Determination of Burning Length for Bunched Cables: Flame Spread Testing, Standards, and Acceptance

Determination of burning length for bunched cables is a core fire-safety evaluation that measures how far flame and charring propagate along a group of vertically installed cables under a standardized ignition source. Governed primarily by the IEC 60332-3 series and its EN and GB counterparts, the test is essential for verifying the flame-retardant classification of cables used in buildings, tunnels, power plants, and rail systems. This article explains the principles, standards, sample requirements, procedures, and acceptance criteria of bunched-cable burning-length determination.

Technical Overview

Bunched cable burning length determination, also known as the vertical flame-spread test for grouped cables, evaluates the ability of a bundle of cables mounted vertically on a ladder to resist flame propagation when exposed to a calibrated propane burner. Because cables in real installations are routed in groups on trays and in shafts, the total combustible mass per meter is far greater than that of a single cable, and the accumulated heat release can sustain flame spread even along cables that individually pass a small-scale flame test. The bunched test therefore reproduces the worst-case grouped installation condition and uses the resulting charred (damaged) length as the primary pass/fail indicator.

The internationally recognized method is defined in the IEC 60332-3 series, with equivalent adoptions worldwide. Key reference standards include:

• IEC 60332-3-10 – apparatus and general requirements for the vertical bunched-cable test
• IEC 60332-3-21 / -22 / -23 / -24 – test categories B, C, D, and A respectively
• EN 50266-2 series – European adoptions of the bunched-cable flame-spread test
• GB/T 18380.3 series – Chinese national standards technically equivalent to the IEC 60332-3 series
• UL 1685 and IEEE 1202 (FT4) – North American vertical tray flame tests with their own char-length criteria

Under IEC 60332-3, the test category (A, B, C, or D) defines the nominal volume of combustible material per meter of the test bundle: 7 L/m for Category A, 3.5 L/m for Category B, 1.5 L/m for Category C, and 0.5 L/m for Category D. Category A represents the most severe loading condition combined with the longest burner application time.

Testing Samples

Test specimens consist of finished cables cut to a minimum length of 3.5 m. The number of cables per test is calculated by dividing the category volume per meter by the volume of combustible material per meter of a single cable, rounded to the nearest whole number. Typical product families subjected to bunched-cable burning-length determination include:

Power Distribution Cables

Low-, medium-, and high-voltage power cables, including XLPE-insulated and PVC-insulated constructions with flame-retardant or low-smoke zero-halogen (LSZH) sheaths intended for grouped installation on trays and in risers.

Control and Instrumentation Cables

Multicore control cables, shielded instrument cables, and paired signal cables used in industrial process plants, where bundled routing along long tray runs is standard practice.

Communication and Data Cables

Structured-wiring category cables, coaxial cables, and optical-fiber cables installed in large bundles within buildings, plenum spaces, and data-center cable managers.

Flame-Retardant and Fire-Resistant Cables

Specialty LSZH, fire-resistant, and circuit-integrity cables whose declared flame-retardant category (A, B, C, or D) must be substantiated by objective test evidence for certification or tender compliance.

Testing Items

The principal measurable and observational outputs of the test are:

• Maximum charred (burning) length, measured from the lower edge of the burner to the highest damaged point on the cable bundle
• Flame-propagation behavior, including whether flaming ceases after removal of the ignition source and the observed after-flame time
• Verification of the declared flame-retardant category of the cable design under the selected loading condition
• Recording of falling debris and secondary effects where required by the product specification

Complementary fire-performance tests frequently performed together with burning-length determination include smoke density measurement (IEC 61034) and halogen-acid gas evolution (IEC 60754). Together these support full reaction-to-fire classification, such as the European classes under EN 13501-6.

Testing Methods

Test Apparatus and Setup

The test is conducted in a vertical chamber approximately 4 m high, equipped with a controlled air inlet and extraction system. A ladder-type mounting frame, typically 3.5 m high, supports the cable bundle on its front face. A belt-type propane burner of standardized design, with a nominal thermal output of approximately 20.5 kW, is positioned horizontally so that its flames impinge on the lower portion of the bundle. Airflow through the chamber is regulated throughout the exposure to ensure reproducible combustion conditions.

