Ignition by a hot surface is relatively straightforward. However, things get slightly more complicated when looking at other ignition sources, is energy being discharged into the area usually considered in terms of a spark this also encompasses other energy forms, light sources (LED) and electromagnetic waves such as radio waves.
Putting spark energy in electrical terms, the ignition energy required to ignite critical mixtures of each gas and air have been determined experimentally using an electrically generated spark as the ignition source.
The results being shown in this graph for a simple resistive load are known as “the gas curves”.
If the voltage and current is below the relevant gas group curve then ignition cannot occur from a spark, this is the basis for the intrinsic safety protection method.
From the curves it is evident that, in theory, a low voltage high current (e.g. <10v >3A) or high voltage low current (e.g. >200v <10mA) spark would not cause ignition. However, these are just the curves for pure resistance, there are further graphs for capacitance and inductance which would limit the paractical use for high voltage or current.
intrinsic safety is generally used in low voltage, low current (instrumentation) circuits i.e. 24v or less.
There is no direct relationship between Gas groups and ignition temperature as can be shown when looking at the characteristics of Hydrogen and Propane.
Hydrogen, which is well known as a highly explosive gas is IIC as its ignition energy is as low as 17µJ whereas Propane requires over 10 times the energy (180µJ). However Hydrogen needs a much higher 'hot surface' temperature to ignite 560°C whereas propane is 470°C.
Most people would be surprised to know Hydrogen is used as a cooling agent for turbo generators particularly in the Nuclear industry due to its high ignition temperature and high thermal conductivity.
As well as low energy to ignite, Hydrogen is explosive over a wide range of concentrations from 4 - 77% v/v whereas Propane is explosive only in a narrow mixture range (approximately 2 - 10% v/v) making it easier to use.
This one of the reasons we use propane (LPG) for BBQ’s and not Hydrogen.
From this it can clearly be seen that Ignition energy and ignition temperature are not necessarily related.
Gas Groups
| IEC & CENELEC Gas Groups |
Representative Gas |
Ignition Energy |
|
|---|---|---|---|
| I | Methane | 180µJ | Mining |
| IIA | Propane | 180µJ | |
| IIB | Ethylene | 60µJ | |
| IIC | Hydrogen | 20µJ |
The above is the traditional method of explaining spark ignition parameters and is easily understandable in terms of electrical parameters.
The way that the group members are decided, like most aspects of hazardous area assessement, is by testing. (see EN 60079-20-1) Gases and vapours are classified according to their Maximum Experimental Safe Gaps (MESG).
Spark Tests
maximum gap between the two parts of the interior chamber which, under specific test conditions1, prevents ignition of the external gas mixture through a 25 mm long flame path when the internal mixture is ignited, for all concentrations of the tested gas or vapour in air
and/or their Minimum Ignition Current (MIC)
Minimum current in resistive or inductive circuits that causes the ignition of the explosive test mixture in the spark-test apparatus according to IEC 60079-11
| Group | MESG | MIC Ration | One Determination | Both MESG and MIC |
|---|---|---|---|---|
| IIA | ≥ 0.9mm | > 0.8 | MESG ≥ 0.9 mm or MIC > 0.9 |
0.8 ≤ MIC ≤ 0.9 need to confirm by MESG |
| IIB | > 0.5 and < 0.9 mm |
≥ 0,45 and ≤ 0.8 |
0.55 mm ≤ MESG < 0,9 mm or 0.5 ≤ MIC ≤ 0.8 |
0.45 ≤ MIC ≤ 0.5 need to confirm by MESG |
| IIC | ≤ 0.5 | < 0.45 | MESG < 0.5 mm or MIC < 0.45 |
0.5 ≤ MESG < 0.55 need to confirm by MIC |
For a mix of gases there are a number of ways of determining characteristics based on known gases, refere to EN 60079-20-1 clauses 4.5 and 4.6
Notes
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Detailed in EN 60079-20-1 Explosive atmospheres. Material characteristics for gas and vapour classification. Test methods and data ↩