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This UPS typically switches to battery backup mode when voltage drops so low that incandescent bulbs dim significantly. That is what it should do. Temporary power. However these things are made so cheaply as to be fooled even by electrical noise. In your case, its battery was defective. If AC line noise caused the APC to switch to batteries, then batteries could not provide sufficient power to the computer.

A resulting lockout was cleared by pressing its reset button. Does not matter if sufficient AC power was incoming. That UPS had completely disconnected from AC. And then locked because its battery was nearly dead.

Plug a laser printer directly into the same duplex receptacle that powered the UPS and computer. Electrically same as when plugged into the UPS 'surge protected' outlets. Power demands, as demonstrated by Gene's numbers, are well below what would cause problems on properly wired circuits.

Hi jimmie,

Here's my guess. a UPS will have 2 ratings. One is for the available power when on battery backup. This may be, say, 300 watts (watt=volt X amp, so 300 watts = 300/120 or about 2.5 amps.)

The second is total output when on AC power, which would be the total of what is plugged into the battery backup, and the surge protector outlets. Lets say this rating is 12 amps. If the computer and monitor take 2 amps, and the laser printer 10 amps, then the 12 amps could be met or exceeded. Then an internal circuit breaker (in the UPS) could have tripped.

So why at that particular time? Either one of the devices took more current, or the "trip curve" of the circuit breaker would allow a certain current for a certain period of time, and the time was exceeded.

In either case, an internal circuit breaker would open, and any devices plugged into either side of the UPS would go dead.

The circuit breaker sometimes resets automatically, sometimes there is a button for you to press to reset.

Or, the UPS is starting to go bad.

So the computer is an "innocent bystander."

Make sense?

Gene

westom and Gene,

I have never taken the time to understand this type of gadgetry, but with a disrupted install of Dragon my attention was grabbed, so I thought the topic might be pertinent to fellow AC users.

My initial intent was to warn others of avoiding the mistake I made.

I spent over an hour last night trying to google my last question above before I posed it here so your answers are once again much appreciated and thanks for the rational well written responses, from a guy who is trying to rapidly catch up on things not necessarily learned along the way.

That's why engineers exist. A 15 amp breaker may take 2 hours before it trips on a 20 amp load. We dumb numbers down. For example, we provide a ballpark number to associate a 14 gauge wire with a 15 amp breaker; a 12 gauge wire with a 20 amp breaker.

Turn five 100 watt incandescent bulbs on at the same time on a 15 amp circuit. How much do they demand on power on? Maybe 30 amps. Why does that not trip a 15 amp breaker? Electrical concepts say why. We dumb it down. Five 100 watt bulbs consume 4.2 amps continuously. We just forget to mention the initial 30 amp consumption is normal for a 15 amp breaker and 14 gauge wire. Discussing such details confuse layman.

We know why a UPS is not recommended for a laser printer (obviously not to be confused with an inkjet printer). Motorized appliances do not like the 'dirty' power provided by a typical UPS in battery backup mode.

Even noise could have tripped that UPS into battery backup mode. Then the UPS discovered its battery was defective in about 2 seconds. So it locked out all power - crashing your computer. No circuit breaker need be involved since other electronic circuits can cause the resulting power off and Alarm.

What would happen if five 100 watt bulbs were powered from surge protected outlets? A resulting maybe 28 amp demand would not trip any circuit breaker.

We know a UPS powering a motorized appliance is not recommended because UPS power can be so 'dirty'. Not undersized (insufficient wattage). 'Dirty' power is ideal for electronics and potentially harmful to motorize appliances and power strip protectors. Reality is always this much more complex.

Here's the issue. Not all are grounded to the same potential. Like I mentioned earlier, in some devices, the chassis ground is not the same as the Earth ground. In some devices which are highly susceptible to interference, the chassis ground is kept at a different potential. That's where the term floating ground comes from. It's kept above the potential of the Earth ground. And if you have a devices with different grounds, that leads to interference.

An example is a stereo system that "hums". This is a result of the chassis ground and the Earth ground being at a different potential. Or components having different ground reference points. It's also referred to as a ground loop. Assuming you had plugged such a device into your UPS and then pulled the power, the results would not be pretty if you were to unplug the UPS to test it.

Stereo humming is one of the reasons that certain devices have a chassis ground ("fixed" ground) that is above the Earth ground to prevent issues from things like ground loops. There's a good chance in the medical profession that you encounter such devices which need protection from this type of interference.


That is correct, but the issue is not after it's on battery power or even before. The issue is the transition. During that instant you unplug the UPS is where the problem is. The ground was expected to be there by the UPS so that current can leave via the ground wire during the switch. Once you remove that path to the ground, that could leave through you or one of your devices. This is why you hear random stories of people's UPS blowing up when they do the unplugging test.

As to why the computer shutoff why you still had electricity in the office is simply due to overload protection built into the UPS. Almost all of them have it nowadays. If it's pulling too much power, it will shutoff to protect the device.

Gene is probably right with the first theory about the internal circuit breaker. If you look up the specs, you'll the APC 600 has internal circuit breaker with a reset button to reset the breaker on the UPS. Some UPS's just have fuses instead of the circuit breaker.