Unfortunately not mine, no idea how this has happened. Anyway prvoked my head to some questions:
That's too bad. Knowing about the fuses, suggests you may have met the owner.
- Majority of damage is caused by this fused resistor, it fired PCB, internal plastic protection and even the case. Hmm... this looks like this resistor was too slow to cut connection and made huge explosion and fire. Shuould fused resisitor really explode? Is plastic protection a correct protection from fire?
Even with my small transients and the camera running at close to 1ms frame rates, it's no where fast enough to capture an event like this. I see a lot of people post about the input protection circuit is for safety. As you can see from the picture, the case plays a big part.
If you exceed the design limits of any component, I am guessing you could see it fail in the same manor. PhotonicInduction made a few videos where he placed some HRC fuses across a large high voltage capacitor and was able to get them to rupture. These were 10KA+ break currents. So even a safety fuse will have it's limits.
That was a very interesting channel to watch. Nothing I would ever attempt at home.
- PTC was compeltely melted and changed to cloud of smoke. Maybe it shuld be heatshrinked to react faster on overrload (heat cumuates faster when PTC is shrinked?)
I doubt heat shrunk tube would do much in this case. I've seen it used a fair amount on the smaller PTCs but not the larger ones. During my tests, I've damaged a fair number of PTCs in low end meters. They often use 5mm parts which don't always hold up very well.
- MOVs exploded, but PCB trace to MOVs surrived intact, hmmm ... why?
Higher resistance, more power dissipated?
- None of fuses exploded or broke connection, hmm ...all were just fine. No they were not repalced after damage, all were nicely smoked.
Why would the fuses be effected. It's obvious that what ever happened, it was on the voltage circuit of the meter, not the current.
I would say in this case, trying to make a meter survive what ever happened is not solving the root problem. With so little damage done to the meter, I doubt the person was harmed and may not have learned anything from it. Next time, they may not be so lucky.