The bunched cable burning test is a standardized fire-performance evaluation that measures vertical flame propagation along wires and cables installed in bundles on a vertical ladder. Specified in the IEC 60332-3 series and adopted by EN and GB standards, it is a core requirement for flame-retardant cable certification in buildings, rail transit, ships, power plants, and data centers. This article explains the test principle, sample requirements, test categories, procedures, acceptance criteria, and typical applications.
Technical Overview
The bunched cable burning test, formally described as the test for vertical flame propagation on vertically mounted bunched wires or cables, is a large-scale laboratory fire test designed to evaluate how effectively a cable restricts flame spread when installed in groups. Because installed cables are rarely routed singly, the test reproduces the concentrated fuel load of grouped cable runs on vertical trays and ladders, providing a far more severe and realistic assessment of flame-retardant performance than single-cable tests such as IEC 60332-1-2.
The test is governed internationally by the IEC 60332-3 series, with corresponding regional adoptions by EN and GB standards bodies:
- IEC 60332-3-10 — test apparatus, calibration, and general verification requirements
- IEC 60332-3-21 — Category A F/R, with samples attached to both faces of the ladder
- IEC 60332-3-22 — Category A, the most severe loading condition
- IEC 60332-3-23 — Category B
- IEC 60332-3-24 — Category C
- IEC 60332-3-25 — Category D, generally intended for small-diameter cables
- EN 60332-3 series and EN 50266 series — European adoptions
- GB/T 18380.3x series — Chinese national adoptions of the same test method
- UL 1685 and IEEE 1202 — comparable North American vertical tray flame tests
The categories differ in the total volume of combustible material loaded per meter of ladder length and in the duration of flame application. Category A loads 7 liters per meter with flame applied for 40 minutes, Category B loads 3.5 liters per meter for 40 minutes, Category C loads 1.5 liters per meter for 20 minutes, and Category D loads 0.5 liters per meter for 20 minutes. In China, the flame-retardant designations ZA, ZB, ZC, and ZD defined in GB/T 19666 correspond directly to these four categories.
Testing Samples
Power Cables
Low-voltage and medium-voltage power cables with XLPE or PVC insulation, sheathed with PVC, polyethylene, or low-smoke halogen-free compounds, are the most common subjects of the test. These cables carry significant energy and are installed in large aggregations, so they are typically qualified to Category A, B, or C depending on the installation environment.
Control and Instrumentation Cables
Multicore control cables, shielded instrumentation cables, and signal cables used in industrial plants, substations, and process facilities are tested to verify that flame spread along grouped runs remains limited, normally to Category B or C.
Data and Communication Cables
Small-diameter twisted-pair, coaxial, optical, and electronic wires are frequently evaluated under Category D because their individual combustible volume is low, yet large bundles in cable trays and risers still represent a significant fire load.
Sample Preparation Requirements
Test samples consist of multiple cable lengths, each approximately 3.5 meters long. The required number of samples is calculated from the measured volume of combustible material per meter of the cable so that the total loading meets the target category value. Adjacent samples are separated with metal spacers or grouped according to the spacing rules of the applicable standard, mounted on the front face of the ladder (and on both faces for Category A F/R). Samples are preconditioned in a controlled environment before the test to ensure reproducible results.
Testing Items
The primary quantity measured in the bunched cable burning test is the extent of flame propagation along the mounted bundle, expressed as the length of charred or fire-affected insulation and sheath material. The evaluation covers:
- The vertical extent of charring after a defined ignition exposure
- Whether combustion continues or self-extinguishes after removal of the ignition source
- Compliance with the maximum permitted damaged length in both the upward and downward directions
- Optional supporting measurements frequently performed together with the burning test, including smoke density to IEC 61034-2, halogen acid gas evolution to IEC 60754-2, and flame propagation under the single-cable method of IEC 60332-1-2
A test report from an accredited third-party laboratory records the cable construction, calculated sample quantity, category applied, flame application duration, observations during the test, and the measured extent of damage.
