Cable insulation and sheathing protect conductors throughout everyday operation. When exposed to fire, however, these materials also influence how much smoke develops, which gases are released and how flames spread.

These characteristics matter in metro tunnels, high-rise buildings, data centers and other enclosed spaces where smoke removal or evacuation can be difficult. They also deserve attention in railway and marine transport, public buildings and critical infrastructure.

Low smoke zero halogen, usually abbreviated to LSZH, describes specified smoke and halogen-related characteristics. Understanding what the term means-and how it relates to XLPO and XLPE-helps buyers move from a material label to a suitable cable specification.

01 / What does "low smoke zero halogen" mean?

LSZH combines two distinct requirements.

Low smoke concerns the smoke produced under defined burning conditions. Smoke can obscure escape routes and make emergency response more difficult. Cable smoke testing evaluates this behavior under controlled conditions; it does not mean that a cable produces no smoke in every fire.

Zero halogen concerns halogens in the material system and the applicable test requirements. Halogens include fluorine, chlorine and bromine. PVC, for example, contains chlorine as part of its polymer structure.

Halogen-free formulations help avoid particular sources of halogen acid gases during combustion. However, "zero halogen" should be interpreted against specified test methods and limits. It does not mean that all combustion gases are harmless.

LSZH, LSOH and LS0H are common market abbreviations. The wording alone is insufficient to establish a product's performance: the relevant specification and supporting test evidence remain essential.

02 / How are LSZH cable compounds made?

An LSZH compound is not one fixed plastic recipe. It is an engineered combination of a base polymer, a flame-retardant system and other additives.

Polyolefin-based formulations are common. The polymer or polymer blend provides the foundation for properties such as flexibility, electrical insulation and mechanical strength. Additives then adjust the compound for its intended role.

Aluminum hydroxide, also known as alumina trihydrate or ATH, and magnesium hydroxide, or MDH, are established mineral flame retardants used in halogen-free cable compounds. When heated sufficiently, these materials absorb heat as they decompose and release water vapor. Their mineral residues can also contribute to protective layers, depending on the formulation. Technical reference: mineral flame retardants in cable compounds.

The challenge is balancing these effects with other requirements. Filler loading and dispersion influence flexibility, tensile properties and processing. Stabilizers and other additives must also suit the expected service environment.

Consequently, two compounds described as LSZH may differ considerably in handling, durability and application suitability.

03 / How does LSZH relate to XLPO and XLPE?

These terms describe different aspects of a material.

Polyethylene belongs to the polyolefin family, so XLPE falls within the broader XLPO classification. In commercial specifications, however, XLPO may describe blends or copolymer systems that should not automatically be renamed XLPE.

A compound can be both XLPO and LSZH. Crosslinking describes its polymer structure; LSZH describes other characteristics that must be established through the appropriate evaluation. Our article on XLPO in solar cables explains the material family and the role of crosslinking in more detail.

Similarly, XLPE insulation does not by itself establish that a finished cable is LSZH. An XLPE-insulated cable may have a PVC sheath, while another may use a halogen-free sheathing system. Fillers, tapes and other non-metallic components also matter. For the electrical and thermal role of the insulation layer, see XLPE insulation in AC power cables.

04 / Four different fire-performance properties

Low smoke, halogen-free, flame-retardant and fire-resistant descriptions are often grouped together, but they answer different questions.

For example, IEC 60332-1-2 evaluates vertical flame propagation on a single insulated conductor or cable. Passing this test does not establish continued electrical operation during a fire.

Circuit integrity is a separate subject. Tests such as IEC 60331-1 assess cables required to maintain circuit integrity when exposed to fire and mechanical shock under specified conditions.

05 / Where are LSZH requirements especially relevant?

LSZH requirements are particularly relevant where smoke accumulation, difficult evacuation, equipment sensitivity or the consequences of service interruption increase the importance of cable fire performance. The exact requirements depend on the location, circuit function, applicable rules and project specification.

Metro systems, high-rise buildings and enclosed routes

Underground stations, metro tunnels, enclosed corridors and cable shafts can present difficult smoke-management conditions. High-rise buildings add long evacuation routes and vertical service spaces.

These characteristics make smoke generation, combustion gases and flame propagation important specification topics. Railway cable guidance also distinguishes the demands of tunnels and stations from those of open-air installations. Railway infrastructure reference.

Data centers and 5G supporting infrastructure

Data centers contain concentrated cable installations and sensitive electronic equipment. Smoke and corrosive combustion products can extend damage beyond the immediate fire area, making cable material selection relevant to both personnel protection and equipment resilience. Data-center cable reference.

Similar considerations can apply to 5G equipment rooms, indoor telecommunications distribution and associated power supplies. The installation environment determines the requirements; the presence of 5G technology alone does not define a cable fire classification.

Data centers generally have substantial cooling systems, but cooling airflow should not be confused with effective smoke extraction during a fire.

Railway and marine transport

Passenger trains and ships combine enclosed spaces with potentially constrained evacuation. Cable selection may therefore address smoke, combustion-gas characteristics and flame propagation alongside vibration, moisture, oil exposure and mechanical durability.

Railway infrastructure, rolling stock and shipboard installations have different qualification requirements. A general-purpose LSZH cable should not be assumed suitable for all three.

Public buildings and densely occupied spaces

Hospitals, schools, airports, shopping centers and other public buildings bring together large numbers of people, including occupants who may need assistance evacuating.

LSZH requirements can form part of the fire-safety specification for these environments. Where cables serve emergency systems, the required circuit-integrity performance must also be identified separately.

Nuclear facilities and mines

Nuclear facilities and underground mines can also have project-specific low-smoke and halogen-free requirements. These environments demand broader qualification: depending on the installation, considerations may include radiation, environmental aging, moisture, mechanical damage or hazardous atmospheres.

LSZH is therefore one part of cable suitability, rather than a complete qualification for nuclear or mining service.

06 / How is performance verified?

Different tests address different claims.

The scope of each method matters. IEC 61034-2 addresses smoke density, while IEC 60754-1 and IEC 60754-2 address different combustion-gas characteristics. None alone establishes every aspect of fire performance.

07 / Applying the material knowledge to AC cables

In a low-voltage power cable, insulation provides the electrical barrier around the conductor, while the sheath protects the underlying construction. These layers can use different materials to meet their respective requirements.

When selecting a variant, confirm the rated voltage, conductor size, installation conditions and required fire-test performance. Specialist transport, nuclear, mining or emergency-circuit applications require evidence specific to those uses.

08 / Frequently asked questions

Is every XLPE cable LSZH?

No. XLPE identifies the insulation material. The sheath and other components must also be considered.

Can a material be both XLPO and LSZH?

Yes. The terms describe different characteristics and can apply to the same compound when supported by the relevant evidence.

Does LSZH mean a cable cannot burn?

No. LSZH does not mean non-combustible. Flame propagation and circuit integrity require separate evaluation.

Is an LSZH sheath enough to classify the complete cable?

A sheath description alone is insufficient. Complete-cable claims should be supported by evidence covering the relevant materials, construction and tests.