Why varistors-surge protective device cannot be used for overvoltage protection?

When there is current flow through the surge protective device, the the powerful inrush voltage drop on the resistance of the conduction voltage is to be called Up, which will cause SPD fever. When the fever temperature rise exceeds the limit of metal oxide varistors, there will be irreversible damage. Because surge voltage duration very short, the fever is insufficient to exceed the limit; but the current supply by overvoltage continues for a long time, MOV quickly increasing up to the damage limit value. Therefore surge protective device cannot be used for over-voltage protection.