Low Smoke Zero Halogen (LSZH) Cable Combustion Testing: Standards, Methods, and Compliance Requireme

Low smoke zero halogen (LSZH/LSOH) cable combustion testing verifies that cables installed in confined and public spaces meet strict requirements for flame retardance, smoke emission, halogen-free gas evolution, and acidity of combustion gases. Accredited third-party laboratories conduct these tests in accordance with IEC, EN, GB/T, ASTM, UL, ISO, and JIS standards to support product certification, regulatory compliance, and fire-safety engineering. This article explains the key test items, sample requirements, test methods, acceptance criteria, and typical applications of LSZH cable combustion testing.

Technical Overview

Low smoke zero halogen (LSZH, also written LSOH or LSHF) cables are constructed with insulation, bedding, and sheathing compounds that contain no intentional halogenated polymers. Instead of PVC, chlorinated polyethylene, or fluoropolymers, these cables typically use polyolefin-based compounds—such as cross-linked polyethylene (XLPE), ethylene-vinyl acetate (EVA), or thermoplastic polyolefins—filled with metal hydroxide flame retardants like aluminum trihydrate (ATH) and magnesium hydroxide (MDH).

In a fire, conventional halogenated cables release hydrogen chloride (HCl) and hydrogen bromide (HBr), which are highly toxic and corrosive, together with dense black smoke that severely reduces visibility and hampers evacuation. LSZH cables are engineered to limit flame propagation, produce minimal smoke, and evolve gases with low acidity, thereby protecting occupants, escape routes, and sensitive electronic equipment.

Combustion testing is the technical means of demonstrating these properties. The fire performance of a cable is evaluated across four dimensions: flame retardance (resistance to ignition and flame spread), smoke density (light transmittance through smoke), halogen content of evolved gases, and acidity/conductivity of the gas dissolution. Regulatory frameworks such as the EU Construction Products Regulation with fire classification standard EN 13501-6, and the Chinese cable fire-hazard classification standard GB 31247, rely directly on these combustion test results.

Testing Samples

Finished Cable Samples

Flame propagation and smoke density tests are performed on complete cable samples, including all insulation, fillers, inner coverings, and outer sheaths, because the composite construction determines real fire behavior. Typical sample requirements include:

• Single vertical flame test (IEC 60332-1 series): one sample of 600 mm ± 25 mm length.
• Vertical bunched cable flame test (IEC 60332-3 series): multiple samples, each 3.5 m long, mounted on a vertical ladder in a defined loading configuration.
• Smoke density test (IEC 61034-2): multiple samples of 1 m length, the quantity depending on the calculated cable volume per meter.
• EN 50399 (CPR flame spread with heat release measurement): samples of approximately 3.5 m, bunched on a vertical ladder.

Material Samples

Halogen content and acidity tests are conducted on compounded material taken from the cable. Approximately 0.5 g to 1 g of material is cut or powdered from the insulation, sheath, or other polymeric components. For material-level flammability screening, molded bars are prepared for oxygen index and temperature index testing.

Sample Conditioning

Prior to testing, samples are conditioned in a controlled environment, typically at 23 °C ± 5 °C for a minimum of 16 hours, to ensure reproducible ignition and burning behavior. Detailed conditioning parameters are specified in each individual test standard.

Testing Items

Depending on the product specification, certification scheme, and target market, LSZH cable combustion testing generally covers the following items:

• Flame propagation of a single insulated wire or cable (vertical flame test).
• Flame spread of vertically mounted bunched cables (categories A, B, C, and D).
• Smoke density under defined fire conditions, expressed as minimum light transmittance.
• Halogen acid gas evolution (HCl, HBr, HF) from burning polymeric materials.
• Acidity of evolved gases, expressed as pH value and conductivity.
• Oxygen index and temperature index of polymeric materials.
• Heat release rate, flame spread distance, and flaming droplets/particles for CPR classification.
• Optional toxic gas emission analysis (e.g., CO, HCN) using gas analysis techniques for specialized applications such as railway rolling stock.

Testing Methods

Halogen Acid Gas Content — IEC 60754-1 / EN 60754-1 / GB/T 17650.1

A weighed specimen of approximately 0.5–1 g of polymeric material is heated in a combustion tube furnace through which a controlled stream of air is passed. The evolved gases are absorbed in an aqueous solution, and the halogen acid content is determined analytically (by titration or ion chromatography). For a cable to qualify as halogen-free, the evolved halogen acid gas content must be less than 0.5%, corresponding to 5 mg/g of material.

Acidity (pH and Conductivity) — IEC 60754-2 / EN 60754-2 / GB/T 17650.2

The combustion gases from a pre-weighed material specimen are passed through deionized water, and the resulting solution is evaluated for pH value and conductivity. Commonly specified acceptance criteria for low-acidity (halogen-free) cables are pH not less than 4.3 and conductivity not greater than 10 µS/mm. These values also underpin the acidity classifications (a1, a2, a3) used in EN 13501-6.

Smoke Density — IEC 61034-2 / EN 61034-2 / GB/T 17651.2

The test is conducted in a closed chamber of 3 m × 3 m × 3 m (27 m³). Cable samples are mounted horizontally on a ladder and exposed to a standardized alcohol tray fire source. A horizontal light path with a defined light source and photometric receiver measures light transmittance during and after burning. Typical acceptance criteria require a minimum light transmittance of 60%, with more stringent projects specifying 80% or higher. Lower transmittance corresponds to denser, more hazardous smoke.