Specimen Preparation

Cables are secured to the ladder at close to touching spacing; where the calculated number of cables is small, a spacing of about half a cable diameter is applied, and where the number exceeds the usable ladder width, additional layers are arranged behind the front layer in the geometry defined by the standard. The combustible volume per meter of each cable is determined from its construction, which establishes the specimen count for the target category.

Ignition and Test Procedure

After the burner and chamber airflow have stabilized, the burner is applied to the specimen for a defined duration:

• Category A and Category B tests: burner applied for 40 minutes
• Category C and Category D tests: burner applied for 20 minutes

The burner is then removed and the extraction rate is increased to purge smoke from the chamber. During the test, flame propagation along the bundle, the time at which flaming ceases, and any falling burning debris are observed and recorded. The specimen is allowed to cool completely before post-test assessment.

Measurement of Burning Length

After cooling, the cable surfaces are gently wiped with a dry cloth to remove soot and loose deposits. The burning (charred) length is measured as the maximum vertical distance between the lower edge of the burner and the highest point at which the cable exhibits carbonization, melting, or other permanent thermal damage. Superficial sooting that can be wiped away is excluded from the assessment, in line with the standard's definition of the affected part.

Acceptance Criteria

Under IEC 60332-3 and its EN and GB equivalents, the cable passes the selected category when the measured charred length does not exceed 2.5 m above the lower edge of the burner. North American tray tests such as UL 1685 and IEEE 1202 specify their own shorter limits (approximately 2.4 m and 1.5 m respectively), so the applicable regional specification must be confirmed before testing.

Applications

Bunched-cable burning-length determination supports design, certification, and quality assurance across a wide range of sectors:

• Buildings and high-rise construction – demonstrating compliance of riser and distribution cables with fire codes and European reaction-to-fire classes (EN 13501-6, classes B2ca, Cca, Dca, Eca)
• Rail transit and infrastructure – qualification of cables for metro lines, tunnels, stations, and depot installations
• Power generation and substations – validation of large grouped power runs in cable trays and galleries
• Petrochemical and offshore installations – fire-safety qualification of cabling in high-hazard process areas
• Data centers and telecommunications – verification of large communication-cable bundles in confined spaces
• Manufacturer R&D and type testing – development of flame-retardant materials and independent verification at accredited third-party laboratories

FAQ

What is the pass criterion for burning length in the bunched cable test?

Under IEC 60332-3, and the equivalent EN 50266-2 and GB/T 18380.3 standards, the charred length measured from the lower edge of the burner must not exceed 2.5 m for the cable to pass the tested category.

How do Categories A, B, C, and D differ?

The categories specify the volume of combustible material per meter of the test bundle: 7 L/m for A, 3.5 L/m for B, 1.5 L/m for C, and 0.5 L/m for D. Categories A and B additionally use a 40-minute burner application versus 20 minutes for C and D, making Category A the most demanding condition.

How many cable samples are required for a test?

The specimen count equals the category volume per meter divided by the combustible volume per meter of a single cable, rounded to the nearest whole number, with each specimen at least 3.5 m long. A thin communication cable may require dozens of samples, while one large power cable can satisfy the bundle on its own.

Why test cables in bunches when single-cable flame tests already exist?

Single-cable tests (IEC 60332-1 and IEC 60332-2 series) apply a small burner to one specimen and cannot reproduce the cumulative heat release of grouped installations. Bunched testing with a high-output burner realistically simulates tray installations and prevents cable designs from being qualified under unrealistically mild conditions.

Where should bunched cable burning-length tests be performed?

Testing should be carried out at accredited third-party laboratories or qualified in-house facilities equipped with a compliant IEC 60332-3-10 test chamber, a calibrated burner, controlled ventilation, and traceable measurement instrumentation for determining the resulting charred length.