Metal Oxide Varistor (MOV) and thermal tripping protection device are core components in voltagelimiting SPDs, which directly determine the safety performance. It is necessary to summarize the factors that are considered in the design of components by analyzing the failure modes of these two parts in depth. In this paper, the three failure modes of MOV are studied, including the elevated working temperature under 〖WTBX〗U〖WTBZ〗c, over large pulse current, and transient overvoltage. The performance of thermal tripping protection device was tested under the MOV fault mode. The measures of improving the safety of SPDs are summarized, choosing the appropriate MOV (including the capacity of current and residual voltage, high temperature stability, the endurance of power frequency under overlarge impulse current, the tolerance under transient overvoltage), designing the right thermal tripping protection with reliable technical parameters (including material properties, hot spots, tripping distance, tripping force and welding process, adding over current protection device, etc.).