If you have a high resistance on a neutral in a 3 phase system, even if no items are 3 phase the common neutral bar becomes a central point for all things connected, so a current will go up from one phase off through a load back down the neutral then up the next item on a diffrent phase (the lowest resistance path) If an item on the second phase is a high load (low resistance)(or inductive)this will produce a high voltage across the other items even up to 400v if the neutral is shot.
