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So I understand that high current through a conductor generates heat, and too high heat can melt things or cause components to fail. Short circuits are frequent causes of overcurrent, no?

Overvoltage is bad because it can cause overcurrent, or in the case of capacitors, trigger a breakdown voltage.

From what I understand, some components 'blow' (or inflate, explode, otherwise fail aside from direct melting) from dielectric breakdown - other components like diodes can fail for the same reason.

But is it actually the voltage that's causing these things to expand and pop like they do, or is it actually the rapid transit of electrons as a result of the excess voltage triggering an overheat and igniting something? I'm not sure my understanding of all this is correct.

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    \$\begingroup\$ A high voltage by itself may or may not cause failures. Think of common static electricity...a very high voltage but a low current. It is dangerous for some electronics but your normal mains wiring laughs it off. If you also provide high current...as in lightning...then you have a very dangerous combination. \$\endgroup\$ – Elliot Alderson Aug 22 at 20:57
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Most failures are ohmic, meaning materials heat up and degrade, and melt. This can be caused by overvoltage or overcurrent depending on the component.

In my experience, part failure occurs most when the voltage is too high and it leads to breakdown and heating. Most of the time it's heat that kills a part, which is why when powering circuits on for the first time, if there is a problem with the design, and if you can switch on the circuit and turn it off before the parts heat beyond their ratings, the circuit can be saved. (I usually power on and off for 1 second and look at it with a heat camera)

Under extreme circumstances with arcing, other interesting things like wielding can occur with ESD where the arc creates a plasma and etches materials away. (look for esd damage pics, it's surprising).

When things explode it is usually from the materials in the components being vaporized creating a gas which in turn causes a mechanical failure/explosion. The gas can be created by a very fast temperature rise or arc. In the case of capacitors, the dielectric can create gasses.

I once had a relay explode on me (early on in my career, stupidity on my part, I ignored a rating) and the case hit me right between the eyes, a little to the left or the right and it could have been bad.

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  • \$\begingroup\$ I also know that the bad batch of Chinese capacitors back in the 90's had a bad electrolyte formula that created hydrogen gas in the capacitors and also helped the explosions. There were some power supplies that I had to replace that had a cap that would go off like a firecraker \$\endgroup\$ – Voltage Spike Aug 22 at 20:34
  • \$\begingroup\$ Y'know, I completely forgot things expand when you heat them. I suppose heating something fast enough would cause it to expand fast too, and if one material expands/heats faster than another encasing it it'll pop the whole thing. \$\endgroup\$ – RidleyPrime Aug 22 at 20:48
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    \$\begingroup\$ And don't forget that wet electrolytic capacitors contain a liquid that starts boiling if it gets too hot. The current through an electrolytic cap flows through the electrolyte and directly heats it. \$\endgroup\$ – Michael Karcher Aug 22 at 21:45

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