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 there are a number of reasons why this is impractical therefore 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.
|IEC & CENELEC