Views: 0 Author: Site Editor Publish Time: 2025-09-03 Origin: Site
· Mechanism: This is primarily an environmental and manufacturing-related failure. MOV blocks are typically encapsulated within an epoxy resin or, for higher-quality units, housed in a hermetic ceramic or glass package. If the sealing integrity is compromised—either due to manufacturing defects, mechanical damage, or prolonged exposure to harsh environments—moisture can permeate the housing. This moisture directly affects the sintered zinc oxide disk, leading to a decrease in its electrical insulation resistance along the surface and through the bulk material.
· Process of Failure: The presence of moisture, often combined with contaminants and operating voltage, facilitates electrochemical reactions and leakage current paths. This can cause a gradual decrease in the varistor voltage (V1mA) and a significant increase in leakage current. Under an applied voltage, this can lead to localized heating, further carbonization of the paths, and ultimately, a thermal runaway event triggered by the moisture. The initial failure point is often the seal.
· Physical: Visible cracks or breaches in the epoxy coating or ceramic housing. Signs of corrosion on internal electrodes or external terminals. In severe cases, the epoxy may be bulged or ruptured from internal pressure generated by vaporized moisture.
· Electrical: Pre-failure symptoms include a measurable drop in insulation resistance and an increase in leakage current. The final failure often presents as a low-impedance short circuit, but the root cause is the compromised seal, not energy overload.
· Mechanism: This is an intrinsic, wear-out mechanism related to the MOV's fundamental operation. Each time an MOV suppresses a transient—even within its rated energy capability—microscopic damage occurs at the grain boundaries within the ZnO ceramic. The high current flow during conduction causes localized heating and melting of the Bi₂O₃-rich intergranular layers, which are critical to the varistor's nonlinear characteristics.
· Process of Failure: With repeated electrical transients or prolonged exposure to temporary overvoltages (TOVs), this damage accumulates. The varistor voltage (V1mA) gradually decreases, and the leakage current steadily increases. The device's ability to handle subsequent transients diminishes. Eventually, the leakage current becomes high enough to cause sufficient Joule heating under normal operating voltage, leading to thermal instability.
· Physical: The MOV body often appears intact with no visible external damage. The housing is not breached.
· Electrical: The primary indicator is a significant shift in electrical parameters before catastrophic failure. Diagnostic testing would reveal a lower-than-specified V1mA and elevated leakage current. The final failure typically results in thermal runaway, converting the device into a short circuit. This mode is common in areas with frequent, low-energy surges.
· Mechanism: This is an catastrophic, sudden failure caused by a single transient event that exceeds the MOV's energy absorption capability (Maximum Single Pulse Energy Rating - I²t) or peak current rating (Iₚ). The immense energy dumped into the MOV disk generates heat faster than it can be dissipated, causing a rapid and extreme temperature rise throughout the bulk material.
· Process of Failure: The intense heat can vaporize the metallic electrodes, melt the ZnO grains and internal connections, and generate extreme internal pressures. This process is extremely rapid, occurring in milliseconds.
· Physical: This is often the most dramatic failure. The MOV package may be violently ruptured, shattered, or charred. There may be signs of exploded internal components, splattered metallization, and blackened epoxy. The printed circuit board (PCB) around the device may show severe thermal damage.
· Electrical: The device usually fails as a short circuit, but in extreme cases, the internal arc can vaporize the conductive path, resulting in an open circuit. The failure is instantaneous with the overvoltage event.
Distinguishing between these failure modes requires a systematic approach:
Failure Mode Primary Cause Key Physical Evidence Key Electrical Evidence
Moisture Ingress Compromised sealing Cracked/blistered housing, corrosion Gradual parameter shift before failure, short circuit
Progressive Aging Repeated transients / TOVs Housing often intact Gradual decline in V1mA, increase in leakage current
Thermal Overstress Single excessive surge Violent rupture, charring, splattering Sudden short or open circuit; no prior warning
· Against Moisture: Specify MOVs with robust hermetic sealing (ceramic/glass) for critical or harsh environment applications. Ensure proper conformal coating on PCBs. Avoid mechanical stress during installation.
· Against Aging: Carefully model the expected surge environment and select an MOV with an energy rating that includes a significant safety margin for the lifetime of the product. Implement routine maintenance checks in critical systems to measure leakage current and identify degrading MOVs before they fail short.
· Against Overstress: Perform a detailed risk assessment for the maximum possible surge (e.g., direct lightning strike induction). Use a coordinated protection scheme where a high-energy MOV (e.g., at a service entrance) absorbs the bulk of the energy, followed by secondary MOVs with lower clamping voltages downstream. Consider combining MOVs with gas discharge tubes (GDTs) or spark gaps for very high-energy events.
Conclusion While the final state of a failed MOV is often a short circuit,the underlying causes—moisture ingress, progressive aging, or single-event overstress—are distinct. A careful forensic examination of the physical evidence, combined with knowledge of the device's electrical history and operating environment, is indispensable for accurate diagnosis. Implementing MOVs with appropriate ratings, robust packaging, and within a coordinated protection strategy is key to maximizing their protective lifespan and ensuring the reliability of the equipment they safeguard.
