Electronic delay detonators
AXXIS GII electronic delay detonators have four protection barriers against surge currents and spikes caused by lightning strikes :
• Sparking gaps at the plug end of the detonator circuit to divert high voltage currents to ground ;
• Surge protectors to protect the detonator circuitry against lower voltage spikes that are not grounded via the spark gaps ;
• In-line resistors to block high currents from reaching the electronic circuitry . The in-line resistors act as fuses that fail to open-circuit when subjected to very high currents ; and
• A thermal barrier in the form of a fully encapsulated electronic board inside each detonator . Should any component in the circuit fail at high temperature , the heat would be contained by the encapsulating jacket and will not reach the fusehead or detonator charge .
Tests carried out on GII detonators using high voltage Marx generators and high-current capacitor circuits have placed safety limits of the AXXIS GII detonators at above 600kV or 40kA . It is , therefore , unlikely that an electronic detonator with similar protection barriers will be initiated by a negative CG lightning strike . However , detonators subjected to the high voltage and current levels from negative CG strikes will not necessarily remain functional . It is probable that all brands of electronic delay detonator will initiate at the extremely high voltages and currents when subjected to a strike by the less frequent positive CG strikes .
Boosters and explosives
At the highly elevated currents and the related heat and pressures generated by positive CG strikes , a direct hit on explosives or boosters is likely to initiate these products . However , in most instances of a direct lightning strike on a charged shot , most of the shot holes will be stemmed . A direct strike on a stemmed hole is less likely to result in a transfer of heat or shock energy to the charge . The literature only lists initiation of exposed explosives charges and munitions to lightning strikes .
Fly rock risk
When using electronic detonators , if a direct lightning strike occurs that is high enough in energy to initiate the detonator , the initiation will be confined only to the detonators directly affected . Even if the surface lines are connected , there won ’ t be a propagation of initiation to the other connected holes , because electronic detonators will only initiate on a specific coded signal and not on a plain surge in current . This is not the case with electric detonators or non-electric detonators connected on surface to each other . Therefore , the safest initiation system to use in lightning risk areas is electronic , which is particularly robust for this purpose because of the protection barriers built into the units .
Fly rock range
If only the holes that are directly impacted by a direct strike initiate , the amount of energy released in the detonation will be significantly lower than the energy released by the entire blast firing in an uncontrolled way . In the latter , extreme fly rock can be expected and full evacuation during a thunderstorm is sensible . However , with electronic initiation systems , an un-commanded initiation from a direct highenergy lightning strike will be confined to one or only a few holes initiating in the immediate vicinity of a strike . If these holes are confined
( not along a free face ) the fly rock will eject vertically in a crater and plume with the lateral extend of the fly rock not extending further than a few tens of metres from the hole . In the case where an unplanned initiation occurs at a free face , there is a higher risk of fly rock extending to a range of a few hundred metres if the charged shot holes are under-burdened .
Lightning prediction
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 be evacuated when an electric storm approaches . This is a standard requirement worldwide and when ignored has resulted in deaths associated with detonations caused by lightning .
Most of the mines use warning systems that measure raised levels of static charge in the atmosphere . These units are not reliable predictors of an impending lightning strike and are unfortunately usually in the hands of blasters or mine security personnel who pay little regard to the units . Centralised warning systems are much better , as the systems are properly maintained and the warning goes out to senior personnel who will react effectively to clear a pit and ensure safety .
A warning service is provided by the South African Weather Service ( SAWS ), which is based on a countrywide network of lightning sensors . The service provides for contractual monitoring of a cell covering a mining or quarrying operation and is able to provide a warning of approaching lightning storms and the severity of measured lightning strikes in the storm . An example of warnings from the SAWS is given in Table 1 , which shows the various warnings for impending lightning and the strike current risk .
16 _ QUARRY SA | March 2017