Bunched cable flame propagation testing, standardized under the IEC 60332-3 series and its regional equivalents such as EN 50266, GB/T 18380, and UL 1685, evaluates the ability of grouped cables to resist flame spread when installed vertically in trays or ladders. This article explains the test principle, sample requirements, test categories, procedures, pass criteria, and typical applications, helping manufacturers, specifiers, and engineers correctly verify and select flame-retardant cable products.
Technical Overview
Bunched cable flame propagation testing simulates the most common real-world installation condition in which multiple cables are routed together in cable trays, conduits, or shafts. When a fire affects one cable in a dense bundle, heat and burning material can propagate along the group far more severely than along a single isolated cable. The purpose of the test is therefore to determine whether a vertically installed bundle of cables will limit flame spread to an acceptable extent under a defined external ignition source.
The internationally recognized method is defined in the IEC 60332-3 series, which is adopted in Europe as EN 60332-3 (previously EN 50266) and in China as the GB/T 18380.3 series. Comparable North American methods include the UL 1685 vertical tray fire-propagation and smoke-release test and the IEEE 1202 (FT4-type) vertical tray flame test. Depending on the total volume of combustible non-metallic material loaded onto the test ladder, the IEC 60332-3 method is divided into four categories:
- Category A (IEC 60332-3-21 / GB/T 18380.32): 7 liters of non-metallic material per meter of sample length
- Category B (IEC 60332-3-22 / GB/T 18380.33): 3.5 liters per meter
- Category C (IEC 60332-3-23 / GB/T 18380.34): 1.5 liters per meter
- Category D (IEC 60332-3-24 / GB/T 18380.35): 0.5 liters per meter, typically applied to smaller cables in low-density installations
The test result is a pass/fail determination based on the extent of the charred or damaged portion of the samples after exposure to a calibrated propane burner. Higher categories represent a more severe loading condition and are generally specified for high-density installations such as power plants, tunnels, and industrial facilities.
Testing Samples
Sample Configuration and Length
Each test uses multiple cable specimens, each approximately 3.5 meters long, cut from production-length cable. The number of specimens is calculated from the volume of non-metallic (combustible) material contained in one meter of the cable, such that the total non-metallic volume per meter of test length equals the value required by the selected category, with the result rounded up to the next whole cable.
Sample Mounting
The specimens are secured to the rungs of a vertical steel ladder using metal binding wire, with adjacent cables bunched close together on the front face of the ladder in accordance with the mounting rules of the applicable standard. For categories with high loading, or where the calculated number of cables exceeds a single layer, the standard specifies how the remaining cables are arranged so that the test reproduces a realistic, closely grouped cable run.
Sample Conditioning
Prior to testing, samples are conditioned for at least 16 hours at a temperature of 23 ± 5 °C. For cables whose flame-retardant properties are known to be humidity-sensitive, additional conditioning in a controlled-humidity environment may be specified by the relevant product standard or by agreement between the customer and the accredited third-party laboratory.
Testing Items
The core scope of bunched cable burning tests includes the following assessed parameters:
- Vertical flame propagation characteristics of bunched cables under a standardized ignition source
- Maximum height of the charred or affected portion of the samples, measured after the test
- Determination of compliance with the acceptance criterion (damaged length limited to 2.5 m above the burner reference line for IEC 60332-3 categories)
- Optional smoke release measurement, including peak smoke production rate and total smoke released, when testing to the smoke-measurement versions of UL 1685 or related tray tests
- Supporting fire-performance items frequently tested together with the bunched cable test, such as smoke density per IEC 61034-2 / GB/T 17651.2, halogen acid gas evolution and corrosivity per IEC 60754 / GB/T 17650, and single-cable vertical flame tests per IEC 60332-1-2 / GB/T 18380.12
Testing Methods
Test Apparatus
The test is performed in a dedicated vertical burning chamber with internal dimensions of approximately 1 m deep, 2 m wide, and 4 m high, equipped with an air inlet at the base and an exhaust hood at the top. A calibrated vertical air flow of approximately 5 m/s is maintained through the chamber during the test to ensure reproducible oxygen supply and heat removal conditions. The steel ladder carrying the samples is mounted inside the chamber, and instrumentation is verified periodically against the calibration requirements of IEC 60332-3-10 / GB/T 18380.31.
