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Douglas C. Smith

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Technical Tidbit - July 2013
Human Metal ESD Characteristics, Size of Metal Object
(Intensity of ESD from a metal object in a human hand is not significantly affected by the size of the metal object)

Test setup

Figure 1.
Test Setup to Measure the Effect of the Size of a Metal Piece in a Human Hand on ESD Intensity

Abstract: The intensity of an ESD event from a human hand is affected strongly by the presence of metal held in the hand. Data is presented to show that the peak amplitude of the EMI generated by a discharge directly from a metal object in a human hand is not strongly affected by the size of the metal object held. Any size piece of metal intensifies the ESD event significantly.

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.

Discharge from  along metal pointer

Figure 2.
Human Metal ESD From a Long Metal Pointer

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.

EMI pickup in probe from long metal pointer discharge

Figure 3.
Voltage Induced in 1163A Shorted Scope Probe by HMM ESD - Long Metal Pointer
(Vertical Scale = 2 V/div, Horizontal Scale = 10 ns/div)

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.

Discharge from a collapsed metal pointer

Figure 4.
Human Body ESD From a Collapsed Metal Pointer

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.

Probe pickup from a collapsed metal pointer

Figure 5. Voltage Induced in 1163A Shorted Scope Probe by HBM ESD - Collapsed Metal Pointer
(Vertical Scale = 2 V/div, Horizontal Scale = 10 ns/div)

Figure 6 shows the same experiment, but this time a small metal coin was used, a US dime. Charge was developed the same way and a typical resulting waveform is shown in Figure 7.

Discharge from a coin in hand

Figure 6. Human Body ESD from a Small Coin

Probe pickup from coin discharge in hand

Figure 7. Voltage Induced in 1163A Shorted Scope Probe by HBM ESD - Coin
(Vertical Scale = 2 V/div, Horizontal Scale = 10 ns/div)

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!

Summary: There is a large increase in the intensity of  an ESD event from a human hand when a piece of metal is added. The measured intensity of the radiated EMI from the ESD event shows that once metal is added to the human hand, the increased intensity of the event is not a significant function of the size of the piece of metal.

Acknowledgment:  Thanks to David Smith, my youngest son, who held the metal pieces and generated the ESD events. He is currently studying Computer Science in college.

An article on this website related to this topic is:
  1. April 2013, Human Metal vs. Human Body ESD
    (Metal in a Human Hand significantly increases ESD intensity)
Equipment used in this Technical Tidbit:
  1. Agilent 1163A, 1 GHz 500 Ohm resistive passive probe
  2. Agilent DSO5054A

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Copyright 2013 Douglas C. Smith