ISO 5660 heat release rate testing is the internationally recognized cone calorimeter method for quantifying the fire performance of materials and products by measuring heat release, smoke production, and mass loss under controlled radiant heating. This standard provides critical data for fire hazard assessment, product certification, and fire safety engineering across building, transportation, and consumer goods industries. In this article, we explain the technical principles, sample requirements, test procedures, measured parameters, and practical applications of ISO 5660 testing.
Technical Overview
ISO 5660 is an international fire test standard that specifies methods for measuring the response of materials exposed to controlled levels of radiant heating, with heat release rate (HRR) being the primary quantity determined. The standard is published in two parts: ISO 5660-1, which covers the cone calorimeter method based on oxygen consumption calorimetry, and ISO 5660-2, which addresses smoke production rate measurement using an optical density technique. The cone calorimeter is widely regarded as one of the most significant advances in fire testing instrumentation because it measures the single property most closely correlated with fire hazard: the rate at which heat is released by a burning specimen.
The fundamental principle underlying ISO 5660-1 is the oxygen consumption technique, derived from Thornton's discovery that, for most organic materials, approximately 13.1 MJ of heat is released per kilogram of oxygen consumed, regardless of the specific fuel. By accurately measuring the oxygen concentration in the exhaust duct, along with gas flow rate, the tester can compute the instantaneous heat release rate with high precision. A truncated conical radiant heater uniformly irradiates a horizontally or vertically oriented specimen at preset heat fluxes, and an electric spark igniter initiates flaming combustion when combustible volatiles are evolved.
Because the test captures the complete burning history of a specimen, from piloted ignition through flaming combustion to extinction, the resulting data are far more informative than single-point pass/fail metrics. Regulators, fire engineers, and material scientists rely on ISO 5660 data to compare materials quantitatively, validate fire models, and support performance-based design decisions.
Testing Samples
Specimen Dimensions and Preparation
Standard specimens for ISO 5660-1 testing are square plaques measuring 100 mm × 100 mm, with a preferred thickness between 6 mm and 50 mm. Thinner materials may be layered to achieve an adequate thickness, provided the layers are representative of the end-use configuration and are not artificially bonded in a way that alters combustion behavior. Specimens must be conditioned to constant mass at 23 °C and 50% relative humidity in accordance with ISO 291 before testing.
Substrates and Edge Treatment
Specimens are typically mounted with the exposed face upward on a holder that incorporates a retainer frame leaving an exposed area of approximately 88.4 mm × 88.4 mm. For materials normally applied to a substrate, the actual substrate, such as gypsum board, particleboard, or metal, must be used to reproduce realistic heat transfer and back-face insulation conditions. Aluminum foil is commonly wrapped around the underside and edges of the specimen to prevent edge burning, dripping, and molten flow unless such behavior is itself the subject of evaluation.
Material Types Suitable for Testing
- Rigid thermoplastics, thermosets, and polymer composites
- Building products such as wall linings, ceiling panels, flooring, and insulation
- Railway and aviation interior components, including seat cushions and wall panels
- Electrical cables, wire insulation, and switchgear housings
- Furniture foams, upholstery fabrics, and mattress composites
- Wood-based products, coatings, paints, and intumescent formulations
- Textiles, laminates, adhesives, and additive concentrates
Testing Items
An ISO 5660-1 cone calorimeter test yields a comprehensive set of quantifiable fire performance parameters, each recorded as a function of time throughout the exposure period. The key measured and derived items include:
- Time to sustained flaming ignition (TTI), recorded in seconds under the applied irradiance
- Heat release rate (HRR), expressed in kilowatts per square meter and plotted continuously
- Peak heat release rate (pHRR), the maximum instantaneous HRR, a key indicator of flashover potential
- Average heat release rate over defined intervals, typically 60 s, 180 s, and 300 s after ignition
- Total heat released (THR), the integrated energy output in megajoules per square meter
- Effective heat of combustion (EHC), the ratio of heat released to specimen mass loss in megajoules per kilogram
- Mass loss rate (MLR), measured continuously by a load cell beneath the specimen holder
- Smoke production rate (SPR) and total smoke production (TSP), determined from optical density measurements in the exhaust duct
- CO and CO₂ yield, when gas analysis instrumentation is fitted
- Residual mass fraction and visual observations of phenomena such as dripping, bubbling, cracking, and char formation
Under ISO 5660-2, the optical smoke measurement system is validated and applied to determine specific extinction area and smoke production values, complementing the heat release data obtained under Part 1.
