The vertical tray flame test apparatus is a critical tool for ensuring the fire safety of electrical and fiber optic cables, and Wewon Environmental Chambers Co., Ltd. is proud to offer a solution that meets the highest standards of accuracy, compliance, and reliability. Our equipment supports both Method 1-Vertical Tray and Method 2-FT4 tests, complies with UL 2556, UL 1685, CAN/CSA C22.2 No. 2556, and UL 1581, and is designed to serve the needs of cable manufacturers, testing laboratories, and regulatory bodies worldwide.
By choosing Wewon’s vertical tray flame test apparatus, you’re investing in equipment that not only helps you meet global safety standards but also provides the data needed to develop safer, more reliable cables. In an industry where fire safety can mean the difference between life and death, our commitment to quality ensures that your testing processes are in good hands. For more information about our vertical tray flame test apparatus or to discuss your specific testing needs, contact Wewon Environmental Chambers Co., Ltd. today. We’re here to support your mission to enhance cable safety and compliance.
In the realm of electrical and fiber optic cable safety, ensuring resistance to flame propagation and controlling smoke release is paramount. These cables, which form the backbone of modern infrastructure from commercial buildings to industrial facilities must meet stringent standards to prevent fire-related hazards. One critical tool in verifying their fire performance is the vertical tray flame test apparatus. Designed to simulate real-world fire conditions, this equipment evaluates how cables behave when exposed to flames while mounted on vertical trays – a common installation scenario.
At Wewon Environmental Chambers Co., Ltd., we specialize in manufacturing high-precision vertical tray flame test apparatus that adheres to globally recognized standards, including UL 2556, UL 1685, and CAN/CSA C22.2 No. 2556. Our instruments are engineered to deliver accurate, repeatable results, making them indispensable for cable manufacturers, testing laboratories, and regulatory bodies tasked with ensuring product safety. In this article, we’ll explore the key features of our vertical tray flame test apparatus, the test methods it supports (including Method 1-Vertical Tray and Method 2-FT4), its applications, and its compliance with international standards.
| This test establishes the method for determining the resistance of a wire, cable, or cord to the propagation of flame, while installed in a vertical tray. | 9.6.1 Scope | UL 2556 9.6 Vertical Tray Flame Test (Method 1 – Vertical tray and Method 2- FT4) | |
| Materials and reactants shall consist of | 9.6.2 Materials and reactants | ||
| a) propane at a minimum 95% purity, having a heating value of 94 ± 2 MJ/m3 (2540 ± 50 Btu/ft3); and | |||
| b) compressed air. | |||
| Note: Propane gas defined as Special Duty Propane in ASTM D 1835 or HD-5 Propane as defined in GPA Standard 2140 is recommended. | |||
| Caution: Propane gas is denser than air and can settle and become an explosion hazard. Consult the gas supplier for special precautions to be taken. | |||
| The test apparatus shall consist of | 9.6.3 Apparatus | ||
 Figure H.1 Flame Test Enclosure and Exhaust Duct |
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 Figure H.2 Typical Draft Fan Arrangement |
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 Figure H.3 Bi-directional Probe |
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| a) a test enclosure and exhaust duct (see Annex H, Clauses H.1 and H.2, and Figures H.1 to H.3), located in a test building that has vents for the discharge of combustion products and provisions for fresh-air intake; | |||
| b) an exhaust fan and baffe system capable of producing an exhaust flow rate of 0.65 ± 0.5 m3/s (23.0 ± 1.8 ft3/s) or 5 ± 0.4 m/s (16.4 ± 1.31 ft/s) in the duct (see Figure H.2) | |||
| c) velocity-measuring equipment (see Figure H.1) such as: i) bi-directional probe (see Figure H.3); ii) temperature probe – Type K with an inconel sheath; ii) low pressure transducer (manometer) having a minimum resolution of 0.025 Pa (0.001 Ib/in²); and iv) computer controls/data acquisition equipment; |
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 Figure H.7 Schematic of Gas Train |
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| d) a gas train system (see Figure H.7) arranged such that the
air and propane are mixed through the venturi mixer prior to entering the
burner and composed of: i) propane flow meter capable of measuring a flow rate of 2.3 x 10-4 m3/s (29 ft3/hr) to an accuracy of ± 3% (see Note 2); ii) airflow meter capable of measuring a flow rate of 13.3 x 10-4 m3/s (170 ft3/hr) to an accuracy of 士 3% (see Note 3); iii) needle valves for controlling the flow of propane and air; iv) a venturi mixer (see Note 4); v) a ribbon burner (see Note 4) having a flame-producing surface (face) consisting of a flat metal plate that is 341 mm (13-7/16 in) long and 30 mm (1-5/32 in) wide. The plate shall have an array of 242 holes drilled in it (see Figure H.5); and vi) pressure regulators for propane and compressed air |
