Single Cable Vertical Burning Test: Standards, Procedures, and Pass Criteria

The single cable vertical burning test is a core fire-safety evaluation used to determine whether an individual wire or cable can resist flame propagation and self-extinguish after exposure to a controlled ignition source. Performed by manufacturers and accredited third-party laboratories in accordance with IEC 60332-1-2, GB/T 18380.12, EN 60332-1-2, and the UL 1581 VW-1 method, this test is a mandatory step in the certification and quality control of flame-retardant cables for buildings, equipment, energy, and transport applications. This article explains the test principle, sample requirements, procedures, evaluation criteria, and typical applications.

Technical Overview

The single cable vertical burning test, also known as the single vertical insulated wire or cable flame test, evaluates the flame-retardant performance of one insulated conductor or cable specimen when a standardized propane flame is applied to its lower portion while the specimen hangs vertically. The purpose is to verify that, once the ignition source is removed, the specimen extinguishes itself and that combustion does not spread along the cable to an unacceptable extent. It simulates an incipient fire condition in which a single cable is exposed to an external flame, and it is one of the most widely performed fire tests in the wire and cable industry.

The test is governed by an internationally harmonized family of standards. The primary method is specified in IEC 60332-1-2 (test on a single vertical insulated wire or cable with a 1 kW pre-mixed flame), together with IEC 60332-1-1, which defines the apparatus. In China, the identical national adoption is GB/T 18380.12, which replaced the older GB/T 12666.2 method. In Europe, the harmonized method EN 60332-1-2 supersedes the former EN 50265-2-1. For smaller cross-sections, IEC 60332-2-2 and GB/T 18380.22 define a scaled-down procedure for small single wires. In North America, the equivalent vertical wire flame test is the VW-1 method described in UL 1581 and UL 2556, while JIS C 3005 provides corresponding requirements in Japan.

• Evaluates self-extinguishing behavior and vertical flame propagation of a single specimen
• Uses a calibrated 1 kW pre-mixed propane flame as the ignition source
• Provides repeatable, comparative results suitable for certification and routine quality control
• Forms the baseline flame-retardancy requirement in most product standards for wires and cables

Testing Samples

For the IEC 60332-1-2 / GB/T 18380.12 method, specimens are cut from finished cable or wire, typically to a length of 600 mm ± 25 mm, straightened as far as practicable, and conditioned for a minimum of 16 hours at a standard laboratory temperature of 23 °C ± 5 °C before testing. The number of specimens is defined by the applicable product standard so that the result is representative of the production lot. The test can be applied to a broad range of specimen types:

Power and Distribution Cables

Single-core and multicore insulated power cables, including 450/750 V rated cables manufactured to IEC 60227, IEC 60502, GB/T 5023, and GB/T 12706, are routinely subjected to this test to demonstrate basic flame retardance.

Building Wiring and Fixed Installation Cables

Cables intended for permanent installation in residential, commercial, and industrial premises must satisfy the single-cable flame test as part of the fire-performance requirements referenced by regional wiring rules and cable certification schemes.

Control, Instrumentation, and Data Cables

Multicore control cables, screened instrumentation cables, and low-voltage data cables are tested to confirm that their sheathing and insulation compounds do not propagate flame along raceways and trays in the event of localized ignition.

Flexible Cords and Equipment Wiring

Flexible cords used on appliances, tools, and machinery, as well as appliance wiring material styles evaluated under UL frameworks, are commonly verified using either the IEC 60332-1-2 method or the VW-1 vertical flame test.

Small-Section Wires

Wires with small conductor cross-sections, such as automotive harness wire and electronic equipment wiring, are assessed using the reduced-scale single vertical flame procedure of IEC 60332-2-2 / GB/T 18380.22 or the VW-1 method.

Testing Items

During the test, the laboratory records and evaluates the following parameters, which together determine whether the specimen complies with the acceptance criteria of the specified standard:

• Extent of charring: the distance from the upper support to the highest point of the charred or affected region, measured after the specimen has cooled and been cleaned
• Afterflame time: the duration for which the specimen continues to burn after each removal of the test flame
• Flaming droplets and particles: whether burning material falls from the specimen during the test
• Ignition of the indicator paper: whether flaming drops ignite the paper placed beneath the specimen on the base of the enclosure
• Afterglow behavior: any continued glowing combustion without a visible flame after flame removal
• VW-1 specific items: duration of afterflame after each flame application, percentage of kraft paper flag consumed, and ignition of the cotton batting placed below the specimen

Testing Methods

Apparatus

The IEC 60332-1-2 / GB/T 18380.12 apparatus consists of a metal enclosure approximately 1200 mm high, 300 mm wide, and 450 mm deep with an open front, mounted under a suitable extraction hood that remains off during flame application to avoid disturbing combustion. The specimen is held vertically between an upper clamp and a lower support. Ignition is provided by a propane burner producing a nominal 1 kW pre-mixed blue flame, verified through gas flow control and periodic calibration in accordance with IEC 60332-1-1 / GB/T 18380.11. A layer of indicator paper is placed on the base of the enclosure below the specimen to detect ignition by flaming droplets.

