The purpose of determining the electrical properties of explosives is to provide the input necessary for modeling of the electrical circuit formed by the system or sub system. This is of special interest in situations where electrical charges may accumulate in the system due to process parameters. Hazardous conditions may develop if electrostatic charges are allowed to accumulate unchecked, or through dynamic current flow.
Triboelectric Charging and Sliding Charge Generation
One means of inducing electrostatic charge on a material or system is by triboelectrification. This may occur in many ways, for example, by mere contact and separation of two dissimilar materials or as ingredients feed through a tube or fall through the air. These test measure the amount of charge generated by the operation in question. These tests are generally tailored to the process or situation of most interest.
Airvey Charge Generation
Another form of triboelectrification of special interest to any one considering airveying of explosives or explosive dusts.
This test is used to rank propellant sensitivity to dielectric breakdown due to electrostatic discharge phenomena. It is also used in electrical modeling of propellant and explosives and their systems. Voltages up to 60 Kilo-volts are used. Both the sudden discharge of electrons through an explosive and the build-up of charge can lead to initiation of the material and effect the safety of the system.
French (ESD) Test
This is a “go”, “no-go” series of tests to determine the susceptibility of a propellant to ignition by electrostatic discharge/dielectric breakdown in a typical processing operation. Three 90 mm x 100 mm cylindrical samples are prepared and subjected to 30 discharges each of a predetermined energy and voltage. Tests at operational limits as well as ambient temperature are typically recommended. The propellant passes the test if none of the 90 trials in a series results in bursts of flame, explosions, ignition or fragmentation of the sample.
Volume and Surface Resistivity, Dielectric Constant
To determine the electrical properties of explosives and non-explosives, SMS performs a volume and/or surface resistivity test, and a dielectric constant test. The ability of a material to conduct, store and dissipate charge can then be evaluated for the specific conditions of processing or use for the material. These values are most often used to model the process or condition so as to determine whether hazardous levels of charge and/or voltage can be produced or stored in the system. This is also a basis for tests of the degree of electrical shielding provided by materials in the system or process.
Reference: JANNAF Propulsion Systems Hazards, ESD Subcommittee Report, CPIA….