Most of us know the basics of fire safety. We know that wood burns (Class A) and flammable liquids burn (Class B). However, there is a category of fire that is far hotter, far more reactive, and far more dangerous if mishandled.
This is the Class D Fire.
Class D fires involve combustible metals. To the untrained eye, metal does not look like fuel. We build engines and buildings out of it because it is strong and heat-resistant. Yet, under the right conditions, certain metals can ignite and burn with a terrifying intensity.
The most critical thing to know is that using the wrong extinguisher on a metal fire can be fatal. Pouring water on burning magnesium, for example, results in an explosion that can destroy a building.
At Fire Response Ltd, we supply the specialist fire extinguishers required to tackle these rare but deadly hazards. Whether you run a machine shop, a laboratory, or an aerospace facility, understanding Class D risks is essential for your safety compliance.
The Quick Answer
A Class D Fire is a fire involving combustible metals such as Magnesium, Titanium, Potassium, or Lithium. These fires burn at incredibly high temperatures (often exceeding 2,000°C). They require a specialist M28 or L2 Dry Powder extinguisher. Crucially, you must never use water or standard foam extinguishers on a Class D fire. Water reacts with the burning metal to create Hydrogen gas, leading to an immediate and massive explosion.
What Metals Can Burn?
It is rare for a solid block of steel to catch fire. Class D fires usually occur when the metal is in a specific form: shavings, dust, or “swarf” created during machining processes. When the surface area is high and the mass is low, these metals become highly flammable.
The most common culprits in the UK industry are:
1. Magnesium
Used in lightweight car wheels (alloy wheels), aerospace components, and pyrotechnics. It burns with a blinding, brilliant white light.
2. Lithium
Found in pure form in batteries and chemical labs. Note that while Lithium-Ion battery fires are a major risk, pure Lithium metal fires are distinct and fall strictly under Class D.
3. Titanium
Used in high-performance engineering and medical implants. Titanium dust from grinding is highly explosive.
4. Aluminium
While your drinks can not burn, fine aluminium powder or swarf from a lathe is highly combustible and acts as an accelerant. This is why “good housekeeping” in factories is a critical fire prevention measure.
The Chemistry of Danger: Why Water Explodes
Why are we so terrified of putting water on a metal fire?
It comes down to simple chemistry. At the extreme temperatures of a Class D fire (often 2000°C+), water molecules (H2O) break apart instantly. The oxygen feeds the fire, making it burn hotter. The hydrogen is released as a gas, which is highly explosive.
Essentially, by adding water, you are adding rocket fuel. This reaction is violent and instantaneous, capable of blowing out windows and causing severe injury to the operator. This is why standard office extinguishers (Water, Foam, or CO2) are strictly forbidden in areas with metal risks.
How to Extinguish a Class D Fire
You need a specialised tool for this job. You cannot simply cool the fire down; you have to smother it.
The solution is a Specialist Powder Extinguisher. These usually come in a distinct canister with a low-velocity applicator (a long lance) to allow the user to stand back and apply the powder gently without blowing the burning metal dust into the air.
There are two main types of powder:
1. M28 Powder (General Purpose)
This is typically sodium chloride-based (salt).
How it works: The heat of the fire melts the powder. It fuses to form a crust or a “skin” over the burning metal. This skin cuts off the oxygen supply (smothering) and absorbs the heat.
Use: Effective on Magnesium, Sodium, and Aluminium swarf.
2. L2 Powder (Lithium Specialist)
This contains graphite or copper compounds.
How it works: It acts as a heat sink to cool the metal down while also smothering it.
Use: specifically designed for Lithium metal fires, which burn even hotter than magnesium.
Identifying a Class D Hazard Zone
If you are a landlord or a health and safety manager, you need to identify if your premises are at risk. Class D hazards are most commonly found in:
Laboratories: Storing pure alkali metals for research.
Engineering Workshops: Where CNC machines create piles of metal shavings.
Automotive Factories: Handling alloy components.
Aerospace Facilities: Working with titanium and aluminium airframes.
Under the Regulatory Reform (Fire Safety) Order 2005, if you have these materials on site, you have a legal obligation to provide the correct extinguisher. Relying on a standard red water extinguisher in a magnesium workshop would be considered a severe breach of safety regulations.
Safety Training is Vital
Having the right blue or yellow labelled extinguisher on the wall is only half the battle. Your staff must know how to use it.
Fighting a metal fire is terrifying. The light is blinding, and the heat is intense. If an untrained employee panics and grabs the nearest water hose, the results could be catastrophic. At Fire Response Ltd, we recommend specific Fire Warden Training for any staff working in industrial environments. They need to know to:
Sound the alarm immediately.
Select the M28 or L2 extinguisher.
Apply the powder gently to avoid spreading the burning debris.
Leave the crust alone to cool for hours (metal fires can reignite easily).
Conclusion
Class D fires are the heavyweights of the fire world. They are rare, but they are unforgiving.
If your business involves machining, storing, or transporting combustible metals, you cannot rely on standard fire safety advice. You need a specialist approach. By ensuring you have the correct Class D extinguishers and, more importantly, that your staff know exactly what not to do, you can protect your facility from disaster.
If you are unsure about the specific risks in your workshop, contact Fire Response UK. Our engineers can assess your materials and recommend the exact suppression system required to keep your business safe.
