A bad ballast has the effect of 'grounding' one leg of the 'ungrounded' delta. If there's another incidental ground somewhere, you have a fault. If the fault is a high-impedence type of fault, you don't trip breakers or light up the pahse monitors ... but your line voltages swing all over the place. That, in turn, means that the electronics of the induction equipment don't work right.
For those unfamiliar with 'induction welding,' that is a process quite a bit different from anything you'll see in a workshop. There is no electrode, rod, or flux. Instead, steel is passed through a magnetic field that cycles very fast .... we're talking about thousands of cycles per second. This filed makes the metal withing it red-hot. The red-hot metal is then squeezed together, where it mixes with the other red-hot metal and forms a weld. This is how pipe is made (including EMT).
The induction welding equipment has no transformers; it relies upon very rapid switching of the AC supply to create both the high frequency and the higher voltages used. If your AC supply coming in is not a clean waveform, the electronics (SCR's and IGBT's most often) won't work right.
Of course, the opposite also applies: the induction equipment does introduce harmonics to the AC supply. Not as bad as a variable speed drive, but there is the effect.
Hence, my desire to isolate the lighting from the power.