Shock
Tube
Systems, Inc.
Main Menu
Electric v.
Non-Electric
Unfortunately, the leg wires can also act as antennas. They can pick up electromagnetic energy from a number of sources that can induce a current flow through the bridgewire or cause a spark to jump from the bridgewire to the device's case. The bridgewire heats up regardless of whether the electric current was supplied deliberately or inadvertently and so inadvertent initiation can occur. Some of the sources of energy that can cause inadvertent initiation are Radio Frequency (RF), Electroststic Discharge (ESD), Electromagnatec Induction (EMI), and Electromagnetic Pulse (EMP).
There are several approaches which can be used to enhance the safety of electrically initiated devices. One approach to reduce the initiation sensitivity of a HWI is to increase the diameter of the bridgewire and thereby reduce its resistance. This however requires higher voltage and current to fire the unit and requires larger and more costly leads to carry the current, especially at long distances. Another way is to increase the surface to volume ratio of the bridgewire in order to increase its heat transfer to the ignition mix and thus reduce its rate of heating. This is accomplished in several ways to commonly achieve a detonator that can withstand one full ampere of current flow through its bridge without initiation. These devices usually utilize a bridgewire of about one ohm resistance and therefore one ampere produces one watt of power. These devices are thus known as "one amp, one watt" devices. While greatly reducing the sensitivity of the HWI, the one amp one watt devices are much more expensive.
One of the first commonly available low sensitivity initiators was the Exploding Bridgewire Detonator (EBW). In contrast to the HWI, the EBW uses the shock generated by the explosion of a bridgewire to directly initiate PETN. The bridgewire design is such that when a pulse of current from a discharging capacitor is sent through the wire it actually explodes causing the device to function. If a current less than the threshold required for initiation is supplied, the bridgewire will either do nothing or burn out. The bridge may explode weakly, but it will not produce a high enough shock pressure to detonate the PETN. In this case a dud results, but no inadvertent initiation. For the bridgewire to be caused to explode, it takes 500 volts DC or greater and a rise rate of greater than 100 ampere per microsecond is needed. To achieve the high current flow, special, low internal-impedance, high voltage capacitors and coax cables are needed. These drive up the cost of the system.
Other schemes have been developed over the years to reduce the chance for accidental initiation of electric initiators. These include Exploding Foil Initiators (EFI), slapper detonators, and Semiconductor Bridgewire (SCB) detonators. These all share the requirements for specialized, expensive fire sets and cables. For this reason, the shock tube detonator system was developed. Shock tube based systems were first available commercially in the late 1970s. During the 1980s and 1990s they steadily replaced electric initiators for most commercial blasting and demolition. Developed as a safer detonator, they also turned out to be much easier to use in the field than electric systems.
Military personnel also use blasting caps for demolition, engineering, and explosive ordnance disposal. Due to the vagaries of Government procurement processes and the need for new explosive items to pass stringent safety requirements, the military was later than the commercial world to adopt the new shock tube technology. The first service to adopt shock tube detonator assemblies was the US Navy. The Navy has a distinct situation onboard its aircraft carriers where it has large quantities of explosive ordnance in close proximity to very high power radar and other electromagnetic emitting devices. The Navy pioneered the study of how to deal with the Hazards of Electromagnetic Radiation to Ordnance (HERO). So it was no surprise that in the mid 1980s the Navy was the first military service in the world to qualify a shock tube based non-electric detonator system. This system was the MK 120 series of detonator assemblies. STS personnel are proud to have developed these systems and supported the Navy during their Type Classification.
Shock tube based detonator systems are far less sensitive to the effects of RF, EMI, ESD and EMP than electric initiators. All Shock Tube Systems, Inc. designs are tested to withstand a 25,000 volt electrostatic discharge without functioning. Shock Tube Systems, Inc., shock tube based detonator assemblies give the user a versatile tool that has proven to be the safest, most reliable initiation system available.