🚿 Fire Suppression Terminology Glossary: 55 Essential Terms Explained

Why These Terms Matter

Fire suppression engineering draws its vocabulary from NFPA standards, insurance underwriting practice, and decades of fire dynamics research. A fire protection engineer, sprinkler designer, or AHJ (authority having jurisdiction) uses these terms with precise, code-defined meanings that determine hydraulic design criteria, agent selection, and life-safety outcomes. Getting a term wrong — confusing "density" with "flow," or "dry pipe" with "preaction" — can lead to an under-designed system, a failed plan review, or worse, a system that does not perform as intended during an actual fire.

This glossary covers 55 of the most important terms spanning water-based sprinkler systems, standpipes, fire pumps, clean agent and foam suppression, kitchen hood protection, water mist, and core fire dynamics concepts. Terms are organized alphabetically with the governing NFPA standard referenced wherever applicable.

A

Actuation Device — NFPA 17/17A

The component (fusible link, heat detector, or manual pull station) that initiates discharge of a fire suppression system. In kitchen hood systems, actuation is most commonly a fusible link rated to melt at a specific temperature, releasing a mechanical linkage that trips the cylinder valve.

AFFF (Aqueous Film-Forming Foam) — NFPA 11

A synthetic foam concentrate that spreads a thin aqueous film across the surface of a hydrocarbon fuel, suppressing vapor release and cooling the fuel. AFFF has historically contained PFAS (per- and polyfluoroalkyl substances) and is being phased out in many jurisdictions in favor of fluorine-free foam (F3) formulations.

Alarm Check Valve — NFPA 13

A check valve installed in a wet-pipe sprinkler riser that permits water flow in one direction (toward the sprinklers) while diverting a portion of that flow to a waterflow alarm device (water motor gong or electric pressure switch) when a sprinkler activates.

Antifreeze System — NFPA 13

A wet-pipe sprinkler system in which listed antifreeze solution, rather than plain water, fills piping subject to freezing. Following NFPA 13's post-2011 requirements, only pre-mixed, factory-listed antifreeze solutions at listed concentrations are permitted — site-mixed glycerin or propylene glycol solutions are no longer allowed due to documented fire tests showing they can ignite and accelerate fire spread.

Area of Sprinkler Operation — NFPA 13

The design area used for hydraulic calculations — the portion of the sprinkler system, expressed in square feet, assumed to operate simultaneously in a fire, against which the design density is applied. Larger design areas are required for higher-hazard occupancies (e.g., 2,500 sq ft for Extra Hazard vs. 1,500 sq ft for Ordinary Hazard).

ASHRAE

American Society of Heating, Refrigerating and Air-Conditioning Engineers — while primarily an HVAC standards body, ASHRAE data (occupied space heat release, smoke control calculations) frequently intersects with fire dynamics analysis used in fire suppression and smoke management design.

B

Backflow Preventer — NFPA 13, 24

A device installed on the water supply connection to a fire protection system that prevents contaminated water in the fire system from flowing back into the potable water supply. Double check detector assemblies (DCDA) and reduced pressure zone (RPZ) assemblies are the most common types used on fire sprinkler services.

Backup Battery / Standby Power — NFPA 20

An alternate power source (batteries, generator) required for electric fire pump controllers to ensure pump operation continues during a normal power outage, per NFPA 20's requirements for a reliable power supply.

Bourdon Gauge — NFPA 25

A pressure gauge using a curved, flattened tube that straightens under internal pressure to drive a needle indicator. Used on sprinkler system risers, fire pump discharge/suction, and standpipe outlets to display system pressure for inspection and testing per NFPA 25.

Branch Line — NFPA 13

The piping in a sprinkler system that directly supplies sprinklers, typically fed from a cross main. Branch lines are the smallest-diameter pipe in the hydraulic tree, sized based on the number and spacing of sprinklers they serve.

Bulb-Type Sprinkler — NFPA 13

A sprinkler head that uses a liquid-filled glass bulb as its thermal-sensitive element. The bulb shatters at its rated temperature (e.g., 155°F/68°C for standard-response ordinary temperature), releasing the cap and allowing water discharge. Distinguished from fusible-element (soldered link) sprinklers by faster response and cleaner activation.

