Pixabay . com
When lightning strikes
Despite the fact that lightning affects drill and blast operations regularly , few in the industry fully understand its impact on explosives or on people working in the vicinity of a lightning strike , writes Tony Rorke .
Lightning is a phenomenon that almost daily affects drill and blast operations globally during the summer rainfall months . When electric detonators were still commonly used for initiating blast holes or for initiating surface lines in non-electric blasts , lightning used to be a serious hazard to blasting operations in the pit . It was for this reason — and for trying to avoid workers being struck by lightning — that the standard procedure for clearing the blast to safe distances when a thunderstorm approaches was introduced .
Lightning has a major impact on productivity , especially when false alarms cause the unnecessary clearing of a pit . Lightning prediction systems need to be multifaceted to achieve a high level of confidence in the prediction of lightning .
Lightning types and current
Many types of lightning occur , but the type that impacts mines and blasting activities most are the cloud to ground strikes , commonly referred to as CG strikes or CG flashes . Most CG strikes are negative strikes that occur from the negatively charged base of a cloud to the positively charged ground . A lightning strike releases a large amount of energy . In Queensland , Australia , the average lightning strike is between 30 and 40kA . These strikes are all negative CG strikes .
The perilous strike of the lightning family is the positive flash — one that lowers positive charge to earth . These are much less common than negative CG flashes , but carry as much as 10 times the current of negative CGs and often last longer . They emerge from the cirrus anvils that sweep downwind of thunderstorms , rather than from a storm ’ s core .
Leaders form in the anvil of the cumulonimbus and may travel horizontally for several kilometres before veering towards the ground . A positive lightning bolt can strike anywhere within several kilometres of the anvil of the thunderstorm , often in areas experiencing clear or only slightly cloudy skies ; they are also known as ‘ bolts from the blue ’ for this reason . Positive lightning typically constitutes less than 5 % of all lightning strikes .
Some storms feature many more positive flashes than usual . The presence of smoke , dust , or pollution in storm clouds seems to encourage the development of positive flashes . As a result of their greater power , as well as lack of warning , positive lightning strikes are considerably more dangerous . Positive lightning strikes are likely to initiate explosives charges ( detonator , booster , or bulk column charge ) in blastholes , and an adequate warning system needs to be in place to counter the risk .
Lightning strike rate
The measured rate of lightning strikes per square kilometre per year in Queensland is between one and two as measured by the Australian Government Bureau of Meteorology
14 _ QUARRY SA | March 2017