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At the photo you may see a small dent on the aluminum shell of a run capacitor type cbb65 (metalized polypropylene film capacitor). I have been told that this small dent may not affect the function of the capacitor or cause it a premature fail. But if the dent was deeper so it will squish the insulator (PP) and the metal film, I assume that it will affect its function. My question is, what will be the symptoms in that situation? (too deep dent) Will the capacitance change/decrease? May a leakage to the ground occur (through the aluminum shell)? What is the proper test to check that the dent doesn't affect the function of the capacitor or cause it a premature fail?

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    \$\begingroup\$ The component is clearly physically damaged and this takes higher precedence over arguments about whether it might be internally damaged or have reduced performance. Replace it is my strong advice. A test that says it's OK doesn't rule out that it will fail in a year, a month or a week. \$\endgroup\$
    – Andy aka
    Commented Sep 28, 2023 at 15:29
  • \$\begingroup\$ It's one of the reasons why most start / run capacitors are protected by the mounting chassis. As Andy says, replace. \$\endgroup\$ Commented Sep 28, 2023 at 17:50
  • \$\begingroup\$ Thanks for the comments.The capacitor is probably need to be replaced,but my question was:"What are the symptoms of a dent on a run capacitor metal shell? Whether it needs to be replaced or not is not the issue.I would like to learn/understand this situation.How this dent may affect the capacitor while the metal shell is squishing the metal film/insulator inside the cap?what will be the symptoms? \$\endgroup\$
    – Dan
    Commented Sep 28, 2023 at 18:34

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I can speak only for film capacitors assuming they are rectangular shaped and force is applied uniformly across the surface. The capacitance will increase/decrease (very slightly) depending on which side is "squished". When force is removed, capacitance returns to normal. There, it's even unlikely it will be damaged.

But a dent like this, in a can shaped capacitor filled with oil no less, is totally unpredictable. Even with a datasheet showing the exact position and orientation of the dielectric sheets and electrode sheets it would be nearly impossible to tell what would happen. It could produce a dead short (which can randomly happen in the future without warning), or show no symptoms at all.

I would not use that capacitor.

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  • \$\begingroup\$ In regards to the possible dead short that it could produced(as you mentioned),Does it include this type of metalized film capacitor,which are"self-healing"?and can it short/leak to the ground(touching and leak current through the outer aluminum shell)randomly in the future? \$\endgroup\$
    – Dan
    Commented Sep 30, 2023 at 13:36
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    \$\begingroup\$ I don't know what's different in "this" capacitor, but I have seen "self-healing" metallized film capacitors explode (in a tesla coil) so all bets are off. Generally they are definitely more resistant than most other capacitor types, but not immune. About the leak to ground, I'm not sure.. if the shell was plastic then no. But the aluminium shell would make me wary. \$\endgroup\$ Commented Oct 1, 2023 at 15:10

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