What is a class C fire? flammable gases, LEL/UEL, the golden rule and the right extinguisher

Quick answer: A class C fire is a fire fuelled by flammable gases including LPG, CNG, natural gas, propane, butane, hydrogen, and acetylene. The fire burns at the point where gas escapes into air. The gas supply is the fuel source. Shutting off that supply is always the first response before any extinguisher.

An LPG fitting in a commercial kitchen had been leaking for 45 minutes. The chef smelled it but assumed it was the pilot light. He reached for a lighter to relight the burner.

The explosion demolished the kitchen and injured three people. The fire that followed was a class C fire but the explosion came first.

That is what separates gas fires from every other fire class. The fuel is invisible. It accumulates silently. And it can detonate before it ever becomes a visible flame.

This guide covers everything you need to respond correctly to a class C fire: the science of when gas ignites, every common fuel and where it appears in India, two completely different scenarios that require two completely different responses, the right extinguisher, and a naming conflict between Indian and US standards that trips up safety professionals worldwide.

For the full overview of all 6 fire classes, see the classes of fire guide.

What is a class C fire? definition

A class C fire is any fire fuelled by a flammable gas including LPG, natural gas, propane, butane, methane, hydrogen, and acetylene where combustion occurs at the point where the gas is released into air.

Under Indian Standard IS 15683:2018, class C is defined as: fires involving flammable gases under pressure including liquefied gases. The same classification applies under European EN 2, Australian AS/NZS 1841, and ISO 3941.

Three facts separate class C fires from every other class:

• The fuel is invisible, you cannot see it pooling or spreading
• The fire is continuously fed from a supply source that no extinguisher can stop
• The pre ignition gas cloud is often more dangerous than the visible fire itself

This is why shutting off the gas supply is not just important, it is the entire response strategy. Everything else is secondary.

The science behind class C fires: LEL, UEL, and the flammable range

A flammable gas does not ignite at any concentration. It only ignites when the gas air mixture falls inside a specific window, bounded by two limits.

LEL (Lower Explosive Limit): the minimum gas concentration in air at which the mixture can ignite. Below LEL, there is not enough fuel. The mixture is too lean.

UEL (Upper Explosive Limit): the maximum concentration at which it can ignite. Above UEL, there is not enough oxygen. The mixture is too rich.

Between LEL and UEL is the flammable range, any ignition source here will trigger a fire or explosion.

LEL and UEL for common class C gases

Hydrogen ignites across 71% of possible air concentrations. LPG ignites across only 7.7%. That difference is why hydrogen is far more hazardous than LPG per unit of leak, as almost any hydrogen air mixture is within the flammable range.

Acetylene’s range of 2.5% to 100% means nearly any mixture of acetylene and air can ignite.

How a leak becomes a fire: step by step

1. Gas leak begins. Concentration is zero, below LEL, no ignition risk yet.
2. Leak continues. Concentration rises toward LEL.
3. Concentration crosses LEL and enters the flammable range.
4. At this point, any ignition source triggers the fire, such as an open flame, a light switch arc, a phone screen activating, or static from a synthetic shirt.
5. If no ignition source is present, the gas continues accumulating. Above UEL, it is temporarily too rich to ignite, but as the cloud edges disperse, those edges re enter the flammable range continuously.

The practical rule: when you smell gas in an enclosed space, do not operate any electrical switch, on or off. Both actions produce an arc. Do not use a phone. Do not strike a match to find the leak. Evacuate and shut off the supply from outside.

Class C fire fuels: gases, their properties, and where they appear in India

LPG: liquefied petroleum gas

LPG is a blend of propane and butane, stored as a liquid under pressure and vaporised at atmospheric pressure for use. With 320+ million domestic connections, LPG is India’s most common class C fire fuel.

The specific danger of LPG is that it is heavier than air. Leaked LPG sinks, settles at floor level, accumulates in drains, basements, and low corners. A small kitchen leak in an enclosed space can reach its LEL of 1.8% within minutes, entirely invisibly.

