Combustion Performance Analysis of Locomotive and Rolling Stock Cables

Combustion performance analysis of locomotive cables is a critical step in verifying the fire safety of railway vehicles, where flame retardancy, smoke emission, halogen content, and combustion gas toxicity directly determine passenger survivability and evacuation time. This article explains the scope of combustion testing for rolling stock cables in accordance with EN 45545-2, EN 50305, IEC 60332, IEC 61034, and IEC 60754, covering sample categories, test items, methodologies, acceptance criteria, and typical engineering applications.

Technical Overview

Locomotive and rolling stock cables operate in one of the most demanding fire-safety environments in electrical engineering. A railway vehicle is a confined, high-occupancy enclosure that frequently passes through long tunnels and underground sections, so a cable fault must not propagate fire along the harness, generate dense smoke that obscures escape routes, or release corrosive and toxic gases that incapacitate passengers and destroy electronic equipment. Combustion performance analysis therefore evaluates the complete fire behavior of cable systems, from ignition resistance and flame propagation to smoke density, halogen emission, gas toxicity, and circuit integrity.

In Europe, the governing framework is EN 45545-2, which harmonized former national specifications such as BS 6853, DIN 5510-2, and NF F 16-101 into a single classification system. The standard defines three hazard levels, HL1, HL2, and HL3, according to vehicle category and operating scenario, and assigns electric cables to requirement sets R15, R16, and R17 depending on their function. Product standards such as EN 50264 and EN 50306 define the construction of halogen-free railway cables, while EN 50305 provides the dedicated fire test methods. In China, equivalent requirements are covered by GB/T 12528, GB/T 19666, GB/T 18380, GB/T 17650, and GB/T 17651. Accredited third-party laboratories perform these tests for type approval, design validation, and routine conformity assessment.

Testing Samples

Low Smoke Zero Halogen Power Cables

Standard halogen-free power cables manufactured to the EN 50264 series, rated 0.6/1 kV or 1.8/3 kV, with EPR or XLPE insulation, halogen-free cross-linked or thermoplastic (HFFR) sheaths, and single-core or multi-core constructions with optional screens. These cables form the backbone of traction auxiliary, lighting, and distribution circuits on board.

Thin-Wall Data and Signal Cables

Thin-wall cables to EN 50306 and shielded bus or Ethernet cables used for train control networks, communication, door control, and passenger information systems. Their small conductor sizes and thin insulation make smoke and toxicity behavior particularly sensitive to sheath compound formulation.

Fire-Resistant and Circuit Integrity Cables

Cables serving emergency lighting, public address, fire detection, and evacuation systems that must continue to function during a fire. These designs combine mica-glass tape barriers with LSZH insulation and are classified for circuit integrity under EN 50200, BS 6387, or IEC 60331.

Specialty and High-Temperature Cables

Silicone rubber insulated cables, cross-linked compounds, and cables installed near traction equipment or in engine rooms where sustained operating temperatures exceed 90 degrees Celsius. Thermal aging before fire testing may be required to simulate end-of-life condition.

Testing Items

A comprehensive combustion performance program for locomotive cables typically includes the following items:

• Single-cable vertical flame propagation (flame retardancy of individual wires and cables)
• Bunched-cable vertical flame spread under a large ignition source
• Smoke density determination by light transmittance in a 3 m cubic chamber
• Halogen acid gas evolution (HCl, HBr, HF content of combustion products)
• Corrosivity of combustion gases (pH value and conductivity of the absorption solution)
• Toxicity of combustion gases (CO, CO2, HCl, HBr, HCN, HF, NOx, SO2)
• Conventional toxicity index calculation for hazard level assessment
• Limiting oxygen index (LOI) and temperature index of insulation and sheath compounds
• Fire resistance and circuit integrity under flame, mechanical shock, and water spray

Testing Methods

Flame Propagation and Flame Spread

Single cables are evaluated to IEC 60332-1-2 (EN 60332-1-2) by applying a 1 kW propane burner for 60 seconds to a 600 mm vertical sample; the charred portion must not approach the upper support within specified limits. EN 50305 clause 9.1 applies a modified procedure for railway cables, including a supplementary burner for larger cross-sections. Bunched installations are assessed to IEC 60332-3-24, in which a 20.5 kW burner impinges on vertically bundled cables for 20 minutes and the maximum char height above the burner is measured. GB/T 18380 series methods are technically equivalent to the IEC 60332 procedures.