Testing Methods
Calibration and Instrument Verification
Before testing, the cone calorimeter must be calibrated in accordance with the standard. The irradiance uniformity of the conical heater is verified across the specimen plane, the heat release measuring system is calibrated using a methane gas burner of known flow, and the load cell, oxygen analyzer, and smoke obscuration system are checked against reference values. Daily calibration with methane of certified purity establishes the C-factor used in the oxygen consumption calculations.
Exposure Conditions
The radiant heat flux is selected to reproduce the thermal environment relevant to the fire scenario under study. Common irradiance levels include 25 kW/m², 35 kW/m², 50 kW/m², and 65 kW/m², with 50 kW/m² representing conditions similar to a well-developed room fire and 25 kW/m² used for slowly developing fires. The exhaust flow rate through the duct, typically 0.024 m³/s, and the ambient conditions of temperature, humidity, and pressure are recorded for incorporation into the calculations.
Test Procedure
- Condition and weigh the specimen, wrap it in aluminum foil, and place it in the holder with the retainer frame
- Position the assembly on the load cell beneath the pre-stabilized conical heater at the set irradiance
- Apply the spark igniter and start data acquisition, recording time to ignition when sustained flaming occurs
- Continue the test until all flaming has ceased and the mass loss rate has returned to a negligible value, or for the maximum specified duration
- Analyze exhaust gases continuously for O₂, CO, and CO₂ concentrations and duct optical density
- Process the data to compute HRR, THR, MLR, EHC, and smoke parameters using the equations specified in the standard
- Report the full time-history curves and tabulated characteristic values for each specimen, with a minimum of three replicate specimens recommended for statistical validity
Expression of Results
Results are normalized per unit exposed area of 0.0088 m². The heat release rate is calculated from the oxygen depletion, corrected for instrument delays and background drift. Reports must include the irradiance level, specimen orientation, ignition time, all HRR metrics, mass data, smoke data, and observations of specimen behavior. Repeatability limits stated in the standard allow laboratories to assess the acceptability of replicate results.
Applications
ISO 5660 heat release rate testing serves a broad range of industries and regulatory contexts. In construction, the data support classification under performance-based building codes and provide inputs for reaction-to-fire assessments in Europe, the Middle East, and Asia-Pacific markets. In the railway sector, the standard underpins material qualification programs aligned with EN 45545 series requirements, where peak HRR and total heat release values feed into hazard analysis for rolling stock. Aviation and marine applications use cone calorimeter data to evaluate cabin and interior materials against established acceptance criteria.
For manufacturers, the method is an indispensable research and development tool. It enables quantitative comparison of flame retardant formulations, evaluation of additive effectiveness, and screening of candidate materials long before full-scale testing. Fire protection engineers use ISO 5660 results as direct inputs for zone models and computational fluid dynamics simulations, where ignition temperature, heat of combustion, and HRR curves drive scenario modeling. Insurance assessors, forensic laboratories, and academic institutions similarly depend on the standard for fire investigation, benchmarking, and peer-reviewed research.
FAQ
What is the difference between ISO 5660-1 and ISO 5660-2?
ISO 5660-1 specifies the cone calorimeter method for determining heat release rate using oxygen consumption calorimetry, while ISO 5660-2 specifies the method for measuring smoke production rate based on optical density in the exhaust duct. Both parts are frequently performed on the same instrument during a single test campaign.
Which heat flux level should be selected for testing?
The choice depends on the intended fire scenario. Irradiances of 25 to 35 kW/m² represent slowly developing or pre-flashover exposure, whereas 50 kW/m² approximates the radiant environment of a developed room fire and is the most commonly reported condition. Higher levels up to 75 kW/m² are used for severe exposure assessments such as certain transportation applications.
How many specimens are required?
At least three replicate specimens should be tested for each condition to characterize variability, and additional replicates are advisable for heterogeneous products such as composites, upholstered assemblies, or materials exhibiting intumescence or dripping.
Can products be tested vertically?
Yes, the standard permits both horizontal and vertical specimen orientations. Vertical orientation is often preferred for wall lining products and materials that melt and drip, as it better represents end-use mounting and affects measured ignition and burning behavior.
How does ISO 5660 relate to other fire test standards?
ISO 5660 is methodologically equivalent to ASTM E1354 and shares the cone calorimeter principle with tests referenced in EN and IMO frameworks. Its quantitative output complements classification tests such as EN 13501-1, UL 94, and FAR 25.853 by providing the underlying heat release data used for engineering analysis and product development.
