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In Reply to: No, silver-mica caps are not safe for AC line use. posted by Al Sekela on April 13, 2007 at 15:54:10:
The only caps that are safe are those rated "X," X2," or "Y." These are designed to be self-healing after breakdown, without causing a resistive leak.It was suggested to me quite a long time ago (by an electrical technician who works for a big name aerospace company) that carefully enclosing the cap with heatshrink (which is normally quite thick) would provide that "extra" margin of safety in the rare case of a pinprick material failure, thereby creating or approximating the outer material thickness with which an X or Y cap may be made. If I were to wrap a cap, I would probably use Kapton, which if memory serves, has a heat-resistance rating of 500F as well.
In both cases, wrapping the cap also creates the benefit of additional resonance control. Your thoughts on all of this?
Follow Ups:
Non-X- or Y-rated caps may develop resistive leakage paths internal to the cap body upon exposure to AC line spikes. Repeated spikes can and will decrease the resistance to the point where significant power is dissipated by the leak. The cap will get hot and eventually catch fire or set something nearby on fire.Your friend's tweak would be a good way to improve the outer insulation of a cap that had to be used at a high voltage with respect to ground, and may improve the sonics by reducing the acoustic response. However, it will not affect the internal leakage issue.
However, it will not affect the internal leakage issue.Wouldn't that depend on the build quality of the cap in question? And if a particular company addresses that potential failure in its intrinsic design?
The right choice of insulator and metal film are necessary, but so are test procedures. This is not a "defect" issue per se, as any capacitor dielectric will break down if stressed with a sufficiently high voltage. The issue is how well the dielectric + conductor film system heals itself after the breakdown event.With the best of design intentions and knowledge, and the best uniformity of materials, a capacitor made on a manufacturing line that does not perform testing for the self-healing requirements would be at risk for failure in a highly dangerous application. Whoever makes Auricaps may well use the test methodology as well as a self-healing design, but if they don't get third-party approval and mark the capacitors as X- or Y-rated, then how are we to know?
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