CNG: compressed natural gas

CNG (primarily methane) fuels vehicles, industrial burners, and city gas networks across Indian cities. Methane is lighter than air, so outdoor leaks disperse upward. Indoor leaks are serious because methane accumulates at ceiling level.

CNG cylinders are stored at 200+ bar. A cylinder involved in fire is a BLEVE (Boiling Liquid Expanding Vapour Explosion) hazard. Cooling pressurised cylinders with water during a class C fire is essential to prevent catastrophic rupture. LEL is 5% with a narrow 10% flammable range.

Natural gas, propane, and butane

Natural gas (methane) supplies industrial burners, hospital boilers, and city gas networks. Propane performs better at low temperatures and dominates commercial LPG applications. Butane is the primary component of domestic LPG cylinders in India. Each has the specific LEL/UEL characteristics shown in the table above.

Hydrogen: the emerging class C hazard

Hydrogen’s flammable range runs from 4% to 75%, the widest of any common industrial gas. Three properties make it particularly hazardous:

• Invisible flame in daylight: a burning hydrogen leak can go undetected visually
• Extreme flammable range: almost any gas air ratio between 4% and 75% ignites
• Hydrogen embrittlement: long term hydrogen exposure degrades steel storage vessels, creating leak pathways

Green hydrogen infrastructure is expanding in India for fuel cells, industrial energy, and refinery desulphurisation. Facilities that did not previously handle hydrogen class C fire risk increasingly do now.

Acetylene: the most dangerous class C gas in industrial use

Acetylene has a flammable range of 2.5% to 100%. Beyond its fire risk, acetylene is chemically unstable when compressed above 1.5 bar and can detonate without any oxidant. Industrial cylinders store it dissolved in acetone to manage this instability.

Any acetylene cylinder involved in a fire represents a detonation risk independent of the normal class C fire hazard. Acetylene is used in oxy acetylene welding and cutting across Indian industry, making it one of the most hazardous class C fuels in routine daily use.

Biogas

Biogas (50-65% methane, with CO2, hydrogen sulphide, and trace gases) is generated at agricultural biogas plants, sewage treatment facilities, and landfill sites, all expanding sectors in India. Its fire behaviour follows methane, but the presence of hydrogen sulphide (H2S) adds an acute toxic hazard at concentrations far below its own LEL of 4%.

Ammonia

Ammonia is a refrigerant in cold storage, food processing, and fertiliser plants across India. LEL 15%, UEL 28%. Its pungent odour gives earlier warning than most gases, but ammonia is acutely toxic at 300 ppm (its IDLH value), which is far below its flammable range. An ammonia class C fire is simultaneously a toxic gas incident and requires specialist DCP formulations and full PPE for responders.

What causes a class C fire?

Equipment failure

Human factors

• Poorly connected LPG fittings, the leading cause of domestic kitchen gas fires in India
• Leaving gas appliances running without ignition (unburned gas accumulation)
• Storing LPG cylinders in enclosed, low lying spaces where heavier than air gas accumulates
• Hot work near gas lines without adequate clearance
• Ignoring odour warnings, mercaptan is added to LPG and CNG as a warning agent at sub LEL concentrations. Dismissing that smell is how silent 45 minute leaks happen.

Why class C fires often start with an explosion

Many class C fire incidents do not begin with a visible flame. An unignited gas accumulation reaches LEL, finds an ignition source, and detonates. The explosion phase happens first. The sustained jet flame at the supply point, the actual class C fire, follows.

This is why explosion damage at a class C fire scene often exceeds the burn damage. The pre ignition gas cloud can detonate throughout an entire room. Understanding this sequence is the foundation of the two scenario response framework below.

The two scenarios you face and why they need different responses

Every class C fire encounter is one of two completely different situations. Responding to one with the other’s protocol can turn a manageable hazard into a fatal one.

Scenario A: unignited gas leak (no visible fire)

Gas is escaping but has not ignited. The gas air mixture is approaching or has entered the flammable range. This is potentially more dangerous than a burning fire, as an unconfined gas cloud can detonate uniformly throughout its entire volume.

