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 source, and the 'T' value for Gas and Dust are both arrived at and displayed differently.
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 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.
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.
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 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 dust 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°C1 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 220.127.116.11) figure 1.
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 <T234°C rating is used it would be suitable for a dust of MIT 350°C and LIT of 309°C assuming layering is no more than 5mm2
In practice the use of the temperature concept is very straightforward. When applied to the fuel it means it will not ignite until at least the specified temperature is reached (including safety margins). For equipment it is marked with the corresponding Temperature rating to say it will not reach that temperature.
There is a slight caveat for dust in that, unless specified otherwise, the standard certification will is only valid for up to a 5mm layer of dust on the equipment. An additional safety margin must be added if dust is allowed to build up beyond the specified thickness.
This is not because the dust in a layer presents a greater hazard (layered dust is generally not an explosive hazard) but that it can insulate the equipment causing a greater than 'tested' temperature rise.
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 18.104.22.168) figure 1 ↩
5mm Layering is the default for certification unless stipulated differently in the Type certificate. ↩