Bunched Cable Burning Test: Flame Propagation Testing to IEC 60332-3 and GB/T 18380

The bunched cable burning test is a large-scale fire performance evaluation that measures the vertical flame propagation characteristics of cables and wires mounted in bundles on a vertical ladder, simulating real installed cable tray conditions. Conducted in accordance with the IEC 60332-3 series, EN 60332-3, and GB/T 18380.3x standards, this test is the primary basis for classifying flame-retardant cables into Categories A, B, C, and D (ZA, ZB, ZC, ZD) and is widely required for power, control, data, and fire-safety cables in buildings, rail transit, energy infrastructure, and industrial plants.

Technical Overview

A bunched cable burning test, also known as a vertical flame propagation test on bunched cables, evaluates how far flames spread along a group of cables installed close together in a vertical configuration. Unlike single-cable flame tests, which only assess one insulated wire or cable in isolation, the bunched test reproduces the conditions found in real installations such as cable trays, conduits, shafts, and risers, where grouped cables can propagate fire far more aggressively than a single cable.

In the test, multiple cable lengths are attached to a vertical steel ladder inside a purpose-built combustion chamber. A high-capacity propane burner applies a controlled flame source to the lower section of the bundle for a specified duration. After the flame source is removed, the extent of charring and damage along the cables is measured to determine whether flame propagation stayed within the acceptance limits defined by the standard.

The governing international standard is the IEC 60332-3 series, which is adopted in Europe as EN 60332-3 and in China as the GB/T 18380.3x series of national standards. The test is divided into four severity categories according to the volume of combustible cable material loaded per metre of ladder:

• Category A (IEC 60332-3-22 / GB/T 18380.31): 7 litres of cable material per metre, the most severe loading, with cables mounted on both faces of the ladder.
• Category B (IEC 60332-3-23 / GB/T 18380.32): 3.5 litres per metre, typically used for heavily loaded power and control circuits.
• Category C (IEC 60332-3-24 / GB/T 18380.33): 1.5 litres per metre, the most commonly specified category for general-purpose flame-retardant cables.
• Category D (IEC 60332-3-25 / GB/T 18380.34): 0.5 litres per metre, intended for small-diameter cables such as data, signal, and instrumentation cables, using a smaller burner with approximately 1 kW output.

Test results directly support product classification. In China, cables passing the respective categories are designated with the prefixes ZA, ZB, ZC, and ZD under GB/T 19666, and the test also forms part of the combustion performance grading system of GB 31247 for cables used in construction projects.

Testing Samples

Flame-Retardant Power Cables

Low-voltage and medium-voltage power cables with PVC, XLPE, or halogen-free (LSZH/PO) insulation and sheath materials, such as cables rated 0.6/1 kV and above, are tested to verify their ZA, ZB, or ZC flame-retardant ratings. Sample lengths of at least 3.5 metres are required, and the number of parallel lengths is calculated from the specified loading volume per metre.

Control and Instrumentation Cables

Multi-core control cables, shielded instrument cables, and cables for industrial automation are tested to confirm that grouped installations in control rooms, substations, and plant cable racks will not propagate fire beyond acceptable limits. Depending on size and application, these products are usually evaluated under Category B, C, or D.

Data, Communication, and Signal Cables

Category 5e/6/6A local area network cables, telephone cables, coaxial cables, and fire alarm system cables are typically small in diameter and installed in large bundles. Category D of the bunched burning test was developed specifically for these products, reflecting the lower fuel load but high cable density typical of data centres and communication buildings.

Fire-Resistant and Low-Smoke Cables

Cables intended for emergency circuits, such as fire pump supplies, emergency lighting, and evacuation systems, combine flame-retardant performance with circuit integrity. The bunched burning test verifies the flame propagation component of their performance, which is often supplemented by smoke density testing per IEC 61034 / GB/T 17651 and circuit integrity testing under IEC 60331 / GB/T 19216.

Testing Items

The core parameters evaluated and recorded during a bunched cable burning test include:

• Vertical flame propagation behaviour of the bunched cable assembly during and after burner exposure.
• Maximum upward extent of charred or affected cable material, measured from the burner reference position.
• Downward extent of damage below the flame application zone, confirming that the fire did not spread down the bundle.
• Combustion behaviour after removal of the ignition source, including whether burning ceases and any dripping of flaming debris.
• Optional smoke production measurement, which can be integrated into the test chamber in accordance with EN 50399 for European classification purposes.
• Confirmation of the calculated cable loading volume against the category requirement, ensuring the test severity is valid.

The test does not produce a numerical flame spread index; it is a pass/fail evaluation based on the measured extent of damage relative to the acceptance criteria.

