--- title: 'Hot surface Ignition' url: 'https://hazardex.com/define/hot-surface' markdown: 'https://hazardex.com/define/hot-surface.md' date: '2026-09-15' description: "Gas⤵️ \_Dust⤵️ Hot surface ignition There\_are two sources of ignition, a spark or a hot surface As with most parameters used with Atex the hot surface ignition temperature is arrived at by physical testing rather than theoretically calculations, therefore values can vary depending on the data sourc…" taxonomy: category: - docs --- # Hot surface Ignition [](#) [ Hazardex ](https://hazardex.com/) [ Define the Hazard ](https://hazardex.com/define) Hot surface Ignition [Gas⤵️](#gas) [Dust⤵️](#dust) ## Hot surface ignition ![Hotplate](https://hazardex.com/user/pages/02.define/01.hot-surface/hotplate.webp)There are two sources of ignition, a [spark](https://hazardex.com/define/spark) or a **hot surface** As with most parameters used with Atex the hot surface ignition temperature is arrived at by physical testing rather than theoretically calculations, therefore values can vary depending on the data source, and the ['**T**' value for Gas](#gas) and [Dust](#dust) are both arrived at and displayed differently. **Auto-ignition Temperature AIT** lowest temperature (of a hot surface) at which, under specified conditions (detailed in EN 60079-20-1), an ignition of a flammable gas or vapour in mixture with air/inert gas occurs. Also known as **Minimum Ignition Temperature** although this term is generally used in the context of dust. Auto-ignition temperature should not be confused with with the flash point temperature which is generally a lower value. **Flash Point Temperature** Lowest liquid temperature at which, under certain standardized conditions, a liquid gives off vapours in quantity such as to be capable of forming an ignitable vapour/air mixture. The hazardous area regulations do not apply to liquids with a flashpoint of greater than 60°C[1](#fn:1)The above parameters are well known for most individual gases. . However, if there is an unknown gas such as vapours from a liquid mix then either a worst case scenario should be assumed or the vapour tested according to EN 60079‑20‑1 --- [Dust⤵️](#dust) [🔝](#top) ## Gas 'T' Rating ![thermometer](https://hazardex.com/user/pages/02.define/01.hot-surface/t-gas.svg) Auto Ignition temperatures ![thermometer](https://hazardex.com/user/pages/02.define/01.hot-surface/t-gas.svg "Autoignition temperatures")Autoignition temperatures [Autoignition temperatures](#gastemp) In order to simplify the issue, for gas ignition the gases are banded into 6 groups, T1 - T6 based on the autoignition temperature of the gas. The ignition temperature is looked at and the gas allocated to a 'T' group **lower** than the autoignition temperature e.g. Butane with an auto ignition of 365°C would be designated T2 gas (>300 °C) Taking a list of the 169 most common potentially explosive gases found the split is: T1 – 62, T2 – 68, T3 – 28, T4 – 8. T6 group consists of Carbon Disulphide and Ethyl Nitrate. The autoignition value for Ethyl Nitrate sometimes listed as high as 100°C but erring on the safe side a figure of 95°C is usually accepted, making it a T6 gas. Phosphine is a highly explosive gas. However, with an ignition temperature of <40°C i.e. below T6 and will potential ignite on a hot day, it is outside the scope of standard certification and special conditions protection methods must be applied.[2](#fn:2) For Oil and Gas applications T3 is usually sufficient, T4 to be on the safe side. However, higher T ratings are often specified without due thought to the issues that may present. --- [Gas⤴️](#gas) [🔝](#top) ### Dust Ignition Temperature ![thermometer](https://hazardex.com/user/pages/02.define/01.hot-surface/t-dust.svg)Dust Minimum Ignition Temperature ![thermometer](https://hazardex.com/user/pages/02.define/01.hot-surface/t-dust.svg "MIT Dust")MIT Dust [MIT Dust](#dusttemp) The ignition figures for Dust are arrived at by test methods according to EN 50281-2-1:1999 the results can vary significantly and often figures are published which use different test methods. Unlike Gas **Dust** 'T' figures are not grouped or banded and the specific ignition temperature should be used e.g. T200°C, in addition there are two temperatures to take into consideration **Minimum Ignition Temperature** of cloud (**MIT**) and the **Layer Ignition Temperature**, (**LIT**). When the term 'auto-ignition temperature, or just ignition temperature is used it generally refers to the MIT. **Minimum ignition temperature of a dust cloud**: Lowest temperature of the hot inner wall of a furnace at which ignition occurs in a dust cloud in air contained therein. **Minimum Ignition Temperature of a dust layer**: Lowest temperature of a hot surface at which ignition occurs in a dust layer of specified thickness on this hot surface. The ignition temperature of dusts are based on standard (typical) particle size and this can vary significantly a good example being the generic term sugar, which depending on type (and particle size) can have an ignition temperature anywhere in the range associated with food stuff. The final '**T**' rating is calculated from the lowest value of either **2/3 MIT** or **75°C[3](#fn:3) below LIT**. The 75°C reduction is based on a standard assumption of no more than 5mm layer of dust. Should there be a likelihood of more than 5mm of dust accumulating then the reduction should be greater as detailed in EN 60079–14 (Section 5.6.3.3) figure 1. For recently certified equipment EN 60079-0 now calls for the [label](https://hazardex.com/labels#dust-temperature) to include details of the layer thickness that the certification was based on e.g. T100 300°C for equipment with a maximum surface temperature of 300°C tested at 100mm thickness of dust. If no thickness is specified it is assumed dust layering was not considered for the 'T' rating and therefore the site must take any potential dust layering into consideration. Often an ignition temperature for the dust is quoted without identifying whether it is MIT or LIT, in most cases this is the MIT. As an example (Wheat) flour MIT has been quoted as anywhere between 380°C 430°C depending on data source, the 430°C is the accepted figure for wheat grain. The most common figure appears to be an MIT of 400°C which in this case is the same as the generally accepted LIT. However, an LIT of 200°C has been published for certain types of wheat flour dust. This can create issues with finding equipment, particularly motors, with T125°C or less. For a dust with MIT 350°C and LIT 400°C we get: using MIT 350°C x 2/3 = 234°C using LIT: 400°C - 75 = 375°C In this case a T234°C as the lower of the two would be quoted for the area. These figures are for example only. Each site is responsible for selecting the appropriate temperatures to work with. As can be seen here most foodstuffs fall within a band between 350°C & 450°C. if equipment with