Testing Methods
Test Apparatus and Conditions
The test is conducted in a vertical enclosure approximately 4 meters tall, inside which a metal ladder about 3.5 meters high is installed. The prepared sample bundle is attached to the ladder so that the cables hang vertically. Air is drawn through the enclosure and exhaust stack at a controlled rate of approximately 5 cubic meters per minute, and the apparatus is periodically verified using standard reference cables to confirm that the severity of the rig remains within specification.
Ignition Source
A belt-type propane burner equipped with a Venturi air mixer, approximately 257 mm wide, is positioned to impinge on the lower portion of the mounted samples. The burner delivers a nominal thermal output of 20.5 kW, producing a consistent ribbon of flame across the full width of the bundle. The flame is applied continuously for 20 minutes under Categories C and D, and for 40 minutes under Categories A, A F/R, and B.
Test Procedure
- Measure the cable dimensions and calculate the volume of combustible material per meter to determine the number of sample lengths required
- Mount the samples on the ladder with the prescribed spacing and orientation
- Position the burner and verify gas and air flow settings
- Apply the ignition source for the duration defined by the selected category
- Withdraw the burner, allow any residual flaming to cease, and ventilate the enclosure
- Clean the samples and determine the uppermost and lowest points of charred or affected material
- Compare the damaged extent with the acceptance criteria and issue the test report
Acceptance Criteria
The test is passed when the charred portion does not approach closer than 50 mm below the top support of the ladder, and when the lowest point of damage remains at least 540 mm above the upper edge of the bottom rung. In other words, flame must neither reach the top of the bundle nor descend appreciably along the lower length of the samples. These dual distance requirements confirm that the cable construction restricts both upward flame spread and downward fire growth.
Category Selection Guidance
- Category A / ZA — high fuel density installations such as power plants, tunnels, metro systems, and high-rise risers
- Category B / ZB — dense grouped installations in commercial and public buildings
- Category C / ZC — general building wiring and common tray installations, the most widely specified level
- Category D / ZD — small-diameter data, communication, and electronic cables
Applications
Passing the bunched cable burning test is a prerequisite for marketing flame-retardant cables in most regulated markets. The test underpins flame-retardant class markings under GB/T 19666 in China, supports the fire-performance declarations required by EN 50575 under the European Construction Products Regulation when combined with EN 50399 heat-release measurements, and aligns with the vertical tray requirements of UL 1685 and IEEE 1202 in North America.
Typical end-use sectors include building electrical distribution, rail transit and rolling stock, shipboard and offshore installations, power generation and transmission facilities, road and rail tunnels, data centers, and industrial process plants. Manufacturers also use the test during compound development and routine qualification to validate flame-retardant sheath formulations before full-scale certification.
FAQ
What is the difference between the single-cable test and the bunched burning test?
The single vertical wire test of IEC 60332-1-2 applies a small burner to one cable, whereas the bunched test of IEC 60332-3 mounts many cable lengths on a ladder and exposes them to a 20.5 kW ribbon burner. The bunched method reproduces the combined fuel load of grouped cables and is significantly more demanding, which is why it is the basis for most flame-retardant ratings.
How is the number of test samples determined?
The laboratory measures the cable diameter and material distribution to calculate the volume of combustible matter per meter. The required sample count is the category loading value, for example 1.5 liters per meter for Category C, divided by that volume, rounded to a whole number of 3.5-meter lengths.
Does passing this test mean the cable is low-smoke and halogen-free?
No. The bunched burning test evaluates only flame propagation. Low-smoke and halogen-free performance must be demonstrated separately through smoke density testing to IEC 61034-2 and halogen acid gas testing to IEC 60754-2, which are typically bundled with the burning test in a complete fire-performance evaluation.
How does the test relate to European fire classification?
The IEC 60332-3 result corresponds historically to the flame-spread classes F1 and F2, while the current European Euroclasses for cables (B2ca through Eca) are determined by the large-scale vertical tray test of EN 50399, which extends the bunched burning configuration with measurements of heat release, smoke production, and flaming droplets.
How often must the test apparatus be verified?
The rig is validated at prescribed intervals with standard verification cables in accordance with IEC 60332-3-10, ensuring that the burner output, air flow, and overall severity remain consistent so that results from accredited third-party laboratories remain comparable over time.