UPS electrical problem

[Benjamin Scott]

On Tue, Jun 29, 2010 at 9:33 PM, Greg Rundlett (freephile)
<greg at freephile.com> wrote:
> Within the past week, when the household thermostat kicks on or off the
> central A/C system, the PC shuts off instantaneously.

  What happens if the electrical power supply to the UPS is
disconnected?  (For example, flip the circuit breaker for the outlet.
Don't test a UPS by unplugging it, that cuts the ground which can
cause other problems.)

  Some possible causes, in roughly decreasing order of likelihood:

  As others have suggested, the battery is the most likely culprit.
UPS batteries are generally only good for a few years at best.  Heavy
battery usage can shorten that further.  For example, if your UPS has
been kicking on and off battery every time your air conditioner stars
or stops...

  It could be a UPS overload.  Overloads usually trigger an alarm or
trip a UPS's built-in circuit breaker, but not always.  I've seen
overloads manifest as "the UPS drops the load".  Note that a worn-out
battery and an overload can have similar symptoms -- dropping a
regular load, but a tiny load seems okay.  (This is one reason I
suggest a clean disconnect test: If the UPS is fine on a clean
disconnect, then it's not an overload.)

  As others have said, it may be a question of the low-voltage
threshold being set wrong, or simply not being sensitive enough in the
design.  Air conditioner causes a voltage sag, UPS thinks everything
is okay, but PC disagrees.  (Another reason for the clean disconnect
test.  The UPS can't argue about zero volts.)

  It's possible for the UPS's power or control electronics to fail.
Like anything else.

  Another possibility is the UPS is allowing power transients through
which your load doesn't like.  Many UPSes are just a glorified surge
suppressor when they're not on battery.  When they sense trouble, they
switch over.  That switch takes time.  That's long enough let some
kinds of power transients through, which is enough to piss off some
loads.

  Better UPSes include some kind of power
conditioning/filtering/regulator/magic to address switching delays and
line transients.  APC calls their magic "line interactive", which I've
never seen a convincing technical explanation of.

  Also, cheaper UPSes will not output a "true" AC sine wave -- they'll
use a square or stepped approximation.  Some loads *really* hate that
-- AC motors, for example.  Computers are almost always okay, though.

  The best UPSes always run off the battery+inverter, so there's no AC
switching or AC transient possible.  Names for this include
"continuous" or "double conversion" or "on-line".  They are expensive,
both up-front and over time.

-- Ben