Ignition Source and Flame Application
The ignition source is a ribbon-type (belt) propane gas burner approximately 500 mm wide, delivering a total heat output of approximately 20.5 kW, positioned so that the flame impinges on the lower portion of the cable bundle. The flame application time depends on the test category:
- Category A and Category B: 40 minutes
- Category C and Category D: 20 minutes
Test Procedure
After mounting, measuring, and conditioning, the samples are installed in the chamber and the exhaust airflow is stabilized. The calibrated burner is ignited, verified, and then applied to the samples for the specified duration. Observers record flame behavior, including afterflame and afterglow, during and after the exposure. After the samples have cooled and been cleaned, the extent of charring or damage is measured relative to the bottom edge of the burner contact position.
Assessment and Pass Criteria
For IEC 60332-3 / GB/T 18380 categories, the test is considered passed when the charred or affected portion of the samples does not extend more than 2.5 meters above the bottom edge of the burner. For North American tray tests such as UL 1685 and IEEE 1202 (FT4-type), the typical acceptance criterion limits the damaged length to approximately 2.4 m (8 ft) above the burner, and the smoke-measurement version additionally limits peak smoke release rate and cumulative smoke production. When combined with EN 50399 / GB/T 31248 evaluation, additional parameters such as flame spread distance, heat release rate (FIGRA), total heat release, and total smoke production are used for classification under EN 13501-6 or GB 31247.
Applications
Bunched cable flame propagation test results are widely used for product qualification, market access, and engineering specification in safety-critical installations. Typical applications include:
- Qualification of flame-retardant and low-smoke zero-halogen (LSZH) power, control, and instrumentation cables for buildings, infrastructure, and industry
- Verification of compliance for CE marking under the EU Construction Products Regulation, supported by EN 50399 classification to EN 13501-6 classes B2ca, Cca, Dca, and Eca
- Support for Chinese market access and classification under GB 31247, which combines bunched cable or EN 50399-type testing with smoke density and corrosive gas requirements
- Acceptance testing for metro and railway tunnels, road tunnels, airports, hospitals, high-rise buildings, data centers, power plants, and substations, where cable routes are densely bundled
- Marine and offshore installations, where classification societies' requirements typically reference IEC 60332-3 categories
- Research and development of flame-retardant insulation, sheathing compounds, and cable construction designs
FAQ
What is the difference between IEC 60332-1 and IEC 60332-3?
IEC 60332-1 (and IEC 60332-2 for small cables) evaluates a single vertical cable specimen exposed to a small burner, representing an isolated cable failure. IEC 60332-3 evaluates a vertically bunched group of cables loaded onto a ladder and exposed to a much more powerful 20.5 kW ignition source, which reproduces the severe conditions of a fire involving a dense cable installation and is therefore the key test for flame-retardant cable qualification.
How do I select the appropriate test category?
Category selection depends on the cable loading density expected in service and on the governing specification. Category A (7 L/m) is normally specified for power plants, tunnels, and other high-density installations; Category B (3.5 L/m) for heavy industrial and infrastructure projects; Category C (1.5 L/m) for general commercial and public buildings; and Category D (0.5 L/m) for smaller cables in lower-risk installations. Project specifications, national regulations, or product standards should always be consulted.
What are the pass criteria for the bunched cable test?
For all IEC 60332-3 / GB/T 18380.3 categories, the test is passed when the charred or affected portion of the cable bundle does not extend more than 2.5 meters above the bottom edge of the burner after the specified flame application time of 20 or 40 minutes.
How many samples are required for testing?
The required quantity is calculated from the volume of non-metallic material per meter of the cable being tested. Each specimen is approximately 3.5 meters long, and the number of specimens is chosen so the total non-metallic loading per meter matches the category requirement, rounded up to the next whole cable.
Does the bunched cable test measure smoke and toxic gas emissions?
The IEC 60332-3 method itself assesses only flame propagation and char extent. Smoke density is evaluated separately using IEC 61034-2 / GB/T 17651.2, and halogen acid gas and corrosivity using IEC 60754 / GB/T 17650. Smoke release can be measured within the UL 1685 smoke version, and comprehensive heat and smoke parameters are obtained under EN 50399 / GB/T 31248 for higher-level classifications.
Is bunched cable testing mandatory for market access?
Requirements vary by market. In the EU, cables placed on the market for permanent installation in construction works must be classified under EN 13501-6, which relies on EN 60332-1-2 for the Eca class and EN 50399 for higher classes. In China, GB 31247 defines the burning behavior classes B1, B2, and B3 supported by GB/T 18380 series tests. Many utilities, railway operators, and industrial end users also impose category A, B, C, or D requirements contractually, and manufacturers typically demonstrate compliance through accredited third-party laboratories.