IEC/TS 60695 1 20 Edition 1 0 2008 02 TECHNICAL SPECIFICATION SPÉCIFICATION TECHNIQUE Fire hazard testing – Part 1 20 Guidance for assessing the fire hazard of electrotechnical products – Ignitability[.]
Gases
Flammability limits
Flame propagation in a fuel/air gas mixture is inhibited when the fuel concentration is either too low or too high, defined by the lower flammability limit (LFL) and upper flammability limit (UFL) These limits are crucial because flames require a minimum temperature to sustain combustion, and an excess of air or fuel disrupts the ability to maintain this temperature Flammability limits are typically represented as the percentage of fuel by volume in the mixture.
Arc fires
Faults in electrical equipment like junction boxes and power transformers can lead to disruptive electric arcs, causing insulation materials to pyrolyse and generate high-temperature combustible gases These gases can expand rapidly and, when mixed with air, pose a significant explosion risk.
Liquids
Introduction
With the exception of some unstable or reactive substances, liquids do not generally ignite
Combustible vapor, generated through the vaporization of liquids, is typically what ignites The vaporization process is influenced by both the temperature and the chemical composition of the liquid involved.
Ignition parameters
Temperature plays a crucial role in determining the ignitability of liquids, with three key temperatures to consider: auto ignition temperature, fire point, and flash point Auto ignition occurs when a substance ignites without any localized heat source.
MECON Limited is licensed for internal use in Ranchi and Bangalore, with materials supplied by the Book Supply Bureau It is important to understand that the flash point refers to the temperature at which a substance can ignite momentarily, while the fire point indicates the temperature at which sustained combustion occurs after ignition.
Various testing methods are employed to assess characteristic temperatures, with the measured temperature influenced by the specific details of the testing apparatus Consequently, it is crucial to specify the test method when referencing these parameters.
Insulating liquids
ISO 2719, known as the Pensky-Martens closed cup method, is referenced in IEC standards for determining the flash point of insulating liquids This method is designed to measure the flash point within a confined space, effectively identifying small quantities of volatile substances An alternative to this method is ISO 2592.
(Cleveland open cup) which is used to measure the flash point over an open liquid surface
The flash point measured by ISO 2592 is significantly lower than that measured by ISO 2719
IEC 60695-8-3 is focused on quantifying the heat released from burning insulating liquids In this standard, test specimens are subjected to a consistent heat flux alongside a piloted ignition source Key ignition-related properties include the time to ignition at a designated heat flux and the minimum incident heat flux necessary to sustain ignition.
Solids
Introduction
Generally, solids do not ignite on their own; instead, it is the combustible vapor produced through pyrolysis that ignites This vaporization process relies on both the temperature and the chemical composition of the solid material.
The exceptions to this general statement are:
– some other non-metallic elements, for example carbon (see 4.3.4), sulphur and phosphorous;
– certain reactive substances (see 4.3.5); and
Parameters affecting ignition
The production of flammable volatiles in solids depends on the material's temperature, which is influenced by the type of heat input This heat can come from various sources, including radiant, convective, and conductive heat fluxes, as well as imposed flames or hot wires, either individually or in combination.
The ease of ignition will also depend on the chemical nature of the flammable volatiles, which in turn will depend on the chemical nature of the solid
The rate of heating of the material is a function of a number of properties of the solid: a) thickness; b) thermal conductivity, (k); c) density, (ρ); d) specific heat, (c); e) absorptivity (in the case of radiative heating)
In thick test specimens, the material beneath the surface effectively conducts heat away, which reduces surface heating and enhances ignition resistance Conversely, in thin specimens, this heat conduction is limited, resulting in lower resistance to ignition.
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Most thermoplastic materials do not adhere to the general rule regarding ignition resistance, as they tend to melt away from heat sources like flames or hot wires, often leading to non-ignition This behavior can create a misleading perception of their ignition resistance Therefore, special considerations must be taken when testing the ignitability of thermoplastics The challenges associated with testing thermoplastics in standard fire tests are addressed in ISO 10840.
Thermal inertia, represented by the product kρc, plays a crucial role in determining how quickly a material heats up Materials with high thermal inertia, such as solid metals, experience a slower rate of surface heating, resulting in a longer time to reach ignition temperature Conversely, materials with low thermal inertia, like certain foamed plastics or low-density combustibles, heat up more rapidly, leading to a shorter time to ignition.
Upon ignition of the test specimen, flame propagation occurs when the flame emits enough heat flux, primarily through thermal radiation, to sustain pyrolysis and ignition at an adequate rate ahead of the pyrolysis front.
