Computation of Coupled Voltage to an Unshielded Cable Due to Transient EM Fields Radiated by ESD

The increasing development of dispersed but highly interconnected systems of telecommunication centers, computers and control equipments leads to an extensive use of cables to avoid interference problems. When an external electromagnetic field interacts with an imperfectly shielded cable, the interconnecting cables act as collectors of energy and considerable amount of energy can be coupled into the systems. The induced current and voltage transients may then cause damage or malfunction of sensitive electronic circuits. Simulation of structural current due to an Electrostatic discharge (ESD) event involves passing large transient currents through the object under test. This causes radiation of transient electromagnetic fields in the environment and induced transient voltages into the cables and equipments. A coupling model is developed to estimate the extent of coupling of transient EM fields to cables. A coupling model is presented to predict induced terminal voltages on an unshielded cable placed above a ground reference plane due to transient electromagnetic fields radiated by Electrostatic discharge (ESD). The unshielded cable is exposed to the free space-radiating field due to IEC 61000-4-2 ESD waveform. The transient current is represented by Hertzian dipole elements and the time domain expressions for the resultant electric field intensity at any point are obtained. The frequency spectrum of the electric field is obtained using Discrete Fourier Transform. Assuming that the incident transient electric field is parallel to the cable termination, the frequency spectrum of the coupled voltage at the terminations is computed. The time domain representation of the terminal induced voltage is determined using Inverse Discrete Fourier Transform (IDFT). Computed results are presented for the contact discharge IEC 61000-4-2 ESD current waveform at 8 kV and air discharge IEC 61000-4-2 ESD current waveform at 16 kV. The coupled voltage into an unshielded cable is computed for resistive termination and RC shunt termination. The coupled voltage reduces from 6.25V for ESD air discharge and 625V for ESD contact discharge for a resistive termination to 0.325 mV for ESD air discharge and 7.8 mV for ESD contact discharge for a RC shunt termination.