Low voltage cable combustion testing is a critical fire safety evaluation that measures the flame propagation, smoke emission, and toxic gas release characteristics of power cables rated up to 0.6/1 kV under controlled laboratory conditions. This article provides a comprehensive overview of applicable international standards, sample preparation requirements, test procedures, pass criteria, and typical applications, helping cable manufacturers, specifiers, and testing laboratories ensure full compliance with fire safety regulations.
Technical Overview
Low voltage cable combustion testing refers to the systematic evaluation of the fire performance of finished power cables rated 0.6/1 kV and below when exposed to standardized ignition sources and fire conditions. Because polymeric insulation and sheathing materials such as PVC, XLPE, PE, and LSZH (low smoke zero halogen) compounds are combustible, installed cable runs can act as pathways for flame propagation and as sources of dense smoke, corrosive gases, and toxic combustion products during a fire event. Combustion testing verifies that cables meet flame-retardant, low-smoke, halogen-free, and fire-resistance requirements before installation in buildings, infrastructure, and industrial facilities.
The core dimensions of a complete combustion test program are summarized below:
- Flame propagation behavior of a single cable under direct impingement
- Vertical flame spread of bunched cable installations simulating real tray conditions
- Smoke density and light transmittance reduction in an enclosed space
- Halogen acid gas evolution and combustion gas acidity
- Circuit integrity (fire resistance) of energized cables under flame, mechanical shock, and water spray
The most widely applied standards include IEC 60332-1-2, IEC 60332-3-21/-22/-23/-24/-25, IEC 61034-2, IEC 60754-1 and -2, IEC 60331, and BS 6387, together with their Chinese national equivalents GB/T 18380, GB/T 17651, GB/T 17650, and GB/T 19216, as well as the European classification standards EN 50399 and EN 13501-6 under the EU Construction Products Regulation (CPR).
Testing Samples
Single Vertical Cable Samples
For the single cable flame propagation test (IEC 60332-1-2 / GB/T 18380.12), representative specimens are cut from finished cable to a length of approximately 600 mm. The specimens must be straightened, cleaned, and conditioned in a controlled environment before testing. The flame application time is determined by the overall cable diameter: 60 seconds for cables up to 25 mm, 120 seconds for cables above 25 mm and up to 50 mm, and 240 seconds for cables above 50 mm.
Bunched Cable Samples
For the vertical bunched cable test (IEC 60332-3 series / GB/T 18380.3 series), finished cable samples approximately 3.5 m in length are mounted on a standard vertical steel ladder. The total cable volume per meter of ladder length determines the test category: Category A requires 7 liters per meter, Category B requires 3.5 liters per meter, Category C requires 1.5 liters per meter, and Category D requires 0.5 liters per meter. Category A samples are mounted on both the front and rear faces of the ladder, while other categories use the front face only. These categories correspond to the Chinese flame-retardant grades ZA, ZB, ZC, and ZD respectively.
Low Smoke Zero Halogen Material Samples
For halogen content and acidity testing (IEC 60754 / GB/T 17650), material specimens of approximately 1 gram are taken from the insulation and sheath compounds, chopped into small pieces, and combusted in a tube furnace. For smoke density testing (IEC 61034-2 / GB/T 17651.2), finished cable samples of approximately 1 meter in length are placed horizontally on the test ladder inside the smoke chamber.
Fire Resistant Cable Samples
For fire resistance tests (IEC 60331 / GB/T 19216 / BS 6387), samples of finished fire-resistant cable are fitted with electrical terminations at both ends and energized at rated voltage during the test. The sample configuration must preserve the installed structure, including any mica-glass tape fire barrier layers, to evaluate genuine circuit integrity performance.
Testing Items
A complete low voltage cable combustion test program typically includes the following items, which may be selected individually or combined depending on the product specification and certification requirements:
- Single vertical cable flame propagation test (flame retardancy of an individual cable)
- Vertical bunched cable flame spread test, Categories A, B, C, or D
- Smoke density test with light transmittance measurement
- Determination of halogen acid gas evolution (HCl, HBr, HF content)
- Determination of combustion gas acidity by pH value and conductivity
- Fire resistance test under flame only, or under combined flame, mechanical shock, and water spray
- Optional supplementary items such as oxygen index and temperature index of materials
Testing Methods
Single Vertical Cable Flame Test
The specimen is clamped vertically with its upper end fixed to a support. A standardized 1 kW premixed propane burner producing a blue flame is applied to the specimen at an angle of 45 degrees at a defined position below the upper support. After the specified flame application time, the burner is removed and the extent of charring is measured. The pass criteria require that the charred portion does not extend to within 50 mm of the upper support, and that the distance between the lower end of the charred portion and the top of the burner is greater than 540 mm.
