In my opinion, any device that shifts from a high impedance to a low impedance to protect an attached, parallel load, needs another part.
The additional part is a series energy limiter, examples a PTC device, an inductor, a resistor, a fuse, etc. The resulting circuit is nothing more than a shunt regulator with the load and breakover device at the same voltage, the remainder across the series limiter and the mains impedance. The mains impedance tends towards 1/2% per unit----a very small number. This point was made several years ago when an ambitious marketer brought a voltage suppressor comprised of several MOV's in parallel across the mains, and nothing else. We gave it single simulated lightning disturbance---when the smoke cleared all that remained of the suppressor was a dark stain on the test bench. [email protected]
