Electrical Resistivity Measurement
In addition to the ESD risk evaluation, SMS can perform detailed surveys on electrical continuity (grounding and bonding) of process facilities and equipment. SMS has the capability to measure the resistivity of conductive floors, tabletops, and mats or runners and can perform the testing in accordance with DOD 4145.26-M C126.96.36.199.5 Test Procedure. For DOD product manufacturers, conductive floors and table tops must be tested at least annually and all test records kept for a minimum of 5 years. SMS will provide a written report with an assessment of the current electrostatic controls based on the data obtained through the survey. As required, recommendations for corrective actions or improvements of the electrostatic control strategies will be provided.
ESD Risk Evaluation
SMS has developed the capability to quantitatively assess the risk of electrostatic discharge (ESD) from persons working with static sensitive materials such as explosives, flammable vapors and gases, Electro-Explosive Devices (EEDs), and electronic components or articles. The approach for this quantitative study includes:
- Measurement and documentation of the maximum electric potential found on operators under their various operating conditions
- Quantification and documentation of ESD discharge current traces under these various conditions
With this information, SMS compares these actual measured energies against the ESD sensitivity of the material receptor(s) (e.g. explosives, flammables, articles, etc…). From this comparison, an assessment of risk can be determined.
Electrostatic sensitive articles and substances can be initiated from human ESD through a contact or air discharge. As the charged individual approaches the article or substance, the difference in potential can cause the air to become ionized and thereby reduce the resistance sufficiently to favor an air discharge event over a contact discharge. A contact discharge is where the charge accumulated on the individual isn’t transferred until contact. Air discharges are more likely at higher electric potentials and result in a smaller energy transfer than a contact discharge due to the energy diffused in the spark gap. Most risk evaluations assume a contact discharge always occurs including the Human Body Model.
For example, shown below is a plot of the energy and power required for the ignition of multiple similar devices (shown as points at the right of the plot). On the left of the plot is shown the energy and power measured for human electrostatic discharge events. The line with black diamonds is for the Human Body Model (which assumes discharge of the electrostatic charge upon contact). The maroon diamonds are the energy and power of actual human discharges measured at a production facility at various charge levels. All of the measured discharge events were sparks across an air gap to the charge collection device. The blue triangles indicate the calculated power and energy of the human discharge events if the measured spark discharges were actually contact discharges.
As shown in the figure an individual would have to be charged excessively to impart sufficient energy to initiate a device. The likelihood of a human ESD event resulting in initiation of the example devices is extremely unlikely as there is a significant difference between the required energy to ignite the device and the energy of a human ESD event.
The process for identifying the risk of initiation includes comparing the energy required for initiation to the energy of a human ESD event under the conditions at the processing facility. The following steps are completed to quantify the ESD risk:
- Quantify the maximum electric potential of the operators in the scenario of interest using an electrostatic field meter
- Actively charge and discharge a person under similar circumstances and record the discharge current
- Quantify the energy and power of the discharge events
- Compare the energies of the human ESD discharge event to the required initiation energy at a given power