IEC 60695-4 2012 Fire hazard testing -- Part 4: Terminology concerning fire tests for electrotechnical products IEC 60695-8-2 - Fire hazard testing -- Part 8-2: Heat release - Summary an
Complete combustion measured by the oxygen bomb calorimeter
The adiabatic constant volume bomb calorimeter is the key device for measuring heats of combustion It features a robust central vessel, or "bomb," designed to withstand high pressures while maintaining a constant internal volume This bomb is placed in a stirred water bath, forming the calorimeter, which is further surrounded by an outer water bath During combustion reactions, the temperatures of both the calorimeter water and the outer water bath are continuously monitored and adjusted using electrical heating to maintain the same temperature, ensuring that there is no heat loss to the surroundings and that the calorimeter remains adiabatic.
To perform a measurement, a specific mass of the sample is placed in a bomb with an electrical ignition wire The bomb is filled with pressurized oxygen, sealed, and allowed to reach thermal equilibrium The sample is ignited with a precise amount of energy, ensuring complete combustion due to the excess of high-pressure oxygen.
The heat released is calculated from the known heat capacity of the calorimeter and the rise of temperature which occurs as a result of the combustion reaction
The experiment gives the heat released at constant volume, i.e the change in internal energy,
∆U The gross heat of combustion is the enthalpy change, ∆H, where:
∆ = + where ∆ (PV) is calculated using the ideal gas law;
∆ NOTE Bomb calorimeter measurement of the heat of combustion of building products is described in ISO 1716 [4].
Incomplete combustion
Measurement techniques
Fires typically involve combustion in air at atmospheric pressure, leading to incomplete combustion As a result, the heat released during these fires is generally lower than the total heat of combustion of the materials involved.
The heat released can be determined indirectly using one of the following techniques: a) oxygen consumption; b) carbon dioxide generation; c) gas temperature increase.
Heat release by oxygen consumption
A significant amount of heat is consistently released per unit of oxygen consumed for many organic fuels, with an average value of 13.1 kJ/kg of oxygen This constant is commonly applied in both small-scale and large-scale testing Consequently, measuring the oxygen consumption and the mass flow rate in the exhaust duct is sufficient to determine the heat release in a combustion system.
Table 1 presents net heat of combustion values, revealing that, except for ethene, ethyne, and poly(oxymethylene), the calculated heats of combustion per gram of oxygen consumed range from 12.5 kJ to 13.6 kJ These values assume complete combustion; however, Huggett discusses the implications of incomplete combustion and provides calculations for various scenarios For instance, cellulose combustion results in a 9:1 ratio of CO2 to CO.
(C 6 H 10 O 5 ) + 5,7 O 2 → 5,4 CO 2 + 0,6 CO + 5 H 2 O ∆H c = −13,37 kJãg −1 of O 2 or burning to give an appreciable amount of carbonaceous char:
(C 6 H 10 O 5 ) + 3 O 2 → 3 CO 2 + 3 C + 5 H 2 O ∆H c = −13,91 kJãg −1 of O 2 compared with complete combustion:
Huggett discusses several other examples and concludes that the assumption of a constant heat release per unit of oxygen consumed will be sufficiently accurate for most applications
If the correct value of ∆H c per gram of O 2 is known for a particular material then this shall be used instead of the approximate value
Table 2 lists some heat of combustion values for insulating liquids
There is a variety of fire tests that use the oxygen consumption method They vary from the micro-scale, e.g ASTM D 7309 [7], to the large-scale, e.g EN 50289-4-11 [8]
Heat release fire tests that are of relevance to the testing of electrotechnical products are described in IEC 60695-8-2.
Heat release by carbon dioxide generation
This technique relies on the principle that the energy released during combustion is roughly proportional to the amount of carbon dioxide produced, assuming complete or nearly complete combustion with minimal CO/CO₂ ratios The average proportionality constant is 13.3 kJ/g of carbon dioxide generated For more precise calculations of heat release, a specific value for the material or product should be utilized.
In general, heat release values determined by carbon dioxide generation agree well with heat release rate values determined by oxygen consumption.
