It's not the design, it is the device. A transient pulse needs to be dissipated. To do that, the MOV becomes a short and then when the voltage decreases, it can become a high resistance path again. However, when the conduction time is beyond a certain amount of energy, heating occurs and the crystalline structure of the MOV can crack, melt, etc. and the device has protected it's last time.

I dug up an old circuit of a surge suppressor we fielded:

This has both three and two lead MOVs. It also has redundancy since there are slight variations in performance of MOVs. Also, all the combinations of transient possibilities are protected.

This one is designed to let the MOV shorting out cause the isolation relay to disengage.

On Sat, May 18, 2024 at 5:09?PM davesmith1800 via <davesmith1=[email protected]> wrote:

In technical terms the Varistor shorts out? (when a voltage spikes happen) flipping the breaker or burning out the fuse.?

So if not designed right you easly have fire because it gets very hot.?

Dave?



On Sat, May 18, 2024 at 04:58 PM, mike allen wrote:

Surge protectors aren't all equal . They said on the news just now the
fire was started at one of the surge protectors for a 3D printer I believe .

--
Buffalo John