Single Burning Item (SBI) Test Heat Release Rate Analysis: Measurement Principles, Key Parameters an

The Single Burning Item (SBI) test, specified in EN 13823 and its Chinese counterpart GB/T 20284, is the cornerstone intermediate-scale method for evaluating the heat release rate of building products exposed to a fire in a room corner. This article provides a professional analysis of how heat release rate parameters such as FIGRA and THR600s are measured, calculated, and applied to fire performance classification under EN 13501-1 and GB 8624. Understanding this analysis enables manufacturers to optimize material formulations and secure compliant fire classification through accredited third-party laboratories.

Technical Overview

The Single Burning Item (SBI) test is an intermediate-scale reaction-to-fire test defined in EN 13823 (harmonized in China as GB/T 20284). It simulates a single burning item, such as a waste-paper basket, placed in the corner of a room and thermally attacking the wall and ceiling linings of a building product. Heat release rate (HRR) is the central measurand of the test, because it quantifies the energy a product feeds back into a developing fire, which directly governs flashover potential and occupant escape time.

Heat release rate in the SBI test is determined by oxygen consumption calorimetry, based on Huggett's principle that approximately 13.1 MJ of heat is released per kilogram of oxygen consumed, regardless of the fuel. Exhaust gases are continuously analyzed for oxygen, carbon dioxide, and carbon monoxide, allowing the total heat release of the specimen to be derived with high accuracy without direct thermal measurement.

The raw HRR curve is processed into derived indices that feed directly into the fire classification systems of EN 13501-1 (Euroclasses) and GB 8624-2012 in China. The principal heat release parameters are:

• FIGRA0.2MJ and FIGRA0.4MJ – Fire Growth Rate Indices (W/s), reflecting the speed of fire development
• THR600s – Total Heat Release (MJ) over the first 600 seconds of exposure
• Peak HRR and the full time-resolved HRR curve of the specimen
• LFS – Lateral Flame Spread along the long wing
• SMOGRA and TSP600s – smoke production indices measured simultaneously
• Observation of flaming droplets and particles

Testing Samples

Specimen Geometry and Configuration

The test specimen consists of two vertical wings mounted at a right angle to form an internal corner. The long wing measures 1000 mm in width and 1500 mm in height, and the short wing measures 495 mm in width and 1500 mm in height. Products are mounted on the trolley exactly as intended in end use, including any substrate, backing board, air gap, joints, and mechanical fixings, following the general substrate and conditioning rules of EN 13238. Vertical joints are incorporated at defined positions to represent realistic installation, and the exposed top and outer edges of the long wing are protected by a U-shaped profile.

Typical Product Categories

• Wall and ceiling linings, including decorative panels and laminates
• Thermal insulation products such as PIR, EPS, XPS, phenolic foam, and mineral wool boards
• Metal-faced sandwich panels and aluminium composite panels
• Gypsum boards, cement boards, and wood-based panels with or without coatings
• Pipes, ducts, and linear products evaluated in defined end-use configurations

Conditioning and Number of Specimens

Specimens are conditioned to constant mass at (23 ± 2) °C and (50 ± 5) % relative humidity prior to testing. A minimum of three replicate specimens is tested for the formal assessment, optionally preceded by one preliminary specimen. Because heat release results are sensitive to moisture content, joint design, and fixing details, faithful reproduction of the declared end-use application is essential for a valid and reproducible classification.

Testing Items

The heat release analysis of the Single Burning Item test reports the following items, all derived from the measured HRR curve:

• FIGRA0.2MJ (W/s): the maximum of HRR averaged over 30 seconds divided by elapsed exposure time, considering only times after total heat release has exceeded 0.2 MJ; this index governs classes A2 and B
• FIGRA0.4MJ (W/s): the equivalent index using a 0.4 MJ threshold; this index governs classes C and D
• THR600s (MJ): the total heat release integrated over the first 600 seconds of burner exposure, with class limits of 7.5 MJ for A2/B and 15 MJ for C
• Peak HRR (kW): the maximum 30-second averaged heat release rate of the specimen
• LFS: verification of whether flame spread reaches the far edge of the long wing
• SMOGRA (m²/s²) and TSP600s (m²): smoke growth rate and total smoke production recorded concurrently
• Flaming droplets and particles: behavioral observations used for the d0/d1/d2 subclasses

FIGRA captures how rapidly a product contributes to fire growth, whereas THR600s captures the cumulative energy contribution. Together they discriminate between products that flash over quickly and those that burn slowly but energetically over a long period.