Heat release by increase of gas temperature
The gas temperature technique operates under the premise that there are no heat losses, with all heat from the fire contributing to the temperature increase of the hot air and fire effluent mixture If thermal radiation losses are minimal, this method, also known as the thermopile technique, can yield heat release values comparable to those obtained through oxygen consumption or carbon dioxide generation methods Heat release is calculated by measuring the temperature rise of the gases at the thermopile relative to a reference temperature, typically the ambient temperature This measurement is then converted to heat release by assessing the total flow of the air and fire effluent mixture, using the specific heat of the mixture at the relevant air temperature, or through calibration with a known heat release material like methane.
Heat release values obtained through temperature measurement are typically lower than those derived from calorimetry techniques that measure oxygen consumption or carbon dioxide generation, primarily due to significant heat losses In small-scale tests, these heat losses can be minimized by carefully designing the system to be as adiabatic as possible.
A new method for determining heat release values through temperature measurement has been established as ISO 13927 This method utilizes the same heating and specimen mounting systems as ISO 5660-1, making it suitable for production control and research comparisons The test apparatus is user-friendly and cost-effective.
Table 1 – Relationship between heat of combustion expressed in units of kJãg −1 of fuel burned and kJãg −1 of oxygen consumed for a variety of fuels
Fuel Formula ∆ H c a kJãg −1 of fuel kJãg −1 of O 2
NOTE 2 Most of the values in column 3 are calculated from thermodynamic data The values in column 4 are calculated from those in column 3 assuming complete combustion
NOTE 3 For values calculated from thermodynamic data, carbon is assumed to be converted to carbon dioxide, hydrogen to water, nitrogen to nitrogen dioxide and chlorine to hydrogen chloride a Reactants and products at 25 °C, all products gaseous
Table 2 – Relationship between heat of combustion expressed in units of kJãg −1 of fuel burned and kJãg −1 of oxygen consumed for a variety of insulating liquids
Insulating liquids Formula ∆ H c a kJãg −1 of fuel kJãg −1 of O 2
Mixture of mono- and dibenzyl toluene (3) – 39,5 b
(1) Silicone transformer liquid, type T1, IEC 60836 [9]
(4) Transformer and switchgear mineral oil, IEC 60296 [12]
Technical Committee 10 has identified varying values for the heat of combustion of silicone oil, ranging from 25 kJ⋅g\(^{-1}\) to 27 kJ⋅g\(^{-1}\) The reactants and products are considered at 25 °C, with all products being in a gaseous state Currently, there is no available data on this topic.
5 Parameters used to report heat release data
Heat of combustion (gross and net)
The standard heat of combustion is the enthalpy change associated with the complete combustion of one mole of a substance under standard conditions In fire science, this is often called the "gross heat of combustion," and it is measured in energy per unit mass instead of energy per mole.
NOTE Older terms, now deprecated, are “calorific potential” and “gross calorific value”
Water produced during combustion is classified as a liquid In the case of a carbon and hydrogen compound, complete combustion results in the transformation of all carbon into carbon dioxide gas and all hydrogen into liquid water.
Gross heat of combustion is determined using oxygen bomb calorimetry, which ensures complete conversion of the sample into fully oxidized products However, in actual fire scenarios, this complete conversion is uncommon, as some combustible materials may remain as char, and combustion products are frequently only partially oxidized, such as soot particles in smoke and carbon monoxide.
The net heat of combustion differs from the gross heat of combustion in that it assumes any water produced is in the vapor state This distinction accounts for the latent heat of vaporization of water.
The net heat of combustion, measured at 298 K and equivalent to 2.40 kJ/g, is always less than the gross heat of combustion In the context of flames and fire, water exists as vapor, making it more suitable to utilize net heat of combustion values.
Heat release rate (HRR)
The heat release rate is defined as the thermal energy emitted per unit time during a fire or fire test This parameter is essential for quantifying the intensity of a fire.
