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Some (electrolytic) capacitors I have in a kit have vents, some do not (not on the top, not on the bottom).

The vents are there to safely let the gas out instead of letting the capacitor shoot.

So why don't all the capacitors have these? If they would fail (you never know): aren't the vented capacitors safer to use?

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    \$\begingroup\$ Besides that maybe you just can't see the vent mechanism, or they don't have liquid electrolytes and thus don't need vents, it can just be that they are old ones where it wasn't invented, or cheap ones where somebody saved 0.00001¢ per manufactured cap. \$\endgroup\$ – PlasmaHH Mar 9 '17 at 15:54
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    \$\begingroup\$ If it blows up inside a sealed appliance, it probably won't have enough energy to hurt you. Larger caps have more potential for disaster. High capacitance leads to thinner insulating layer that can more easily fail. High voltage leads to more current if the cap fails. \$\endgroup\$ – Oskar Skog Mar 9 '17 at 16:16
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    \$\begingroup\$ @KevinVanRyckegem: Depends on many factors. A spark may be likely at overvoltage conditions. Fuel + oxygen + enough heat or a spark = flame, if it sparks and it has an organic electrolyte you may get a flame. \$\endgroup\$ – Oskar Skog Mar 9 '17 at 16:24
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    \$\begingroup\$ @KevinVanRyckegem: The threat isn't big enough, I think. Electrolytics in consumer appliances seldom fail violently. Consumer appliances are seldom near clothes (except for washing machines etc). Class II appliances are sealed so a small flame inside them will soon die. And unless you have fluffy or gasoline soaked clothes, they won't be /that/ easy to ignite, especially for the small electrolytics. \$\endgroup\$ – Oskar Skog Mar 9 '17 at 16:28
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    \$\begingroup\$ I think the weakened metal and vents are only used for letting out gas that builds up relatively slowly. If the cap remains intact after having let out the steam, it won't spark or burst into flames. \$\endgroup\$ – Oskar Skog Mar 9 '17 at 16:32
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Do you mean the "T", "X" or "K" on top of most electrolytic capacitors ?

enter image description here

Actually these vents are not vents but a deliberately made weak-point in the housing of the capacitor.

The vents are only needed for Capacitors which contain some electrolytic fluid which could start to boil and create pressure.

Not all capacitors contain electrolytic fluid, for example "Solid electrolytic capacitors" or "Polymer capacitors" don't.

enter image description here

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  • \$\begingroup\$ Hm, but there is no vent and no weak-point in the housing. The capacitor looks like this: goo.gl/PwA0N1 however, the top is entirely flat; there is no weak-point. I thought this is an electrolytic capacitor and it should have the weak-point; in order to avoid them building up too much pressure during a failure. -- I should state that I bought these from a Chinese website, so I'm not sure about the quality. But is it right to assume that it is less safe not to have that? It seems like an important safety feature for such a capacitor. \$\endgroup\$ – Kevin Van Ryckegem Mar 9 '17 at 16:03
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    \$\begingroup\$ Unless there's a respectable brandname on the capacitor, I would not trust it. It would be OK for some hobby use under and experiments but not in a hot power supply. \$\endgroup\$ – Bimpelrekkie Mar 9 '17 at 16:21
  • \$\begingroup\$ So I wonder, do some brands also have the left capacitor you show without the weakened (T/X thing) housing? Or is that just because I have gotten bad capacitors? Out of curiosity: what could be the worst case of such a capacitor failing? Thanks for the info! \$\endgroup\$ – Kevin Van Ryckegem Mar 9 '17 at 16:25
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    \$\begingroup\$ Worst case failing? Along the lines of youtube.com/watch?v=0OIGz9gHCRI \$\endgroup\$ – Matt Sieker Mar 9 '17 at 18:17
  • \$\begingroup\$ do some brands also have the left capacitor you show without the weakened (T/X thing) housing? Yes, the smaller value capacitors, older types (when doing this was not so common), Axial shaped capacitors (wires coming out on opposite sites). I have plenty in my parts drawer which do not have the weakened top. \$\endgroup\$ – Bimpelrekkie Mar 9 '17 at 20:29

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