when a UPS on bypass is a bad idea

Under most normal circumstances, a UPS might seem like pretty good insurance for your vital computer systems. Pity we all work under conditions that make a war zone seem mundane.

So you have your UPS. Where do you think would be a good location for it. A clean air conditioned shack well away from any hazards. Not ours. For convenience it is located deep in a motor room basement where it is kept toasty warm by the excess heat from numerous rows of switching cabinets. But that's OK for when it rains, the overflow from the leaking gutters runs down the walls, splashes into the room containing the UPS and keeps it all cool.

So the UPS sits there quietly humming away, improving the quality of the feed voltage for delivery to the critical systems in the computer room. All is well until one day it overheats. No worries you say, it will simply switch to bypass. A little dirtier electricity but still good. The systems all stay active and the production line doesn't stop.

If only it were that simple. This week on the first scorching day prior to summer, the UPS overheated but instead of switching to bypass, it shut down completely. Seems it monitors the input voltage and only switches to bypass when this input voltage is within the "safe" levels preset in its config. Our input voltage fluctuates wildly due to the heavy loads imposed by 4 pretty big motors (1200V+10000A). It is usually well below the nominal 240V when production is active and can go 250V+ on weekends. On Tuesday, it was below the lower threshold so we ended up with an Unusable Power Supply not once but twice. Arrggghhhh.

Why do these things always seem to happen at the end of a day on my day off?