Vertical Burning Testing for Metro Materials: Standards, Test Methods and Compliance Requirements

Vertical burning testing is a cornerstone of fire-safety qualification for non-metallic materials used in metro vehicles and station systems, evaluating ignitability, flame spread, afterflame time and dripping behavior under worst-case vertical orientation. This technical guide explains the governing standards — including EN 45545-2, UL 94 / IEC 60695-11-10, EN ISO 11925-2 and the IEC 60332 cable series — the sample requirements, measured parameters and acceptance criteria typically applied by accredited third-party laboratories.

Metro systems concentrate large numbers of passengers in enclosed, underground environments where heat and smoke cannot dissipate quickly. Because of this, every plastic, elastomer, composite, foam and cable installed in a metro car or station must demonstrate measurable resistance to ignition and flame propagation, and the vertical burning test is one of the most widely specified small-scale methods for demonstrating that performance.

Technical Overview

A vertical burning test mounts a material specimen in a vertical orientation and exposes it to a standardized gas flame for a defined period. The vertical position represents the most severe common-case geometry: as the flame travels upward, combustion gases preheat the unburned material above the flame front, accelerating flame spread compared with horizontal testing. Engineers use the results to classify materials, compare formulations and demonstrate compliance with rail-vehicle fire-safety specifications.

For metro applications, vertical burning data is rarely used in isolation. It normally forms part of a multi-parameter fire-safety dossier that also covers flame spread, heat release rate, smoke density and smoke toxicity. In Europe, the controlling framework is EN 45545-2 (Railway applications — Fire protection on railway vehicles — Requirements for fire behaviour of materials and components), which assigns materials to requirement sets R1 through R26 depending on product category and location in the vehicle. Hazard levels HL1, HL2 and HL3 are defined in EN 45545-1 according to the design and operating category of the line; fully underground and automated metro operations generally demand the most stringent level, HL3.

• UL 94 and IEC 60695-11-10 — 20 mm (50 W) vertical burn classification V-0, V-1, V-2 for plastics, adopted in China as GB/T 5169.16.
• UL 94, IEC 60695-11-20 and ASTM D5048 — 500 W vertical burn classification 5VA and 5VB for more demanding components.
• EN ISO 11925-2 — single-flame-source ignitability test referenced within EN 45545-2 requirement sets and in building-product classification schemes such as EN 13501-1.
• EN 60332-1-2 / IEC 60332-1-2 and EN 60332-3-24 / IEC 60332-3-24 — vertical flame propagation tests for single cables and bunched cable installations.
• ASTM D3801 — North American vertical burn method equivalent in principle to the UL 94 V procedure.
• Legacy and regional specifications such as DIN 5510-2, BS 6853 and NF F 16-101 for older European fleets, and the Chinese railway industry standard TB/T 3138, which specifies small-flame flame-retardancy determination for non-metallic rolling-stock materials and remains widely referenced on metro projects alongside EN 45545-2.

Testing Samples

Interior Trim and Structural Panels

Wall, ceiling and partition panels, windshields, driver cab linings and equipment covers are tested as representative coupons of the finished laminate. Typical constructions include phenolic or aluminum honeycomb sandwich panels, glass-fiber-reinforced plastics (GRP), polycarbonate/ABS alloys and painted or laminated decorative surfaces. Specimens must include the production coating, film or lamination, because finishes can materially change ignition behavior.

Seats and Upholstery Components

Seat shells, cushions, headrests and armrests are sampled so that each functional layer is evaluated: flexible polyurethane or composite foam cores, fire-blocking interlayers, and textile or artificial-leather cover materials. Complete seat assemblies are additionally subjected to full-scale furniture tests under EN 45545-2, but coupon-level vertical burn screening is used during material selection.

Elastomers, Sealants, Insulation and Ducting

Rubber door and window seals, grommets, hose, thermal insulation, air-distribution ducting and vibration-damping pads are tested in their as-installed thickness and density. Cellular and foamed materials warrant particular attention because low-density structures ignite and drip more readily than solid equivalents.

