The tripping of a surge protective device (SPD) is a response to a fault condition by its built-in safety mechanism. Essentially, it disconnects the device from the circuit through mechanical or electronic means to prevent the fault from escalating. Its tripping behavior is directly related to the characteristics of the SPD's core components, surge parameters, system operating conditions, and environmental conditions. The core reasons for this are as follows:

1. Surge energy overload

When surge current (such as the 10/350μs or 8/20μs waveform generated by direct lightning or strong induced lightning) exceeds the tolerance threshold (such as Iimp, Imax) of the SPD core components (such as MOV, GDT), or the accumulated energy exceeds the component tolerance, it will cause component breakdown and short circuit, triggering the thermal trip device (such as bimetallic strip, thermal fuse) to operate, cutting off the circuit to prevent the fault from expanding.

2. Power frequency continuous loss control

After a surge, if the SPD component (such as an aged MOV or a GDT with arc extinguishing failure) cannot return to a high-resistance state and continues to conduct the grid's power frequency current (forming a "freeze current"), and the freeze current value exceeds the component's tolerance limit (such as the freeze current If of the MOV or the arc extinguishing capability of the GDT), it will cause overheating or overcurrent, triggering the electromagnetic release or thermal release device to cut off the connection.

3. Component aging and degradation

During long-term operation of an SPD, core components (such as MOVs) may experience performance degradation due to repeated surge shocks. This can manifest as increased leakage current (from μA to mA), increased power consumption, and continuous heat generation that causes the temperature to exceed the threshold (usually 70-120°C). Alternatively, oxidation of the GDT electrodes and gas leakage may cause characteristic drift, ultimately triggering a thermal trip. This is a normal response to the end of life.

4. Environmental and installation defects

Excessively high ambient temperature (exceeding the rated range of -40 to +85°C), excessive humidity, or the presence of corrosive media can accelerate component degradation or cause surface discharge. Loose wiring during installation (excessive contact resistance) or improper coordination of higher-level protection (such as excessively high rated current of the circuit breaker) can cause local overheating or overcurrent, indirectly triggering a trip.