To understand how an SPD works, the customer needs to look at MOVs. MOVs are typically made of zinc oxide and are connected in parallel. MOVs are bi-directional and will begin conducting when a threshold voltage is achieved. When below the voltage, resistance is high and a trivial amount of current will flow through the MOV. Once the threshold voltage is met (typically 115% rated voltage), the resistance drops very low to almost 0 ohms and voltage will flow through the MOV as current, rather than the load. When the voltage drops below the threshold, the MOV resistance increases and stops clamping the overvoltage.