Electronics Factory ESD Practice Case
Q: We check the ESD control panel weekly to ensure it's properly grounded through a 1MΩ resistor. What are the common causes of unstable resistance readings between the ESD (hard ground) and the conductive work surface ground (soft ground)?
A: One reason for unstable resistance readings between the power ground (ESD ground) and the conductive work surface is ground current. If the footpad has multiple grounding points, ground current may flow, causing ohmmeter readings to be inaccurate. Other possible causes include: low instrument battery voltage, ground loops between the instrument's AC power supply and the ground being measured, etc. It's best to disconnect the ground wire from the instrument's power plug. Some digital instruments are sensitive to surrounding magnetic or strong electric fields; during measurements, carefully check the power distribution cable, EMI sources, etc., and try to perform multiple measurements repeatedly within a shielded area to determine if the problem has been eliminated.