Cables and Wiring

Power, traction, control and data cables are tested as complete constructions, including insulation, fillers, inner sheath and outer jacket. EN 60332-1-2 evaluates a single vertical cable length against a 1 kW propane flame, while EN 60332-3-24 evaluates bunched cables on a vertical ladder to simulate a routed cable bundle.

Engineering Plastics and Small Components

Connector housings, switch bodies, relay cases, terminal blocks, fan impellers and electronic equipment enclosures are classified using the UL 94 vertical method at the minimum marketed wall thickness, since thinner sections ignite and burn faster than thick ones.

General specimen preparation requirements include conditioning to equilibrium at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 48 hours before testing, with edges cut cleanly and free of flash, cracks or delamination. For UL 94 programs, an additional oven-aged specimen set (168 hours at 70 °C) is tested when long-term thermal exposure is part of the qualification.

Testing Items

A vertical burning test program for metro materials measures and reports the following parameters, which together determine the classification and pass or fail decision.

• Afterflame time after the first flame application (t1) and after the second application (t2), recorded for each specimen individually.
• Total afterflame time summed across all specimens in a set, which caps the classification level.
• Afterglow time (t3) — persistence of glowing combustion without a flame after the flame source is removed.
• Flaming drips and flaming particles, detected by ignition of the dry cotton indicator (UL 94) or filter paper (EN ISO 11925-2) placed beneath the specimen.
• Flame spread distance and the time for the flame tip to reach the 150 mm gauge mark in EN ISO 11925-2.
• Char length, damaged length and burn-through of the specimen, including whether the flame reaches the upper clamp or support.
• For cables, the extent of charring along the vertical sample and whether it approaches the upper support within the permitted distance.
• Classification output: V-0, V-1, V-2, 5VA or 5VB for plastics; pass or fail against the specified parameters for cable and small-flame methods.

Testing Methods

UL 94 / IEC 60695-11-10 Vertical Burn (V-0, V-1, V-2)

Specimens of 125 mm × 13 mm are clamped vertically at the upper end. A methane or natural-gas burner adjusted to a nominal 20 mm blue flame is applied for 10 seconds to the lower edge, then removed while the afterflame time t1 is recorded; the flame is applied a second time for 10 seconds and t2 and afterglow are recorded. A pad of dry absorbent cotton positioned below the specimen captures flaming drips. Two sets of five specimens are tested.

• V-0: each afterflame ≤ 10 s, total for both applications across the set ≤ 50 s, and no cotton ignition.
• V-1: each afterflame ≤ 30 s, total ≤ 250 s, and no cotton ignition.
• V-2: timing identical to V-1, but flaming drips that ignite the cotton are permitted.

UL 94 / IEC 60695-11-20 and ASTM D5048 500 W Test (5VA, 5VB)

A 125 mm high flame is applied five times for 5 seconds each to vertical bars and, separately, to horizontal plaques. To pass, combustion must cease within 60 seconds after the fifth application with no flaming drips; 5VA additionally requires that the plaque is not burned through, while 5VB permits a hole only if all other criteria are met. These grades are typically specified for components located near potential ignition sources such as electrotechnical equipment.

EN ISO 11925-2 Single Flame Source Test

A 250 mm × 90 mm specimen is attacked by a 15 second or 30 second propane flame applied at 45° to the bottom edge (edge attack) or to the face (surface attack). The specimen passes when the flame tip does not reach the 150 mm gauge mark within the time limit specified by the applicable classification scheme and flaming droplets do not ignite the filter paper beneath the specimen. Within EN 45545-2 practice, this method serves as the small-flame ignitability element for defined product categories and is also the basis of Class E classification under EN 13501-1 for station building materials.

