Least that's what I think had happened. Think the poco feeds us 277VAC in a Y configuration, and that directly powers the ceiling florescent lights. About a third of them went dead. And about 2/3 the 120V outlets looked to be delivering very low voltage, maybe about 60V. A few incandescent desk lamps glowed dimly, and many computer power supplies barely kept alive. I suppose that in an electrical closet there's a transformer to convert the 480V delta to 120V/208v Y, and a dead phase (open circuit?) would cause 2 of the 3 phase 120V to go to around 60V. If B is dead, primary of A to B and B to C would end up in series across A and C, both those primaries seeing half the usual voltage, and then their secondaries will be at half their usual voltage. But that mid point, B, would end up at around 120V above ground? But not enough to drive the 277V florescent lights on the B phase. The building elevators shut down, but I think they have a relay that cuts all its power when a phase is lost. But apparently not the 480V delta to 120V Y transformer.
Theories are fine, what you need now is data. Start working your way upstream with your meter. Check the feed to each panel in turn. Eventually you'll get to the buildings' main disconnect.
Ultimately, you'll kill power at that main disconnect, to check your incoming feed with no load from the building. If your voltages are suddenly OK, isolate that transformer and check it with a megger. A short within that transformer could be overloading the PoCo feed, and bringing the voltage down.
We had this EXACT problem here a few years ago, 60V on one phase of a 120Y/208 secondary, and wierd things afoot because of it (some equipment on an outlet working fine, others not, computers that seemed powered up until you tried to turn them on, etc). In our case, it turned out to be a lost phase in the utility line. I unfortunately can't recall the details of the utility line problem, but it should be easy enough to determine if the problem is on the poco side- are the neighbors complaining, too?
Definitely sounds like a Single Phasing issue. Could be a result of a Single Fuse blown on the 480V side - likely at the Service's Main Disconnect. May also be a failed Contact at the Service Disconnect, either across a Circuit Breaker or Fused Switch.
If your Tenant Space is the only Occupant fed from the Service, and the problem exists universally throughout the Building, the issue most likely exists at the Service Equipment - between the Load Side of the Service Disconnect and the Line Side of the Distribution Section's Feeder devices.
A failed Primary Fuse Link on the Utility Transformer's Primary side would have similar results, however the Lighting would "Bounce Around" between the "Dead" Phase and the remaining Two live Phases. The "Bouncing" becomes more dramatic when 3 Phase Motors are running under load.
You can verify if a Primary Fuse link has opened on the Utility's Primary side, if the Transformers or Primary Feeders are Pole Mounted. Just look for the Fuse Cartridge that is hanging down.
Most of the time under the condition of lost Primary Fuse, 3 Phase Motors connected to Static loads will not fully start, but will attempt starting by rotating slow enough to keep the input Amperes within range of the OCPD.
Normal Single Phasing by loss of a Phase on the Secondary side will cause most loaded 3 Phase Motors to remain in Locked rotor condition. This scenario is accompanied by heavy vibrations at 120 hz, no shaft rotation, and a corresponding trip of an OCPD on the Motor's Circuit.
If the problem is localized, or you are fed from a Multi-Tenant Service Section, the problem may exist at a single location outside the Service Disconnect.
For Separately Metered Tenant Spaces, the problem could begin at the Line Side of the KWh Meter.
The SDS Transformer is reacting normally to the lost phase issue.
Before I go into a bunch of descriptions, please verify the extents of the problems, and if other Tenants are experiencing the same problems.
Reply back with some observed information. There is no need to enter any Equipment for voltage tests, as the readings will be incorrect. Besides, these verifications are more visual and audible types.
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!
Single-Phasing makes it impossible for 3-phase motors to START without direct assistance -- you'd have to mechanically flip them over under low-load conditions -- then they can pull-in.
If they are ALREADY running when a leg is lost they will continue to run -- but the surviving Single-Phase will be forced to provide ALL of the EMF. The apparent voltage at the phase lost -- as read at the motor feed/fused disconnect is due to back EMF driven by the whirling magnetic field of the rotor. It's enough to feed your test instruments but does not represent energy flowing INTO the motor.
Once Single-Phasing is evident you must IMMEDIATELY shut down all 3-phase motors as you are cooking the windings.
If you dig around you'll find stories of entire refrigeration systems destroyed (costing $$$$$ in lost ice cream, etc. plus operational shut down for total motor replacement) all due to high amp Service fuses failure to blow and the 'maintenance' staff un-aware of their dire situation. Instead of placing panic calls they left the stuff running until the morning crew arrived.
When the morning came, the mourning came: everything was toast.
Single-Phasing is also brutal on anyone running 480V high-bay lighting -- for the same reason.
In short: if Single-Phasing occurs: PANIC...
Shut down everything ASAP and call a real electrician.