C

Cross Main — NFPA 13

The pipe supplying branch lines, either directly or through a feed main, typically the largest pipe in a sprinkler zone after the riser itself. Cross mains run perpendicular to branch lines in a typical gridded or tree sprinkler layout.

Clean Agent — NFPA 2001

An electrically nonconducting, volatile, or gaseous fire extinguishant that does not leave a residue upon evaporation. Clean agents include halocarbon agents (FM-200/HFC-227ea, Novec 1230) and inert gas agents (IG-541, IG-100, IG-55) — used to protect data centers, archives, and electrical rooms where water damage or residue from dry chemical agents would be unacceptable.

Concentration (Design) — NFPA 2001

The percentage, by volume, of clean agent required in a protected space's atmosphere to extinguish a specific fuel's fire, typically expressed with a safety margin above the minimum extinguishing concentration determined by cup burner testing.

Cup Burner Test — NFPA 2001

A standardized laboratory test method used to determine the minimum extinguishing concentration of a gaseous clean agent against a specific fuel, forming the technical basis for design concentration requirements in NFPA 2001.

Cylinder (Agent Storage) — NFPA 12, 2001

A pressure vessel used to store clean agent, CO2, or dry chemical extinguishing agent under pressure prior to release. Cylinders are sized and quantified based on the hazard's agent concentration requirement and enclosure volume.

D

Deflagration

A combustion process that propagates through a fuel-air mixture at subsonic speed, driven by heat transfer, as opposed to detonation which propagates via shockwave at supersonic speed. Most building fires and dust explosions involve deflagration; understanding this distinction matters for explosion venting and suppression system design.

Density (Design Density) — NFPA 13

The rate of water application, expressed in gallons per minute per square foot (gpm/sq ft), required over the design area of a sprinkler system. Density requirements increase with hazard classification: Light Hazard is typically 0.10 gpm/sq ft, Ordinary Hazard Group 2 is 0.20 gpm/sq ft, and Extra Hazard can exceed 0.40 gpm/sq ft.

Density/Area Method — NFPA 13

The primary hydraulic design methodology for sprinkler systems, in which a required density (gpm/sq ft) is applied uniformly across a specified design area, with the density/area curve for the occupancy hazard classification defining acceptable combinations.

Detector Check Valve

A check valve assembly with a bypass metered flow connection used to detect small, continuous water usage (indicating a leak or unauthorized tap) on a combined domestic/fire water service, distinguishing it from the larger flow associated with sprinkler activation.

Discharge Time — NFPA 2001, 12

The maximum permitted time for a clean agent or CO2 system to reach its design concentration after actuation, typically 10 seconds for halocarbon agents and up to 60 seconds for inert gas agents in NFPA 2001, chosen to outpace fire growth while limiting turbulence and agent loss through openings.

Double Interlock (Preaction) — NFPA 13

A preaction sprinkler system configuration requiring both a fire detection signal (from smoke or heat detectors) AND sprinkler head activation (fusible element melting) before water is admitted to the piping. Provides the highest level of protection against accidental water discharge, used for freezer warehouses and sensitive equipment spaces.

Dry Chemical Agent — NFPA 17

A fire-extinguishing powder (typically sodium bicarbonate, potassium bicarbonate, or monoammonium phosphate) discharged from a pre-engineered system, most commonly used in older commercial kitchen hood systems (largely superseded by wet chemical systems per NFPA 96 for cooking appliance protection).

Dry Pipe System — NFPA 13

A sprinkler system in which piping is filled with pressurized air or nitrogen instead of water, used in unheated spaces (parking garages, freezers, attics) subject to freezing. When a sprinkler activates, air pressure drops, the dry pipe valve trips, and water fills the piping — introducing a water delivery time delay that must be accounted for in system design per NFPA 13's trip time requirements.

E

ERRC (Extended Coverage) Sprinkler — NFPA 13

A sprinkler designed and listed to protect a larger floor area per head than standard-coverage sprinklers — commonly up to 24 ft × 24 ft (576 sq ft) or more — reducing the total number of sprinklers and branch line piping required in a given space.

Extinguishing Concentration — NFPA 2001

The minimum percentage by volume of a clean agent in air required to extinguish a flaming fire involving a specific fuel, as established through cup burner testing; design concentration is set with a safety factor above this value.

Extra Hazard Occupancy — NFPA 13

The highest sprinkler hazard classification, applied to occupancies with high fuel loads or rapid fire development potential — such as flammable liquid processing, plastics manufacturing, or textile mills — requiring the greatest design density and design area among the standard hazard classifications.

