LSZH vs FRLS Cable: Fire Performance, Smoke Toxicity & Where You Use Them

When specifying fire-safe cables for public buildings, tunnels, metro stations, solar carports or densely populated facilities, electrical contractors and project engineers often run into two popular low-fire cable grades: LSZH and FRLS. On the surface, both sound like “low smoke cables”, but their definitions, test criteria, smoke toxicity, flame spread limits and mandatory use scenarios are not interchangeable. Choosing the wrong one can lead to failed local inspections, rejected SEC approvals in Chile, non-compliance with IEC specifications, or worse, safety risks during a real fire event.
Many buyers mix them up: some treat FRLS as equivalent to LSZH, while others assume LSZH automatically means flame retardant. Neither assumption is fully correct.
In this guide from CHUNBIN CABLE, we clearly separate LSZH and FRLS cables. We will cover core definitions, fire test requirements, smoke density, halogen content, toxicity, typical standards, and which cable you must select for different project types across Latin America, Africa and international IEC projects.
Basic Definition of LSZH and FRLS
LSZH stands for Low Smoke Zero Halogen. This refers to the insulation and sheath compound, not automatically a flame-retardant property.
Key characteristics:

• Zero halogen (no chlorine, bromine, fluorine, iodine or astatine) in the polymer formulation.
• When burned, it releases very low smoke density and no corrosive halogen acid gas.
• Halogen acid is dangerous because it forms hydrochloric acid when mixed with fire water; it damages electronics, corrodes structural steel and causes severe respiratory injury.
• Important note: LSZH only describes smoke and halogen emission. A raw LSZH compound may or may not be flame retardant. You must check if it also carries FR (flame retardant) classification.

Common standards referenced for LSZH: IEC 60754-1 / 60754-2 (halogen acid gas), IEC 61034 (smoke density).
FRLS stands for Flame Retardant Low Smoke. This is a combined performance grade.
Key characteristics:

• The cable compound is formulated to resist flame propagation (flame retardant).
• It produces low smoke under fire conditions, but it is not guaranteed zero halogen.
• This is the biggest difference: FRLS can still contain halogens. Many FRLS compounds use halogen-based flame retardants to achieve flame-retardant ratings.
• Its core design goal: stop fire from spreading along the cable route, while limiting smoke output.

Common standards: IEC 60332 series (flame retardancy), paired with smoke density tests.
Quick core difference: LSZH = No halogen + low smoke (flame retardancy optional) FRLS = Flame retardant + low smoke (halogen may be present)
Technical Comparison

Parameter	LSZH Cable	FRLS Cable
Full Name	Low Smoke Zero Halogen	Flame Retardant Low Smoke
Halogen Content	Zero halogen	May contain halogens
Main focus	Non-corrosive, low toxic fumes	Stop flame spread, low smoke
Flame retardant	Not inherent; must add FR grade	Inherent flame retardant
Smoke density	Very low	Low (usually higher smoke than pure LSZH)
Corrosive gas during fire	None	Possible halogen acid gas
Typical standard tests	IEC 60754-1/-2, IEC 61034	IEC 60332-1 / -3 + smoke test
Cost	Higher material cost	Generally cheaper than FR-LSZH
Key risk if misused	LSZH non-FR version can spread fire	FRLS releases corrosive acid gas in fire

Fire Performance & Smoke Toxicity Breakdown
FRLS cables are engineered first for flame retardancy. Under IEC 60332-3 category A/B/C, bunched cables will self-extinguish within the required time and prevent fire travelling along cable trays. This is critical for vertical risers, cable ducts and multi-cable bundles.
Standard LSZH without FR additive has no flame-retardant capability. If ignited, it can continue burning and propagate fire to adjacent cables. That is why you will often see the combined specification FR-LSZH, meaning flame retardant + low smoke zero halogen. This is the premium grade for high-risk public spaces.
Both are “low smoke” compared to conventional PVC cables. PVC burns with dense, black smoke that quickly obscures escape routes.

• LSZH: Smoke density is the lowest of the three. Visibility remains better for evacuation.
• FRLS: Low smoke, but many formulations produce thicker smoke than LSZH, especially halogenated FRLS grades.

Smoke density is measured via IEC 61034: light transmission percentage inside a closed smoke chamber. Higher light transmission = safer for evacuation.

Toxicity & Corrosivity

This is the decisive factor for emergency evacuation spaces. When PVC or halogenated FRLS burns, halogen gases combine with moisture in lungs and air to form acid vapours. These vapours:

1. Irritate and damage the respiratory tract;
2. Corrode CCTV, fire alarm panels, emergency lighting and communication systems;
3. Destroy steel supports and electrical equipment.

LSZH, by definition, cannot release halogen acid gas. Toxicity is limited mainly to carbon monoxide, same as most polymer combustion, but without corrosive acid.
For tunnel, metro, underground car park and hospital projects, many national codes explicitly ban halogen-containing cables for this reason
Where You Use LSZH / FR-LSZH, and Where FRLS Works

Mandatory LSZH / FR-LSZH Zones (Zero Halogen Required)

• Underground tunnels, road tunnels, railway and metro stations
• Hospitals, schools, shopping malls, theatres and stadiums
• Elevator shafts, emergency exit routes, escape stairwells
• Data centres, telecom rooms, fire control panels
• Marine vessels, offshore platforms
• Chile SEC high-occupancy public buildings, many Latin American public infrastructure bids
• Battery energy storage systems (BESS), indoor solar inverter rooms

The priority here: preserve visibility for evacuation and protect life safety & critical electronic systems from corrosive fumes.

Suitable Applications for FRLS Cables

• General commercial wiring where zero halogen is not specified
• Industrial workshops, factory power circuits
• Outdoor cable trenches, overhead auxiliary cables
• Non-public utility buildings, warehouses with limited occupancy
• Low-budget residential complexes where local codes accept FRLS

Important reminder: Always cross-check with your project bill of quantities and local electrical code. Some African standards and Chile SEC project documents explicitly separate FRLS and LSZH requirements.

Conclusion

LSZH and FRLS both belong to low-smoke cable families, but they solve different fire safety problems. FRLS is built to stop fire spreading, at a moderate cost, but it may release corrosive halogen gases. LSZH (especially FR-LSZH) eliminates halogen acid gas and delivers minimal smoke, which is essential for occupied evacuation routes and critical infrastructure, though it carries a higher material cost.
The safest specification workflow is: match cable grade to local code and project risk level, rather than choosing based only on price. Mixing these two cable types incorrectly can lead to compliance failure and safety liability for contractors.