AC unit blows cap

Based on *my* central AC unit, what *could* happen is that the cap could either fail open or fail shorted. (Mine failed open). And every time the compressor tried to run, it would draw *lots* of current (dim the lights) for perhaps five seconds, until a thermal limiter opened, and then a few minutes later it would do the same thing. (This was because there was only the single phase -- not the shifted phase needed to start the motor, so it was stalled and drawing a lot of current.

Now -- if the cap shorted instead, it would draw current through both windings, but with no phase shift, so it would still not start spinning -- and the current through the cap (especially if it were not a

*full* short) would heat it up, and eventually burst the housing with boiling oil. Any spark at that time would likely set the oil on fire.

And that thermal switch I *think* is only in series with the main winding, not with the cap shifted winding, so a shorted cap would keep drawing current even after the thermal switch opened.

But this is all speculation.

No doubt you don't have three phase available. If you did, you could run both the compressor and the external cooling fan from three phase and not have to worry about run caps. (Obviously, the compressor and the cooling fan would have to be *made* for three phase. :-)

I hope that the fire was confined to inside the housing, and did not other damage.

I don't know of any maintenance which could have prevented this -- unless the spade connectors on the cap were corroding and heating up, and regular inspection might have spotted that.

Perhaps replace the cap proactively every few years?

Hopefully, a professional AC person will chime in to give more complete information.

Good Luck, DoN.

Or, use of the wrong kind of capacitor. Motor-rated caps come in two flavors, X and Y (if memory serves), and one of them is guaranteed to fail open.

I'd check that the cap used matches what the AV manufacturer specified.

Joe Gwinn

The oil filled caps tend to leak when aging, so it likely spilled oil down the side of the unit. Then, if it shorted and exploded, any spark present had lots of oil to start a fire, melting the wiring.

Luckily, a couple years ago when my A/C went on the fritz, the cap had failed OPEN. No fire, just lack of function. Apparently, you weren't as lucky.

Just make sure that it wasn't the wires rubbing through their insulation on the sheetmetal housing which caused the fire. Ensure that all pass-thru holes for wiring are lined with plastic or rubber bulkhead grommets. Sheetmetal has nasty-sharp edges.

The good caps are "self protecting" and fail open if the case pressure gets too high. The idea is the top of the can bulges and actually tears off an internal connection to the cap, preventing the case rupture, mess and possible fire.

Then of course many run caps lack this feature and just get hot and burst.

Capacitor bank failures in large datacenter UPSes (100kVA and up) tend to be pretty spectacular.