开云体育

1- MOV failure mode is short circuit, not open circuit. Never put such a device on the fridge or a freezer line !

2- Use exclusively MOVs on AC lines

3- Clamping voltage is not a big problem: Electronic devices plugged to the grid are designed for withstanding at least 1500V peak. For indirect shocks on AC lines, typical surge waveform is very short: 8?s rise time & 20?S overall duration and, in home distribution circuits, it rarely exceeds this value. There are other schemes, depending on the nature of the shock (direct or indirect) and the nature of the upstream protection.
Of course, the earthing circuit is of a prime importance: the the surge path must have a very low stray inductance and its resistance be compliant to the standards.

As a matter of example: At home, located in a quite high keraunic area, all the grid distribution cables are buried and it's a favorable situation. I have a 40y-old 220Vac protection still alive, with 3 varistors (phase-neutral, phase-earth and neutral-hearth) 400V clamping voltage @ 1mA, with a limitation of 700V for a 10Amp shock, 1200V for a 2500A shock, 1400V for 5kA or 1500V for 10kA. Response time is better than 50ns. The device has it's own earth 1m below with a straight cable, the protection is placed at the head of the distribution cabinet located 5m after the company's counter. my earth ground network resistance is well below 50 Ohms when the soil is dry. Since it's been installed, the standard voltage has been establish to 230V and I didn't see any difference.
I never had any issue for 40 years. It's not the same in my neighborhood !!

Then at the computer level, just because I'm paranoiac (!), i have the same scheme but with 275 V service varistors I replaced the original 250Vac by Chinese components because they were too close to the peak value of the grid voltage. The earth path is shorter than 1 meter.

Read this:
and for those who can read French Language. This book is very rich hence highly recommended, written both in worker language ans engineer's one ; worth to be translated with Googletrans.