F

Fire Department Connection (FDC) — NFPA 13, 14

A siamese-type fitting on the exterior of a building that allows fire apparatus to pump supplemental water into a building's sprinkler or standpipe system, supplementing or substituting for the automatic water supply.

Fire Load

The total quantity of combustible material present in a space, expressed as heat energy released per unit floor area (typically Btu/sq ft or MJ/sq m), used to estimate potential fire severity and to inform hazard classification and structural fire resistance requirements.

Fire Point

The lowest temperature at which a liquid fuel generates sufficient vapor to sustain continuous combustion after ignition (distinguished from flash point, at which vapor ignites but does not sustain burning). Fire point is typically a few degrees above flash point.

Fire Pump — NFPA 20

A pump (electric or diesel driven) dedicated to boosting water pressure and flow for fire protection systems when the available municipal or gravity supply is insufficient. Fire pumps are rated at 100%, 150%, and 200% of rated flow points on their characteristic curve and must be listed and installed per NFPA 20.

Flame Detector

A detection device sensing infrared (IR), ultraviolet (UV), or combined UV/IR radiation characteristic of open flame, used to actuate suppression systems rapidly in high-hazard applications (aircraft hangars, turbine enclosures) where response speed is critical.

Flame Spread Index — ASTM E84

A numerical rating derived from the Steiner tunnel test describing how quickly flame spreads across a material's surface relative to red oak (rated 100), used in building and fire codes to classify interior finish materials.

Flashover

The near-simultaneous transition to full-room involvement in fire, in which radiant heat from a growing fire ignites all exposed combustible surfaces in a compartment nearly instantaneously. Flashover typically occurs when upper-layer gas temperatures reach approximately 1,100°F (600°C) and represents a critical threshold in fire dynamics and life-safety egress timing.

Foam Concentrate — NFPA 11

The undiluted foaming agent stored for a foam suppression system, mixed with water at a specified proportioning rate (commonly 1%, 3%, or 6%) to form foam solution before being aerated into finished foam at the discharge device.

Foam Proportioner — NFPA 11

A device that meters and mixes foam concentrate into the water stream at the correct ratio, using methods such as in-line eductors, balanced pressure pumps, or bladder tanks, to produce foam solution at the design proportioning rate.

Fusible Link — NFPA 17A, 96

A two-piece metal link held together by solder that melts at a rated temperature, used to actuate kitchen hood suppression systems and hold open fire/smoke dampers until a fire condition releases them.

H

Halon — NFPA 2001

A family of halogenated hydrocarbon fire suppression agents (Halon 1301, Halon 1211) that were highly effective clean agents but are ozone-depleting substances banned from new production under the Montreal Protocol. Existing Halon 1301 systems may remain in service and be recharged from recycled stock, but new installations use replacement agents like FM-200 or inert gases.

Heat Release Rate (HRR)

The rate at which a fire releases thermal energy, expressed in kilowatts (kW) or Btu/second, a fundamental parameter in fire dynamics used to model smoke production, sprinkler/detector response time, and structural fire exposure — commonly characterized using t-squared fire growth curves (slow, medium, fast, ultra-fast).

Hydraulic Design — NFPA 13

The engineering calculation method for sizing sprinkler system piping to deliver the required density and flow at the required residual pressure to the hydraulically most remote sprinklers, accounting for friction loss (via the Hazen-Williams equation), elevation changes, and available water supply.

I

Igniter (Pilot Flame)

A continuously burning small flame or spark source (as opposed to the main fire source) used in some industrial process hazard analyses and fire suppression test scenarios to represent an ignition source that must be suppressed along with the main fuel fire.

Inert Gas Agent — NFPA 2001

A clean agent (IG-541, IG-100, IG-55, IG-01) composed of nitrogen, argon, and/or carbon dioxide that extinguishes fire by reducing atmospheric oxygen concentration below the level needed to sustain combustion, rather than through chemical flame inhibition. Requires larger storage volumes than halocarbon agents but produces zero ozone depletion and lower global warming potential.

Inspector's Test Connection — NFPA 13, 25

A test valve located at the hydraulically most remote point of a sprinkler system, used to simulate the flow of a single sprinkler activation to verify waterflow alarm operation during acceptance testing and periodic inspection per NFPA 25.

