Zap or Trap? The Fiery Dilemma of Zinc Oxide Surge Protectors

Surge protective devices (SPDs) are commonly used to guard electronics against transient voltage spikes. However, few are aware that these devices can fail catastrophically, heating up until ignition. The surge-suppressing component in most modern SPDs is a metal oxide varistor (MOV).

The term “varistor” is a portmanteau of the words “varying” and “resistor,” as resistance of the device changes in response to applied voltage. During a voltage spike, the MOV conducts current to dissipate a portion of the surge as heat, protecting connected electronics. However, the very protective properties that make MOVs effective surge protectors can lead to hazardous failures as repeated surges decrease the breakdown voltage—the voltage at which the MOV starts dissipating energy. The breakdown voltage can decrease to nominal supply voltages (e.g. 120 VAC), causing the MOV to conduct current constantly, resulting in resistive failure and overheating.

MOVs consist of a wafer of sintered zinc oxide (ZnO) particles between two electrodes. The particle size, wafer geometry, and doping elements determine the breakdown voltage. The heat dissipated by the MOV during a surge can cause ZnO grain growth, reducing grain boundary surface area and degrading the MOV’s breakdown voltage. Understanding microstructural changes is crucial when investigating whether MOVs from fire scenes were causal to the fire.

This study examines the microstructure of ZnO varistors under three conditions: 1) typical service, 2) abusive services, and 3) simulated fire exposure. Utilizing scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), we explore the structural changes that occur before and after these conditions. Our findings provide new material insights into MOV failure mechanisms and their role in fire ignition, with significant implications for the safety and reliability of SPDs.