Combustion Performance Evaluation of Battery Box Materials for Metro Vehicles

Battery boxes are safety-critical onboard equipment in metro vehicles, and the non-metallic materials used in their enclosures, insulation, sealing and wiring systems must undergo rigorous combustion evaluation before entering service. This article explains the applicable fire safety standards, typical test samples, key testing items, laboratory methods and acceptance criteria used to evaluate the combustion performance of metro battery box materials, supporting material selection, type approval and compliance with EN 45545-2, TB/T 3138, NFPA 130 and related frameworks.

Technical Overview

Battery boxes on metro vehicles—whether housing nickel-cadmium or lead-acid auxiliary batteries, or lithium-ion traction and energy-storage packs—contain significant stored electrical energy and are typically mounted beneath the car body, on the roof, or inside technical cabinets. A fire or thermal event involving these enclosures in a tunnel environment poses severe risks to passenger evacuation and infrastructure, so the polymeric and composite materials used in and around the battery box must be systematically evaluated for ignitability, flame spread, heat release, smoke production and smoke toxicity.

The dominant evaluation framework for international metro projects is EN 45545-2 (Railway applications – Fire protection on railway vehicles – Requirements for fire behaviour of materials and components), which assigns each material to a requirement set and links acceptance criteria to hazard levels HL1 to HL3 defined in EN 45545-1. Underground metro operation generally corresponds to the higher hazard levels, commonly HL2 or HL3, depending on the operating and design categories of the vehicle. Externally mounted equipment such as underframe battery boxes is commonly assessed under the requirement set for external and underframe components (for example R17), which focuses on small-flame ignitability and smoke density, while components installed inside the passenger compartment follow the more severe interior requirement sets.

For projects in the Chinese domestic market, TB/T 3138 specifies flame-retardant requirements for non-metallic materials used on locomotives and rolling stock, while North American projects typically reference NFPA 130. Product-level standards such as IEC 62928 for lithium-ion traction batteries and IEC 62619 for industrial lithium cells add enclosure and material-level fire provisions. Electrical component materials inside the battery box are additionally screened using the glow-wire test methods of the IEC 60695-2 series and the UL 94 classifications aligned with IEC 60695-11-10. Testing is normally performed by ISO/IEC 17025 accredited third-party laboratories, and the resulting reports support design verification, type approval and supplier material qualification.

Testing Samples

The scope of sampling covers every non-metallic material that could contribute fuel to a fire originating in or spreading from the battery box. Materials should be submitted at the thickness and color used in the actual application, since test results are thickness-dependent, and multiple colors of the same polymer often require separate evaluation.

Enclosure and Housing Materials

These include polymeric and composite housings such as polycarbonate, ABS, modified PPE, sheet molding compound (SMC) and bulk molding compound (BMC), as well as organic coatings, paint systems and powder coatings applied to metallic enclosures. Although stainless steel or aluminum enclosures are themselves non-combustible, their coatings and internal liners remain within the evaluation scope.

Insulation, Buffer and Potting Materials

Thermal management and cell-fixing materials such as polyurethane and EPDM foam pads, mica sheets, aerogel blankets, ceramic fiber boards, epoxy or silicone potting compounds, and structural adhesives are evaluated because they are located closest to potential ignition sources within the battery system.

Electrical Insulation and Wiring Materials

This group covers cable insulation and sheathing compounds (such as XLPO and low-smoke zero-halogen formulations), busbar insulation sleeves, connector housings, terminal blocks, fuse holders, printed circuit board substrates and battery management system enclosures.

Sealing, Bonding and Auxiliary Materials

Silicone gaskets, foam sealing strips, double-sided adhesive tapes, cable transit blocks, nameplates, labels, protective films and ventilation membranes are included as small but numerous fuel sources that can propagate flame across the assembly.

Testing Items

Depending on the governing standard, requirement set and hazard level, the combustion evaluation of metro battery box materials typically includes the following items:

• Limiting oxygen index (LOI) to characterize intrinsic flame retardancy of the material.
• Small-flame ignitability using a single flame source, including assessment of flaming drips.
• Lateral flame spread behavior under radiant heating, including critical flux at extinguishment.
• Heat release rate behavior and the maximum average rate of heat emission (MARHE).
• Smoke production, expressed as maximum specific optical density (Ds,max) and the cumulative smoke parameter VOF4.
• Smoke toxicity, expressed as the conventional toxicity index (CIT) derived from quantified gas yields.
• Glow-wire ignition temperature (GWIT) and glow-wire flammability index (GWFI) for electrical components.
• UL 94 flammability classifications such as V-0, V-1, V-2, 5VA and HB for polymeric parts.
• Flame spread index and specific optical density per ASTM E162 and ASTM E662 for NFPA 130 projects.

Testing Methods

Each test item is performed under controlled laboratory conditions using internationally standardized methods. Specimens are conditioned to equilibrium in a standard atmosphere of approximately 23 °C and 50 % relative humidity before testing, and typically three to six specimens per condition are tested, plus spares, to account for material variability.

Ignitability – EN ISO 11925-2

A small propane flame is applied to the bottom edge or surface of a vertically or horizontally oriented specimen for 15 or 30 seconds. The material passes if the flame tip does not reach the 150 mm gauge mark within the specified time and if flaming drops do not ignite filter paper placed beneath the specimen. This is the primary screening test for underframe and roof-mounted equipment.