K

K-Factor — NFPA 13

A sprinkler's discharge coefficient relating flow rate to pressure, expressed in the formula Q = K√P, where Q is flow in gpm and P is pressure in psi at the sprinkler. Standard K-factors range from K-5.6 (standard orifice) to K-25.2 (large orifice/ESFR), with higher K-factors delivering more water at lower pressure.

L

Light Hazard Occupancy — NFPA 13

The lowest sprinkler hazard classification, applied to occupancies with low quantities and combustibility of contents — offices, schools, churches, and residential areas — requiring the lowest design density (typically 0.10 gpm/sq ft over 1,500 sq ft).

Lower Explosive Limit (LEL)

The minimum concentration of a flammable gas or vapor in air below which the mixture cannot ignite or explode, a key parameter in gas detection systems used to trigger ventilation or suppression before a combustible atmosphere can form.

N

NFPA 13

The Standard for the Installation of Sprinkler Systems — the foundational document governing water-based fire sprinkler system design, hydraulic calculation, hazard classification, and component listing requirements in the United States.

NFPA 14

The Standard for the Installation of Standpipe and Hose Systems — governs Class I (fire department use, 2.5-inch), Class II (occupant use, 1.5-inch), and Class III (combined) standpipe systems, including required flow rates and pressures at outlets.

NFPA 20

The Standard for the Installation of Stationary Pumps for Fire Protection — governs fire pump selection, sizing, controller requirements, testing, and acceptance for centrifugal and positive displacement fire pumps.

NFPA 25

The Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems — establishes the required frequency and procedures for inspecting sprinkler systems, standpipes, fire pumps, and water storage tanks after initial installation.

NFPA 72

The National Fire Alarm and Signaling Code — while primarily a fire alarm document, NFPA 72 governs the detection and notification components that often interface with and initiate fire suppression system actuation (preaction releasing panels, clean agent releasing panels).

NFPA 96

The Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations — governs the design of kitchen hood exhaust systems and the wet chemical fire suppression systems (per UL 300) that protect them.

NFPA 750

The Standard on Water Mist Fire Protection Systems — governs the design, installation, and testing of water mist systems, which use very fine water droplets to suppress fire through both cooling and localized oxygen displacement while using dramatically less water than conventional sprinklers.

Novec 1230 — NFPA 2001

A fluoroketone clean agent (chemical name FK-5-1-12) with a global warming potential near zero and an atmospheric lifetime of days rather than years, marketed as a low-environmental-impact alternative to HFC-based clean agents like FM-200.

NOAEL/LOAEL — NFPA 2001

No-Observed-Adverse-Effect-Level and Lowest-Observed-Adverse-Effect-Level — toxicological thresholds used to set safe human exposure concentration limits for clean agents in normally occupied spaces, factoring into design concentration and egress time requirements.

O

Ordinary Hazard Occupancy — NFPA 13

A mid-range sprinkler hazard classification divided into Group 1 (light manufacturing, laundries) and Group 2 (higher combustible storage, mercantile), each with progressively higher design density requirements than Light Hazard but lower than Extra Hazard.

Orifice Size — NFPA 13

The diameter of a sprinkler's discharge opening, directly related to its K-factor; larger orifice sprinklers deliver higher flow at a given pressure and are used for high-challenge storage and ESFR applications.

P

Pendant Sprinkler — NFPA 13

A sprinkler installed with its frame arms pointing downward from a ceiling-mounted branch line, discharging water downward against a deflector — the most common sprinkler orientation in finished ceiling spaces.

Preaction System — NFPA 13

A sprinkler system using a preaction valve that holds piping normally dry (or under supervisory air pressure) until a fire detection signal opens the valve, admitting water into the piping in advance of sprinkler activation — used to protect water-sensitive spaces like data centers and museums while retaining automatic sprinkler protection.

Purple K

A potassium bicarbonate-based dry chemical extinguishing agent especially effective on flammable liquid (Class B) fires, commonly used in portable extinguishers and vehicle/aircraft fueling area fixed suppression systems.

Q

Quick Response Sprinkler — NFPA 13

A sprinkler with a fast thermal response element (response time index, or RTI, of 50 or less), designed to activate significantly faster than standard-response sprinklers to control fire in its early growth stage — required in most light hazard occupancies including residential and healthcare spaces.