Heat release rate is commonly reported in the form of a graph against time A heat release rate curve is shown in Figure 1
Figure 1 – Heat release rate (HRR) curve
Heat release (HR)
Heat release (see 3.15) is defined as the thermal energy that is produced in a fire or fire test
Heat release is a crucial parameter for quantifying fire size, typically calculated by integrating heat release rate data over time As illustrated in Figure 2, this calculation is derived from the data presented in Figure 1 However, it is common practice to report only the total heat release.
Figure 2 – Heat release (HR) curve
Heat release rate per unit area (HRR*)
In the context of flat test specimens, the heat release rate is often expressed as the rate of heat release per unit area of the exposed surface, typically measured in kW/m².
NOTE Data from the cone calorimeter [13] are usually reported in this way [14]
A heat release rate per unit area curve is shown in Figure 3 (It is based on the curve of Figure 1 assuming an exposed surface area of 100 cm 2 )
Figure 3 – Heat release rate per unit area (HRR*) curve
Total heat release
Total heat release represents the cumulative heat released at the conclusion of a specified time period This value is determined by integrating the heat release rate, typically from ignition until the fire test concludes It serves as a crucial metric for quantifying the magnitude of a fire.
The total heat release in the curve of Figure 2 is 900 kJ.
Peak heat release rate
The peak heat release rate represents the highest heat release rate recorded during a fire test and is useful for evaluating the effectiveness of various flame retardant treatments However, caution is advised when interpreting this value, especially in scenarios where the heat release rate curve exhibits multiple peaks.
The peak heat release rate in the curve of Figure 1 is 3 kW.
Time to peak heat release rate
As well as the amount of heat produced, the time it takes for the heat to be produced is important
This guide focuses on the peak heat release rate, but caution is advised when interpreting cases with multiple maxima in the heat release rate curve.
The time to peak heat release rate in the curve of Figure 1 is 300 s.
Effective heat of combustion
Measurement and calculation
The effective heat of combustion is defined as the heat released from a burning test specimen over a specific time interval, divided by the mass lost during that same period This measure indicates the heat released per unit mass of the volatile fuel produced from the specimen Typically, the effective heat of combustion differs from the net heat of combustion, with the two being equal only when the entire test specimen is consumed and all combustion products are fully oxidized.
To calculate the effective heat of combustion using heat release rate data, it is essential to measure the mass loss rate of the test specimen This measurement is achieved by mounting the specimen appropriately.
HRR */ k W ⋅ m –2 the test specimen holder on a load cell so that mass measurements can be recorded as a function of time
If the mass loss curve associated with the data shown in Figure 1 has the form shown in Figure 4, the effective heat of combustion will have a constant value of 25 kJãg −1
The effective heat of combustion remains relatively constant during the burning of a test specimen, indicating a stable combustion mechanism However, combustion mechanisms can vary at different stages of a fire, leading to changes in the effective heat of combustion These variations can serve as valuable indicators of the effectiveness of flame retardants.
At the beginning and end of a fire test, when mass loss rates are minimal, errors from division by zero or near-zero can result in illogical values for the effective heat of combustion.
Examples
The following examples illustrate the difference between net heat of combustion and effective heat of combustion
The net heat of combustion of toluene is 40,99 kJãg −1 and is a measure of the thermal energy released by the chemical reaction:
C 7 H 8 (liquid) + 9 O 2 (gas) → 7 CO 2 (gas) + 4 H 2 O (gas), T = 298 K
When toluene is burned in a cone calorimeter, it exhibits inefficient combustion, resulting in the formation of soot, carbon monoxide, and other partially oxidized products The effective heat of combustion for toluene, without external heat flux, is approximately 36 kJ/g, indicating incomplete combustion In this scenario, the entire test specimen volatilizes, meaning the effective heat of combustion of the volatile fuel matches that of the test specimen This would differ if any residue from the test specimen remained.