Testing Methods

Test Apparatus

The test is performed in a ventilated vertical combustion chamber equipped with a steel ladder approximately 3.5 metres high and 0.5 metres wide on which the cable samples are mounted. For Categories A, B, and C, ignition is provided by a flat ribbon-type propane burner delivering a thermal output of approximately 73.7 MJ/h (about 20.5 kW), with a controlled air supply maintaining a stable total airflow of approximately 5,000 litres per minute through the chamber. Category D uses a smaller burner of approximately 1 kW output applied for the same exposure period.

Specimen Preparation

Sample cables are cut to a minimum length of approximately 3.5 metres and conditioned before testing. The number of cable lengths is calculated from the specified volume of combustible material per metre for the target category, based on the overall cross-section of each cable minus its metallic cross-section. The calculated bundle is secured to the ladder rungs at defined intervals so that the cables hang vertically with adjacent surfaces in contact, reproducing a worst-case grouped installation.

Test Procedure

The burner is positioned at the lower portion of the cable bundle and ignited with a calibrated propane-air mixture. The flame is applied continuously for 20 minutes for Categories A, B, C, and D. Observations of flame spread, smoke emission, and flaming drips are recorded throughout the exposure. After the burner is extinguished, the ventilation is maintained until combustion has completely ceased and smoke production has abated. The bundle is then dismantled and each cable is examined to determine the extent of charring, carbonisation, or other heat damage.

Pass Criteria

The test is considered passed when the following conditions are met on the tested bundle:

• The extent of the charred or affected portion does not reach within 50 mm of the top support of the ladder.
• The lower limit of the affected portion remains at least 500 mm above the bottom edge of the burner.
• The measured damage limits apply to all cables in the bundle, and the required number of valid test repetitions achieves the criteria.

If damage approaches the top of the ladder or extends too far in any direction, the cable fails the category and a lower severity category may need to be evaluated, or the cable design must be improved.

Applications

Bunched cable burning test reports and category certifications are used across many industries and regulatory contexts:

• Construction and real estate projects: supporting flame-retardant grade declarations under GB 31247 for cables installed in high-rise buildings, shopping centres, hospitals, schools, and tunnels.
• Rail transit and metro systems: qualification of power, control, and communication cables installed in dense trunking along tunnels and stations.
• Power generation, substations, and grid infrastructure: verification of cable tray installations in thermal power plants, nuclear facilities, and renewable energy stations.
• Data centres and telecommunications: confirmation of Category D performance for large bundles of structured cabling.
• Industrial plants, petrochemical facilities, and ships: assessment of grouped cable runs where fire propagation between cable routes presents a major hazard.
• Product certification and tender compliance: manufacturers use category A/B/C/D results as technical evidence in product datasheets, tender documents, and regulatory submissions.

In Europe, the same test platform, extended with heat release and smoke production measurements under EN 50399, feeds into the Construction Products Regulation classification of cables (B2ca, Cca, Dca, etc.), demonstrating the global importance of the bunched burning test methodology.

FAQ

What is the difference between a single cable flame test and a bunched cable burning test?

Single cable tests such as IEC 60332-1 / GB/T 18380.12 apply a small burner to one vertical wire or cable and are suitable for small products. The bunched test under IEC 60332-3 / GB/T 18380.3x mounts many cables together on a ladder with a much larger ignition source, evaluating flame spread in realistic grouped installations. Almost all power, control, and data cables are ultimately rated by the bunched test categories.

How are Categories A, B, C, and D selected?

The category is chosen based on the expected installation density and fire safety requirements of the project. Category A represents the heaviest loading (7 L/m) and is required for critical or densely packed installations, Category C (1.5 L/m) is the common baseline for flame-retardant cables in China designated ZC, and Category D (0.5 L/m) is reserved for small data and signal cables. Specifiers, design institutes, or end-user standards typically define the required category.

How many samples are needed for one test?

The quantity is not fixed; it depends on cable diameter and construction. The standard calculates the number of lengths from the required volume of non-metallic material per metre divided by the volume of one cable. Several metres of production cable, typically taken from a single manufacturing batch, are needed to build one bundle, and repeat bundles may be required to complete the assessment.

Does passing the bunched burning test mean the cable is halogen-free or low-smoke?

No. The bunched burning test only evaluates flame propagation. Halogen content and smoke density are verified by separate tests such as IEC 60754 (halogen acid gas emission) and IEC 61034 / GB/T 17651 (smoke density). Low-smoke zero-halogen flame-retardant cables must satisfy all relevant tests in combination.

How long does the test take?

The flame application itself lasts 20 minutes, but a complete evaluation includes sample preparation, loading calculation, conditioning, chamber stabilisation, post-test observation, and damage measurement. A full test campaign for one cable type, including any required repetitions, typically takes several working days at an accredited third-party laboratory.

What should be done if a cable fails the test?

Failure indicates that flame spread exceeded the allowed limits for that category. Manufacturers can retest at a lower category appropriate to the product, or improve the compound formulation, increase flame-retardant filler content, or modify the sheath design to achieve the target category.