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An electronic device ESD sensitivity or susceptibility is determined by testing using ESD simulators to determine its Withstand Voltage classification. So ideally the closed product should be tested.
Many companies assume that the enclosed product is not ESD sensitive. This is likely often correct, but there are some concerns.
The electronic components are controlled by the use of electrical signals. This signal can be overridden with an ESD event following the same electrical path (power rails, Input/Output traces/pins/cables/etc.).
As an ESDS component goes through the manufacturing cycle, it does not become less sensitive. However, the probability that an ESD event will strike the ESDS item may decrease.