Single Cable Flame Propagation — IEC 60332-1 / GB/T 18380 series

A single cable sample is clamped vertically and exposed to a 1 kW premixed propane burner, with the flame application time depending on the cable diameter. The sample passes if the charred portion does not come within 50 mm of the upper support and no flaming reaches the top of the sample. A lower-energy variant (IEC 60332-2) applies to small wires and cables of reduced cross-section.

Bunched Cable Flame Spread — IEC 60332-3 / EN 50266 / GB/T 18380.3x

Multiple cable samples are secured to the front of a vertical ladder and exposed to a high-energy ribbon burner for a defined duration. The categories differ in cable loading volume and flame application time:

• Category A: 7 L/m of cable volume, 40 min flame application.
• Category B: 3.5 L/m of cable volume, 40 min flame application.
• Category C: 1.5 L/m of cable volume, 20 min flame application.
• Category D: 0.5 L/m of cable volume, 20 min flame application.

The cable passes if the charred portion does not extend more than 2.5 m above the lower edge of the burner.

CPR Fire Classification — EN 50399 and EN 13501-6

For the European market, flame spread, heat release rate, and flaming droplets are measured using EN 50399, combined with EN 60332-1-2, EN 61034-2, and EN 60754-2 results to assign a Euroclass for cables (B2ca, Cca, Dca, Eca, Fca) with additional classifications for smoke (s1a, s1b, s2, s3), acidity (a1, a2, a3), and droplets (d0, d1, d2). This classification is mandatory for cables permanently installed in buildings in the EU.

Supplementary and Market-Specific Methods

• Oxygen index: ISO 4589-2, ASTM D2863, GB/T 2406.2 — measures the minimum oxygen concentration required to support flaming combustion of the material.
• Specific optical density of smoke: ASTM E662 (NBS smoke chamber) — an alternative material-level smoke measurement used in some specifications.
• Vertical tray flame test: UL 1685 with smoke measurement option, conducted per UL 2556 test methods — common for North American market specifications.
• Japanese flame retardancy requirements: JIS C 3005 flame tests for wires and cables.
• Railway applications: EN 45545-2 fire safety requirements, which integrate smoke density and gas toxicity analysis for rolling stock components.

Applications

LSZH cable combustion testing serves a wide range of industries and purposes:

• Rail transit, subways, road tunnels, and airports, where evacuation time and visibility are critical and where specifications such as EN 45545-2 or LSZH public-tender requirements apply.
• Hospitals, schools, shopping centers, theaters, and high-rise buildings, where building codes increasingly mandate reduced-smoke, halogen-free wiring.
• Data centers and telecommunication facilities, where corrosive hydrogen chloride can destroy electronic equipment even after a small fire.
• Ships, offshore platforms, and petrochemical plants, where personnel must escape through confined spaces.
• Nuclear power plants and other critical infrastructure with rigorous fire-safety qualification programs.
• Manufacturer R&D and routine quality control of flame-retardant compound formulations and cable designs.
• Certification support, Declaration of Performance issuance under the EU CPR, GB compliance in China, and verification testing by accredited third-party laboratories for buyers and project owners.

FAQ

What is the difference between flame-retardant and low smoke zero halogen?

Flame-retardant cables resist ignition and limit flame spread, but may still contain halogenated materials that release acidic gases and heavy smoke. LSZH cables combine flame retardance with halogen-free materials, low smoke emission, and low gas acidity, verified through both sets of combustion tests.

What are the typical acceptance criteria for an LSZH cable?

Common criteria are halogen acid gas content below 0.5% (IEC 60754-1), pH ≥ 4.3 and conductivity ≤ 10 µS/mm (IEC 60754-2), minimum light transmittance of 60% in the 3 m cube smoke test (IEC 61034-2), and compliance with the specified flame test category in IEC 60332-1 or IEC 60332-3. Individual project specifications may impose stricter limits.

How do Chinese standards correspond to IEC standards?

The Chinese national standards are largely harmonized with IEC: GB/T 17650 corresponds to IEC 60754 (halogen and acidity), GB/T 17651 corresponds to IEC 61034 (smoke density), GB/T 18380 corresponds to IEC 60332 (flame propagation), and GB/T 19666 specifies general requirements for flame-retardant and fire-resistant cables. GB 31247 establishes the Chinese fire-hazard classification system for cables.

How should laboratories performing these tests be qualified?

Testing should be performed by accredited third-party laboratories operating in accordance with ISO/IEC 17025, with the relevant IEC, EN, GB/T, ASTM, UL, or JIS methods within their scope of accreditation.

How many samples are required and how long does testing take?

Sample quantities depend on cable size and the standards selected—for example, one 600 mm sample for a single flame test versus several 3.5 m samples for a bunched cable test. After sample preparation and conditioning, individual tests typically require from one day (halogen and acidity tests) to several days (bunched flame and smoke density tests), with reporting following laboratory review.

Why is smoke transmittance measured as a percentage?

Light transmittance quantifies visibility through the smoke generated in the 3 m cube chamber. A transmittance of 60% means that 60% of the light passes through the smoke; higher values indicate clearer air along escape routes and safer conditions during evacuation.