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80 E&MJ; • SEPTEMBER 2017 www.e-mj.com OPERATING STRATEGIES Explosives supplier BME recently called attention to an ongoing mining-industry transition from non-electric to electronic delay detonators (EDDs), a trend that the company said has brought many benefi ts to mining productivity. However, accord- ing to BME, there are still plenty of ad- vantages to be gained by blasters who are prepared to think "outside the box." That was the message from BME's technical director, Tony Rorke, who ad- dressed the focus in recent years on the fl exibility in blast timing that EDDs allow — and more specifi cally on the value of longer timing delays. "When we were fi rst exploring the capa- bility of EDDs, we were impressed by the accuracy that blasting with this technology allowed us," he said. "This accuracy and reliability was a huge step forward, ensuring that there were no out-of-sequence blasts." When using non-electric initiation systems, there were certain traditional timing conventions that were employed. These were based on the fact that accura- cy decreased signifi cantly as the length of the pyrotechnic delay elements increased and that the downlines may be severed (cutoffs) during a blast before the fi ring signal reaches the detonators, leading to dangerous misfi res. "To get the detonators to fi re in some acceptable sequence, therefore, delay periods were in the past kept as short as possible," he said. "The longer down- hole delay has the dominant impact of fi ring time accuracy, so the risk of out- of-sequence initiation is a function of the in-hole detonator. Keeping this short as possible led to delay periods of between 350 milliseconds and 550 milliseconds becoming standard for in-hole delays in surface blasting." The result of these in-hole delays was that surface delays had to be notably shorter, to ensure an adequate "burning front" — so standard delays most com- monly used today for surface connections are 17, 25, 42 and 67 milliseconds. These delay periods are generally suitable for hard rock, but the shorter periods (17 and 25 milliseconds) increase the risk of out-of-sequence fi ring or blast chok- ing, due to the inaccuracy of the longer in-hole periods. However, if the surface delay periods are increased to try and prevent non-sequential fi ring, there is a greater risk of cable cutoffs caused by short- burning fronts. "None of these problems exist with EDDs," said Rorke. "Their accuracy al- lows for short delays with little risk of non-sequential fi ring. Furthermore, each detonator counts down independent- ly — irrespective of whether the cable attached to it is cut. This means that blast planners can program long delays without worrying about misfi res caused by downline cutoffs." Blasters Realizing Value of Longer Delays Allowed by Electronics Although there is a distinct trend throughout the mining industry to adopt electronic delay detonators (EDD), blast engineers and supervisors may need to look at blast pattern layout in new, different ways to achieve all of the benefi ts offered by EDD.