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 Figure H.5 Burner Holes |
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| e) an ignition system; | |||
| f) steel cable trays (see Figure H.4) | |||
 Figure H.4 Steel Cable Tray |
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| g) wire (copper or steel) ties not larger than 2.1 mm² (14 AWG) | |||
| h) a flame height gauge (optional, see Figure H.1) | |||
| i) a hand-held vane anemometer capable of measuring wind speeds of 1 m/s (3.3 ft/s); and | |||
| j) a length-measuring device accurate to 0.1% of length measured. | |||
| Note 1: Provisions for the filtering of discharged gases and for compliance with safety and local environmental codes are not addressed in this Standard. | |||
| Note 2: A propane flow meter (catalog no. 58-162788-00), available from Dwyer Instruments, 102 Indiana Hwy. 212, Michigan City, IN 46361-0373, USA, has been found to be suitable. Equivalent apparatus is also acceptable. | |||
| Note 3: An airflow meter (catalog no. RMC-103), available from Dwyer Instruments, 102 Indiana Hwy. 212, Michigan City, IN 46361-0373, USA, has been found to be suitable. Equivalent apparatus is also acceptable. | |||
| Note 4: A ribbon burner (catalog no. 10L 11-55) and venturi mixer (catalog no. 14-18) available from the AGF Burner Inc., 1955 Swarthmore Ave., Lakewood, NJ o8701, USA, have been found to be suitable. Equivalent apparatus are also acceptable. | |||
| Samples shall be taken from finished wire, cable, or cord. | 9.6.4.1 General | 9.6.4 Preparation of specimen | |
| Provisions shall be made within the chamber for securing the tray in the vertical position in the center of the enclosure after specimens have been attached. | |||
 Figure H.8 Cable Tray |
9.6.4.2 Method 1 – Vertical tray specimen mounting | ||
| The sample shall be cut into specimens 2440 mm (96 in) nominal in length. Specimens shall be straightened and fastened to the tray in a single layer by means of wire ties at their upper and lower ends and at two other equally spaced points along their lengths (see Figure H.8). Specimens shall be installed in the center 150 mm (6 in) of the tray, one-half cable diameter apart. The lower end of each specimen shall be located not more than 100 mm (4 in) above the bottom end of the cable tray. The number of specimens shall be determined using the following formula: | |||
| N = (102/Dmm) + 0.33 or N=(4/Din) + 0.33 where N = number of cables (rounded up to the nearest whole number, e.g., 3.14 = 4 specimens) D = diameter of the cable, mm (in) |
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| For a flat cable, the equivalent cable diameter shall be
calculated as D = 1.128 x (TW)1/2 where T = length of the minor axis of the flat cable, mm (in) W = length of the major axis of the flat cable, mm (in) |
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| For ribbon cables, the spacing between adjacent cables shall not be less than 3.2 mm (1/8 in). The full width of the tray may be utilized. | |||
| Depending upon the outside diameter of the individual cable, the test specimens shall be either separate, individual lengths or a bundle of individual lengths and shall consist of multiple 2440 mm (96 in) lengths of the finished cable. The specimens or specimen bundles shall be centered between the side rails in a single layer (see Figure H.8). The lower end of each specimen or specimen bundle shall be located not more than 100 mm (4 in) above the bottom end of the cable tray. Each specimen or specimen bundle shall be separately attached to each rung of the cable tray using one wrap of copper or steel wire not larger than 2.1 mm² (14 AWG). | 9.6.4.3 Method 2-FT4 specimen mounting | ||
| For cables smaller than 13 mm (0.51 in), the specimens shall be grouped into untwisted bundles (nominally circular; see Figure H.8) as indicated in Table 5. The bundles shall be spaced one-half bundle diameter apart on the cable tray as measured at the point of attachment to the cable tray. | |||
| For cables 13 mm (0.51 inch) and larger in diameter, each specimen shall be individually attached to the cable tray with a separation of one-half cable diameter or 15 mm (0.59 in) (whichever is less) between specimens. The tray loading shall comply with Table 6. | |||
| For a flat cable, the equivalent cable diameter shall be
calculated using the following formula: D = 1.128 x (TW)1/2 Where D = calculated cable diameter, mm (in) T = length of the minor axis of the cable, mm (in) W = length of the major axis of the cable, mm (in) |