Limiting Oxygen Index – EN ISO 4589-2

The specimen is burned in a vertical glass chimney under a controlled oxygen and nitrogen mixture. The minimum oxygen concentration that sustains candle-like burning for a defined time or length is reported as the LOI; higher values indicate better flame retardancy. LOI is widely used for material screening and for acceptance under TB/T 3138.

Lateral Flame Spread – EN ISO 5658-2

A vertically oriented specimen is exposed to a gas-fired radiant panel, and a pilot flame ignites the hot end. The spread of the flame front along the specimen is recorded, and the critical heat flux at which the flame extinguishes is calculated. This method applies to larger interior and structural components at higher hazard levels.

Heat Release – EN ISO 5660-1 Cone Calorimeter

A 100 mm × 100 mm specimen is exposed to a radiant flux of typically 50 kW/m² with piloted ignition. Oxygen consumption calorimetry yields the heat release rate curve, from which the maximum average rate of heat emission (MARHE) is derived. MARHE must not exceed the limit assigned by the applicable requirement set and hazard level.

Smoke Density – EN ISO 5659-2

Specimens are burned in a closed smoke chamber under both flaming and non-flaming modes at 25 kW/m² irradiance. Light transmittance measurements across the chamber produce the specific optical density curve, summarized as Ds,max and VOF4. Underground metro applications impose strict smoke density limits because smoke obscuration directly affects evacuation times in tunnels.

Smoke Toxicity – EN ISO 5659-2 with FTIR Analysis per EN 17084

Gas effluents generated in the smoke chamber are transferred to a Fourier-transform infrared gas analyzer, and concentrations of CO, CO₂, HCN, HCl, HBr, HF, SO₂ and NOₓ are quantified at defined exposure intervals, typically four minutes. The results are converted into the conventional toxicity index (CIT), which must remain below the limit specified for the requirement set and hazard level.

Glow-Wire Tests – IEC 60695-2-10, -2-12 and -2-13

A heated glow wire at temperatures between 550 °C and 960 °C is applied to the specimen. The glow-wire flammability index (GWFI) determines whether the part self-extinguishes without igniting surrounding materials, while the glow-wire ignition temperature (GWIT) identifies the temperature at which the material no longer ignites. These tests are mandatory for live electrical parts and unattended equipment inside the battery box.

Vertical and Horizontal Burning – IEC 60695-11-10 (UL 94)

Specimens are exposed to a 50 W gas flame in horizontal or vertical orientation. Vertical ratings from V-2 to V-0 classify materials by afterflame time, afterglow behavior and flaming drips; for example, V-0 requires each afterflame time to be no longer than 10 seconds and prohibits ignition of the cotton indicator. UL 94 data are widely used for component-level qualification of connectors, housings and circuit board materials.

North American Methods – ASTM E162, E662 and E1354

For NFPA 130 projects, the flame spread index is measured with the radiant panel method of ASTM E162, smoke density with the NBS smoke chamber method of ASTM E662, and heat release optionally with the cone calorimeter method of ASTM E1354. Typical interior material acceptance limits cited by NFPA 130 include a flame spread index not exceeding 25 and specific optical density values of Ds(1.5) not exceeding 100 and Ds(4) not exceeding 200.

Applications

Combustion evaluation of battery box materials is applied throughout the life cycle of metro rolling stock programs, from research and development to in-service requalification. Typical application scenarios include:

• Type approval of auxiliary battery boxes and emergency power enclosures on metro electric multiple units and coaches.
• Onboard lithium-ion energy storage systems for catenary-free trams and metro sections, including battery boxes for traction power and regenerative energy absorption.
• Battery systems supplying emergency ventilation, lighting and door operation during power failures in tunnels.
• Export projects requiring EN 45545-2 compliance, domestic projects referencing TB/T 3138, and North American projects referencing NFPA 130.
• Material screening during development, verification of flame-retardant reformulations, supplier change management and periodic requalification of serial parts.
• Technical support for battery system design verification against IEC 62928 and IEC 62619 enclosure and material fire requirements.

FAQ

Which hazard level applies to metro battery box materials?

The hazard level is determined by the operating category and design category of the vehicle according to EN 45545-1. Underground metro operation generally results in HL2 or HL3 classification, which imposes stricter ignitability, smoke density and toxicity limits than HL1. The applicable requirement set also differs for underframe, roof-mounted and interior-installed equipment.

Do metallic battery box enclosures require testing?

Metallic enclosures such as stainless steel or aluminum housings are themselves non-combustible and generally do not require flame testing. However, organic coatings applied to the metal, internal polymeric liners, gaskets, cable entries and all non-metallic components inside the enclosure remain fully within the evaluation scope.

What sample quantities and dimensions are needed?

Sample requirements depend on the selected test methods. Common specimen sizes include 100 mm × 100 mm plates for cone calorimeter testing, 75 mm × 75 mm plaques for smoke density and toxicity testing, and small bars for oxygen index, ignitability, glow-wire and UL 94 testing. Typically three to six specimens per condition are required, and materials should be supplied at production thickness.

How long does a complete evaluation take?

A full test program covering ignitability, smoke density, toxicity and component-level tests usually takes four to eight weeks, including specimen conditioning, laboratory scheduling and report preparation. Screening with LOI and UL 94 can often be completed within one to two weeks.

What happens if a material fails the acceptance criteria?

Failed materials must be reformulated, replaced with flame-retardant alternatives, or protected with fire barriers and redesigned layouts, after which the relevant tests are repeated. Early screening with LOI, UL 94 and cone calorimeter methods during the development phase significantly reduces the risk of failure during formal type testing at accredited third-party laboratories.