Whitepaper

ESD Testing for Class II Medical Devices

Published by Astrodyne TDI

This whitepaper from Astrodyne TDI, authored by Principal Engineer David Love, addresses a specific and consequential testing problem introduced by the 4th edition of medical EMC standard IEC 60601-1-2, which extended the requirement for Level 4 ESD testing — 15 kV air discharge and 8 kV contact discharge — to virtually all medical devices rather than only life-critical ones. The central issue is charge ratcheting: when sequential same-polarity ESD pulses are applied to an ungrounded Class II device without discharging accumulated charge between pulses, voltage builds incrementally across the isolation barrier with each event, potentially reaching destructive levels for components such as optocouplers and transformers that typically fail above 10 kV. The document explains why the IEC 61000-4-2 standard's guidance on charge removal lacks clarity for plastic-housed devices, why carbon brush sweeping is insufficient, and why self-discharge through the isolation barrier is impractically slow. Two practical remedies are presented: physically contacting the ESD injection point with a thin metal probe connected through 470 kΩ resistors to bleed accumulated charge between pulses, and an alternating polarity injection method the author developed, which applies pulses in alternating positive and negative sequence to keep isolation barrier voltage oscillating near zero rather than ratcheting upward. The document also clarifies the distinction between Class I and Class II power supplies with functional earth connections, and recommends documenting any protocol modifications in the EMC test plan.