Smoke Density Measurement

Smoke production is measured to IEC 61034-2 (EN 61034-2, GB/T 17651.2) in a 3 m cubic chamber. Cable samples are mounted horizontally above a standard alcohol fire source, and the attenuation of a horizontal light beam is recorded during and after combustion. EN 45545-2 prescribes minimum light transmittance limits, typically between 60 and 90 percent depending on the requirement set and hazard level, ensuring that escape routes remain visible during evacuation.

Halogen and Corrosivity Determination

Halogen acid gas content is determined to IEC 60754-1, in which combusted sample gases are absorbed in water and quantified; halogen-free materials must evolve no more than 0.5 percent halogen acid (approximately 5 mg/g). Corrosivity is assessed to IEC 60754-2 by measuring the pH and conductivity of the gas absorption solution, with halogen-free acceptance criteria of pH not less than 4.3 and conductivity not exceeding 10 microsiemens per millimeter. These limits protect signaling and control electronics from acid damage.

Toxicity of Combustion Gases

EN 50305 clause 9.2 specifies a tube-furnace method in which the sample is combusted in a controlled air stream and the evolved gases are collected and analyzed for CO, CO2, HCl, HBr, HCN, HF, NOx, and SO2. A conventional toxicity index is calculated by comparing each concentration with reference danger values for a 30-minute exposure. Alternatively, EN 45545-2 permits the ISO 5659-2 smoke chamber operating at radiant fluxes of 25 or 50 kW/m2 combined with FTIR gas analysis, from which the conventional index of toxicity (CIT) is derived using weighted summation of the individual gas contributions.

Oxygen Index and Temperature Index

Material-level flammability is characterized to ISO 4589-2 by determining the limiting oxygen index, the minimum oxygen concentration required to support candle-like combustion. Modern halogen-free flame-retardant railway compounds typically achieve LOI values of 30 percent or higher. ISO 4589-3 extends this to a temperature index, the temperature at which the LOI falls to the ambient oxygen concentration, providing a margin assessment for hot environments.

Fire Resistance and Circuit Integrity

Cables required to maintain function during fire are tested energized under direct flame at temperatures of 750 to 950 degrees Celsius for 30 to 120 minutes, with circuit continuity continuously monitored. EN 50200 establishes the PH30, PH60, and PH120 classifications for small unprotected cables, while BS 6387 category CWZ and IEC 60331-31 add mechanical shock and water spray during and after flame exposure to simulate firefighting conditions.

Applications

• Type approval and EN 45545-2 hazard level (HL1, HL2, HL3) certification of new cable designs for rolling stock manufacturers
• LSZH compound development and formulation optimization for insulation and sheath materials
• Compliance demonstration against project specifications for metro, tram, EMU, high-speed train, and locomotive programs
• Incoming inspection and batch conformity control of cable deliveries to rolling stock and harness manufacturers
• Failure analysis of field-returned cables and root cause investigation after thermal events
• Technical support for vehicle-level fire safety engineering, hazard analysis, and tender documentation

FAQ

What do hazard levels HL1, HL2, and HL3 mean?

They are fire safety categories defined in EN 45545-2 according to vehicle design and operation. HL1 applies to vehicles with low occupancy and easy evacuation, HL2 to vehicles such as double-deck and sleeping cars with more difficult egress, and HL3 to automated driverless trains where evacuation support is limited. Higher hazard levels impose stricter flame spread, smoke density, and toxicity limits.

Why is smoke density especially critical for railway cables?

Unlike buildings, railway vehicles operate in tunnels where smoke cannot dissipate quickly. Dense smoke reduces visibility along escape paths and accelerates incapacitation through irritant gases, so EN 45545-2 imposes strict minimum light transmittance requirements measured to IEC 61034-2.

What qualifies a cable as low smoke zero halogen?

The material must satisfy the halogen-free criteria of IEC 60754, meaning halogen acid evolution below 0.5 percent, pH not less than 4.3, and conductivity not exceeding 10 microsiemens per millimeter, combined with high light transmittance in the smoke chamber test. Typical constructions use EPR or XLPE insulation with cross-linked polyolefin HFFR sheaths.

How should samples be prepared for testing?

Samples should be representative of series production, supplied in the lengths and quantities specified by each method, and conditioned in a standard atmosphere of 23 plus or minus 5 degrees Celsius before testing. For large cross-section power cables, supplementary burner tests per EN 50305 clause 9.1 may require additional sample lengths.

Are EN, IEC, and GB test results mutually recognized?

Many GB/T methods, such as GB/T 18380, GB/T 17650, and GB/T 17651, are technically aligned with their IEC counterparts, so data are generally comparable. However, EN 45545-2 and EN 50305 incorporate railway-specific procedures and classification logic, so full market access in Europe requires testing directly against the EN framework in accredited third-party laboratories.