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Douglas C. Smith

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Technical Tidbit - March 2012
Air Discharge ESD Current Waveforms Revisited
(high frequency ESD current measurements on system cables)

nasty air discharge current waveform

Figure 1.
Air Discharge ESD Current Measured on System Cable
(Vertical scale = 1 Amp/div, Horizontal Scale = 50 ns/div)

Abstract: Electrostatic Discharge, ESD, in air can result in highly variable current waveforms in both the discharge itself and in cables of equipment subjected to ESD. Examples of some nasty current waveforms induced in system cables are given and the implications discussed.

Discussion: Figure 1 shows a current waveform induced in a system cable of equipment that was subjected to an air discharge ESD event above 10 kV. Most of the current waveforms collected looked like a smooth version of the low frequency damped sine wave with a period of about 75 ns in Figure 1. But once in a while the waveform of Figure 1 resulted, and when it did the system responded in a undesirable way.

Note the high frequency "hash" on the rising edge of the damped sine wave. It is a pretty nasty waveform with a high di/dt of about 10 A/ns (estimated). The hash persists for the first 100 ns of the damped sine wave. It addition, note the small burst that arrives about 60 ns before the main event.

In Figure 2, the main event occurs about 500 ns after the initial small burst raising the possibility of a secondary air discharge in the equipment itself as the source of the "main event" on the waveform.

Figure 2.
Another Air Discharge ESD Current Measured on System Cable
(Vertical scale = 2 Amp/div, Horizontal Scale = 100 ns/div)

The problem with air discharge is that individual discharges are highly variable, especially at high voltages. The result is that a large number of air ESD events must be applied to cover most of the possible current waveforms and system effects that might occur. In addition, characteristics of the discharge, and thus the current waveforms generated, are a function of air pressure, humidity, and other factors.

Measuring current waveforms of ESD events is a great way to understand what is happening. The equipment on which the waveforms in the figures above were measured did not react to the ESD events that produced smooth current waveforms. But when current waveforms with a lot of hash appeared, as illustrated above, the equipment did react to those events. The current measurement resulted in the understanding that the occasional equipment failure was due to a noisy ESD event not because of a time dependency in the system state as can sometimes be the case with ESD.

Summary: Current measurements are a handy way to understand ESD effects and were used in this article to show how high voltage ESD in air can produce some nasty current waveforms and that these currents often affect equipment.

Equipment used in this article:

Additional articles on this website related to this topic are:

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Copyright 2012 Douglas C. Smith