I've got a few SurgeX units that I've taken apart. It's an inductor in line with the hot, and sense circuitry that shuts off a downstream relay when the input v

I've got a few SurgeX units that I've taken apart. It's an inductor in line with the hot, and sense circuitry that shuts off a downstream relay when the input voltage rises. I don't remember sizing up the capacitance but surely there has to be some, and presumably they've done the math so that the voltage rise downstream of the inductor is slower than the relay switching speed. > a hot-only surge seems pretty unlikely What do you mean? N/G should be bonded and at the same voltage with exterior surges, assuming a dedicated circuit. Across L-N/G is the only place a surge can show up - barring parasitic coupling of the equipment to ground/environment, or erroneously connecting a wire to unprotected equipment. And compared to TVS ones that pass all the lines through, relying on their ability to shunt overvoltage/current, with the active element actually dissipating much of the surge power? The series mode of operation at least seems analytically tractable.