To investigate electrostatic discharge (ESD) immunity testing for wearable electronic devices, the worst scenario i.e., an ESD event occurs when the body-mounted device approaches a grounded conductor is focused in this paper. Discharge currents caused by air discharges from a charged human through a hand-held metal bar or through a semi-sphere metal attached to the head, arm or waist in lieu of actual wearable devices are measured. As a result, it is found that at a human charge voltage of 1kV, the peak current from the semi-sphere metal is large in order of the attachment of the waist (15.4A), arm (12.8A) and head (12.2A), whereas the peak current (10.0A) from the hand-held metal bar is the smallest. It is also found that the discharge currents through the semi-sphere metals decrease to zero at around 50ns regardless of the attachment positions, although the current through the hand-held metal bar continues to flow at over 90ns. These discharge currents are further characterized by the discharge resistance, the charge storage capacitance and the discharge time constant newly derived from the waveform energy, which are validated from the body impedance measured through the hand-held and body-mounted metals. The above finding suggests that ESD immunity test methods for wearable devices require test specifications entirely different from the conventional ESD immunity testing.

This paper proposes a method to predict and control noise voltage caused by electrostatic discharge (ESD) to electronic equipment. The relationship of grounding system configurations for a typical set of equipment to ESD immunity has been derived using a mechanism of ground potential variations. The equivalent circuit representing ground elements as lumped constants enables us to predict the transient ground potential differences between PCB (Printed Circuit Board) ground planes connected via signal cables and induced noise voltage at the receiving end. The calculation shows that the contribution of ground potential differences to noise voltage is comparable to that of the electromagnetic coupling between the discharge current on the enclosure and the circuit loops. The calculation also shows some characteristic results, such as; the induced noise voltage is remarkably dependent on the unbalance in ground cable lengths and on the impedance of ground conductors connecting PCBs, especially when the equipment uses a single-point grounding system. These characteristics were confirmed by measurements of induced ground potential differences, noise voltage and immunity levels. Thus the proposed method is shown to be very effective to analyze the dependency of grounding conditions on ESD immunity and to improve ESD immunity in equipment design.