Consider a 100 g sample of wood that burns to leave a carbonaceous char of mass 20 g and that releases 960 kJ of heat The effective heat of combustion will be 12 kJãg −1 (i.e
The heat released per gram when 80 g of volatile degradation products is burned is 960 kJ, equating to 12 kJ/g In contrast, the heat released per gram of the test specimen is 9.6 kJ/g (i.e., 960 kJ/100 g) It's important to highlight that the net heat of combustion for wood is significantly higher, typically ranging from 16 kJ/g to 19 kJ/g, reflecting the complete combustion of wood into fully oxidized products.
FIGRA index
FIGRA, or Fire Growth Rate, measures the size and growth rate of a fire Fires that are large and grow quickly exhibit a high FIGRA index, indicating their danger, while smaller, slower-growing fires have a lower FIGRA index The FIGRA index is determined by the maximum value on a specific graph.
HRR(t) is the heat release rate at time t, and t-t o is the elapsed time, at time t, after a defined start time, t o
NOTE 1 The FIGRA index was devised in the development of EN 13823 [15] which is an intermediate scale corner test used for the regulation of building products in Europe
As a single value parameter for regulatory purposes, some consider that the FIGRA index gives a better indication of the severity of a fire than total heat release or peak heat release
NOTE 2 In EN 13823 the HRR value is a 30 s moving average
Figure 5 shows the FIGRA curve derived from the heat release rate data of Figure 1 The FIGRA index is 0,011 4 kWãs −1 (at 223 s)
Figure 5 – FIGRA curve derived from Figure 1
The FIGRA index is a valuable metric for evaluating fire hazards, as it integrates the heat release rate with the time taken to achieve it Importantly, the FIGRA index is always calculated for a duration shorter than the time of maximum heat release rate, which is 223 seconds compared to 300 seconds in the provided curves.
The FIGRA index must be approached with caution when there is an early rapid but low heat release, as the slope of the heat release rate (HRR) versus time curve can be steeper than expected This discrepancy can render the FIGRA index irrelevant and potentially misleading.
Time (s) FIGRA index = 0,011 4 kW ⋅ s –1 HRR ( t ) ( t- t 0 ) kW ⋅ s –1
The HRR curve depicted in Figure 6 resembles that of Figure 1, with a notable small HRR peak of approximately 0.58 kW occurring around 33 seconds.
The FIGRA curve obtained from these data is shown in Figure 7
Figure 7 – FIGRA curve derived from Figure 6
The FIGRA index is measured at 0.0208 kW/s at 23 seconds, which is approximately double the value derived from the significant portion of the curve, despite the early peak accounting for less than 2.2% of the total heat release.
ARHE and MARHE
ARHE, or Average Rate of Heat Emission, is determined by dividing the total heat release (THR) at a specific time \( t \) by the elapsed time from a defined starting point \( t_0 \).
MARHE, or Maximum Average Rate of Heat Emission, represents the peak ARHE during a specified testing period This value is influenced by the fire's size and growth rate, with larger and rapidly spreading fires resulting in higher MARHE values, while smaller, slower fires yield lower MARHE values.
NOTE The MARHE parameter was devised in the development of EN 45545-2 [16] which is concerned with the fire safety of railway rolling stock in Europe
The MARHE index is viewed by some as a more effective measure of fire severity compared to total heat release or peak heat release, similar to how the FIGRA index serves as a single value parameter for regulatory purposes.
Figure 8 shows the ARHE curve derived from the heat release rate data of Figure 1 The MARHE is 1,826 kW (at 429 s)
Figure 8 – ARHE curve derived from Figure 1
Figure 9 – ARHE curve derived from Figure 6
The MARHE index is less sensitive to early small peaks in the HRR curve compared to the FIGRA index, making it a more valuable parameter for analysis As illustrated in Figure 9, the ARHE curve derived from the HRR data shows a MARHE value of 1,861 kW at 427 seconds, which is only slightly different from the value obtained from the data in Figure 1.
Time (s) MARHE = 1,861 kW kW THR ( t - t 0 )
6 Considerations for the selection of test methods
Ignition sources
Ignition sources must be selected to ensure they are both reproducible and reflective of the relevant fire scenario This entails that the ignition source should accurately represent exposure to either a) atypical localized internal energy sources within the electrotechnical equipment or system, or b) external heat or flame sources outside the electrotechnical equipment or system.
