The single wire burning length test, formally known as the test for vertical flame propagation for a single insulated wire or cable, measures the charred length and self-extinguishing behavior of an insulated conductor after exposure to a standardized flame. Based on standards such as IEC 60332-1-2, IEC 60332-2-2, GB/T 18380, and the VW-1 procedure in UL 1581, it is widely used by manufacturers and accredited third-party laboratories to verify the flame-retardant performance of wires and cables.
Technical Overview
The single wire burning length test determines whether a single insulated conductor can resist the spread of flame along its length and self-extinguish once an external ignition source is removed. In the test, a straight length of wire is suspended vertically and exposed to a controlled propane flame. After the flame is withdrawn, the technician records the afterflame and afterglow behavior and measures the axial extent of the charred, thermally damaged region of the insulation, which is reported as the charred, or burning, length.
The test is specified in the IEC 60332-1 and IEC 60332-2 series, adopted in China as the GB/T 18380 series, and has close counterparts in North American requirements such as the VW-1 flame test of UL 1581. Because fire performance is a decisive safety property for wires installed in buildings, appliances, vehicles, and electronic equipment, this test is a routine element of product certification, factory quality control, and research verification, and is commonly performed by accredited third-party laboratories.
Testing Samples
General-Use Insulated Wires and Cables
Extruded-insulation products, including PVC, XLPE, polyolefin, and rubber insulated wires, such as general-purpose wiring rated up to 450/750 V, fixed installation wires, and internal wiring of electrical equipment. Specimens are typically cut to a length of 600 mm plus or minus 25 mm as required by IEC 60332-1-2 and GB/T 18380.12, straightened, and conditioned in a standard laboratory atmosphere before testing.
Small-Diameter Insulated Conductors
Fine wires with a small overall diameter, for example wires used in electronic appliances, instruments, and lightweight harnesses, are tested under IEC 60332-2-2 and GB/T 18380.22, which specifies a milder diffusion-flame procedure appropriate to smaller conductor sizes and thinner insulation.
Flexible Cords and Equipment Wires
Flexible cords, connecting wires for household appliances, internal wiring of information technology equipment, and low-voltage automotive-type cables can also be evaluated using the single-wire vertical burning procedure where the relevant product standard requires it.
Testing Items
The principal measured and observed quantities of the test include:
- Charred length, that is, the axial extent of carbonized or thermally damaged insulation, expressed as the length of the damaged region along the specimen or as the distance between the upper support and the upper edge of the charred portion.
- Afterflame time, the duration for which the specimen continues to burn with a visible flame after the test flame is removed.
- Afterglow time, the duration of glowing combustion without a visible flame after flaming has ceased.
- Behavior of falling debris, including whether flaming drops or particles detach and whether they ignite the indicator paper placed beneath the specimen.
- Condition of the insulation, including dripping, cracking, or shrinkage beyond the charred zone, recorded as supplementary observations.
Testing Methods
Test Apparatus and Conditions
The standard apparatus comprises a metal enclosure that shields the specimen from draughts, a vertical support that clamps the specimen at the top, a weight attached to the lower end to keep the wire straight, a propane burner producing a 1 kW premixed blue-cone flame for the IEC 60332-1-2 procedure, timing instrumentation, and indicator paper positioned beneath the specimen. Ambient conditions, gas supply, and flame geometry are verified before each test series.
Test Procedure
- Cut the specimen to the specified length, typically 600 mm plus or minus 25 mm, straighten it, condition it, and mark the reference points.
- Clamp the specimen vertically with its upper end supported and the specified weight suspended from the lower end.
- Position the burner at the defined angle so that the flame contacts the specimen at the prescribed point of application.
- Apply the test flame once for the time specified in the standard, commonly 60 s for the 1 kW procedure of IEC 60332-1-2 and GB/T 18380.12, and a shorter duration for small wires under IEC 60332-2-2.
- Withdraw the burner and record the afterflame and afterglow times, together with the behavior of any falling debris.
- After the specimen has cooled, gently wipe away soot and identify the boundary of the charred portion, aided by light bending, which reveals cracking of the carbonized insulation.
- Measure the charred length and its distance from the upper support, repeat the test on the number of specimens required by the product standard, and report all individual results.
Evaluation of Results
Under IEC 60332-1-2 and GB/T 18380.12, the test is considered passed when the upper edge of the charred portion does not approach within 50 mm of the top support, meaning the burning length is confined to the lower part of the specimen, and any specified indicator-paper requirement is met. For the VW-1 test under UL 1581, the specimen must stop burning within 60 s after each flame application, no more than 25 percent of the indicator flags may be charred, and the cotton or indicator material below the specimen must not be ignited. Failure to meet any single criterion on any specimen constitutes a failure of the test.
Applications
The single wire burning length test is applied throughout the wire and cable industry wherever the flame-retardant performance of an individual insulated conductor must be demonstrated:
- Product certification and market access for building wires, flexible cords, and equipment wiring under national and international certification schemes.
- Factory quality control, verifying that insulation and sheathing compounds retain their designed flame-retardant properties in routine production.
- Research and development of flame-retardant PVC, polyolefin, and low smoke zero halogen compounds, where charred length serves as a key comparative metric.
- Acceptance testing performed by accredited third-party laboratories on behalf of manufacturers, distributors, and end users.
- Failure analysis, in which the burning length of a returned field sample is compared against benchmark material.
FAQ
What is the pass criterion for the single wire burning length test?
In the IEC 60332-1-2 and GB/T 18380.12 procedure, the charred portion must not reach within 50 mm of the upper support, and the requirements concerning falling debris must be satisfied. The measured charred length therefore serves as a direct, quantitative indicator of the wire's ability to limit flame propagation.
Which standards specify this test?
The principal standards are the IEC 60332-1 series for the 1 kW premixed flame procedure, the IEC 60332-2 series for small wires, the corresponding GB/T 18380 series, EN 60332, and the VW-1 flame test within UL 1581 and UL 2556. Product standards such as the IEC 60227 and GB/T 5023 series reference these flame tests as routine requirements.
What specimen length and preparation are required?
A specimen length of about 600 mm plus or minus 25 mm is typical. The specimen is straightened and conditioned in a standard laboratory atmosphere before the test, and a weight is attached to the lower end so that the wire hangs perfectly straight during flame exposure.
How is the charred length actually measured?
After the specimen has cooled and burning has fully ceased, soot is wiped away and the boundary between charred and undamaged insulation is identified. Bending the wire lightly helps reveal where carbonized material cracks. The axial extent of the damaged region, and its distance from the upper support, is then measured with a ruler or scale and reported in millimeters.
How does this test differ from bunched cable burning tests?
The single wire test evaluates one insulated conductor in isolation, whereas bunched tests such as the IEC 60332-3 series assess flame spread across a group of cables mounted on a vertical ladder, simulating a larger installed cable bundle. Single-wire results are a precondition in most product standards, while bunched testing addresses fire propagation in installed configurations.
Can the test be outsourced to laboratories?
Yes. Accredited third-party laboratories routinely perform the single wire burning length test under IEC 60332-1-2, GB/T 18380, and related procedures, and issue test reports that are accepted for certification, regulatory compliance, and customer verification purposes.