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| For ribbon cables, the spacing between adjacent cables shall not be less than 3.2 mm (1/8 in). | |||
| The mounted test specimens shall be conditioned for at least 3 h at RooM TEMPERATuRE immediately before the test is started. | 9.6.4.4 Conditioning | ||
| At the start of the test, the apparatus and the air in the test area shall be at a temperature of at least 5 °C. | 9.6.5.1 Startup | 9.6.5 Procedure | |
| Power shall be applied to the control system. | |||
| The propane flow shall be set to 220 ± 8 cm3/s (28 ± 1 ft3/h) when corrected to standard temperature and pressure (20 °C, 101 kPa (14.7 psi). The airflow shall be set to 1280 ± 80 cm3/s (163 ± 10 ft3/h) when corrected to standard temperature and pressure, resulting in a calculated heat output of 20.5 kW (70,000 BTU/h). The burner shall then be shut off. | |||
| In order to establish and maintain a nominal exhaust flow rate of 0.65 ± 0.05 m3/s (23.0 ± 1.8 ft3/s) in the duct, the velocity in the duct shall be maintained at 5 m/s (16.4 ft/s) by using either manual or automated control baffles in the duct, or by adjusting the exhaust fan speed. | |||
| The prepared tray shall be firmly secured in the vertical position (see Figure H.1). | 9.6.5.2 Method 1 – Vertical tray | ||
 Figure H.6 Burner Placement Details |
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| The burner shall then be positioned on the side opposite from where the specimens are mounted, with the burner face vertical and its long dimension horizontal. The burner face shall be placed 76 ± 5 mm (3.0 ± 0.25 in) horizontally from the nearest surface of the specimens, parallel to the tray rungs and centered midway between the side rails of the tray. The center-point of the burner face shall be positioned 457 ± 6 mm (18 ± 0.25 in) above the botom end of the tray and midway between two tray rungs (see Figure H.6, Horizontal). | |||
| The door of the chamber shall be closed and remain closed for the duration of the test. | |||
| The burner shall be re-ignited and the gas and airflows shall be adjusted to the values indicated in Clause 9.6.5.1. The burner flame shall be applied continuously for 20 min or until the specimen burns to the top of the tray. | |||
| After 20 minutes, the burner flame shall be extinguished and the specimen fire (if any) shall be allowed to burn itself out. | |||
| A record shall be kept of the specimen flame height (cm or in) at one-minute intervals during the 20 min test, as well as the time in minutes/seconds that specimens continue to flame following removal of the burner flame. | |||
| The prepared tray shall be firmly secured in the vertical position (see Figure H.1). | 9.6.5.3 Method 2-FT4 | ||
| The burner shall then be positioned on the same side of the tray where the specimens are mounted, with the burner face angled 20° ± 2° upward from the horizontal (see Figure H.6, Angled). The top of the burner shall be located 305 ± 25 mm (12 ± 1 in) above the base of the tray, parallel to the tray rungs and centered midway between the side rails of the tray. The leading edge of the burner shall be placed 76 ± 5 mm (3.0 ± 0.25 in) horizontally from the nearest surface of the specimens. | |||
| The door of the chamber shall be closed and remain closed for the duration of the test. | |||
| The burner shall be re-ignited and the gas and airflows shall be adjusted to the values indicated in Clause 9.6.5.1. The burner flame shall be applied continuously for 20 min or until the specimen burns to the top of the tray. | |||
| After 20min, the burner flame shallbe extinguished and thespecimen fire(if any) shall be allowed to burn itself out. | |||
| A record shall be kept of the specimen flame height (cm or in) at one-minute intervals during the 20 min test, as well as the time in minutes/seconds that specimens continue to flame following removal of the burner flame. | |||
| The exhaust speed data system shall measure the exhaust speed every 5 s and generate a graph of velocity for the duration of the test (see Figure H.9 for a typical plot of velocity graph). | 9.6.6.1 Data acquisition | 9.6.6 Results and calculations | |
 Figure H.9 Typical Exhaust Velocity Graph｜Exhaust Flow Rate (m/s) |
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| After burning has ceased and the specimens are allowed to cool, they shall be wiped clean, and the maximum extent of char determined. Soot that can be removed with a cloth shall be ignored. | 9.6.6.2 Evaluation of damage | ||
| Note: For information, other damage such as melt or blistering may also be recorded. | |||
| The limit of charring shall be determined by pressing against the specimen surface with a sharp object. Where the surface of the specimen changes from a resilient surface to a brittle or crumbling surface, the limit of charring has been determined. | |||
