Do HVAC capacitors have a safety rating?

Question

If I place a capacitor directly on the 120V home power line, someone has probably been careful in choosing its safety rating. The most important thing to avoid is end-of-life damage in which the capacitor shorts out (because shorting could cause a fire or electrical shock).

HVAC ("Heating, Ventilation, and Air Conditioning") fans/compressors, however, usually place a capacitor on the ground side of home power, so I don't see any safety ratings on these capacitors. But, everyone anticipates some sort of end-of-life damage after about 60,000 hours of use.

Do these HVAC capacitors usually fail as an open or short? I am concerned that a short could then damage my series-connected fan/compressor motor. Is that a real concern, or does a shorted capacitor just cause poor HVAC energy efficiency? If damaging the motor is possible, but would require a very low-resistance short which usually does not happen, please elaborate.

My exact capacitor is here. It seems to meet IEC 60252-1:2001 and UL Listing E185318 standards, but I have no idea what these mean for safety (and I cannot even confirm either standard with the manufacturer). So, how do you expect most of these capacitors to fail after 60,000 hours of use?

Answer 1

usually place a capacitor on the ground side of home power, so I don't see any safety ratings on these capacitors

Those run capacitors are between a motor winding and one of the phases. They are not across the phases. They are always in series with a winding. If the capacitor shorts out, the motor's thermal or stall protection should kick in and turn the motor off. The overall design is such that if the capacitor fails shorted, it'll be handled. These capacitors rarely fail shorted though. They mostly fail open, eventually leading to a stalled motor.

I am concerned that a short could then damage my series-connected fan/compressor motor.

The motor has to be thermally protected anyway, with the assumption of a stalled rotor. Whether the rotor stalls due to mechanical breakdown (e.g. a stuck compressor piston) or due to loss of capacitively-derived phase, it still has to survive this. If the motor design had the capacitor connected to a very low-impedance winding, such that a shorted capacitor would make the motor draw excessive current, the overcurrent protection should take care of it.

Whether it's a concern or not can be checked by replacing the capacitor with a short and measuring the motor's winding currents when supplied by a variac, so that the currents can be kept in check by starting with a low voltage and slowly ramping up while monitoring them.

If damaging the motor is possible, but would require a very low-resistance short which usually does not happen, please elaborate.

To a motor, usually, a shorted run capacitor, an open run capacitor, and a stalled rotor have similar thermal implications - at least well within an order of magnitude, so the protection is one and the same. Keep the windings within the insulation system's temperature rating and all will be good.

But you can and should measure all this. No need to hypothesize - it can all be measured using a relatively inexpensive setup.