Desco Europe ESD - Questions And Answers

How are you, greetings from Malaysia. I would very much appreciate if you could please answer this questions below. I came across a new experience yesterday but I could not figure out the reason why? Electrostatic charge on a plastic part container reads 3 kV. When my assistant holds with one hand reading drops to approx 600V. Holding with both hands now the reading increases to 3 kV. This is where I don't understand the theory. Next question..............When I measure the plastic container measuring 13"x15" reading varies at different points of the Tub. I understand the plastic tub is a non-conductive material but clipping it with ground wire to a grounding point will the charge be discharge to ground? Lastly, if this non-conductive plastic container is placed inside an antistatic tub as a safety measure will this prevent the container from being electrically charged ?

Your first question has many unknown variables, any of which could explain the phenomena you and your assistant are experiencing. My first assumption is that the container is truly insulative, i.e., it will not conduct current at any level. If this is correct, then what will cause the deviation of the electric field readings is the location of measurement device to the surface being measured. Since an insulative surface will not allow equal distribution of charge imbalance, but rather isolated islands of charge imbalance caused by say triboelectric generation at those specific areas, then moving the measurement device several inches from the initial location of measurement could yield a several thousand volt/inch field measurement deviation.Still basing the logic on the above assumption, then answers to your second and third question is no, clipping a ground wire onto an insulative tub will not discharge the charge imbalance and if the tub already has a charge on it, placing it into an antistatic safe enclosure will not prevent it from being charged, since it already is charged. It will however, minimize additional charging to the tub. Assuming the tub is dissipative, i.e., electrically conductive but at a high resistance range, say 1 gigohm, then the answers change. In answer to your first question, when your assistant holds the container with one hand, the charge is then redistributed over their body and thereby reduces the overall field strength, yielding a lower voltage reading. Now, here is where the many unknowns make it hard to finish the answer. Assuming the one hand grab is with the tub on a surface, such as a table, and when grabbed with two hands, the tub is lifted off this surface and held by your assistant free standing. This lifting action would charge the container and overall capacitance of the tub and operator yielding a higher voltage reading. If your assistant was to change their stance, (stand instead of sit or vice versa, rock their feet, shift their weight, etc.) this would change the overall capacitance and yield varying field measurements. In answer to question two, yes, by being conductive (even in the dissipative range), the tub would discharge charge when grounded. In answer to question three, placing a neutral tub into an antistatic container would minimize charge generation on the tub. Other possibilities include the following: The body's capacitance may have changed to effect the measured voltage. If the capacitance were to increase, then the total measured voltage will reduce and vice versa. Also, charge suppression may be in effect, where the body suppresses some of the charge, minimizing the overall electric field being measured.

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