1. The UPS that cannot fail dead, will.
2. When you complain to the Power gang about their UPS failing your well-behaved three-phase load, rest assured that when power comes back up, you will discover 75% of your load is on one phase and the other 25% split between the other two.
2.1. The Power boys will be barely able to restrain their snickers when pointing this out.
3. There will be a spare UPS they never told anyone about, tucked in an obscure corner.
4. It will be all charged up, in Bypass and ready to go online...except two phases are flipped between input and output, so if the make-before-break Bypass switch were to get switched, there would be a brief flash and puff of smoke.
4.1 Your guys will be barely able to restrain their snickers when pointing this out.
5. "Ready to go online," even after you restack all the loads so the phases can be swapped, actually means "about two hours' downtime."
5.1 Power will not have told the brass this sad and awkward truth.
5.2 You will have to.
5.3 They won't like it.
5.4 By then it will be too late.
5.5 And it will take even longer than you told them it would.
Yayy, hooray! Good newses for all! We have a new UPS online and all the loads are balanced between the phases on all the UPSes, or at least close enough to keep them from acting up. (If I keep smiling and talking fast, maybe the Chief won't point out it took three days and some graveyard-shift overtime to get back to where we were already supposed to have been).
Oh, the glamor and fame and red-blooded adventure to be found travelling between the stars! Of all the fictional prophets of the Space Age, George O. Smith* turns out to have got it right -- and I think he might've had some inside info.
* I'm sorry to see most of the Web bios barely mention his "day job:" he was a EE, one of the men who made the WW II Radio Proximity Fuze work: it amounts to a tiny, vacuum-tube radar set, loaded in an AA shell and shot from a gun! His fiction is grand fun; if you like my reports from the Engineering Department of the USAS Lupine, you'll enjoy visiting his cast of characters.
UPS: Either "Unlikely to be interrupted Power Supply" or "Unprovably interrupted Power Supply." It depends on what just happened and who saw it.
Three-Phase Power: AC with three "hot" leads and one neutral. The AC waveform on each hot lead is 120 degrees out of phase from the other two and if the load on each one is of identical impedance, the neutral current is zero. For complex reasons having to do with the way toast falls butter-side down, this never happens. We like three-phase power 'cos it makes electric motors happier. Also the kids dig it. Or not.
Restacking The Loads: Rearranging the connection of loads to the circuit breakers in a panelboard ("fusebox") in such wise as to try to get zero neutral current, see above. Close counts, especially since it is the best you can ever do.
My Career: Spent doing things so obscure it requires a glossary just to complain. I should have T-shirts with footnotes on them.
YANKEE RADIO TOOL KIT, #106
4 weeks ago