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.

在某些工业情况下，例如卷材工艺，大平面绝缘表面的充电可能会产生静电放电的风险，这可能会点燃易燃的工艺材料，例如溶剂蒸气。多年来，使用静电场测量来评估材料的电荷以估计表面电荷密度。在平面系统中，静电放电的阈值风险等同于产生超过约 3 MVm ^{-1的场强所需的约 25 μCm -2表面电荷密度}，空气的击穿场强。众所周知，当小半径接地导电物体接近带电绝缘体时，在物体表面会发生场强增强。类似地，场强通过在表面附近的场计的存在而增加。本文通过分析、静电场建模和实验研究了带电绝缘体附近物体对场集中的影响以及导致静电放电的条件。该结果可用于理解和量化对测量结果和安全系数的影响，以评估在这种情况下接地导体引入的静电放电风险。

对球形测量电极开始放电需要电极表面的场强超过击穿场强和其他因素。对于不同的电极直径，这发生在电极上的不同感应电荷水平。与较小间隙相比，在较低的电荷密度下，电极-靶间隙可能会发生放电 10-20 mm。对于直径为 16.6 mm 的较小电极，在较低电荷密度下达到了放电阈值，这在评估 IIC 组易燃材料的点火风险时可能很重要。相反，更高的电荷转移被证实，25.4 mm 电极高于发生放电的阈值。