1 When the equipment using grounding protection of a phase ground fault occurs, due to short-circuit current flows through the equipment grounding pole and neutral grounding pole, the two grounding pole voltage, so that the neutral voltage increases a lot, so that the use of zero-protected equipment outside with high voltage, a hazard to people. So the same low-voltage system, can not be taken to protect the grounding of some take protection to zero.
2 Neutral ungrounded system, when a phase of ground fault occurs, the neutral voltage will rise sharply, this time the use of zero-protected equipment with high voltage, so the neutral ungrounded system equipment is not allowed to use protective zero.