Junction capacitance varistors typically hundreds to thousands of magnitude range Pf

Mov varistor grade ns response time, faster than the gas discharge tube, slightly slower than the TVS tube, under normal circumstances electronic circuit for overvoltage protection to meet the requirements of its response speed. High voltage varistor junction capacitance range is generally on the order of hundreds to thousands of Pf, in many cases should not be applied directly the protection of high-frequency signal lines, and the application of the AC circuit protection, since it will increase the junction capacitance greater leakage current protection circuits designed to take full account. Metal oxide varistors  through-flow capacity is larger, but smaller than the gas discharge tube. Varistors referred VDR, it is a non-linear voltage-sensitive semiconductor element overvoltage protection.

When applied to the varistor voltage is below its threshold, the current flowing through it is extremely small, it is equivalent to a resistance infinite resistance. That is, when a voltage is applied on top of it is below its threshold, which is equivalent to an OFF state of the switch.
When applied to the varistor voltage exceeds its threshold, the surge current flowing through it, which is equivalent to the resistance infinitely small resistance. That is, when added on top of it higher than its threshold voltage, which is equivalent to a closed switch.