire class can worsen the fire or create secondary hazards for the operator.

Class	Fuel Type	Approved Agents	Prohibited Agents	Critical Reason
A	Ordinary combustibles (wood, paper, cloth, rubber, plastics)	Water (APW), ABC dry chem, foam	—	Cooling and smothering both work
B	Flammable/combustible liquids and gases (gasoline, diesel, propane)	CO2, dry chem (BC or ABC), AFFF foam	Water	Water splashes burning liquid; steam explosion in fryer
C	Energized electrical equipment	CO2, dry chem (non-conductive)	Water, foam	Water and foam conduct electricity — electrocution of operator
D	Combustible metals (Mg, Ti, Na, K, Li)	Metal-specific dry powder (Met-L-X, Lith-X, Na-X)	CO2, water, ABC dry chem	CO2 oxidizes burning Na/K; water reacts with Mg/Na (H₂ gas explosion)
K	Cooking oils and fats (commercial kitchen)	Wet chemical (potassium acetate/citrate)	ABC dry chem, CO2	Dry chem fails saponification; CO2 allows reignition as it disperses

Class D critical routing rule: There is no universal Class D extinguisher. The metal-specific agent must match the burning metal. Lith-X (graphite-based) for lithium; Met-L-X (sodium chloride) for magnesium and sodium; Na-X (sodium carbonate) for sodium and potassium. Encoding fire_extinguisher.compatible_metals on Class D agents prevents life-safety routing errors in machine shops and battery manufacturing environments.

UL 711 Extinguisher Ratings — Decoding the Label

The alphanumeric UL rating on a fire extinguisher label indicates suppression capability by class. A rating of 2-A:10-B:C decodes as:

Rating Component	Meaning	Practical Implication
`2-A`	Equivalent to 2.5 gallons of water (1-A = 1.25 gal)	Can suppress a larger Class A fire than 1-A; use to determine coverage area per NFPA 10
`10-B`	Can suppress a 10 sq ft Class B fire (trained operator)	Untrained operators achieve ~50–60% of rated B coverage; size up for uncontrolled environments
`C`	Agent is non-conductive at tested voltage	No numeric rating — C means only that using on energized equipment won't electrocute operator
Class D rating	No standard numeric rating — listed for specific metals	Label must name the metal type; general "Class D" rating does not exist in UL 711
Class K rating	UL 300 listed for commercial cooking	Must be UL 300 listed specifically — not interchangeable with other wet chemical agents

OSHA 1910.157 Travel Distance Requirements

OSHA requires that no employee must travel more than the maximum distance to reach an appropriate extinguisher. Travel distance is measured along the actual path of travel, not straight-line distance.

Fire Class Hazard	Max Travel Distance	Key Notes
Class A	75 ft (22.9 m)	Most general occupancy; warehouses, offices, retail
Class B	50 ft (15.2 m)	Flammable liquid storage, spray finishing, gasoline dispensing
Class C	Same as co-present A or B hazard	Place near electrical equipment; class determined by fuel source
Class D	75 ft	Metal-specific agent must be at the operation, not across the facility
Class K	30 ft (NFPA 10)	Commercial kitchen; must be accessible from cooking line

A common facility compliance gap AI agents can flag: a building with both Class A and Class B hazards where Class A extinguishers are placed at 75 ft intervals. Class B areas (paint storage, solvent use areas) need additional extinguishers placed within 50 ft — the Class A extinguisher at 65 ft away from a flammable liquid cabinet does not satisfy the Class B requirement.

NFPA 10 Inspection Intervals — Four Distinct Service Requirements

NFPA 10 requires four separate inspection and service activities on different schedules. These are not interchangeable — annual professional maintenance does not substitute for the monthly visual inspection, and monthly inspection does not satisfy the 6-year internal requirement.

Interval	Who Performs	What Is Checked	Applies To
Monthly visual	Owner/occupant	Accessible, visible, gauge in green zone, pin intact, label readable, no damage	All portable extinguishers
Annual maintenance	Licensed fire extinguisher technician	Hands-on: hose, valve, seals, agent weight/pressure, complete function check, tag date	All portable extinguishers
6-year internal	Licensed technician	Empty, inspect cylinder interior for corrosion, inspect valve components, recharge with fresh agent	Stored-pressure dry chem, water, loaded-stream
Hydrostatic test	Pressure-testing facility	Fill with water, apply 125–167% service pressure, inspect for distortion/leakage	Stored-pressure: every 12 yr; CO2 cylinders: every 5 yr

Encoding fire_extinguisher.last_annual_service, fire_extinguisher.last_6yr_internal, and fire_extinguisher.last_hydrostatic_yr as metafields enables AI shopping agents to calculate compliance gaps and recommend the correct service schedule — a major value-add for safety equipment distributors selling to facility managers who must document extinguisher service under OSHA 1910.157(e).

