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Resistive ignition curves 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.
Propane v Hydrogen 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. 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. 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 Experimental Safe Gap: and/or their Minimum Ignition Current (MIC)
Minimum Ignition Current: 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
Pure Carbon Monoxide would normally be a IIA gas. However, it is specifically listed in EN 60079-20-1 as IIB gas due to differences in MESG when moisture is present i.e. in air at normal temperature and pressure. Notes
Detailed in EN 60079-20-1 Explosive atmospheres. Material characteristics for gas and vapour classification. Test methods and data (责任编辑：) |
