Evaluating the electrostatic discharge risk between small radius objects and charged planar insulating materials

Abstract

In some industrial situations such as web processes, charging of large planar insulating surfaces can give risk of electrostatic discharges that could ignite flammable process materials such as solvent vapours. Charging of the material has for some years been evaluated using electrostatic field measurements to estimate surface charge densities. In a planar system, the threshold risk of an electrostatic discharge has been equated with a surface charge density of about 25 μCm-2 required to produce a field strength exceeding about 3 MVm-1, the breakdown field strength of air. It is known that where a small radius earthed conducting object approaches a charged insulator, intensification of the field strength occurs at the surface of the object. Similarly, the field strength is increased by the presence of a field meter near the surface. This paper investigates the effect of field concentration by an object near the charged insulator and the conditions leading to electrostatic discharges using analysis, electrostatic field modelling and experiments. The results are applied to understanding and quantification of the effect on measurements and safety factor indicated for evaluation of electrostatic discharge risk introduced by the earthed conductor in this situation.Initiation of a discharge to a spherical measurement electrode requires that the field strength at the surface of the electrode exceeds the breakdown field strength and other factors. This occurs at different induced charge levels on the electrode for different electrode diameter. Discharges could occur with electrode-target gap around 10–20 mm at lower charge density than with smaller gaps. The threshold of discharges occurring was reached at lower charge density for a smaller 16.6 mm diameter electrode, which might be important when evaluating ignition risk for Group IIC flammable materials. Conversely higher charge transfer was confirmed with 25.4 mm electrode above the threshold for discharges occurring.