Most system level ESD test standards, like IEC61000-4-2, model an ESD event from a human hand holding a piece of metal, such as a tool. This is called the Human Metal Model, HMM. The April 2013 Technical Tidbit showed that a piece of metal in a human hand significantly intensifies an ESD event. But, how does the intensity of such an ESD event, and the resulting likelihood of system upset, depend on the size of the metal object?

Figure 1 shows the test setup used to generate data for this Technical Tidbit. A metal plate is used as the discharge target. After each discharge, the plate is discharged to AC mains safety ground before the next discharge. Next to the plate is an Agilent 1163A, 1 GHz 500 Ohm resistive passive probe shorted by its ground lead and connected to an Agilent DSO5054A oscilloscope. The shorted probe acts like a magnetic loop and picks up the EMI radiation from the ESD event to the plate. The displayed waveform on the scope is a measure of the intensity of the ESD event and its ability to disrupt nearby electronic equipment.

Figure 2 shows a discharge applied by a long metal pointer. The person is charged up by rubbing a foot on the carpet and then lifting it. Developing a static charge this way in Boulder City, NV, my office and lab location, is pretty easy because of the dry desert environment.


I took a number of discharges this way and the waveform shown in Figure 3 was typical. Note that peak voltage induced in the probe-ground lead loop was over 8 Volts peak, the top of the screen! No wonder that ESD can disrupt equipment operation.


Figure 4 shows the same experiment but this time the metal pointer was collapsed to its smallest length, about 6 inches, 15 cm. Charge was developed the same way and a typical resulting waveform is shown in Figure 5.


Note that the peak voltage recorded is still well over 8 Volts in Figure 5! The difference is that the low frequency component of the ringing is not present because of the smaller size of the collapsed pointer compared to the extended pointer.

Although the peak voltage is lower in Figure 8, almost 6 volts, is is not smaller in proportion to the size of the metal. The coin is maybe 1/10 the size of the collapsed probe and maybe 100 the size of the extended pointer, but the peak voltage induced in the probe is only a little smaller. The April 2013 Technical Tidbit, shows that a discharge directly from the skin is much, much smaller than from a piece of metal. Thus, any size piece of metal significantly increases the intensity of a static discharge from a human hand!