Vertical Bunched Cable Flame Spread Test
The cable samples are mounted on a 3.5 m vertical steel ladder inside a test enclosure with an exhaust hood. A propane-fired ribbon burner conforming to IEC 60332-3-10 is applied to the lower portion of the cable bunch: a 20 kW burner for 20 minutes for Categories B, C, and D, or a 40 kW burner for 40 minutes for Category A. After the test, the maximum height of the charred region is measured. The pass criterion is that the charred portion does not extend beyond 2.5 m above the bottom edge of the burner.
Smoke Density Test
The cable sample is mounted horizontally on a support inside a 3 m × 3 m × 3 m enclosed test chamber. A defined quantity of alcohol is placed beneath the sample as the ignition source and ignited. A horizontal light path between a stabilized lamp and a photometric cell measures light transmittance continuously during and after combustion. The test result is reported as the minimum light transmittance value. For low smoke halogen-free (WDZ) cables, specifications such as GB/T 19666 and most project specifications require a minimum transmittance of 60 percent or higher.
Halogen Acid Gas Emission Test
In IEC 60754-1 / GB/T 17650.1, the material specimen is combusted in a tube furnace at approximately 800 degrees Celsius in a stream of air, and the evolved gases are absorbed in distilled water. The halogen acid content is determined by silver nitrate titration. Low smoke zero halogen materials must contain less than 0.5 percent halogen acid by mass. In IEC 60754-2 / GB/T 17650.2, the combustion gases are absorbed in water and the acidity is evaluated; typical requirements are a pH value of not less than 4.3 and a conductivity of not more than 10 microsiemens per millimeter.
Fire Resistance Test
For BS 6387, the energized sample is exposed to a tube burner at specified temperatures: Category C requires 950 degrees Celsius for 3 hours, Category W adds water spray at 650 degrees Celsius, and Category Z adds mechanical shock at 950 degrees Celsius. The combined category CWZ represents the most demanding requirement. For IEC 60331-21 / GB/T 19216, the energized sample withstands a 750 degree Celsius flame for 90 minutes, optionally followed by mechanical shock or water spray phases. The pass criterion in all cases is that circuit integrity is maintained, with no operation of the protective fuse or breakdown of the insulation during the test and the subsequent observation period.
Applications
Low voltage cable combustion testing is essential wherever cable fire performance is regulated or where life safety, property protection, and system availability are critical. Typical application areas include:
- High-rise residential and commercial buildings, hospitals, schools, and airports
- Metro systems, railway tunnels, and road tunnels with restricted escape routes
- Power plants, substations, and nuclear facilities requiring circuit integrity
- Data centers and telecommunication facilities where smoke can damage equipment
- Petrochemical plants, offshore platforms, and marine installations
- Fire alarm, emergency lighting, and life safety circuits requiring fire-resistant cable
- CE marking under the EU Construction Products Regulation using EN 13501-6 classes B2ca through Eca
Test reports and certifications derived from these combustion tests are routinely used for product type approval, factory production control, tender compliance, third-party certification, and market surveillance by regulatory authorities.
FAQ
What is the difference between flame-retardant and fire-resistant cables?
Flame-retardant cables are designed to resist the ignition and propagation of fire along the cable, limiting flame spread after the ignition source is removed. Fire-resistant cables go further by maintaining electrical circuit integrity during a fire, using mica-glass tape barriers that keep conductors insulated at high temperature. Flame retardancy is evaluated by the IEC 60332 series, while fire resistance is evaluated by IEC 60331, BS 6387, or GB/T 19216.
Which bunched cable test category should be selected?
The category should match the expected installation density and the applicable code. Category C (1.5 L/m, 20 kW, 20 minutes) is the baseline requirement for general building wiring in many markets and corresponds to grade ZC in China, while Category A (7 L/m, 40 kW, 40 minutes) is required for high-density installations such as power plants and tunnels. Specifiers should always confirm the category stated in the project specification before ordering tests.
What are the typical pass criteria for smoke and halogen tests?
For low smoke halogen-free cables, the commonly specified requirements are a minimum light transmittance of 60 percent in the IEC 61034-2 smoke chamber, halogen acid gas content below 0.5 percent by mass in IEC 60754-1, a pH value of at least 4.3, and a conductivity not exceeding 10 microsiemens per millimeter in IEC 60754-2.
How long does a complete combustion test program take?
Single cable and bunched cable flame tests are typically completed within one to two weeks including conditioning, while a full program combining flame spread, smoke density, gas acidity, and fire resistance tests generally requires three to six weeks depending on laboratory scheduling and whether any retests are needed.
Are combustion tests mandatory for low voltage cables?
Requirements vary by market. In the European Union, cables installed in buildings fall under the Construction Products Regulation and must be classified under EN 13501-6. In China, flame-retardant and fire-resistant grades defined in GB/T 19666 and related standards are widely enforced through project specifications and certification schemes. Even where not legally mandatory, combustion testing is standard industry practice for safety-critical installations.