| In the event that materials do not char upon exposure to flame due to the characteristics of the compounds used, other significant damage in the vicinity of the maximum visible flame height that results in the overall specimen diameter being visibly reduced or increased shall be considered “char”. | |||
| 9.6.6.3.1 Method 1-Vertical tray The char height of all specimens shall be less than 2440 mm (96 in) when measured from the bottom of the tray. |
9.6.6.3 Acceptance criteria | ||
| 9.6.6.3.2 Method 2-FT4 The char height on the most centrally located specimens shall be less than 1500 mm (59 in) when measured from the lower edge of the burner face. |
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| The report shall include, as a minimum, the following: a) description of specimens – that is, wire or cable size, Type designation, component makeup and the number of specimen lengths or bundles; b) test method (vertical tray or FT4); c) char height; d) flame height in 1 min intervals; e) after-burn time; and f) velocity graph. |
9.6.7 Report | ||
Every cable installed in a riser, plenum, or vertical shaft is a potential pathway for flame and smoke. One small ignition source can race upward, destroy adjacent circuits, and fill escape routes with toxic fumes. Regulators in North America therefore insist on rigorous fire-propagation and smoke-release verification for all power, control, data, and fibre-optic cables. The laboratory tool that delivers this verification is the vertical tray flame test apparatus. When engineers, certification bodies, and purchasing managers type “vertical tray flame test apparatus,” “UL 2556 Method 1-Vertical tray,” or “Method 2-FT4” into Google or Bing, they are looking for a single partner that can guarantee repeatable data and seamless compliance with UL 2556, UL 1685, UL 1581, and CSA C22.2 No. 2556. Wewon Environmental Chambers Co., Ltd. has engineered exactly that solution.
In a market where a single non-conforming batch can trigger million-dollar recalls, laboratories need more than a burner on wheels. They need a vertical tray flame test apparatus engineered to UL 2556 Method 1-Vertical tray, UL 1685 Method 2-FT4, and CSA C22.2 No. 2556 – without loopholes, without excuses. Wewon Environmental Chambers Co., Ltd. delivers that apparatus today and supports it for the life of your accreditation. When regulators tighten smoke limits or add new optical measurement requirements, firmware updates arrive over the air, ensuring your investment remains compliant and your cables reach the market faster.
For the UL 2556 Method 2 (FT4) Vertical Flame Chamber, relying solely on a single-layer metal enclosure is non-compliant. Since this test requires a 70,000 Btu/h ribbon burner for continuous combustion and evaluates the cable’s char height in a controlled, draft-free environment, the testing chamber must strictly meet several core physical and thermal specifications (with specific parameters often referencing the homologous IEEE 1202 and UL 1685 standards):
- Thermal Insulation:
The maximum conductive heat flux loss through the chamber walls must not exceed 6.8 W/(m²·K). If a metal sheet is used as the outer casing, a thick insulation layer must be installed inside. Acceptable wall structures listed in the standard even include 152 mm thick concrete masonry units, or gypsum board with an 89 mm thick standard fiberglass insulation layer. - Interior Surface:
The interior surface of the combustion chamber must be painted flat black. This ensures a consistent absorption rate of thermal radiation in the environment. The test area must absolutely not retain the original mirror reflection or gloss of stainless steel or other metals. - Air Intakes & Draft-Free Control:
There must be no random air leaks. Air intakes can only, and must, be located at the bottom of two opposing walls, with the total cross-sectional area strictly limited to 1.45 m² ± 0.03 m² (2250 in² ± 50 in²), and the intake areas on both sides should be approximately equal. The doors and the rest of the walls must be strictly sealed to prevent drafts that could affect the flame shape. - Observation Windows:
To observe the testing process, windows can be installed on the chamber, but the total surface area of all observation windows (including those on doors and walls) must not exceed 1.86 m² (20 ft²), and the glass material must be able to withstand the high temperatures of the open flame inside. - Exhaust System & Smoke Release Test Port:
The top of the chamber needs to be connected to a specifically sized exhaust hood and duct. In many export projects, the FT4 test is often combined with an optional Smoke Release Measurement. If the customer needs to measure the total smoke release and peak smoke release rate, the exhaust duct must also reserve space for and integrate a precision photometer measurement system (including a light source and a photoelectric receiver).
