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The detonation of charged holes in a prepared block during an electric storm is a risk . This is the main reason for the safety requirement that a blast must be evacuated when an electric storm approaches .
Leon Louw between 1990 and 2003 . In the eastern part of South Africa , a recent study ( Gill , 2008 ) indicates the summer rainfall lightning strike density to be as high as 15 strikes per square kilometre per year . Electronic detonators are being safely applied in both areas .
Santis ( 1998 ) has published a simple equation to determine the likelihood of a lightning strike ( years between strikes ) depending on the area of concern ( A in km 2 ) and the annual strike frequency ( f in strikes / km 2 / year ):
Equation 1 electronic detonators can safely withstand . Assuming that the majority of the charged shot holes will be stemmed during a lightning strike and that about 90 % of the strikes will be negative CG strikes , then the probability of a detonation induced by a lightning strike will decrease to about one in 720 years .
Considering a typical open pit with a surface area of about 0.2km 2 located in eastern Mpumalanga or northern KwaZulu- Natal in South Africa where the lightning strike density per year is 12 , the likelihood is that a strike would occur once every five months within the open-pit workings .
Electric detonators
Electric detonators are very different to electronic delay detonators . Whereas electronic delay detonators have circuit boards containing a processor , resistors , and electric surge protectors between the incoming leads and the fusehead , electric detonators have the incoming leads connected directly to the fusehead . Electric detonators are particularly prone to electric currents and will almost certainly initiate from a nearby or direct lightning strike , as they have no protection barriers against surge currents . Electric detonators must not be used in mines during lightning season .
Assuming a blasthole is lightning sensitive to an area of 100m 2 ( 0.000001km 2 ), that the lightning strike density is two per km 2 per year , and that 60 000 holes are charged and exposed per year , then a direct strike on a blasthole would be as follows :
In other words , a direct strike on a charged shot hole can be expected once in eight years . This does not mean a lightning induced detonation every eight years . Shot holes will mostly be stemmed and most lightning strikes will generate currents below the 40kA , which most
Figure 1 : Lightning ground flash density in South Africa in 2006 ( Gill , 2008 ).
QUARRY SA | march 2017 _ 15