Single Cable Burning Test: Flame Propagation Testing of Individual Insulated Wires and Cables

The single cable burning test, also known as the single vertical insulated wire flame test, evaluates the ability of an individual wire or cable to resist flame propagation and self-extinguish after exposure to a standardized propane flame. It is a core fire-safety requirement under standards such as IEC 60332-1-2, IEC 60332-2-2, EN 60332, GB/T 18380, and UL 1581 (VW-1), and is routinely carried out by accredited third-party laboratories for product certification, regulatory compliance, and quality control. This article explains the technical principles, sample requirements, test methods, acceptance criteria, and typical applications of the single cable burning test.

Technical Overview

The single cable burning test is a bench-scale fire performance test in which a vertically mounted insulated wire or cable specimen is exposed to a small, calibrated ignition source. The purpose of the test is to determine whether the specimen will resist flame propagation along its length and whether it will stop burning (self-extinguish) once the ignition source is removed. The test simulates a localized thermal event, such as an electrical fault or an external spark impinging on a single cable, and it forms the baseline requirement for flame-retardant cable design worldwide.

Several internationally recognized standards govern this test. IEC 60332-1-2 specifies the single vertical insulated wire or cable test using a 1 kW pre-mixed propane flame, while IEC 60332-1-3 covers the assessment of flaming combustion particles and droplets. IEC 60332-2-2 addresses small-diameter wires. In Europe these procedures are harmonized as EN 60332, in China they are adopted as the GB/T 18380 series, and in North America the closely related VW-1 flame test is defined under UL 1581 and UL 2556. Japanese products are commonly evaluated against the flame test methods in JIS C 3005, and automotive low-voltage cables follow the flame propagation requirements of ISO 6722.

It is important to distinguish the single cable burning test from bunched cable burning tests such as IEC 60332-3 or EN 50399, which evaluate flame spread on groups of cables installed in vertical trays. The single cable test is the entry-level indicator of flame-retardant performance and is frequently referenced in product specifications, factory approvals, and classification systems such as the Euroclass ratings for construction products under EN 50575.

Testing Samples

Power Distribution and Control Cables

Typical specimens include single-core building wires, fixed-installation power cables, control cables, and flexible cords. Specimens are tested complete, with all insulation, bedding, and sheath layers exactly as supplied, so that the flame response of the finished product rather than an individual material is evaluated.

Electronic, Data, and Optical Fiber Cables

Small-diameter products such as hook-up wire, equipment internal wiring, data and communication cables, and optical fiber cables are also covered. For small overall diameters, the dedicated small-wire procedure of IEC 60332-2-2 is applied, since the reduced specimen mass and heat capacity require an adjusted flame application.

Specimen Preparation and Conditioning

Specimens are cut to a length of 600 mm ± 25 mm and straightened without damaging the insulation. Prior to testing, specimens are conditioned in a standard atmosphere, typically for at least 16 hours at a temperature of 23 °C ± 5 °C. A sufficient number of conditioned specimens is prepared, and laboratories commonly test multiple specimens (for example three) to confirm consistent behavior across the sample lot.

Testing Items

The single cable burning test evaluates the following performance characteristics:

• Vertical flame propagation resistance and the extent of charring along the specimen
• Self-extinguishing capability, expressed as the afterflame time following each application of the test flame
• Formation of flaming droplets or particles and their ability to ignite a cotton indicator placed beneath the specimen
• Minimum undamaged distance between the upper support and the onset of the charred or affected portion
• Overall pass or fail judgment against the flame-retardant requirements of the applicable standard or product specification

Testing Methods

Apparatus and Test Environment

The test is performed inside a sheet-metal enclosure approximately 1200 mm high with controlled ventilation openings to stabilize the flame while shielding it from draughts. A vertical support fixture holds the specimen taut between an upper clamp and a lower anchor. The ignition source is a pre-mixed propane and air burner with a nominal thermal output of 1 kW, producing a calibrated blue flame whose characteristics are verified before each test campaign. The enclosure, burner calibration, and ambient conditions are strictly controlled in accredited third-party laboratories to guarantee repeatability.

IEC 60332-1-2 / GB/T 18380.12 Procedure

The conditioned specimen is mounted vertically, and the burner is inclined at 45 degrees to the specimen axis. The flame is applied to a defined point on the lower portion of the specimen in four separate applications, each lasting 10 seconds, with defined intervals between applications during which the specimen is observed for continued burning. After the final application, the specimen is allowed to burn out or self-extinguish, after which the damaged region is measured. The test is considered passed when the charred or affected portion does not approach within 50 mm of the upper support, demonstrating that the flame did not propagate along the specimen.

