I had a unique situation happen recently that has me puzzled. I have a location that has a 480 ungrounded delta bank that runs some water pumps,conveyors,and other equipment. When I arrived on site, I found a fuse blown on a primary phase conductor feeding this bank. However, one pump was running. Other 480 3-phase equipment on this service was also started with the primary fuse blown prior to noticing the fuse blown.
Now I understand that a motor will run on 2 phases but not start......right. But how did the other equipment start on only 2 phases? When I metered the voltage, I read 480 volts across all three phase. Does the running motor cause a "false" phase allowing other motors to start? Similar to a phase converter?
Losing a single fuse results in "Single Phasing" -- that's the term of art.
Let's say it's leg C that blew... Then the only leg pair that can provide true energy is A to B. B to C is open. A to C is open.
The rotating field of a running 3-phase machine will induce apparent power in the other windings. However, it's pretty much a phantom voltage since any attempt to really load it down causes the load to re-direct back to the sole surviving leg pair.
It's the attempt by the induction motor to get all of its needs via one leg-pair instead of three that cooks the windings/motor feeders.
So you NEVER want single-phasing to continue. It's brutal.
Some years back a janitor noticed something weird about the system in the middle of the night. He left a note to have everything checked. Two out of three lights were acting really weird. By 9:AM all of the critical freezer motors were ruined by single-phasing just 8 hours. All of the ice-cream was lost. The facility took months to fix. Medium voltage compressors are NOT off the shelf items and rebuilding the smoked ones was not acceptable to the insurance company. All for want of a single fuse --- and a lack of a single phasing alarm. ( You can't expect a janitor to recognize single-phasing. I've even seen thirty-year men puzzled by it! )