Type of test specimen
To ensure accurate results, it is essential to minimize variations in the shape, size, and arrangement of test specimens Test specimens are categorized based on equipment capabilities, as specific testing methods can only accommodate certain types of samples, such as product testing.
The test specimen is a manufactured product b) Simulated product testing
The test specimen is a component or representative simulation of a product c) Materials or composite testing
The test specimen is a basic material (solid, liquid or gas), or a simple composite of materials.
Choice of conditions
When designing heat release testing conditions for test specimens in large-scale fires, it is crucial to investigate various factors These include selecting appropriate ignition sources, understanding the compartment geometry—such as the size and placement of the test specimen and ignition source, as well as exhaust capabilities Additionally, the presence of other instruments or products for measuring relevant fire properties and the management of fire ventilation must also be taken into account.
The ventilation of a fire can be adjusted to simulate various conditions, such as well-ventilated or under-ventilated (ventilation-controlled) fires Additionally, small-scale fire tests may focus on assessing heat release under atypical atmospheric conditions, including vitiated atmospheres or high-oxygen environments, like those found in spacecraft, as well as the effects of radiation with elevated oxygen levels.
Test apparatus
General
The testing apparatus must be able to evaluate one of the specified test specimens in either a horizontal or vertical position The selected orientation should be the one that provides the most relevant data for fire safety engineering calculations related to full-scale products and their installation.
Small-scale fire test apparatus
The test apparatus must ensure a uniform radiant heat flux to the exposed surfaces of the test specimen, utilizing electrical radiant heaters made from silicon carbide, tungsten-quartz, or metal coils Additionally, it should include an igniter to initiate the combustion of fire effluent produced by the applied heat flux Commonly used igniters, such as electric sparkers or small premixed gas flames, have proven to be effective for this purpose.
The apparatus must be equipped with an exhaust stack to effectively capture the complete mixture of fire effluent and air Essential measuring instruments include those for mass flow rate and temperature, specifically an oxygen analyzer with high sensitivity for the oxygen consumption technique, as well as carbon dioxide and carbon monoxide analyzers that are sufficiently sensitive for the carbon dioxide generation technique Additionally, a thermocouple or thermopile with adequate sensitivity for measuring gas temperature increases is required It is also crucial to ensure proper calibration of the testing instruments.
Test equipment typically features capabilities for simultaneous and interconnected measurements This includes a load cell for assessing mass loss of samples, an optical system in the exhaust duct for measuring smoke obscuration, gas analyzers for determining combustion product concentrations, a soot collection system for particulate measurement, and temperature and pressure sensors positioned at various locations.
Intermediate and large-scale fire test apparatus
An intermediate or large-scale fire test apparatus must include a well-constructed and equipped exhaust duct with the necessary instruments for measuring heat release Additional instrumentation will vary based on specific test requirements.
It is likely that the same type of instruments described above for small-scale fire tests may be useful additions to intermediate and large-scale fire test instruments.
Comparison between small-scale and intermediate/large-scale fire test
Heat release is a crucial factor in evaluating fire hazards, and it can be measured using various fire test apparatus, including large, intermediate, and small-scale tests By selecting the right external heat flux and conditions, small-scale measurements of heat release and mass loss rate can sometimes be correlated with results from larger-scale fire tests.
Choice of fire tests
The chosen test methods must align with the specific fire scenario being addressed If fire tests are not currently defined and require development or modification for a particular IEC technical committee, this process should be conducted in collaboration with the appropriate technical committee, as outlined in IEC Guide 104.
7 Relevance of heat release data
Contribution to fire hazard
The rate of heat release measures fire intensity, while total heat release quantifies fire size It is the primary factor influencing the fire hazard posed by materials and products.
Heat release data are therefore used as important inputs to both fire hazard assessment and fire safety engineering.
Secondary ignition and flame spread
Flame spread is influenced by the ignition of fuel located away from the fire source, which relies on energy input from the heat released by the fire Research indicates that by measuring the heat release rate and other fire properties in heat release test apparatus, it is possible to estimate the maximum flame spread and potential flame-spread rates using computer fire models or straightforward empirical correlations.