Test Procedure

• Condition the specimen, then mount it vertically with the upper end secured in the clamp and the lower end hanging free
• Position the burner with its axis inclined at approximately 45 degrees to the specimen so that the tip of the blue inner cone contacts the lower portion of the specimen
• Apply the flame to the specimen four times, each application lasting 10 seconds, with intervals of 10 seconds between applications
• Observe and record afterflame duration after each application, any flaming droplets, and any ignition of the indicator paper
• After the specimen has cooled, wipe the surface with a dry cloth and measure the distance from the lower edge of the upper clamp to the highest point of charring

Acceptance Criteria

Under IEC 60332-1-2 / GB/T 18380.12, the specimen is considered to pass if the charred portion does not reach within 50 mm of the lower edge of the upper support, and the indicator paper placed below the specimen is not ignited by flaming drops or particles. Under the UL 1581 VW-1 method, the flame is applied five times for 15 seconds each with defined intervals; the specimen passes if the afterflame does not exceed 60 seconds after any application, no more than 25 percent of the kraft paper flag is charred or consumed, and the cotton batting below the specimen is not ignited by falling flaming particles. Any failure of a specimen constitutes a failure of the test unless the product standard specifies otherwise.

Applications

The single cable vertical burning test is a gateway requirement in nearly every sector where flame-retardant cables are specified. Typical applications include:

• Certification of building wires and power cables against IEC, EN, GB, and JIS product standards prior to market access
• Verification of appliance wiring material and flexible cords for consumer and industrial equipment
• Approval programs for automotive and transport wiring harnesses, where small-section wires are evaluated by the scaled method or the VW-1 procedure
• Marine and offshore cables within the IEC 60092 series, which require demonstrated single-cable flame retardance
• Photovoltaic and renewable-energy system cabling, whose certification standards reference EN 60332-1-2 / IEC 60332-1-2 as the baseline fire test
• Research and development of flame-retardant insulation and sheathing compounds, including halogenated and halogen-free formulations
• Incoming inspection and routine production quality control at cable manufacturers
• Regulatory compliance support, dispute verification, and supplier qualification performed by accredited third-party laboratories

FAQ

What is the purpose of the single cable vertical burning test?

It determines whether a single insulated wire or cable will resist the propagation of flame and extinguish by itself after a defined exposure to a standardized propane flame. It is the most fundamental flame-retardancy test for individual cable specimens and a prerequisite for most cable certifications.

Which standards specify this test?

The main standards are IEC 60332-1-2 with its apparatus standard IEC 60332-1-1, the Chinese adoption GB/T 18380.12, the European harmonized EN 60332-1-2, the small-wire method IEC 60332-2-2 / GB/T 18380.22, the UL 1581 and UL 2556 VW-1 method, and JIS C 3005. Although the apparatus and acceptance details differ slightly, the test principle of applying a controlled flame to a vertically suspended specimen is common to all of them.

What are the pass criteria for IEC 60332-1-2 / GB/T 18380.12?

The specimen passes if the charring does not extend to within 50 mm of the upper support and the paper placed beneath the specimen is not ignited by flaming droplets or debris. The flame is applied four times for 10 seconds each, with 10-second pauses between applications.

How does the VW-1 test differ from the IEC method?

The VW-1 test applies the burner flame five times for 15 seconds each and uses kraft paper flags attached to the specimen plus cotton batting beneath it. Its criteria limit afterflame time to 60 seconds, allow no more than 25 percent of a flag to be consumed, and prohibit ignition of the cotton. The IEC method uses a 1 kW pre-mixed propane burner with four 10-second applications and a 50 mm char-distance criterion.

How does this test differ from the bunched cable burning test?

The single cable test evaluates one specimen in isolation and represents a localized ignition scenario. The bunched cable test under IEC 60332-3 / GB/T 18380.3 mounts multiple cables on a vertical ladder and applies a larger burner for a longer duration, simulating fire spread across grouped cables installed together. Many specifications require both tests, since passing the single-cable test does not guarantee performance in a cable bundle.

Is the test performed on finished cables only?

No. Although the standard method is designed for finished wires and cables, the test is also widely used during compound development to compare the flame-retardant performance of insulation and sheathing materials, and for quality-control comparisons between production batches of finished product.

What should be done if a specimen fails the test?

A failure indicates that flame propagated too far along the specimen, burning persisted after flame removal, or flaming drops ignited the indicator paper. Manufacturers typically respond by improving the flame-retardant formulation, increasing filler loading such as aluminum hydroxide or magnesium hydroxide in halogen-free compounds, adjusting cross-linking or wall thickness, and then retesting through an accredited third-party laboratory to confirm compliance.