Comment by cyberax

These devices are basically autotransformers. So they reduce the noise by providing inductive filtering. But they don't really protect against strong surges by themselves.

So Tripp Lite uses a regular varistor for that, just like any other surge protector. In Europe you'd be far better off buying a voltage relay and adding it to your electrical panel, but it's not usually possible with the non-modular US electrical panels.