R

Residual Pressure — NFPA 13, 20, 291

The water pressure remaining in a system while water is flowing, as opposed to static pressure (no flow). Residual pressure at the hydraulically most remote sprinkler must meet or exceed the minimum operating pressure required to achieve design density and K-factor discharge.

Riser — NFPA 13, 14

The vertical supply pipe of a sprinkler or standpipe system that carries water from the ground floor supply connection up through the building, from which each floor's system branches.

Response Time Index (RTI) — NFPA 13

A numerical measure of a sprinkler's thermal sensitivity derived from plunge tests, quantifying how quickly the heat-sensitive element responds to a given gas temperature and velocity. Lower RTI values indicate faster response — standard-response sprinklers have RTI values of roughly 80–350, while quick-response sprinklers are 50 or below.

S

Signaling Line Circuit (SLC) — NFPA 72

An addressable communication circuit connecting individually identifiable detection and initiating devices to a fire alarm or releasing control panel, commonly used in clean agent and preaction releasing systems to identify which specific detector initiated an alarm or discharge sequence.

Standpipe System — NFPA 14

A network of piping installed in a building to deliver water to hose connections at each floor for use by fire department personnel (and in some cases occupants), classified as Class I, II, or III per NFPA 14 based on hose connection size and intended user.

Static Pressure — NFPA 13, 291

The water pressure in a system when no water is flowing, measured as the baseline condition prior to hydrant flow testing or hydraulic calculation, from which residual pressure drop during flow is measured.

Sidewall Sprinkler — NFPA 13

A sprinkler designed for wall-mounted installation, with a deflector that directs water in a fan-shaped pattern out and along the ceiling rather than downward, used where ceiling-mounted piping is impractical, such as in corridors or hotel rooms.

Suppression (vs. Extinguishment)

Suppression refers to sharply reducing a fire's heat release rate and controlling its spread, which may occur before the fire is fully extinguished; sprinkler systems are generally designed to suppress and control a fire (allowing manual extinguishment by responders), while clean agent and total flooding systems are typically designed to fully extinguish a fire.

T

Total Flooding System — NFPA 2001, 12

A fixed fire suppression system that discharges agent to achieve and maintain a uniform extinguishing concentration throughout an enclosed volume, as opposed to a local application system that discharges agent directly onto a specific hazard without regard to the surrounding enclosure's concentration.

Trip Time — NFPA 13

The elapsed time from sprinkler activation to water reaching the sprinkler orifice in a dry pipe or preaction system, a critical performance parameter tested during commissioning since excessive trip time delays fire control in systems without pre-primed water.

t-Squared Fire

A fire growth model in which heat release rate increases proportionally to the square of elapsed time, categorized as slow, medium, fast, or ultra-fast based on the time to reach 1,055 kW (1,000 Btu/s), widely used in fire dynamics simulation and detector/sprinkler response time calculations.

U

Upright Sprinkler — NFPA 13

A sprinkler installed with its frame arms pointing upward, mounted on top of the branch line piping, with a deflector that directs water upward and outward against the ceiling — commonly used in exposed piping applications such as warehouses and parking garages.

UL 300

The Underwriters Laboratories standard covering fire testing of fire extinguishing systems for protection of commercial cooking equipment, establishing the wet chemical agent testing requirements referenced by NFPA 96 for kitchen hood suppression systems.

W

Waterflow Alarm — NFPA 13, 72

A device (vane-type flow switch or pressure switch on an alarm check valve) that detects water movement in a sprinkler system and transmits a signal to the fire alarm system, distinguishing an actual sprinkler activation from a system at rest.

Water Mist System — NFPA 750

A fixed fire suppression system that discharges very fine water droplets (typically under 1,000 microns) at high pressure, extinguishing fire primarily through rapid heat absorption/steam generation and secondary oxygen displacement, using a fraction of the water volume of conventional sprinklers — increasingly used in machinery spaces, historic buildings, and areas with limited water supply.

Wet Chemical Agent — NFPA 96, UL 300

An aqueous potassium-based extinguishing agent used in commercial kitchen hood suppression systems, which both cools the cooking surface and forms a soap-like saponification layer over grease fires to prevent re-ignition — the current standard replacing older dry chemical kitchen systems.

