Discussion: Figure 1 shows a current induced in a system from an ESD simulator. One way to measure such a current is shown in Figure 2 where the ESD simulator is being discharged through a current probe (in this case into horizontal coupling plane of an ESD test table, not a piece of equipment). The current probe can also be located on system cables either inside or outside the equipment enclosure. The time scale is usually set to a few hundreds or nanoseconds so as to catch the secondary discharge at the expense of detail on the high frequency parts of the waveform. Since most current probes are AC coupled, the area above the zero current line and below it must be equal over time. This leads to the waveform apparently dipping below zero current when it fact it may not be. Current dipping below zero on the right half of Figure 1 is due at least partially to this effect.

Clearly there is a problem shown in Figure 1. After one impulse of current a second larger one occurs about 400 ns later. A secondary discharge, normally inside of the equipment under test, can have a greater impact on system operation than the original discharge because it is often closer to circuits and is sometimes more intense. On the second impulse in Figure 1, you can see a slow rise of current (possibly corona effects) followed by a fast rising edge of the breakdown. Subsequent discharges may look different and are likely to occur at somewhat different delays after the original discharge because of variations in the air discharge that forms the second impulse.




Figure 3 contains another example , but in this case the secondary discharge looks much like the original discharge once the fast breakdown occurs. Effects like this can cause equipment to fail an ESD test where the fix is simply to find and eliminate the secondary discharge.