Low Voltage Cable Combustion Testing: Standards, Methods, and Performance Requirements

Low voltage cable combustion testing evaluates how power, control, and building wires rated up to 0.6/1 kV perform under fire conditions, measuring flame propagation, smoke density, halogen gas release, corrosivity, and circuit integrity. Conducted in accordance with internationally recognized standards such as IEC 60332, IEC 61034, IEC 60754, IEC 60331, and EN 50399, these tests are essential for verifying fire safety compliance in buildings, rail transit, tunnels, and industrial installations.

Technical Overview

Low voltage cables, typically rated at 450/750 V or 0.6/1 kV, are installed in large aggregations inside walls, ceiling voids, cable trays, and tunnels of residential, commercial, and industrial facilities. In the event of a fire, these cable bundles can act as a pathway for flame spread and a major source of smoke and toxic or corrosive gases. Combustion testing is therefore a critical part of cable type testing, factory acceptance testing, and regulatory compliance, and it is used to classify cables as flame retardant, low smoke zero halogen (LSZH), or fire resistant.

Low voltage cable combustion testing covers several distinct fire performance characteristics, each governed by a specific test method:

• Flame propagation and self-extinguishing behavior of single cables and bundled cable installations
• Smoke generation and reduction of visibility in escape routes
• Release of halogen acid gases such as hydrogen chloride (HCl)
• Corrosivity of combustion products, expressed as pH value and conductivity
• Circuit integrity of fire-resistant cables during and after flame exposure
• Heat release rate and flame spread performance for construction product classification

The dominant standards frameworks include the IEC 60332, IEC 61034, and IEC 60754 series, the European EN 50399 and EN 50575 standards linked to the Construction Products Regulation (CPR), and the Chinese national standards GB/T 18380, GB/T 17651, and GB/T 17650, which are largely harmonized with their IEC counterparts. Flame retardant performance under the Chinese system is commonly designated as categories ZA, ZB, ZC, and ZD, while the European system assigns Euroclasses from B2ca down to Fca, with additional classifications for smoke (s), acidity (a), and flaming droplets (d).

Testing Samples

PVC Insulated and Sheathed Cables

Conventional PVC insulated and PVC sheathed power cables, control cables, and building wires represent the largest sample group. PVC contains chlorine, so these cables generally achieve flame retardancy through self-extinguishing behavior but release significant HCl and dense smoke when burned. They are tested to determine their flame spread category and to assess whether halogen-related limits can be met.

XLPE Insulated Cables

Cross-linked polyethylene (XLPE) insulated cables, with or without PVC or polyethylene sheaths, are widely used for 0.6/1 kV distribution. Combustion testing verifies the flame retardant category of the complete cable construction, including the sheath compound, filler, and any armor components.

Low Smoke Zero Halogen (LSZH) Cables

Halogen-free cables with LSZH insulation and sheathing are specifically tested to confirm their halogen-free status and low smoke emission. These samples must demonstrate low HCl evolution, low corrosivity, and high light transmittance in the smoke density test.

Fire-Resistant Cables

Cables designed to maintain circuit integrity under fire, typically using mica-glass tape fire barriers around the conductors, undergo additional flame exposure tests with or without mechanical shock and water spray to verify continuous operation of emergency circuits.

Sample Preparation Requirements

Sample lengths vary with the test method, from approximately 0.6 m for single vertical flame tests to multiple lengths of 3.5 m or longer for bunched cable and smoke density tests. Samples must be representative of normal production, taken from finished cable, and conditioned in a controlled environment, typically at 23 ± 5 °C, for a specified period before testing.

Testing Items

A complete low voltage cable combustion test program generally includes the following items:

• Single cable vertical flame propagation test
• Bunched cables vertical flame spread test (categories A, B, C, D or ZA, ZB, ZC, ZD)
• Smoke density test under defined fire conditions (light transmittance)
• Halogen acid gas evolution test (HCl content determination)
• Determination of acidity of combustion gases by pH and conductivity measurement
• Fire resistance and circuit integrity test under flame, water, and mechanical shock
• Heat release rate, total heat release, and flame spread for CPR classification
• Smoke production rate and total smoke production
• Supporting material tests such as limiting oxygen index (LOI)

Testing Methods

Single Vertical Flame Propagation Test (IEC 60332-1-2 / GB/T 18380.12)

A single cable sample approximately 600 mm long is clamped vertically, and a calibrated 1 kW premixed propane burner is applied to the sample at an inclined angle for a specified duration. After the flame source is removed, the extent of charring is measured. The sample passes if the charred or damaged portion does not extend to within 50 mm of the upper support. A related method, IEC 60332-2-2, applies a reduced flame application to small-diameter cables such as flexible cords and thin building wires.

Bunched Cable Flame Spread Test (IEC 60332-3 / GB/T 18380.3)

This large-scale test simulates cables installed in groups on a ladder rack. Cable samples are mounted on the front face of a vertical steel ladder, and a propane burner with a total output of approximately 20 kW is applied at the base. The test categories differ by the volume of combustible cable material per meter of ladder and the flame application time: category A corresponds to 7 l/m applied for 40 minutes, category B to 3.5 l/m for 40 minutes, category C to 1.5 l/m for 20 minutes, and category D to 0.5 l/m for 20 minutes. The cable passes if the charred region does not extend more than 2.5 m above the burner ignition point. The Chinese ZA, ZB, ZC, and ZD flame retardant categories are defined on the same basis.

