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I notice that some HVAC capacitors have labels printed or sprayed on them like the black "negative circle" label on top of the capacitor in the picture. What is it supposed to indicate?

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    polarity; electrolytic capacitors can explode if inserted backwards. – dandavis Jul 26 '18 at 17:48
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The black circle is not a label; it is a safety vent. If an electrolytic capacitor overheats due to high DC voltage, to a large AC component, to reverse connection or to internal failure, it will build up pressure until something gives... hiss, or sometimes bang.

To prevent B A N G, vents are designed to fail gracefully, or at least not too abruptly. The vents are marked below: a weak thermoplastic disk in the harder thermoset top of the large capacitor, and the score marks to weaken the thin aluminum in the smaller, which has already failed.

There should be clear polarity marks on the electrolytic (unless it's a non-polarized cap), often a black stripe along the negative side, plus mark(s) (+) on the positive, or embossed marks near the terminals. (Barely visible in the photo below is a plus mark below the right terminal of the big blue cap.)

Electrolytic capacitor safety vents

BTW, from the terminals, I might guess that this is a non-polarized motor-starting capacitor, but take that surmise with some circumspection. Check the schematic or parts call-out if it needs to be replaced!

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