The single cable vertical burning test, specified in IEC 60332-1-2, IEC 60332-2-2, and the equivalent GB/T 18380 series, is the fundamental method for evaluating the flame-retardant performance of an individual insulated wire or cable in a vertical orientation. This article explains the test principle, apparatus, sample requirements, pass criteria, and typical applications, providing engineers and quality professionals with a practical reference for compliance testing in accredited third-party laboratories.
Technical Overview
The single cable vertical burning test, also known as the test for vertical flame propagation for a single insulated wire or cable, is a fire-safety performance test widely used in the wire and cable industry. Its purpose is to determine whether a single cable, when ignited by a small standardized flame while suspended vertically, will self-extinguish or allow the flame to propagate along its length. The test provides a basic indication of the flame-retardant capability of the cable insulation, sheath, and filler materials.
The principal test method is defined in IEC 60332-1-2 (vertical flame propagation for a single insulated wire or cable, 1 kW pre-mixed flame procedure), which is adopted in Europe as EN 60332-1-2 and in China as the GB/T 18380 series (GB/T 18380.11, GB/T 18380.12, and GB/T 18380.13). For small-diameter wires, the related procedure IEC 60332-2-2 (diffusion flame) and its GB/T 18380.21/18380.22 counterparts apply. In North America, the comparable small-scale vertical flame test is the VW-1 test described in UL 1581 and UL 2556. Together, these standards form the baseline flame-retardance requirement in most cable product specifications.
In the test, a finished cable sample is clamped vertically inside a metal enclosure, and a propane burner flame is applied at an angle of 45 degrees to the cable surface for a defined duration that depends on the cable diameter. After the flame is removed, the extent of charring along the sample and the burning behavior are evaluated against fixed acceptance limits. The method is a go/no-go compliance test rather than a quantitative measurement, and it is typically performed by accredited third-party laboratories as part of product certification, factory quality control, or research and development.
Testing Samples
The test is applicable to virtually all types of finished insulated wires and cables, including power, control, data, and special-purpose products. Typical sample categories include:
Power and Distribution Cables
Rated-voltage power cables, such as PVC or XLPE insulated distribution cables and low-voltage building cables, are tested to verify basic flame-retardant performance before more demanding bunched-cable tests are considered.
Building Wires and Flexible Cords
Single-core PVC or LSZH insulated wires, hook-up wires, and flexible cords used in fixed installations, panels, and appliances are among the most frequently tested products under the small-cable procedures.
Control and Instrumentation Cables
Multicore control cables, shielded instrument cables, and computer cables used in industrial plants are tested to confirm that a single ignited cable will not propagate flame along a tray or conduit run.
Data, Communication, and Optical Cables
Category cables, telephone cables, and optical fiber cables for indoor installations are commonly assessed using the single vertical burning test as the minimum fire-performance requirement in building codes and procurement specifications.
Special-Purpose Cables
Automotive wires, railway rolling stock cables, shipboard cables, and appliance wiring harnesses may be subjected to this test or to closely related vertical flame procedures specified in sector standards.
Sample preparation requirements are straightforward but strict: the specimen must be a section of finished cable of full cross-section, with a length of 600 mm ± 25 mm, taken from normal production. The overall diameter of the sample determines the flame application time. Samples are conditioned under standard laboratory conditions prior to testing, and the applicable product standard or specification defines how many samples must be tested and whether retests are permitted.
Testing Items
When a single cable vertical burning test is performed, the laboratory evaluates and reports the following items:
- Flame propagation behavior of the single vertical cable sample, including whether the cable self-extinguishes after removal of the ignition source.
- Extent of charring or carbonization along the sample surface, measured after the test and cleaning of loose soot and char.
- Distance between the upper edge of the charred zone and the top support, which must be at least 50 mm.
- Distance between the lower edge of the charred zone and the top support, which must not exceed 540 mm.
- Observation of afterflame and afterglow behavior, including the time for the sample to stop burning.
- Behavior of burning or molten drops and debris falling from the sample, recorded as supplementary information.
- Full test conditions, including sample dimensions, cable diameter, flame application duration, and the applicable standard.
Testing Methods
The test apparatus specified in IEC 60332-1-2 and GB/T 18380 consists of a sheet-metal enclosure approximately 1200 mm high, 300 mm wide, and 450 mm deep, open at the front and fitted with ventilation provisions, placed under an extraction hood. A vertical support fixture holds the cable sample upright, with the upper end clamped and the lower end free. Ignition is provided by a propane gas burner of the Venturi type with a nominal thermal power of 1 kW, producing a pre-mixed blue flame with a clearly defined inner cone of approximately 40 mm.