Step by step response:

1. Do not operate any electrical switch, on or off. Both actions produce an arc.
2. Do not use a phone, radio, or any electronic device inside the space.
3. Evacuate everyone immediately. Do not stop to collect anything.
4. Ventilate only if it requires no electrical switches. Open manual doors and windows by hand.
5. Locate and shut off the gas supply from outside the hazardous zone.
6. Call emergency services from outside the building.
7. Do not re enter until a calibrated gas detector confirms the space is below LEL.

What NOT to do:

• Use a fire extinguisher, there is no fire to extinguish.
• Use any flame or spark to check for the leak.
• Assume the smell has gone because you can no longer detect it. Olfactory fatigue means your nose adapts, not that the gas has cleared.

Scenario B: burning jet flame (gas ignited at source)

Gas is escaping and burning at the point of release. This is an active class C fire. The flame burns the gas as it escapes, which actually prevents accumulation of an unignited cloud while the flame is burning.

In many industrial and commercial settings, the safest response is to not extinguish the jet flame until the gas supply is shut off. Extinguishing the flame converts Scenario B back into Scenario A, replacing a controlled jet flame with a re accumulating unignited gas cloud.

Step by step response:

1. Shut off the gas supply. This is the first action, always above extinguisher use.
2. Cool any adjacent pressurised cylinders or vessels with water to prevent BLEVE.
3. If shutting off the supply is impossible and the fire is small and contained, a DCP extinguisher can knock back the flame, but only if the supply is cut immediately after.
4. Evacuate non essential personnel.
5. Call emergency services.

What happens if you extinguish without cutting the supply

This is the mechanism behind multiple casualty class C incidents, and it is almost never explained.

When dry powder knocks back a jet flame without the supply being shut off, the gas continues to flow. Within seconds to minutes, the accumulating gas air mixture crosses LEL and enters the flammable range. Any ignition source then re ignites it, including the still warm extinguisher surface, static from the powder cloud, or any electrical equipment in the space.

That re ignition does not produce another jet flame. It produces a deflagration, a pressure wave travelling through the entire accumulated gas volume.

The golden rule: shut off the gas supply before or simultaneously with any extinguisher use. The shutoff valve is the first tool. The extinguisher is always the second.

How to extinguish a class C fire: correct agents

DCP / ABC dry powder: the primary agent

Dry chemical powder (DCP) or ABC dry powder extingui is the correct extinguisher for class C fires. It interrupts the chemical chain reaction in the gas flame, making it effective and fast at knocking down a burning gas jet.

The limitation: DCP does not stop the gas supply. It is a flame knockdown tool, not a supply shutdown tool. On a class C fire, DCP is the second step. The gas shutoff is always the first.

Speciality Geochem manufactures BIS certified ABC powder fire extinguishers used across Indian industrial, defence, and commercial facilities for class C and multi class fire protection.

Clean agent suppression systems

Clean agents (FM200, Novec 1230) suppress class C fires in enclosed spaces through oxygen displacement and chain reaction interruption. They are appropriate for enclosed equipment rooms and clean environments where DCP residue would cause secondary damage. For fixed installations in enclosed class C fire risk zones, Speciality Geochem’s automatic suppression tube system is designed for this application.

The same supply shutdown rule applies in every case.

CO2: why it is not the right choice for class C

CO2 displaces oxygen and temporarily suppresses the flame. But CO2 dissipates rapidly in any open or ventilated space. The gas continues to flow from the supply. When CO2 clears, the gas re accumulates and re ignites. A CO2 fire extinguisher is a supplementary agent in sealed enclosed environments, not the primary response to an open class C fire.

What NOT to use on a class C fire

Class C fires in India: LPG, CNG and the specific risk profile

India’s class C fire risk profile is distinct from most countries.

LPG dependency: 320+ million domestic LPG connections make LPG the most common class C fire fuel in Indian homes, hotels, restaurants, and street food operations.

