On 06/14/2016 10:24 PM, Bob Jonkman via talk wrote:
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Alvin Starr wrote:

A lot of UPSs switch from the mains to battery once the power drops
and that delay can be enough to cause some power supplies to lose
enough power to fail

If that happens then your UPS is broken. The whole point of a UPS is
to supply *uninterruptable* power. Even the simplest pass-through
UPSes I know about can switch from line to battery in the space of a
single waveform (ie. within 1/60th of a second). Not much point in
having a UPS if it makes all your equipment reboot when the power goes
out.
The answer "your UPS is broken" is a bit simplistic.

What I had is what you get from buying an inexpensive APC UPS and very heavily loaded server with a badly designed power supply.

I stole this from http://www.mpoweruk.com/ups.htm.

Off Line or Standby UPS
These are simple inexpensive systems providing only basic protection. In normal situations the UPS passes the mains power directly to the load. The mains power provides a single DC line which keeps the battery charged. When the UPS detects a voltage too low, it turns on the inverter to power the load from the battery. The system is relatively slow (longer than 4 ms) and the delay between mains power loss and inverter startup can be long enough to disrupt the operation of some sensitive loads. This technology does not normally provide full time power conditioning but it may use a simple filter to clip spikes and electrical noise.
 
On Line or No Break UPS
These are designed to provide a zero transfer time, with better voltage and frequency regulation than that can be achieved by off line and line interactive UPS. In on line systems the mains power is used to provide two DC power lines which feed both the charger and the inverter which is permanently turned on providing the AC power to the application. When the mains fails, the inverter instantaneously draws its DC supply from the battery instead of the mains.
The method by which the AC load is permanently supplied by the system inverter and not the mains is called "double conversion" since the charger provides the AC-DC conversion and the inverter converts the DC back to AC again.
On line systems typically provide full power conditioning, protecting the load from all forms of power disturbances, including brownouts, blackout, transient surges or sags. In the event of a mains power failure, there is no delay or transfer time to backup power. These systems are however more costly and have both higher power consumption, and higher heat generation.
 
Line Interactive UPS
These systems contain an off-line inverter but also use a transformer to supply to the load. In the event of a mains failure the inverter is started and switched to the transformer to provide the output. The transformer is used to provide line conditioning however it also maintains output on its secondary briefly when a total outage occurs increasing the hold up time of the UPS. This results in a break in the output of a few milliseconds or less and is thus faster than a simple off line UPS.


Also take a look at: http://www.repeater-builder.com/backup-power/ups-test.html

It provides a good look at what is actually going on when your UPS from CanadaComputers,NCIX,BestBuy... switches.
You will notice that the provided voltage lags a bit and depending on the timing you could lose up to a whole cycle.
This was also a test with a clean line drop.
In the case where the peak voltage drops before the power fails completely then you have a more complex set of problems.
I remember reading the specs of one UPS that would kick in if the input voltage dropped below 90V.
90V may be too low for some equipment and will cause power supplies to have even lower hold times.

This is why you need to test your UPS and equipment configuration from time to time and buy online UPS's when ever possible.


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