Vertical Cable Flame Propagation Tests

EN 60332-1-2 / IEC 60332-1-2 mounts a single cable sample vertically and applies an inclined 1 kW propane burner for 60 seconds or 120 seconds depending on the cable's overall diameter; charring must not extend to within 50 mm of the upper support. EN 60332-3-24 subjects bunched cables approximately 3.5 m long on a vertical ladder to a 20.5 kW burner for 20 minutes (Category C), with flame spread required to remain below 2.5 m above the burner. These results are combined with smoke density (EN 61034-2) and toxicity assessments to satisfy the cable requirement sets of EN 45545-2.

Chinese and Regional Practice

GB/T 5169.16 implements the IEC 60695-11-10 procedure identically, allowing UL 94-type vertical data to be exchanged between programs. TB/T 3138 specifies small-flame flame-retardancy determination, including vertical orientation, for non-metallic railway materials and is frequently cited in Chinese metro technical specifications, often in parallel with EN 45545-2 for export or dual-standard fleets.

Applications

Vertical burning test reports from accredited third-party laboratories are used throughout the metro vehicle life cycle.

• Type approval and design validation of new rolling stock, demonstrating compliance of interior and equipment materials with the hazard level (HL1–HL3) assigned by EN 45545-1 for the line's operating category.
• Supplier qualification and first-article inspection, establishing that production materials match the approved formulation and thickness.
• Incoming material quality control and periodic re-verification, particularly after resin, pigment, masterbatch or process changes that can shift flame performance.
• Retrofit and refurbishment projects, where replacement panels, seats, flooring accessories and cables must meet the standard applicable to the original or upgraded specification.
• Research and development screening of flame-retardant formulations, including halogen-free systems selected for lower smoke toxicity in tunnels.
• Procurement and tender compliance documentation, where buyers require traceable classification evidence such as V-0 or 5VA grades for defined part families.
• Failure analysis and incident investigation, comparing the fire behavior of recovered materials against original qualification data.

FAQ

Why is the vertical orientation considered the worst case for metro materials?

In a vertical specimen, hot combustion gases rise along the unburned surface and preheat it ahead of the flame front, so flame spreads faster than in horizontal or inclined tests. This reproduces the geometry of a burning wall panel, duct or cable run inside a vehicle, where upward propagation is the dominant risk.

What is the practical difference between V-0 and V-2?

Both extinguish within similar time limits, but V-2 allows flaming droplets that ignite the cotton indicator. For metro interiors above passenger seating or electrical equipment, specifications usually require V-0 or better, because flaming drips create secondary ignition sources below the original fire.

Is a UL 94 V-0 rating sufficient for a metro vehicle by itself?

No. UL 94 is an ignitability and self-extinguishing classification for the material coupon. EN 45545-2 additionally requires flame spread, heat release, smoke density and smoke toxicity data appropriate to the product category, and the full dossier must meet the hazard level assigned to the line. V-0 remains a valuable design indicator and often appears as a supplementary requirement in metro material specifications.

Which thickness should be submitted for testing?

Materials are tested at the thickness as installed, and plastic components are classified at the minimum wall thickness offered commercially, since thin sections perform worse. If a family of thicknesses is marketed, the worst-case thinnest variant must be evaluated.

Do color, texture or surface finish affect the result?

Yes. Pigments, masterbatches, embossing, paints and laminating films change decomposition behavior and dripping tendency, so the finished construction — not the base resin alone — must be the subject of the test.

How many specimens are tested, and what conditioning applies?

Typical programs use two sets of five specimens conditioned for at least 48 hours at 23 ± 2 °C and 50 ± 5 % relative humidity, with an additional oven-aged set for UL 94 where thermal aging is required. Cables and EN ISO 11925-2 specimens follow the specific sample counts and conditioning stated in each method.

Can vertical burning results predict full-scale fire behavior?

They are screening and classification tools, not standalone predictions. Metro fire safety is demonstrated through the combined EN 45545-2 assessment, and where required, full-scale verification such as the EN ISO 9705 room-corner test for complete seat assemblies. Vertical burning data identifies ignition-prone materials early and anchors the overall qualification package.