Understanding Fire-Performance Cables: Flame-Retardant, Fire-Resistant, and Low-Smoke Zero-Halogen Explained

Understanding Fire-Performance Cables: Flame-Retardant, Fire-Resistant, and Low-Smoke Zero-Halogen Explained

In modern building infrastructure, electrical cables are far more than simple conductors of power—they are critical components in a building’s overall fire safety strategy. While standard cables may function perfectly under normal conditions, they can become significant hazards during a fire, contributing to flame spread, emitting toxic smoke, and failing when power is needed most.

To address these risks, specialized cables have been developed to meet higher safety standards. Among the most commonly specified are flame-retardant, fire-resistant, and low-smoke zero-halogen (LSZH) cables. Though often mentioned together, these terms describe distinct performance characteristics. Understanding the differences is essential for architects, engineers, facility managers, and anyone involved in building safety.

Let’s explore what each type means, how they work, and where they should be used.

Flame-Retardant Cables: Stopping the Fire from Spreading

The primary purpose of a flame-retardant cable is to prevent the spread of fire along the cable itself.

Imagine a bundle of cables running through a vertical shaft or cable tray. If one cable ignites, a non-flame-retardant type could act like a fuse, allowing the fire to travel rapidly between floors. Flame-retardant cables are designed to avoid this scenario.

These cables are manufactured with insulation and jacketing materials—such as PVC or cross-linked polyethylene (XLPE)—that contain flame-retardant additives. When exposed to fire, these additives release substances that suppress combustion, helping the cable self-extinguish once the external flame source is removed.

In practice, flame-retardant cables are tested by bundling multiple cables together and applying a flame. To pass, the fire must not propagate beyond a certain distance, and the cables must stop burning within a specified time after the flame is removed.

Where are they used?  

Flame-retardant cables are widely used in commercial buildings, residential high-rises, and industrial facilities—anywhere building codes require limiting fire propagation. In many regions, they are now considered a baseline requirement for fixed wiring.

Common naming: In many markets, these are labeled with prefixes like "FR" or "ZR" (from Chinese standards), or classified under standards such as IEC 60332 for vertical flame spread.

Fire-Resistant Cables: Keeping Critical Systems Alive

While flame-retardant cables aim to stop fire from spreading, fire-resistant cables have a different mission: to continue operating during a fire.

This is crucial for circuits that power life-saving equipment—such as emergency lighting, fire alarms, smoke extraction systems, and fire pumps. If these systems fail during a fire, evacuation becomes far more dangerous.

Fire-resistant cables achieve this by incorporating a mica tape layer wrapped around the conductor. Mica is a naturally occurring mineral with exceptional thermal stability—it can withstand temperatures above 1,000°C. Even if the outer insulation burns away, the mica layer maintains insulation integrity, allowing current to flow.

These cables are tested under severe conditions: exposed to flames at 750–950°C for 90 minutes (or longer), while remaining energized. They must not short-circuit or lose functionality during the test.

Where are they used?  

Fire-resistant cables are mandatory for emergency and safety circuits in hospitals, airports, tunnels, high-rise buildings, and public assembly spaces. They are not intended for general power distribution, but specifically for systems that must remain operational during an emergency.

Naming conventions: Often marked with "FR" in some regions, but more clearly identified as "Fire Survival" or "Circuit Integrity" cables. In China, the prefix "NH" is commonly used.

Low-Smoke Zero-Halogen (LSZH) Cables: Protecting People and Equipment

One of the deadliest aspects of a fire isn’t the flames—it’s the smoke and toxic gases released by burning materials. Standard PVC cables, when burned, emit dense black smoke and corrosive halogen gases (like hydrochloric acid), which can cause asphyxiation and damage sensitive electronics.

Low-smoke zero-halogen (LSZH) cables are designed to minimize these dangers. As the name suggests, they produce very little smoke and no halogen gases when exposed to fire.

Instead of PVC, LSZH cables use thermoplastic or thermoset compounds based on polyolefins. When burned, these materials release mostly water vapor and carbon dioxide—significantly less toxic than halogenated fumes.

This makes LSZH cables ideal for confined or poorly ventilated spaces, where smoke buildup can block escape routes and toxic gases can quickly become lethal.

Where are they used?  

- Underground stations, tunnels, and subways  

- Aircraft, ships, and mass transit vehicles  

- Data centers and telecommunications facilities (to protect equipment from corrosive gases)  

- Schools, hospitals, and shopping malls  

They are often required by code in public transportation and critical infrastructure projects.

Naming: Known as LSZH, LSZ0H, LSFRO (Low Smoke Free of Halogen, Flame Retardant, Oxygen Index), or under standards like IEC 60754 (halogen gas emission) and IEC 61034 (smoke density).

Can Cables Combine All Three Features?

Yes. In high-risk or high-occupancy environments, the best solution is a cable that integrates all three properties: flame-retardant, fire-resistant, and low-smoke zero-halogen.

For example, a cable might be constructed with:

- LSZH insulation to limit smoke and toxicity

- A mica tape layer for fire resistance

- A flame-retardant jacket to prevent flame propagation

Such cables are often labeled as "Fire Survival LSZH" or "Circuit Integrity LSZH" and are increasingly specified in modern building codes for critical infrastructure.

In Chinese standards, this combination is often denoted as WDZN (W:无卤, D:低烟, Z:阻燃, N:耐火), representing the highest level of fire performance.

Final Thoughts: Safety Is Not Optional

Choosing the right cable is not just about electrical performance—it’s about life safety.

- Use flame-retardant cables to prevent fire from traveling through walls and floors.  

- Use fire-resistant cables to keep emergency systems running when they’re needed most.  

- Use LSZH cables to protect people from smoke and toxic gases, especially in enclosed spaces.  

In many cases, the most responsible choice is a cable that delivers all three benefits.

As building designs become more complex and safety expectations rise, the role of intelligent cable selection grows ever more important. After all, in the event of a fire, the difference between a safe evacuation and a tragedy may come down to the cables hidden in the walls.