Smoke Density Test (IEC 61034-2 / GB/T 17651.2)

Cable samples are arranged horizontally on supports inside an enclosed 3 m cubic chamber, and a defined quantity of alcohol fuel is ignited beneath them to simulate a fire condition. A horizontal light beam and photometric system continuously measure the transmission of light through the accumulated smoke. The lowest recorded transmittance is reported. Low smoke cables are typically required to achieve a minimum transmittance of 60%, and higher-performance specifications may require 80% or more, particularly for transit and enclosed public space applications.

Halogen Acid Gas and Acidity Tests (IEC 60754-1 and IEC 60754-2 / GB/T 17650)

In the halogen acid gas test, a weighed material sample is combusted in a tube furnace and the evolved gases are absorbed in water, after which the hydrochloric acid content is determined by titration. Halogen-free materials are generally required to contain less than 0.5% HCl by mass. In the acidity test, the combustion products are absorbed in demineralized water and the solution is evaluated for pH and conductivity. Low corrosivity materials must typically achieve a pH of 4.3 or higher and a conductivity not exceeding 10 µS/mm.

Fire Resistance and Circuit Integrity Tests (IEC 60331 / BS 6387)

Fire-resistant cable samples are mounted vertically with the conductors connected in a monitored circuit protected by fuses. A controlled flame at approximately 750 °C is applied continuously, commonly for 90 minutes, and the cable passes if the circuit remains intact throughout the exposure. More severe regimes, such as the BS 6387 categories, combine high-temperature flame exposure at up to 950 °C with subsequent direct water spray and periodic mechanical shock to demonstrate performance equivalent to a CWZ rating, which is frequently specified for life safety and firefighting circuits.

Heat Release and Flame Spread Test (EN 50399) and CPR Classification

Under the European Construction Products Regulation, cables sold for permanent installation in buildings must be classified according to EN 13501-6 and declared under EN 50575. The EN 50399 test mounts cable samples on a vertical ladder and applies an approximately 20 kW burner while measuring heat release rate, total heat release, flame spread, smoke production, and flaming droplet behavior. The results determine the Euroclass (B2ca, Cca, Dca, or Eca) together with additional classifications for smoke production (s1a, s1b, s2, s3), acidity (a1, a2, a3), and flaming droplets (d0, d1, d2), which are declared on the CE marking and Declaration of Performance.

Applications

Low voltage cable combustion test results directly determine where a cable may be legally and safely installed, and they are routinely specified in the following sectors:

• High-rise residential and commercial buildings, where flame spread and smoke emission limits protect evacuation routes
• Metro, subway, and railway tunnel systems, which typically mandate the highest flame retardant categories combined with LSZH performance
• Airports, hospitals, schools, and other public occupancy buildings with strict smoke visibility requirements
• Data centers and telecommunication facilities, where corrosive combustion products threaten electronic equipment
• Power plants, petrochemical plants, and industrial facilities with classified hazardous areas
• Emergency life safety systems, including fire alarm circuits, emergency lighting, smoke extraction fans, fire pumps, and elevators, which require proven fire-resistant cables

Designers, contractors, and approving authorities rely on combustion test reports to verify that selected cables meet the flame retardant category, smoke class, halogen content, and circuit integrity requirements applicable to each installation environment.

FAQ

What is the difference between flame-retardant and fire-resistant cables?

Flame-retardant cables are designed to limit the spread of fire along the cable and to self-extinguish once the external flame source is removed; they are evaluated by flame propagation tests such as IEC 60332. Fire-resistant cables go further and are designed to keep functioning during a fire, maintaining circuit integrity under sustained flame exposure, often combined with water and mechanical shock, and are evaluated by tests such as IEC 60331 and BS 6387.

What does LSZH mean and how is it verified?

LSZH stands for low smoke zero halogen. It describes cables whose insulation and sheath materials emit reduced smoke and contain virtually no halogen-based compounds. Verification combines the smoke density test of IEC 61034-2, the halogen acid gas test of IEC 60754-1, and the acidity test of IEC 60754-2, confirming high light transmittance, HCl below 0.5%, and acceptable pH and conductivity values.

What do flame retardant categories ZA, ZB, ZC, and ZD mean?

These categories correspond to the bunched cable flame spread test with progressively increasing cable loading on the test ladder. ZA represents the highest loading and longest flame application, followed by ZB, ZC, and ZD. The higher the category, the greater the quantity of installed cable material that can resist sustained flame propagation.

What is the CPR Euroclass system for cables?

Under the European Construction Products Regulation, cables are classified from B2ca (best) to Fca (worst) based primarily on EN 50399 test data. Additional classifications describe smoke production (s), acidity of combustion gases (a), and flaming droplets (d). The classification must be declared through CE marking under EN 50575 before cables are placed on the European market for permanent installation in buildings.

How long does a typical combustion test program take?

Individual flame tests last from a few minutes for single cable tests to several hours for category A bunched tests and fire resistance regimes. A complete test program covering flame spread, smoke density, halogen content, and corrosivity, including sample conditioning and post-test examination, typically requires one to several weeks depending on the scope and the number of cable types submitted.

Are GB/T and IEC test methods equivalent?

In most cases, yes. GB/T 18380 corresponds to the IEC 60332 series for flame propagation, GB/T 17651 corresponds to IEC 61034 for smoke density, and GB/T 17650 corresponds to IEC 60754 for halogen content and acidity. Test reports from accredited laboratories are generally accepted for both domestic Chinese compliance and international projects, although project specifications should always be reviewed for jurisdiction-specific requirements.