The heat flux magnitude ahead of the pyrolysis front is influenced by the heat release rate of the specimen and any ongoing imposed heat flux Additionally, the resistance to ignition is determined by the specimen's minimum ignition temperature and the surface heating rate.
Metals
When metals combust in air, they produce metal oxides Many metals develop a surface layer of oxide due to low-temperature oxidation, which prevents further combustion since it is already an oxidized product Therefore, for the bulk metal to ignite, this oxide layer must be removed.
Metals can be categorized into three groups based on their ignition characteristics The first group includes metals like iron and magnesium, which ignite at or below their melting point, typically above 650 ºC, and do not form a protective oxide layer The second group consists of metals such as aluminum, lead, tin, and zinc, which ignite after melting, with melting points below 660 ºC, and generally form a protective oxide layer Lastly, the third group comprises low-reactivity metals like mercury, silver, gold, and platinum, which do not ignite.
The ignition ease of metals is influenced by their surface area to volume ratio Thin metal films and finely divided powders ignite more readily than larger metal pieces due to the fact that the heat generated during oxidation is related to the burning surface area, while the initial heat loss through conduction is linked to the metal's volume.
Carbon (graphite) and carbonaceous char
Pure carbon, specifically in its graphite form, can ignite in air at temperatures exceeding 800 ºC Between 800 ºC and 1,200 ºC, it undergoes non-flaming surface combustion, also known as glowing combustion When temperatures rise above 1,200 ºC, flaming combustion occurs, characterized by the presence of a carbon monoxide (CO) flame.
Carbonaceous chars are impure carbon forms characterized by varying ignition temperatures due to their volatile content and porosity Similar to graphite, they can undergo both flaming and non-flaming combustion When carbon-containing materials burn, they often develop a protective char layer on their surface, which can shield the underlying material during the initial stages of a fire.
A correlation exists between ignition resistance, indicated by the limiting oxygen index, and char yield across various organic polymers.
Reactive substances
In many fires, the primary oxidizing agent is the oxygen found in the air However, certain materials contain oxygen as part of their molecular structure or incorporate it as a solid oxidizing agent mixed with the fuel These substances are often intentionally designed to be combustible or explosive.
– “blue touch paper” (cellulose and potassium nitrate);
– gunpowder (carbon, sulphur and potassium nitrate);
– cigarettes (tobacco and potassium nitrate);
Dust clouds
Dust clouds consist of solid aerosols, which are small solid particles suspended in air or other gases Their ignition behavior resembles that of a premixed gas rather than that of a solid material.
5 Consideration for the selection of test methods
Introduction
When selecting a test method, it is crucial to consider the relevant fire scenarios, potential ignition sources, the nature of the test specimen, and the specific test procedures and apparatus involved.
Fire scenario
When selecting test methods for a fire scenario, it is crucial to consider several key parameters These include the geometry of the test specimen, such as its thickness and the presence of edges, corners, or joints Additionally, factors like anisotropy, surface orientation, and the rate and direction of airflow must be evaluated The nature and position of the ignition source, along with the magnitude and position of any external heat flux, are also important Finally, it is essential to determine whether the flammable material involved is a solid or a liquid.
Ignition sources
Internal sources
When assessing an ignition source within a product or component, appropriate test methods should simulate overheating from internal metallic parts, small flames with low heat transfer from combustion, and electrical arcs.
Various test methods are available to assess and characterize the properties of materials, products, components, or apparatuses when exposed to heat and/or flame in controlled laboratory settings.
ISO 871 outlines a laboratory procedure for measuring the flash-ignition and spontaneous-ignition temperatures of plastics using a hot-air furnace This standard is part of various methods employed to assess the ignition resistance of plastic materials.
The glow wire test methods (IEC 60695-2-11, IEC 60695-2-12 and IEC 60695-2-13) simulate the first cause of ignition due to overheating by contact with a heated part, without an open flame
IEC 60695-2-11 (GWT) is applicable solely to components or apparatus, offering a qualitative assessment of ignition behavior It establishes a pass/fail criterion based on the burning duration when subjected to specific temperature conditions, particularly above the minimum ignition temperature.
IEC 60695-2-12 (GWFI) and IEC 60695-2-13 (GWIT) are essential standards for preselecting insulating materials The GWFI test evaluates the maximum temperature at which a material can ignite and burn for a limited time without allowing fire to spread from the specimen In contrast, the GWIT test measures the minimum ignition temperature to determine a material's resistance to ignition.
