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.
Last edited by Tesla; 02/09/10 03:21 PM.
