I have never had a good experience with APC UPSs, my favourite anti-feature is the self-test that ends with power to the load being interrupted. The "shuts off instantly when cable connected" anti-feature bit a colleague of mine recently.
Apparently the APC model that we had installed wasn't live failover (or whatever) and when the batteries went "bad" it just killed the power to everything. And thanks to the weird "serial" cables and non-Linux friendliness of APC we never got the monitoring working correctly so we weren't warned about it ahead of time.
Instead of purchasing new batteries for the APC we got a TrippLite unit installed with no problems. It has a standard USB cable and the software works great with Linux & Windows. I even got a used TrippLite unit off of eBay for my office and they sent me rack mounting hardware for free! I love TrippLite and will never again even pause to think about APC.
tl;dr APC screwed us so we switched to TrippLite and have had no problems.
However, I've never had one catch fire while on battery power unlike the first and last TrippLite I tried...
I imagine the test is to ensure the accuracy of the battery runtime prediction, it transfers your load off line power and onto the inverter & batteries. Unfortunately something happens that causes the UPS to shut down or pause rather than immediately switch back to line power. Often the only evidence is that your servers power-cycled unexpectedly, sometimes the UPS actually switches off entirely during the test, however.