The test procedure is performed as follows:
- The sample is clamped vertically so that the flame will impinge on it at a point 475 mm below the top support, corresponding to a position roughly 125 mm above the lower end of the 600 mm sample.
- The burner is positioned with its axis inclined at 45 degrees to the vertical axis of the sample, with the tip of the inner blue cone just touching the surface of the cable.
- The flame is applied continuously for a duration determined by the measured overall diameter of the sample: 60 seconds for diameters up to and including 25 mm, 120 seconds for diameters over 25 mm up to and including 50 mm, 240 seconds for diameters over 50 mm up to and including 75 mm, and 480 seconds for diameters over 75 mm.
- After the application period, the burner is removed smoothly, and the sample is observed until burning and glowing cease.
- The sample is then cleaned gently to remove loose soot and char, and the extent of carbonization is measured against the 50 mm and 540 mm acceptance limits.
For small insulated wires and cables, typically those with an overall diameter not exceeding 25 mm, the alternative procedure of IEC 60332-2-2 applies a softer diffusion flame for approximately 20 seconds, with acceptance based on the charred zone not approaching the upper support. In the North American VW-1 method of UL 1581, a 500 W Tirrill burner is applied five times for 15 seconds each, and the sample must not char excessively, ignite a kraft paper flag, or set fire to cotton placed beneath it. Although similar in spirit, these methods are not interchangeable, and the correct procedure must be selected according to the governing product standard.
The test is considered passed when the charred portion of the sample remains within the specified limits and the flame does not propagate to the supports. If any specimen fails, retesting and the final evaluation follow the rules of the applicable product standard or certification scheme.
Applications
Results from the single cable vertical burning test are used throughout the cable life cycle:
- Product certification and market access, including CE marking under the European Construction Products Regulation, where EN 60332-1-2 forms the baseline fire-performance requirement for cables entering the EU market.
- Mandatory and voluntary certification schemes for rated cables and wires, where compliance with the GB/T 18380 series or IEC 60332 series is a precondition for certification.
- Factory quality control and batch release testing, allowing manufacturers to verify the consistency of flame-retardant compounds and extrusion processes.
- Research and development of flame-retardant materials, including PVC, polyolefin, and low smoke zero halogen (LSZH) formulations, where the test serves as a fast screening tool.
- Procurement and tender compliance for construction projects, rail transit, marine, power generation, and industrial installations, where specifications commonly require evidence of passing the single-cable flame test.
- Supplier qualification and incoming inspection by OEMs, panel builders, and harness manufacturers.
It should be emphasized that this test evaluates a single cable in isolation. For installed cable bunches on trays or in bundles, the bunched cable burning tests of IEC 60332-3 or the European fire classification tests under EN 50399 are additionally required.
FAQ
What is the difference between the single cable test and the bunched cable test?
The single cable test (IEC 60332-1-2) evaluates one vertically suspended cable ignited by a 1 kW flame, whereas the bunched cable test (IEC 60332-3) evaluates multiple cables mounted together on a vertical ladder with a larger ignition source. Passing the single cable test is a basic requirement and does not demonstrate performance in grouped installations.
What are the pass criteria of IEC 60332-1-2?
The test passes if, after the specified flame application and cleaning of the sample, the upper edge of the charred region is at least 50 mm below the top support and the lower edge of the charred region does not extend more than 540 mm below the top support.
How is the flame application time selected?
The duration depends on the overall diameter of the finished cable sample: 60 seconds up to 25 mm, 120 seconds above 25 mm up to 50 mm, 240 seconds above 50 mm up to 75 mm, and 480 seconds above 75 mm.
Is IEC 60332-1-2 equivalent to the UL VW-1 test?
No. VW-1, described in UL 1581 and UL 2556, uses a different burner, five repeated 15-second flame applications, and additional indicators such as a kraft paper flag and cotton. A cable may pass one test and not the other, so the required method must be confirmed against the target market specification.
How long does testing take, and who should perform it?
A single test run, including conditioning, mounting, flame application, cooling, and measurement, is short, and a complete evaluation of a cable type with reporting is typically completed within a few working days. Testing should be performed by trained personnel in accredited third-party laboratories or properly equipped in-house facilities, since the test involves open flames, combustion products, and precise apparatus calibration.