Determination of self-propagating fire thresholds
The heat release rate is crucial in distinguishing between a controlled fire and one that becomes self-propagating Identifying the specific heat release rate at which self-propagation thresholds occur is essential for effective fire management.
Probability of reaching flash-over
Heat release data can be used in fire models to predict the likelihood of a fire developing to a state of flash-over.
Smoke and toxic gas production
The production of smoke and toxic gases during a fire is directly linked to the rate of heat release from the fuel Consequently, reducing heat release can effectively decrease the generation of both smoke and harmful gases.
The role of heat release testing in research and development
The effective implementation of innovative material formulations, such as incorporating flame retardants or altering essential chemical compositions, along with new product designs that modify the shape or size of electrotechnical products, can significantly enhance fire safety Additionally, optimizing the geometrical arrangement of individual products within a system contributes to this improvement Measuring heat release provides valuable data to support these advancements.
[1] IEC 60695-1-10, Fire hazard testing – Part 1-10: Guidance for assessing the fire hazard of electrotechnical products – General guidelines
[2] IEC 60695-1-11, Fire hazard testing – Part 1-11: Guidance for assessing the fire hazard of electrotechnical products – Fire hazard assessment
[3] IEC 60695-1-12, Fire hazard testing – Part 1-12: Guidance for assessing the fire hazard of electrotechnical products – Fire safety engineering
[4] ISO 1716, Reaction to fire tests for building products – Determination of the heat of combustion
[5] THORNTON, W., The Relation of Oxygen to Heat of Combustion of Organic
Compounds, The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science 33, 196 (1917)
[6] HUGGETT, C., Estimation of Rate of Heat Release by Means of Oxygen Consumption,
Journal of Fire and Flammability, 12, 61-65 (1980)
[7] ASTM D 7309, Standard Test Method for Determining Flammability Characteristics of
Plastics and Other Solid Materials Using Microscale Combustion Calorimetry
[8] EN 50289-4-11, Communication cables Specifications for test methods Environmental test methods A horizontal integrated fire test method
[9] IEC 60836:2015, Specifications for unused silicone insulating liquids for electrotechnical purposes
[10] IEC 61099:2010, Insulating liquids – Specifications for unused synthetic organic esters for electrical purposes
[11] IEC 60867:1993, Insulating liquids – Specifications for unused liquids based on synthetic aromatic hydrocarbons
[12] IEC 60296:2012, Fluids for electrotechnical applications – Unused mineral insulating oils for transformers and switchgear
[13] ISO 5660-1, Reaction-to-fire tests – Heat release, smoke production and mass loss rate – Part 1: Heat release rate (cone calorimeter method) and smoke production rate (dynamic method)
[14] BSI DD 246, Recommendations for the use of the cone calorimeter (1999)
[15] EN 13823, Reaction to fire tests for building products – Building products, excluding floorings, exposed to thermal attack by a single burning item
[16] EN 45545-2, Railway applications – Fire protection on railway vehicles – Part 2:
Requirements for fire behaviour of materials and components
[17] TEWARSON, A., JIANG, F H and MIRIKAWA, T., Ventilation-Controlled Combustion of Polymers, Combustion and Flame, 95, 151-169 (1993)
[18] TEWARSON, A., Generation of Heat and Chemical Compounds in Fires, pp 1-179 to
1-199 in the SFPE Handbook of Fire Protection Engineering, Society of Fire Prevention Engineers, Boston, MA, USA (1988)
[19] BABRAUSKAS, V., and GRAYSON, S J., Heat Release in Fires, Elsevier Applied
[20] DRYSDALE, D D., An Introduction to Fire Dynamics, John Wiley and Sons, New York,
[21] DINENNO, P J et al (Editors), SFPE Handbook of Fire Protection Engineering, 2nd edn., NFPA, Quincy, MA, USA (1995)
[22] ISO 13927, Plastics – Simple heat release test using a conical radiant heater and a thermopile detector