Failure modes of voltage-limiting SPDs (varistors)

As a core component of voltage-limiting SPDs, varistor (MOV) failure modes primarily stem from the material's physical properties and electrical stress. The primary failure mode is short-circuit failure and its derivative thermal breakdown. When subjected to surge currents exceeding their tolerance (a single, large energy event or multiple, cumulative energy events) or prolonged exposure to power-frequency overvoltage (TOV) stress, the varistor's grain boundary structure undergoes irreversible degradation. This can manifest as:

Leakage current increases significantly: The grain boundary barrier is destroyed, resulting in a sharp increase in leakage current at the nominal operating voltage and significant Joule heat generation.

Thermal breakdown: Increased leakage current causes the device temperature to continue to rise, and the temperature increase further reduces the grain boundary resistance, causing the leakage current to increase, forming a vicious positive feedback loop. This process ultimately causes the temperature of the varistor to rise sharply to the melting point or decomposition point of the material.

Short-circuit failure: During or at the end of thermal breakdown, the internal material of the varistor melts, carbonizes, or forms a conductive path, causing its resistance to drop sharply to a near-short-circuit state. At this point, if the SPD's internal thermal protection trip device (such as a thermal fuse or spring release mechanism) fails to promptly and reliably disconnect the varistor from the grid, a continuous power-frequency short-circuit current will flow through the failed varistor.

Fire risk: Continuous high-frequency current flowing through the short-circuit failure point generates extreme heat, causing the varistor or surrounding materials to violently burn, arc, or even explode, posing a significant fire hazard. This is the most dangerous state after a voltage-limiting SPD fails, so a reliable built-in thermal tripping mechanism is crucial.

Failure modes of switchingSPDs (graphite gap)

As a representative of switching SPDs (usually spark gaps), graphite gaps exhibit fundamentally different failure modes from varistors, primarily manifesting as "open circuit failure" or "performance degradation." Failure is primarily driven by electrode erosion during discharge and power frequency continuous current interruption.

Electrode erosion and degradation: Each time a surge current is discharged, the electrodes (especially graphite electrodes) erode (sublimate, oxidize, and melt) under the action of the high-temperature arc. As the number of operations increases, the electrode gap increases, causing the discharge ignition voltage (DC breakdown voltage, impulse breakdown voltage) to gradually increase. This can degrade the SPD's protection level (residual voltage) and reduce its effectiveness. Increased electrode surface roughness can affect discharge stability and consistency. Severe electrode material loss and erosion can lead to electrode thinning or even perforation.

Failure to interrupt power frequency continuous current: This is the core failure risk of switching type SPD. After the surge, power frequency continuous current maintained by the system voltage may form in the gap. The gap must reliably interrupt it at its first current zero crossing point. Severe electrode ablation leads to poor heat dissipation and reduced dissipation capacity; the system short-circuit capacity is too large and the continuous current amplitude is too high; gap design or manufacturing defects (such as poor sealing, changes in gas composition) will result in the inability to interrupt the continuous current within the expected time, and the gap will continue to conduct power frequency current, forming a de facto short circuit.

Open-circuit failure: After the power-frequency continuous current interruption fails, if the SPD's backup protection device (such as a fuse or circuit breaker) operates correctly, it will interrupt the power-frequency continuous current, but it will also disconnect the entire SPD (or the gap branch) from the system, resulting in a permanent open-circuit state and loss of protection. Extremely severe ablation or physical damage (such as electrode perforation or housing cracking) can also directly cause the gap to no longer be able to establish effective discharge, resulting in an open-circuit state.

The fire risk is relatively low (compared to a short-circuit failure): Even if an interruption failure occurs, as long as the backup protection device operates promptly to interrupt the power frequency current, it generally does not produce the persistent high-energy heating point that occurs with a varistor in a short-circuit failure. The open circuit state after failure also means that current no longer flows through the failure point. The main physical risk is the strong arcing and gas jets that may occur during the interruption failure, but the duration is generally short (depending on the backup protection operation time), and the overall fire risk is lower than that of a varistor in a persistent short-circuit state.