A Generalised Basis for the Determination of the Hazard of Ignition of Flammable Gas Mixtures by Radiofrequency Radiation

A new approach to the problem of ignition of flammable atmospheres by sparks occurring on structures irradiated by radio frequency fields is reported. The ignition process can be divided into three main steps; radiation of power from the transmitter; reception of power at the unintended receiving antenna; conversion of power into a spark sufficiently intense to cause ignition of a given flammable gaseous mixture. Analyses of the characteristics of unintended receiving antennas are presented; these are divided into 'electrically large' and 'electrically small’ cases owing to the differing significance of the effects of ohmic losses. Graphs are presented showing the variation of the directivity and impedance of electrically large dipole antennas with varying length and thickness. For electrically small antennas the tuned loop is assumed to be the worst practical case, and an equation for its effective aperture is developed, taking account of skin resistance. The measurement of the spark power required to ignite common flammable mixtures was dealt with experimentally using ignition test cells based on well established designs used for dc intrinsic safety testing. The results obtained for hydrogen/air mixtures, showing a surprisingly marked frequency dependence, are displayed graphically. By combining reasonable worst-case results for each step of the process a value may be derived for the safe field intensity at a given site where flammable substances are handled.