I have a UPS that I just found out was plugged into a supposedly grounded three-prong outlet which is on the same circuit as my air conditioner. I has some work done on the a/c yesterday (maintenance on office and bedroom a/c's). The team manager asked me to switch off the circuit breaker which I did. As I did so outside the room I do remember hearing a subtle pop and also sensing a burning odor.

It was hard to connect this to a problem in my office because I had just turned off another circuit by mistake (which has been done before) and the other UPSs (same model) just beeped as they normally would when power is interrupted. Because we have food vendors around outside I at first assumed it was coming trough an open door. But a little while later the manager alerted me to the odor.

I did test all my UPS's and noticed that the problematic one appeared to be messed up (diagnostic beep, lights, etc.). I unplugged everything on it and the power cord too. When the circuit was back on I plugged it back in .... POP, odor again!

I opened it up at a service center and verified that the transformer looked burned out, and the internal circuit breaker switch had popped out. I'm getting it fixed now, but I'm really concerned about the electrical outlet. Someone said that it might not be properly grounded - this may be so, despite the yellow wire going up the wall! I had this outlet and others installed and made sure we had a good ground spike outside.

Now this happened just before and after the current a/c service, but maybe the last time the a/c was serviced the technician screwed up the grounding? The UPS has handled power outages well during this interim, so I'm really scratching my head as to why anything about the house wiring would cause the unit to blow up. Maybe the UPS is just defective?

UC

It really depends upon what has blown up in the UPS. If it is the transformer then I suggest that it was probably already defective and any surge caused by the aircon man pushed it over the top. Transformers are usually pretty robust items capable of handling large overloads without going bang. I'm assuming that it's the incoming breaker that's popped on the UPS.

Do verify your ground, although I doubt that lack of grounding caused the fry-up in the UPS

Was the air-con running when you operated the circuit breaker? If so, your UPS likely doesn't have enough of a snubber circuit to protect itself from the regenerative power of the motor spinning down. You can short the rectifier diodes or SCRs this way. Unless you manually reset the UPS circuit breaker (that popped out), it shouldn't have tripped again when you re-energized.