CNG vehicle fleet: India operates one of the world’s largest CNG vehicle fleets, including buses, auto rickshaws, taxis, and commercial vehicles across Delhi, Mumbai, Pune, Ahmedabad, and dozens of other cities. Enclosed bus depots, refuelling stations, and parking structures are high frequency class C fire risk environments.

Industrial gas use: Chemical plants, pharmaceutical manufacturers, refineries, hospitals (medical oxygen supports combustion), cold storage facilities (ammonia refrigeration), and biogas plants all handle class C fuels at industrial scale.

Under IS 15683:2018, India’s national fire classification standard, class C covers all flammable gas fires, consistent with European EN 2 and ISO 3941. The complete fire classification guide covers IS 15683 alongside NFPA and EN 2 in detail.

“Class C fire” means something different in the US: the naming conflict

This is one of the most practically dangerous confusions in fire safety, and it goes unexplained in almost every resource on this topic.

In the US NFPA system, flammable gas fires are classified as Class B fire, which in NFPA covers both flammable liquids and flammable gases.

The practical consequence: if you are working from a US sourced safety manual and an Indian or European manual simultaneously, always verify which standard the document follows before applying any extinguisher guidance. A “class C extinguisher” in a US manual is rated for electrical fires. That is not the same as what is needed for a class C gas fire under IS 15683 or EN 2.

This matters in multinational facilities, global supply chain safety roles, and any context where both US and Indian or European fire safety documents are in use.

Preventing class C fires: detection, equipment, and protocols

Gas detectors: what the readings actually mean

Gas detectors measure concentration as a percentage of LEL, not as a percentage of air. This distinction is important for interpreting alarm levels correctly.

At a 25% LEL alarm, there is still margin before actual ignition, but that margin can be consumed in seconds with a significant flow rate. A 25% LEL alarm is an evacuation signal, not a “be cautious” signal.

Intrinsically safe equipment in hazardous gas zones

Standard electrical equipment produces arcs during normal operation. In any area where a gas air mixture above LEL may be present, those arcs are ignition sources.

Intrinsically safe equipment is certified so that no arc or thermal event it produces under normal or fault conditions is sufficient to ignite a defined flammable atmosphere. Certification is under ATEX (European) or IECEx (international) standards.

In any classified hazardous zone, LPG filling stations, CNG refuelling stations, gas handling facilities, and chemical plants, only intrinsically safe or explosion proof equipment may be installed. This is a legal requirement under applicable Indian and international standards, not a design preference.

Class C fire prevention checklist

• Have all gas appliances and pipework inspected by a certified gas engineer at least once a year, or more often in high use commercial settings.
• Replace flexible LPG hose connections every 5 years or as per the manufacturer specification, whichever is sooner.
• Never store LPG cylinders in enclosed, low lying spaces. LPG is heavier than air and will accumulate at floor level.
• Store CNG cylinders vertically in well ventilated enclosures away from heat sources.
• Never use an open flame to check for a gas leak. Use soapy water or a calibrated gas detector instead.
• Ensure all gas zone electrical equipment carries ATEX or IECEx certification.
• Establish a written emergency gas leak procedure and train all relevant staff, including cleaning and facilities personnel who often encounter early gas odour warnings first.
• Know the location of every gas shutoff valve in your facility before any emergency occurs. In a gas incident, there is no time to look for it.

Class C fire vs class A fire vs class B fire

Frequently asked questions: class C fires

Q1: What is a class C fire?

A class C fire is any fire where the fuel is a flammable gas. Under Indian Standard IS 15683:2018, European EN 2, and ISO 3941, class C covers fires involving LPG, CNG, natural gas (methane), propane, butane, hydrogen, acetylene, ammonia, and biogas. The fire burns at the point where gas escapes into air. The gas supply continuously fuels the fire until it is shut off, which is why shutting off the supply is always the first response.

Q2: What gases are involved in class C fires?

Class C fire gases include LPG (a propane butane blend), CNG (methane), natural gas (methane), propane, butane, hydrogen, acetylene, ammonia, and biogas. In India, LPG and CNG are the most commonly encountered class C fire fuels. Hydrogen is an emerging class C hazard as green hydrogen infrastructure expands. Acetylene is the most hazardous class C gas by flammable range (2.5% to 100%).

