Why would an apartments central AC units compressors run cap blow up &
start fire of wiring?
a tenants AC went down & that's what I found when I got up on roof. Is
is a lack of maintenance issue?

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On 2014-06-26, notmcguyver wrote:
Why would an apartments central AC units compressors run cap blow up &
start fire of wiring?
a tenants AC went down & that's what I found when I got up on roof. Is
is a lack of maintenance issue?

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.

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In article s.com,
notmcguyver wrote:

Why would an apartments central AC units compressors run cap blow up &
start fire of wiring?
a tenants AC went down & that's what I found when I got up on roof. Is
is a lack of maintenance issue?

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

On Thu, 26 Jun 2014 03:18:02 +0000, notmcguyver
wrote:

Why would an apartments central AC units compressors run cap blow up &
start fire of wiring?
a tenants AC went down & that's what I found when I got up on roof. Is
is a lack of maintenance issue?

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.

--
Learning to ignore things is one of the great paths to inner peace.
-- Robert J. Sawyer

Larry Jaques wrote:
On Thu, 26 Jun 2014 03:18:02 +0000, notmcguyver
wrote:

Why would an apartments central AC units compressors run cap blow up &
start fire of wiring?
a tenants AC went down & that's what I found when I got up on roof. Is
is a lack of maintenance issue?

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.

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.