IEC 60695-11-5 is designed to simulate ignition using a small flame, making it relevant for electrotechnical equipment, its components, and solid electrical insulating materials or other combustible substances This standard assesses the ignitability of test specimens and evaluates their capacity for self-extinguishment.
IEC 60695-11-10 and IEC 60695-11-20 outline distinct test methods that involve applying an open flame directly to the surface of a test specimen The evaluation of materials is based on the duration of burning or glowing after the flame is removed, as well as the production of flaming droplets.
A test flame ranging from 10 to 50 W is utilized, while IEC 60695-11-20 specifies a test flame that is ten times larger with an extended application time Both testing methods offer classification systems that can be employed for quality assurance and the pre-selection of component materials in products.
NOTE The scopes of IEC 60695-11-10 and IEC 60695-11-20 do not refer to the simulation of either internal or external ignition
IEC 60695-11-11 is suitable to simulate ignition by the heat flux from a small non-contacting flame.
External sources
When assessing an ignition source located outside electrotechnical equipment, appropriate test methods should simulate thermal stress from various sources These include direct flame contact with the equipment's surface, high thermal stress from overheated metallic parts, and indirect thermal heat flux, such as radiant heat.
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The methods described in 5.3.1 can also be used to simulate external ignition as well as internal ignition The difference is the location of the application of the thermal stress
IEC 60695-11-5, which simulates ignition by a small flame (see 5.3.1) has gained acceptance in evaluating external ignition sources such as open candle flames 7
Additional test methods could be:
IEC 60695-11-10 and IEC 60695-11-20 – Both these test methods involve direct contact of an open flame onto the surface of the test specimen
NOTE The scopes of these standards do not refer to the simulation of either internal or external ignition
Materials are evaluated based on their burn duration and the production of flaming droplets after the test flame is removed According to IEC 60695-11-10, a 50 W test flame is utilized, while IEC 60695-11-20 employs a flame that is ten times larger with an extended application time Both standards offer classification systems that can be used for quality assurance and the pre-selection of component materials in products.
Indirect thermal flux, as from an item burning nearby, can be evaluated by the following heat release methods
IEC 60695-11-11 (see 5.3.1) – simulates ignition caused by the heat flux from a small non- contacting flame
ISO 5657 is a small-scale test method primarily designed to evaluate materials rather than finished products However, products with a size of less than 100 mm can be tested directly In this method, a conical electrical resistance heater heats the test specimen, and the heat release rate is measured following ignition.
Arc ignition of materials
Arc ignition of flammable gases requires a minimum energy threshold, a principle utilized in "intrinsically safe" cables These cables are designed with specific voltage and inductance levels to restrict the energy of potential sparks from short circuits or relays, ensuring it remains below the ignition point This same approach is applied to determine the safe voltages and currents in cables used within fuel tanks.
To ignite a flammable gas or aerosol mixture, a high voltage source is typically employed to power ignition devices, such as spark plugs, commonly found in gas or oil furnaces.
For arc ignition to take place, a liquid typically must be volatilized A prime example of this is the high voltage power arc that forms over the air/liquid interface of transformer oil The radiant heat transfer can effectively raise the temperature of the liquid, leading to its volatilization and subsequent ignition.
It is highly desirable to exclude this possibility by design
Arc ignition of solids can occur in both wet and dry environments due to various combinations of high or low current and voltage Numerous tests exist to assess both materials and finished products under suitable conditions.
Test method IEC 60112 [2] is used to evaluate tracking up to 600 V on materials
NOTE IEC 60112 is not a test of ignitability, but If ignition occurs followed by persistent flaming within the test period, this constitutes failure of the test
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Test methods EN 3475-603 [3] and EN 3475-604 [4] are for wires used in the aerospace industry and they simulate wet and dry arc propagation, respectively, in electrical wiring
Momentary short-circuit arcs between a faulty insulated wire and another conductor can cause ohmic heating, leading to the thermal pyrolysis and charring of the insulation This charred insulation becomes conductive, allowing the short-circuit arc to persist and potentially propagate along the wire through continuous pyrolysis, a phenomenon known as arc tracking In cases where the arcing wire is part of a multiple wire bundle, the insulation of adjacent wires may also become thermally charred, resulting in further arc tracking Consequently, arc tracking can result in the complete failure of an entire wire bundle or harness.
The IEC 60587 inclined plane wet tracking test is an appropriate protocol for low current and high voltage applications Notably, PTFE (polytetrafluoroethylene) has been observed to ignite during this test, a phenomenon that would be unexpected in a traditional flame ignition test.
