ESD Event, or Electrostatic Discharge, is an electromagnetic phenomenon. When ESD Event occurs, it generates both electric and magnetic fields. Electric field is generated due to sharp reduction of static voltage that caused ESD Event; and the magnetic field is generated by the current produced during the event. Combined, they form electromagnetic field.

If electrostatic discharge occurs directly into object of interest, then measurement of current to the ground may be sufficient. Please keep in mind that many measurement devices may introduce substantial error by providing parasitic reactance (inductance and capacitance). That added reactance is especially noticeable at high frequencies -- exactly in the domain of ESD Events. In addition, such measurement devices as some types of current probes may act as an antenna picking up the electromagnetic field and actually injecting the current into the very device under test, such as magnetic head. In this case, the magnetic head may be damaged solely by the fact of presence of the current transformer.

If electrostatic discharge happens at some distance from the object of interest, it still can cause substantial damage by creating electromagnetic field. The only accurate way of measuring the impact of ESD Event is by measuring the electromagnetic field strength in the immediate proximity to the object of interest.

ESD Events are very short (down to few nanoseconds), meaning that the frequency spectrum they generate is very broad. This spectrum mostly extends towards high frequencies. An observer using spectrum analyzer can see ESD Events generating frequencies as high as 5GHz or more. For such high frequencies transition region from the near field to the far field happens essentially in immediate proximity to the source of discharge (more on near fields please see upcoming Application Notes). That means that both electric and magnetic field measurements are valid methodology for measuring ESD Events.

Electric field strength is measured in Volts per meter (V/m). For the reference, a local radio station generates less than 1mV/m at your house. A cellular telephone may generate up to 10 or even 20V/m in user's head if the antenna is not moved away. A field strength of 10V/m is considered to be unsafe at higher frequencies.