Specific Dependence on Absolute Humidity and Saturation Deficit of Air Discharge Currents from Electrostatic Discharge Generator with Different Approach Speeds

The International Electro-technical Commission (IEC) specifies air discharge immunity testing for electronic equipment in the standard 61000-4-2 under the climatic conditions of relative humidity from 30 to 60 % and ambient temperature from 15 to 35 degrees Celsius. For aiming to improve the testing reproducibility, to clarify effects of the above climate parameters on air discharge testing, we previously measured air discharge currents from an electrostatic discharge (ESD) generator with an approach speed of 80 mm/s under 6 combinations of relative humidity and temperature. The result showed that the same absolute humidity provides almost similar waveforms of the discharge currents despite different relative humidity and temperature. In this study, to further examine such combined effects, we measure air discharge currents from the ESD generator at a test voltage of 15 kV with three different approach speeds of 20 mm/s, 50 mm/s and 80 mm/s under 9 combinations of relative humidity and temperature inside and outside the IEC specified climate ranges. As a result, air discharge current behavior is affected by not only absolute humidity (AH) but also saturation deficit (SD) or the difference between the AH and the saturated water vapor amount. The humidity dependence differs according to the IEC specified and non-specified climate ranges, while it can well be explained by the SD. Under the IEC specified conditions, at faster approach speeds, the peak currents of air discharges have maximum values at AH7.70 g/m³ and SD15.36 g/m³, whereas they are 1.2 to 1.4 times larger compared to those at almost the same AH7.71 g/m³ and smaller SD5.14 g/m³, respectively. In the IEC non-specified climate range, however, regardless of the approach speeds, the current peaks noticeably reach maximum values at AH3.81 g/m³ and SD19.26 g/m³, which are 3.6 to 5.2 times larger than those at roughly the same AH3.96 g/m³ and larger SD35.64 g/m³, respectively.