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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