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.
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