Testing Methods

Apparatus and Instrumentation

The SBI apparatus comprises a trolley carrying the corner specimen, an ignition source, an exhaust hood and measurement duct, and a data acquisition system. The ignition source is a right-angle (triangular) sand-bed propane burner with a nominal heat output of 30.7 kW, placed against the bottom of the specimen corner. Air enters the enclosure at a velocity of (0.60 ± 0.05) m/s past the wings, and the exhaust duct operates at a volumetric flow between 0.50 and 0.65 m³/s. Instrumentation includes high-resolution oxygen, carbon dioxide, and carbon monoxide analyzers on a conditioned gas sampling line, a bidirectional flow probe with differential pressure measurement, duct temperature sensors, and an optical smoke measurement system (white-light or laser photometer). All channels are recorded at intervals of no more than 3 seconds.

Test Sequence

• 0–120 s: baseline recording with no burner operating
• 120–300 s: auxiliary burner ignited to establish the burner-only heat release baseline
• 300 s: main burner ignited and the specimen exposed to the corner flame attack
• 300–1500 s: 20 minutes of continuous exposure, with the burner heat output subtracted to obtain the net specimen HRR
• 1500–1560 s: post-exposure recording to complete the 26-minute total sequence

Heat Release Rate Calculation

The instantaneous heat release rate is computed from the oxygen depletion of the exhaust gas stream using the oxygen consumption equations, which include the expansion factor of combustion gases and corrections for incomplete combustion products such as CO. The net heat release rate of the specimen equals the total measured value minus the simultaneous burner contribution, the latter being stabilized and precisely known from the calibrated propane mass flow. The resulting HRR curve is then smoothed with a running 30-second average, THR600s is obtained by integration from 300 s to 900 s, and FIGRA is the maximum of the quotient of HRR30s and the elapsed time from burner ignition, evaluated under the applicable 0.2 MJ or 0.4 MJ total-heat thresholds.

Quality Assurance

Accredited third-party laboratories maintain traceability through propane purity verification (minimum 95 %), burner heat output validation via calibrated mass flow measurement, span-gas calibration of analyzers, duct flow verification, and smoke system checks. Interlaboratory comparisons confirm reproducibility of FIGRA and THR600s within the limits published in the standard.

Applications

Heat release rate data from the SBI test underpin a wide range of regulatory and engineering uses:

• Euroclass classification (B, C, D classes) under EN 13501-1 in support of CE marking of construction products under the European construction products regulation
• Fire performance classification under GB 8624-2012 in China, where GB/T 20284 results determine B1 and B2 grades for many product families
• Optimization of flame-retardant formulations, facings, coatings, and core materials during product development
• Compliance demonstrations for facade systems, rail and road tunnel linings, and public building interiors where codes mandate low fire growth
• Batch-to-batch quality control and factory production control verification against initially classified specimens
• Comparative input for fire safety engineering, where FIGRA and THR values inform hazard modeling of interior linings

FAQ

What does FIGRA physically represent?

FIGRA is the maximum ratio of heat release rate to elapsed time, so it expresses how aggressively a product accelerates fire growth during the critical early stage. A low FIGRA indicates slow flame spread and limited heat feedback, which delays flashover.

Why are there two FIGRA thresholds, 0.2 MJ and 0.4 MJ?

The 0.2 MJ threshold makes FIGRA0.2MJ sensitive to very early ignition and heat release, suitable for the stringent A2 and B classes. The 0.4 MJ threshold ignores minute initial transients and is applied to the less demanding C and D classes.

How is heat release measured without calorimetric sensors?

By oxygen consumption calorimetry: because nearly all organic fuels release 13.1 MJ per kilogram of consumed oxygen, measuring oxygen depletion, along with CO2 and CO corrections, in the exhaust duct yields the heat release rate with an uncertainty of a few percent.

Which fire classes require the SBI test?

Classes B, C, and D under EN 13501-1 rely directly on the SBI FIGRA and THR600s limits, combined with the small-flame test of EN ISO 11925-2. Class A2 additionally uses SBI results alongside the non-combustibility furnace and gross heat of combustion tests.

How are the results of the three specimens combined?

The 30-second averaged FIGRA and THR600s values from each specimen are rounded and compared with class limits according to the averaging rules of EN 13501-1, with provisions for borderline or divergent results that may require additional specimens.

What factors most often cause heat release variability?

Common causes include incorrect mounting orientation, joints that do not reflect end use, residual moisture from inadequate conditioning, damaged facings, and uncontrolled air gaps between the product and its substrate, all of which accredited third-party laboratories control through strict pre-test verification.

Is GB/T 20284 equivalent to EN 13823?

GB/T 20284 is the Chinese adoption of the EN 13823 methodology, retaining the same specimen geometry, burner exposure, instrumentation, and FIGRA/THR calculation, so heat release data are technically comparable, although classification limits are stated separately in GB 8624-2012.