Recent Developments in the Understanding of Coupling Paths of ESD through a Metallic Cabinet

Based on recent studies made on ESD event statistics and ESD modelization for furniture and human discharge onto a computer frame, in-depth analysis is made of ESD current routes on a metallic frame and mechanisms of re-radiation inside equipment. Although a metal box should behave as an efficient shield, it is shown that this does not happen because high-frequency spectrum (up to the Gigahertz region) of ESD excites all existing seams and slot leakages. Measured values of E and H fields (inside the cabinet) near the discharge area and some peculiar aspects, like the effect of a discharge on a screw protruding significantly inside, are discussed. The Electric and Magnetic field amplitudes lead to some discussion on the wave impedance of the ESD reradiation inside the box and its near field/far field transition. This, in turn, allows a better prediction of the noise voltages induced in nearby PCB traces or flat cables.