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We have weekly monitoring of [our] ESD work surface to ensure it is properly grounded via 1 Meg resistor. What is the common cause of unstable resistance readings between ESD (hard ground) and conductive work surface ground (soft ground)? - Anonymous, Muntinlupa, Philippines

Answer

One cause for an unstable reading between power ground (ESD ground) and a conductive work surface are ground currents. If the mat is grounded at several locations, then there is the probability for ground currents to flow, which will give false readings to a meter movement when measuring Ohms. Other possible causes would be a low battery in the meter, ground loops between the meter's AC power and the ground it is measuring. Try isolating the ground in the plug to the meter by using a "cheater" or adapter that removes the 3rd prong (ground) in a three-prong plug. Some digital meters are sensitive to magnetic or strong electric fields in the near vicinity. Check for power distribution cables, EMI sources, etc. when taking measurements. Try repeating your measurements in a shielded area to see if your problem is eliminated.