An initiation system starts the detonation of the main charge, after which firings should occur at the exact times that are planned to ensure best results. As discussed earlier, the objective of initiation systems during the black powder and dynamite eras was to
improve the safety of explosives. By 1900, the first electric delay detonator was available. This detonator was a fusehead that started the burning of a black powder fuse connected to an ordinary detonator. The delay time before detonation was about 8 sec, so it improved safety but it also reduced the scatter in firing times of nominally identical initiation systems from about 8,000 to 600 psec. During the 20th century, progressively larger blasts required the sequential detonation of many charges, and research on initiation systems turned toward minimizing the scatter in firing times. The timeline for the main developments was
■ 1920s: Instantaneous electric detonators worked by initiating a tiny amount of a very sensitive primary explosive such as lead azide that, when detonated, initiated a small amount of a sensitive secondary explosive in the main charge. Placing the primary explosive in the main charge would have jeopardized safety but was needed to detonate the secondary explosive. The scatter was reduced to 100 psec.
■ 1950s: Short-delay detonators allowed blasts at intervals of 25 m-sec. Over 10 years the scatter was reduced to about 6 m-sec.
■ 1970s: High-precision blasting caps improved the control of vibration, fragmentation, and damage.
■ 1990s: Electronic detonators can deliver a defined delay time and can be made with a scatter of less than 1 m-sec.
The driving forces to improve initiation systems were safety and the need for more rock to be broken by explosives. In the 1970s, multiple ring blasts or mass blasts were introduced to underground mines and they greatly enhanced productivity by permitting long, uninterrupted campaigns of drilling and blasting. The detonation of huge amounts of explosives in sequence in a single mass blast, however, meant that any initiation problem could cause serious production losses. Significant improvements were required to electronic initiation systems to ensure correct timing, avoid misfires, and maintain an acceptably low level of ground vibration. Consequently, the 1970s and 1980s were periods of intense research into electronic initiation and vibration monitoring, and blast design and associated fragmentation. Parallel R&D programs investigated the application of electronic, computing, and detection systems, and ultra-high-speed photography supported this research. The result was that, by 2000, initiation systems were available that were accurate and controllable.