Friday, October 26, 2007

Why is a high resistance of Megohms suitable for grounding static electricity?

Why is a high resistance of Megohms suitable for grounding static electricity?

Engineers are often surprised by the high levels of resistance that give an adequate ground in static electricity work. The reason is simple - static electricity charge generation is effectively a small current generator in the microamp or nanoamp range. We are usually happy to achieve limitation of voltages to a few volts. Simple consideration of Ohms law shows that for, say, 1 microA current generated (which is average level) a 1 M ohm resistance will only show 1V buildup. Increase that to 109 ohms and it might start to get more problematic, showing 1kV!

You can now start to easily see why with modern insulating materials static charge build-up is common - with a 1012ohm material (which are quite common) only 10nA charge generation rate would give 10kV. Modern polymers can be well over 1013 ohm.

Of course life is not so simple in reality but it gives a good first approximation. The second important parameter is charge storage (capacitance) which with resistance forms a characteristic RC charge decay time. If this gets above about a second or so, static voltages stay around long enough for use to notice them.

So for a static dissipative floor in an ESD Protected area, 109 ohms resistance or below is all that is needed to keep static voltages on chairs, trolleys and other items to a low level. Humans are more problematic as they move around and generate a higher current - someone found that 35 Mohms resistance from body to ground would keep body voltage below 100V in most cases with some margin of safety.

People start feeling shocks if their body voltage goes above about 3-4 kV. This can start to happen if the floor resistance goes much above about 1010 ohms. An average body capacitance might be of the order 100pF, so the time constant is around 1 sec. (People like me who have big feet have higher capacitance). Many modern laminates, glass, plastics, synthetic stone etc have resistance well over 1012 ohms. So the decay time can be hundreds of seconds, and voltage reach tens of kilovolts. Any high voltages generated sty on the body for several minutes under these conditions.

There's some more information on this in my on-line article on Why static charge builds up on people

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