IEC 60332-1-3 Droplet and Particle Assessment

Where the specification also requires control of flaming debris, the complementary procedure of IEC 60332-1-3 is applied. A layer of cotton wool is positioned on a support beneath the specimen. Any molten material, flaming droplets, or incandescent particles falling from the specimen that ignite the cotton indicator constitute a failure, regardless of the measured char distance.

IEC 60332-2-2 Procedure for Small Wires

For small insulated wires and cables of reduced overall diameter, IEC 60332-2-2 (GB/T 18380.22) prescribes an adapted flame application reflecting the lower thermal mass of the specimen. The evaluation principle remains identical: the flame must not propagate to the vicinity of the upper support, and the specimen must demonstrate self-extinguishing behavior.

UL 1581 VW-1 Equivalent

For products intended for the North American market, the VW-1 vertical wire flame test is the customary equivalent. The specimen is exposed repeatedly to a gas burner flame, with a kraft paper flag attached near the top of the specimen and a cotton layer placed below. Acceptance requires that the cotton is not ignited, the indicator flag is not charred beyond the permitted extent, and the specimen flame extinguishes within the time limit specified after each flame application.

Evaluation and Reporting

Upon completion, the laboratory records the following information in the test report:

• Complete specimen identification, including cable type, manufacturer designation, dimensions, and construction
• The applicable standard, edition, and any deviations from the standard procedure
• Number and duration of flame applications and observed afterflame behavior
• Measured distance between the upper support and the start of the charred portion
• Observation of flaming droplets or particles and the condition of the cotton indicator
• An explicit statement of conformity (pass or fail) with the acceptance criteria

Applications

Building and Construction

Under the European Construction Products Regulation, cables must be classified under EN 50575 before being placed on the market. The single cable burning test according to EN 60332-1-2 is the basic requirement for entry-level Euroclass ratings, and equivalent requirements exist in other national building codes through the GB/T 18380 series and related standards.

Rail Transit and Infrastructure

Metro, main-line railway, and tunnel installations commonly specify single cable flame test compliance as part of rolling stock cable and trackside cable specifications, frequently combined with low smoke zero halogen requirements to limit fire hazard in confined spaces.

Ships, Offshore, and Industrial Plants

Shipboard electrical installations reference IEC 60092 series requirements, which include single cable flame propagation testing. Industrial plants, refineries, and power stations similarly rely on this test as a minimum acceptance criterion for flame-retardant instrumentation and control cables.

Electronics, Appliances, and Automotive

Internal wiring of appliances, information technology equipment, and luminaires is widely certified using the VW-1 test, while road vehicle low-voltage cables are validated against the flame propagation provisions of ISO 6722. These tests support manufacturer declarations, safety certifications, and regulatory market surveillance.

Manufacturing Quality Control and R&D

Cable producers apply the single cable burning test during compound development, formulation changes, and routine factory surveillance. Accredited third-party laboratories also perform the test for incoming inspection, supplier qualification, and independent verification in cases of dispute or market compliance checks.

FAQ

What is the difference between the single cable burning test and the bunched cable burning test?

The single cable test (IEC 60332-1-2 / IEC 60332-2-2) evaluates one vertically mounted specimen exposed to a 1 kW flame, whereas bunched cable tests such as IEC 60332-3 and EN 50399 evaluate flame spread across a group of cables mounted on a vertical ladder. Passing the single cable test is normally a prerequisite before bunched cable performance is assessed.

What is the pass criterion for IEC 60332-1-2?

The test is passed when the charred or affected portion of the specimen does not come within 50 mm of the upper support, meaning the flame did not propagate along the full length of the specimen. If flaming droplets or particles are assessed under IEC 60332-1-3, the cotton indicator must not be ignited.

How many specimens are required?

Specimen length is 600 mm ± 25 mm, and the standard test campaign uses a defined set of conditioned specimens, with laboratories commonly evaluating three specimens to verify consistent results. The exact number depends on the applicable standard and any additional requirements in the product specification.

Does passing the single cable burning test satisfy all fire safety requirements?

No. The single cable test only demonstrates basic flame retardancy of an individual cable. Depending on the application, additional assessments may be required, including bunched cable flame spread, smoke density under IEC 61034, halogen acid gas emission, toxicity, and corrosivity testing.

Which products should be tested?

Any insulated wire or cable intended for fixed installation or equipment wiring where flame retardancy is specified should be tested, including power cables, control and instrumentation cables, data and telecom cables, optical fiber cables, and appliance internal wiring. Testing should be conducted on the finished cable construction as supplied to the market.