There are lots of different ESD testing standards out there, and most of them have a schematic similar to the one above from MIL-STD-461G. However, if you look across multiple standards, you’ll see that Rd and Cd can have different values. I recently had an aerospace client who would normally test to CS118 from the MIL-STD. But, due to a particular threat environment, we recommended applying a harsher test from the automotive world. CS118 has a maximum test at ±15 kV, but we thought a ±25 kV test would be appropriate.
In looking at ISO 10605 to remind myself of the parameters of the ±25 kV discharge, I was reminded that Rd and Cd vary with different tests, and I wanted to make sure that I was using the appropriate value. The ISO standard has two options for both the resistor and capacitor. The cap can be 150 or 330 pF, and the resistor can be 330 or 2000 Ω. Thanks to these values being standard in both ISO and IEC 61000-4-2, they can be found in most off-the-shelf ESD guns. So, which values should you use if you’re testing beyond a specific standard?
For the capacitor, if the main threat is from a human interaction, the IEC document recommends 150 pF as the standard. ISO 10605 recommends choosing 330 pF when testing equipment that a human might interact with while in the interior of the vehicle and 150 pF for equipment that may be accessed from the exterior. The larger 330 pF value represents something closer to the capacitance of the human/car system, while the 150 pF value represents a “free floating” (if you’ll forgive me) human.
For the resistor, the standards are clear that 330 Ω represents the discharge occurring from a human through a metal tool to the target. Imagine approaching a piece of hardware with a wrench in hand and a spark jumping from the tip of the wrench to the hardware. Because of the lower resistance allowing for more current to flow, this is considered the harsher test and is usually the default. The 2 kΩ resistor is a better representation of a discharge directly from human skin, such as might occur when a user goes to push a button.
For this particular aerospace client, we were most concerned about a human moving independently of the vehicle and interacting with units through touch interfaces, not using tools. So, we chose a ±25 kV discharge with a 150 pF cap and 2 kΩ resistance. I should note that ISO 10605 recommends a maximum discharge of ±15 kV when using a 330 pF cap, so that’s another argument for choosing the 150 pF test.
As a side note, one of the other options to consider for ESD testing is whether contact or air discharge is appropriate. Annex B of ANSI C63.16 has useful guidance on that point, discussing when air vs. contact is appropriate. The short version is that while contact discharge is more repeatable, air discharge is more representative of ESD events out in the field.
One other standard to keep in mind when designing a custom ESD test is MIL-STD-1541, now officially discontinued (but freely available). It shows the circuit schematic for an ESD test that can provide a much wider range of discharge types than the tests based on IEC 61000-4-2. You can see a recent document using this in NASA-HDBK-4002B, where they have a MIL-STD-1541 setup tailored to represent discharges on the exterior of spacecraft due to the space charging environment.
