The MOV is a highly nonlinear resistor and, unlike the Zener diode, is symmetrical, so it works okay for AC or DC. The resistor is not nearly as nonlinear as a Zener, so once the device turns on, the voltage can still rise quite a bit above the initial threshold - protection is not nearly as precise. The MOV is most often found in power line protection. The traditional MOV, such as used for power strips has not been known for its high speed - acceptable for surge, but too slow for EFT.
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