Whitepaper
Indirect Lightning Testing and the Influence of Couplers
This EMC Partner whitepaper examines the role of inductive couplers in DO-160/ED-14 indirect lightning cable bundle testing and explains how coupler design choices directly affect waveform integrity and system impedance. The paper contrasts capacitive and inductive coupling methods, noting that while capacitive coupling is simpler and more predictable, it is impractical for aircraft power systems and high-energy waveforms. Inductive couplers must be built from the correct core material — ferrite for low-energy, short-duration waveforms and laminated silicon steel for high-energy, long-duration pulses — because saturation of the core material causes significant waveform distortion. The coupler turns ratio is shown to have a direct and counterintuitive effect on virtual system impedance: increasing the turns ratio to reach higher test currents simultaneously reduces virtual impedance, which can make it impossible to achieve both the voltage limit and current limit simultaneously on hybrid cable bundles. Measured waveform data illustrates that inserting a short-circuit cable loop inside a coupler during WF2 application causes voltage distortion and suppresses current well below the required limit level. The paper concludes that hybrid generator/coupler systems can overcome these calibration and impedance issues but are large, heavy, and costly, making practical DO-160 compliance difficult.