Class D Fire Extinguishers Explained: Metal Fires, Risks, and UK Safety Responsibilities

Class D fires are among the most misunderstood fire types. Unlike ordinary fires involving wood, paper, or flammable liquids, metal fires behave unpredictably, burn at extreme temperatures, and can react violently with water or incorrect extinguishing agents.

This guide explains what a Class D fire is, how Class D fire extinguishers work, where they are required, and the legal and safety responsibilities that apply in the UK.

What Is a Class D Fire?

A Class D fire involves combustible metals. These metals do not burn in the same way as common materials and often require specialist fire-fighting techniques.

Typical combustible metals include:

• Magnesium
• Sodium
• Potassium
• Lithium
• Aluminium dust or powder
• Titanium
• Zirconium

When ignited, these metals can burn at extremely high temperatures and may react chemically with air, water, or conventional extinguishing agents. In many cases, applying the wrong fire extinguisher can intensify the fire or cause an explosion.

What Is a Class D Fire Extinguisher

A Class D fire extinguisher is:

• Specifically engineered for metal fires
• Filled with a dry powder agent formulated for particular metals
• Intended for controlled, trained use in industrial environments

A Class D fire extinguisher is NOT suitable for:

• Wood, paper, or textiles (Class A fires)
• Flammable liquids (Class B fires)
• Flammable gases (Class C fires)
• Electrical fires
• Cooking oils and fats (Class F fires)

Using the wrong extinguisher on a metal fire can result in violent reactions, metal splatter, or explosive escalation.

How Class D Fire Extinguishers Work

Class D extinguishers do not smother flames in the same way as water, foam, or CO₂.

They work by:

• Forming a heat-absorbing crust over the burning metal
• Excluding oxygen from the reaction
• Preventing metal vapours from sustaining combustion

There is no universal Class D extinguishing agent. Different metals require different powders, which is why correct risk identification is essential.

Why Water, Foam, and CO₂ Must NEVER Be Used on Metal Fires

This is critical safety information.

Water, foam, and CO₂ extinguishers:

• Can react chemically with burning metals
• May release hydrogen gas, significantly increasing explosion risk
• Can cause molten metal to splatter violently, spreading the fire

Examples:

• Burning sodium + water can result in an explosive reaction
• Burning magnesium can strip oxygen from water itself, allowing the fire to continue

For this reason, attempting to use conventional extinguishers on a suspected metal fire is extremely dangerous.

Where Class D Fire Extinguishers Are Required

Class D fire extinguishers are not general-use fire safety equipment. They are required only where there is a known combustible metal fire risk, such as:

• Engineering and machining workshops
• Laboratories handling reactive metals
• Foundries and metal processing plants
• Battery manufacturing or storage facilities
• Aerospace and defence manufacturing

General offices, schools, retail premises, and domestic properties do not normally require Class D extinguishers.

Employer Responsibilities in the UK

Under UK fire safety legislation, employers and duty holders must take a risk-based approach.

Where combustible metals are present, employers must:

• Identify metal fire risks within the fire risk assessment

• Provide the correct Class D fire extinguisher for the specific metal involved

• Ensure extinguishers are:

  • Clearly labelled

  • Easily accessible

  • Maintained by a competent person

Uncontrolled intervention is a recognised cause of serious injury in metal fire incidents.

Identification and Appearance of Class D Fire Extinguishers

In the UK, Class D fire extinguishers:

1. Often features yellow labelling in accordance with BS EN 3
2. May resemble dry powder extinguishers but are clearly marked “Class D”
3. Are commonly supplied as cartridge-operated or scoop-type systems for controlled application