Wet Pipe System — NFPA 13

The simplest and most common sprinkler system type, in which piping is permanently filled with pressurized water, so that sprinkler activation results in immediate water discharge with no trip delay — used wherever ambient temperatures reliably remain above freezing.

Quick Reference Table — 55 Fire Suppression Terms

Term	Primary Reference	Key Point
Actuation Device	NFPA 17/17A	Fusible link, heat detector, or manual pull
AFFF	NFPA 11	Film-forming foam; being phased out for PFAS
Alarm Check Valve	NFPA 13	One-way flow; triggers waterflow alarm
Antifreeze System	NFPA 13	Listed factory pre-mix only since 2011
Area of Sprinkler Operation	NFPA 13	Design area for hydraulic calculation
Backflow Preventer	NFPA 13, 24	Protects potable water from fire system water
Bourdon Gauge	NFPA 25	Standard pressure gauge type
Branch Line	NFPA 13	Smallest pipe feeding sprinklers directly
Bulb-Type Sprinkler	NFPA 13	Glass bulb shatters at rated temp
Cross Main	NFPA 13	Supplies branch lines
Clean Agent	NFPA 2001	Nonconductive, no residue extinguishant
Cup Burner Test	NFPA 2001	Determines minimum extinguishing concentration
Deflagration	Fire dynamics	Subsonic combustion propagation
Density	NFPA 13	gpm/sq ft application rate
Density/Area Method	NFPA 13	Primary hydraulic design method
Discharge Time	NFPA 2001, 12	Time to reach design concentration
Double Interlock Preaction	NFPA 13	Detection + head activation required
Dry Pipe System	NFPA 13	Air-filled; used where freezing risk exists
ESFR Sprinkler	NFPA 13	Early suppression fast response, high-challenge storage
Extinguishing Concentration	NFPA 2001	Minimum agent % to extinguish fuel
Extra Hazard Occupancy	NFPA 13	Highest density/area classification
Fire Department Connection	NFPA 13, 14	Apparatus supplemental water inlet
Fire Load	Fire dynamics	Combustible fuel quantity per area
Fire Point	Fire dynamics	Temp of sustained combustion
Fire Pump	NFPA 20	Boosts pressure/flow for suppression systems
Flame Detector	NFPA 72	IR/UV radiation sensing
Flashover	Fire dynamics	Full-room fire involvement transition
Foam Concentrate	NFPA 11	Undiluted foaming agent
Foam Proportioner	NFPA 11	Meters concentrate into water stream
Fusible Link	NFPA 17A, 96	Solder link; melts to actuate
Halon	NFPA 2001	Banned ozone-depleting legacy agent
Heat Release Rate	Fire dynamics	Fire's thermal energy output rate
Hydraulic Design	NFPA 13	Piping sizing via friction loss calc
Inert Gas Agent	NFPA 2001	Oxygen displacement clean agent
Inspector's Test Connection	NFPA 13, 25	Simulates single-head flow for testing
K-Factor	NFPA 13	Sprinkler discharge coefficient
Light Hazard Occupancy	NFPA 13	Lowest density/area classification
NFPA 13	—	Sprinkler system installation standard
NFPA 14	—	Standpipe and hose system standard
NFPA 20	—	Fire pump installation standard
NFPA 25	—	Inspection, testing, maintenance standard
NFPA 96	—	Commercial cooking fire protection standard
NFPA 750	—	Water mist system standard
Novec 1230	NFPA 2001	Low-GWP fluoroketone clean agent
Ordinary Hazard Occupancy	NFPA 13	Mid-range Group 1/Group 2 classification
Pendant Sprinkler	NFPA 13	Discharges downward from ceiling
Preaction System	NFPA 13	Detection opens valve before head activation
Quick Response Sprinkler	NFPA 13	RTI ≤ 50; fast activation
Residual Pressure	NFPA 13, 20, 291	Pressure while water is flowing
Response Time Index	NFPA 13	Thermal sensitivity measure
Standpipe System	NFPA 14	Class I/II/III hose connections
Total Flooding System	NFPA 2001, 12	Uniform concentration throughout enclosure
Trip Time	NFPA 13	Activation to water arrival delay
Water Mist System	NFPA 750	Fine droplets; low water volume
Wet Chemical Agent	NFPA 96, UL 300	Saponifies grease fires in kitchen hoods
Wet Pipe System	NFPA 13	Permanently water-filled; no trip delay