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