In all these tests, initial leakage currents are of the order of milliamps
High voltage and high current equipment can experience ignition due to power arcs that reach hundreds or thousands of amperes, producing significant radiant heat and molten droplets This ignition risk is typically assessed through failure testing, where intentional faults are introduced, and rated fault currents are applied to the circuit For instance, IEC 60099-4 addresses surge arrestors, highlighting that a power arc can breach the polymeric housing and potentially ignite it, with a maximum allowable afterburn time of 2 minutes.
5.3.3.4 Arc fires in power transformers
Faults in electrical equipment like junction boxes and power transformers can lead to disruptive electric arcs, causing insulation materials to pyrolyse and generate high-temperature combustible gases These gases expand quickly, and when they come into contact with air, they can trigger an explosion.
Power transformers that use oil for insulation face significant risks, particularly those exceeding 100 MVA Recent tests from high-power laboratories reveal that internal faults leading to short-circuits can trigger pyrolysis of the insulating oil, producing a hazardous gaseous mixture of saturated hydrocarbons This pyrolysis reaction generates gas at high pressure and temperature, often resulting in structural failure and potential explosions of the transformer.
Annex A contains some examples of real accidents caused by arc fires in underground hydroelectric power plants or urban substations.
Types of test specimen
The test specimen can include a manufactured product, a product component, a simulated product that represents part of a manufactured item, specified materials (either solid or liquid), or a composite of various materials.
Variations in the shape, size and arrangement of the test specimen should be limited.
Test procedure and apparatus
The test procedure should ideally be structured to yield results applicable for hazard analysis, although this may not be essential for straightforward tests aimed solely at quality control or regulatory compliance.
The test apparatus should be able to test the actual electrotechnical product, a simulated product, a material or a composite, as described in 5.4
The test apparatus must deliver a consistent heat flux to the test specimen from an external heat source or flame, specifically targeting the area where ignition is expected to take place.
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The test apparatus must effectively ignite the vapor-air mixture released from the test specimen, utilizing either an electrical spark igniter or a premixed gas-air flame, both of which have proven to be effective solutions.
An air flow rate which is relevant to the fire scenario of concern should be used
6 Use and interpretation of results
The ignition process and the potential for sustained burning are influenced by numerous factors Therefore, it is crucial to choose test variables that accurately represent the specific fire scenario under consideration.
The following parameters can be used for fire safety engineering purposes: a) auto ignition temperature, b) fire point, c) flash point, d) ignition temperature, e) upper and lower flammability limits, and f) thermal inertia
Assessing the ease of ignition of electrotechnical products under specific conditions is crucial for understanding their fire hazard The principle guiding this assessment is that increased resistance to ignition correlates with a reduced risk of fire Therefore, a high ignition resistance is always preferred to enhance safety.
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Examples of accidents due to arc fires in underground hydroelectric power plants or urban substations
Gas explosion accidents in underground hydroelectric power plants or urban substations may occur as a consequence of electric faults in oil-insulated components such as transformers
An electric arc inside the component causes the pyrolysis of part of the oil, and the gaseous pyrolysis products can then escape from the component to mix with air
The chemical composition of the mixture can lead to an explosion, resulting in a pressure shock wave If this wave is not adequately contained by blast-resistant barriers, it may spread throughout the power plant or substation.
