For single-phase grounding on the load side of 380 V PC bus in a 1 000 MW unit of power plant, burnout accident of potential transformer(PT) occurs. It is found after a detailed analysis of the accident that the grounding ways of LV auxiliary power system are different for 1 000 MW, and 600 MW or 300 MW unit, where the former applies small current grounding system via resistance grounding, while the latter generally adopts large current system via direct grounding. Therefore, the PT burnout accident is caused by improper selection of equipment, and equipment selection of large capacity unit should depend on actual situation.