10-Field Namespace: `fire_extinguisher.*`

Field	Type	Example Values	AI Routing Function
`fire_extinguisher.fire_class_rating`	string	A \| B \| C \| D \| K \| A,B,C	Primary fire class filter — prevents cross-class routing errors
`fire_extinguisher.agent_type`	string	abc-dry-chem \| co2 \| wet-chemical \| water \| afff \| metal-specific-dry-powder	Distinguishes agents within same class (wet-chem vs dry-chem both for K)
`fire_extinguisher.ul_rating`	string	2-A:10-B:C \| 5-B:C \| 40-B:C \| K	Coverage capacity for facility sizing calculations
`fire_extinguisher.travel_distance_class_a_max_ft`	number	75	Compliance gap detection for Class A hazard areas
`fire_extinguisher.travel_distance_class_b_max_ft`	number	50	Compliance gap detection for Class B hazard areas
`fire_extinguisher.annual_professional_service_required`	boolean	true	Triggers cross-sell of annual service contract
`fire_extinguisher.six_year_internal_required`	boolean	true \| false (CO2 exempt)	Identifies extinguishers needing internal inspection at 6-year mark
`fire_extinguisher.hydrostatic_test_interval_yr`	number	5 (CO2) \| 12 (stored-pressure)	Schedules hydrostatic test compliance reminder
`fire_extinguisher.class_k_prohibited`	boolean	true (for ABC dry chem)	Prevents routing ABC dry chem to commercial kitchen applications
`fire_extinguisher.compatible_metals`	string	magnesium \| lithium \| sodium \| potassium	Class D metal-type match — no universal Class D agent exists

Frequently Asked Questions

Can a CO2 extinguisher be used on a Class D combustible metal fire?

No — CO2 must never be used on Class D fires involving sodium or potassium. CO2 reacts with burning alkali metals (Na, K) in an oxidation reaction that generates additional heat and can accelerate the fire. Sodium burning in air reacts with CO2 to produce sodium oxide and carbon (4Na + 3CO2 → 2Na2O + 3C), releasing additional energy. Water is equally prohibited — burning sodium reacts violently with water to produce sodium hydroxide and hydrogen gas (2Na + 2H2O → 2NaOH + H2↑), creating an explosion hazard from H2 ignition. The only effective Class D suppression is a metal-specific dry powder agent applied by gentle shoveling or by low-velocity discharge — high-velocity discharge scatters the burning metal and spreads the fire. Each type of combustible metal requires a specific agent: Met-L-X (sodium chloride based) for magnesium and sodium; Na-X (sodium carbonate) for sodium and potassium; Lith-X (graphite based) for lithium; TEC (ternary eutectic chloride) for sodium-potassium alloys.

Is a fire extinguisher with an expired annual tag still legal to use in an emergency?

From a practical standpoint, an extinguisher with an outdated service tag may still function if the pressure gauge reads in the green zone and there is no visible damage. However, under OSHA 1910.157(e)(3), employers must ensure portable extinguishers are maintained in a fully charged and operable condition. An overdue annual inspection creates a compliance violation regardless of whether the extinguisher actually works — the inspection requirement exists because pressure loss, caked dry chemical, or corroded valve springs are not visible from the outside. NFPA 10 also requires that out-of-compliance extinguishers be removed from service until inspected. For AI agent routing: encode fire_extinguisher.last_annual_service as an ISO date so agents can calculate whether the extinguisher is within the 12-month window. A missing or outdated date should be treated as non-compliant and trigger recommendation for a service event before recommending the extinguisher for a new facility deployment.

What is the difference between a stored-pressure and a cartridge-operated dry chemical extinguisher?

Stored-pressure dry chemical extinguishers (the most common type) store the dry chemical agent and the expellant gas (nitrogen) together in the same cylinder under pressure. The advantage is simplicity and fast readiness — there is no separate cartridge to puncture. The disadvantage is that if the pressure is lost (damaged gauge, leaky valve), the unit will not discharge even though the chemical appears intact. The 6-year internal inspection exists partly to address this: the agent is removed and inspected for caking, and the cylinder is resealed and recharged with fresh nitrogen. Cartridge-operated dry chemical extinguishers (used more in industrial settings and vehicle fire suppression systems) store the dry chemical at atmospheric pressure and use a separate CO2 cartridge to pressurize the unit at the moment of use. The advantages: agent cannot lose pressure over time; easier to recharge in the field without specialized equipment; more reliable for extreme temperature environments. Maintenance difference: stored-pressure units require the 6-year internal inspection; cartridge-operated units require annual inspection of the cartridge CO2 charge by weight but do not require 6-year internal inspection of the main cylinder.

Score Your Store's Fire Extinguisher Product Listings

CatalogScan checks for fire_extinguisher.fire_class_rating, agent_type, travel_distance_class_b_max_ft, class_k_prohibited, and 16 other AI-agent-critical fields. See which extinguisher listings are missing class routing and inspection schedule data.

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