A.2 Examples which are generally available (not an exhaustive list)
Tonstad, Norway, 1973 - Outside spark-over on the cable porcelain terminal with a flash
Explosion of reactive gases and oil mist: 3 people killed, 1 heavy burn injury
Bardufoss, Norway, 1975 - Short circuit in the control cable connection to one of the unit
Explosion: heavy damages in the powerhouse
Roncovalgrande, Italy, 1988 - Ground discharge in the insulator Explosion of reactive gases and oil mist: damage to equipment and structures
Skjomen, Norway, 1998 - Material and system defects in the control systems Explosion and oil fireball: transformer totally damaged
Aroy, Norway, 2001 - Operational mistake and material weakness in the windings or winding insulation No explosion or fire
A.2.2 Urban substations (not an exhaustive list)
Toronto, Canada, 1999 - Toronto Hydro, Windsor Station
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[1] Van Krevelen, D W., Properties of Polymers, 3 rd edn., Elsevier, 1990, p 732
[2] IEC 60112 :2003, Method for the determination of the proof and the comparative tracking indices of solid insulating materials
[3] EN 3475-603:2002, Aerospace series Cables, electrical, aircraft use Test methods
Resistance to wet arc tracking
[4] EN 3475-604:2002, Aerospace series Cables, electrical, aircraft use Test methods
Resistance to dry arc propagation
[5] IEC 60587 :1984, Test method for evaluating resistance to tracking and erosion of electrical insulating materials used under severe ambient conditions
[6] IEC 60099-4 :2004, Surge arresters – Part 4: Metal-oxide surge arresters without gaps for a.c systems
[7] IEC 60695-11-40, Fire hazard testing – Part 11-40: Confirmatory test – Guidance
[8] Babrauskas, V., Ignition Handbook, Fire Science Publishers, Issaquah, WA (USA),
[9] Beyler, C.L., Flammability Limits of Premixed and Diffusion Flames, Section 2, Chapter
9, pp 2-147 to 2-159 in SFPE Handbook of Fire Protection Engineering, National Fire
Protection Association Press, Quincy, MA (USA), 1995
[10] Drysdale, D., An Introduction to Fire Dynamics, John Wiley and Sons, New York, N.Y
[11] Hilado, C.J., Flammability Test Methods Handbook, Technomic Publishing Co., Inc.,
[12] Kanury, A.M., Ignition of Liquid Fuels, Section 2, Chapter 10, pp 2-160 to 2-170 in
SFPE Handbook of Fire Protection Engineering, National Fire Protection Association
[13] Kanury, A.M., Flaming Ignition of Solid Fuels, Section 2, Chapter 13, pp 2-190 to 2-204 in SFPE Handbook of Fire Protection Engineering, National Fire Protection Association
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4.3.4 Carbone (graphite) et résidu charbonneux 32
5 Considérations pour le choix des méthodes d’essai 32
5.3.3 Allumage par arc de matériaux 35
6 Utilisation et interprétation des résultats 37
Annexe A (informative) Exemples d’accidents dus à des feux d’arc dans les postes hydroélectriques souterrains et les sous-stations urbaines 38
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ESSAIS RELATIFS AUX RISQUES DU FEU –
Partie 1-20: Lignes directrices pour l'évaluation des risques du feu des produits électrotechniques – Allumabilité – Lignes directrices générales
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La tâche principale des comités d’études de la CEI est l’élaboration des Normes internationales Dans des circonstances exceptionnelles, un comité d’études peut proposer la publication d’une spécification technique lorsque
• le soutien nécessaire ne peut pas être obtenu pour la publication d’une Norme internationale, en dépit d’efforts répétés, ou
The topic is still evolving from a technical perspective, or for other reasons, when there is a possibility for an international standard agreement in the future, but not in the immediate term.
Les spécifications techniques sont révisées dans les trois années qui suivent leur publication pour décider si elles peuvent être transformées en Normes internationales
La CEI 60695-1-20, qui est une spécification technique, a été établie par le comité d’études
89 de la CEI: Essais relatifs aux risques du feu
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Elle a le statut de publication fondamentale de sécurité, conformément au Guide CEI 104 et au Guide ISO/CEI 51
Le texte de cette spécification technique est issu des documents suivants:
Projet d'enquête Rapport de vote 89/807/DTS 89/827/RVC
Le rapport de vote indiqué dans le tableau ci-dessus donne toute information sur le vote ayant abouti à l'approbation de cette spécification technique
La présente spécification technique doit être utilisée conjointement avec la CEI 60695-1-21
Cette publication a été rédigée selon les Directives ISO/CEI, Partie 2
A comprehensive list of all parts of the IEC 60695 series, titled "Fire Risk Testing," is available on the IEC website.
La partie 1 comprend les parties suivantes:
Partie 1-10 1 : Lignes directrices pour l’évaluation des risques du feu des produits électrotechiques– Directives générales
Partie 1-11 1 Lignes directrices pour l’évaluation des risques du feu des produits électrotechiques – Evaluation de dangers
Partie 1-20: Lignes directrices pour l’évaluation des risques du feu des produits électrotechiques – Allumabilité – Guide général
Partie 1-21: Lignes directrices pour l’évaluation des risques du feu des produits électrotechiques – Allumabilité – Résumé et pertinence des méthodes d'essais
Partie 1-30: Lignes directrices pour l’évaluation des risques du feu des produits électrotechiques – Utilisation des procédures d’essais de présélection
Partie 1-40: Guide pour l’évaluation des risques du feu des produits électrotechiques –
The committee has decided that the content of this publication will not be modified until the maintenance date specified on the IEC website at "http://webstore.iec.ch" in the data related to the publication in question At that time, the publication will be updated.
• remplacée par une édition révisée, ou
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