Q3: What is the golden rule for a class C fire?

Shut off the gas supply before or simultaneously with any extinguisher use. If the supply is not shut off, extinguishing the flame creates an unignited gas cloud that can re ignite as a deflagration rather than a controlled jet flame. The shutoff valve is the first tool. The extinguisher is always the second.

Q4: What fire extinguisher is used for class C fires?

DCP (dry chemical powder) or ABC powder is the correct extinguisher for class C fires. It interrupts the chemical chain reaction in the burning gas. Clean agent extinguishers (FM200, Novec 1230) are appropriate in enclosed environments where DCP residue would cause damage. CO2 is not recommended as the primary agent because it dissipates rapidly in ventilated spaces, allowing gas to re accumulate.

Q5: Can you use CO2 on a class C fire?

CO2 can temporarily suppress a class C flame by displacing oxygen, but it is not the correct primary agent. CO2 dissipates rapidly in open or ventilated spaces. The gas continues flowing from the supply. When CO2 clears, the gas re accumulates and re ignites. CO2 is appropriate as a supplementary agent only in fully sealed, enclosed environments.

Q6: Can water extinguish a class C fire?

No. Water does not suppress gas combustion and must not be used on a burning gas jet. On LPG fires involving a liquid condensate component, water can spread the liquid fuel. The one valid use of water in a class C fire scenario is cooling adjacent pressurised cylinders or vessels to prevent BLEVE, not to extinguish the gas flame.

Q7: What is LEL and why does it matter for class C fire safety?

LEL (Lower Explosive Limit) is the minimum concentration of flammable gas in air at which the mixture can ignite. Below LEL, the mix is too lean to burn. Above UEL (Upper Explosive Limit), it is too rich. Between LEL and UEL, the flammable range, any ignition source will trigger a fire or explosion. LPG’s LEL is 1.8%. Methane’s is 5%. Hydrogen’s is 4% with a UEL of 75%, giving a flammable range 10 times wider than LPG.

Q8: What is the difference between a class C fire and an unignited gas leak?

An unignited gas leak is potentially more dangerous than a burning class C fire. A jet flame burns gas as it escapes, concentrating the hazard at the supply point. An unignited gas cloud accumulates throughout the space and detonates uniformly across its entire volume when it finds an ignition source. Respond to an unignited leak with evacuation and supply shutdown. Respond to a burning class C fire with supply shutdown first, then extinguisher use if needed.

Q9: Why should you never extinguish a class C fire without shutting off the gas first?

Extinguishing the flame without shutting off the supply converts a controlled jet flame into an unignited gas cloud. The gas continues to flow, the cloud enters the flammable range, and any ignition source triggers re ignition. That re ignition is not another jet flame. It is a deflagration, a pressure wave propagating through the accumulated gas volume. Multiple casualty class C incidents have followed exactly this sequence.

Q10: What happens if you extinguish a class C fire without shutting off the gas?

The gas continues to flow and mix with air. Within seconds to minutes, the gas air mixture crosses LEL and enters the flammable range. Any ignition source present, including a warm extinguisher surface, static from the powder cloud, or electrical equipment, can trigger re ignition as a deflagration. The resulting pressure wave can cause structural damage and casualties far beyond the original jet flame.

Conclusion

Class C fires are uniquely dangerous for one specific reason: the fuel is invisible, it forms explosive mixtures before it ignites, and it continues feeding the fire from a source that no extinguisher can reach.

The extinguisher is the last tool you reach for in a class C fire. The gas shutoff valve is the first. Knowing exactly where that valve is, before any emergency, is the single most effective class C fire safety action any facility manager, kitchen operator, or site safety officer can take today.

Speciality Geochem manufactures BIS certified ABC powder fire extinguishers used across Indian industrial, defence, and commercial facilities for class C and multi class fire protection. For the complete reference on all fire classes, including how class C fits alongside class A fire , B, D, E, and F under IS 15683, NFPA, and EN 2, see the complete fire classification guide.