Abstract: During the power test（COC53） for the 125 V DC power supply system（LBA） of Unit 3 of Ling’ao PhaseⅡNuclear Power Station, due to the loss of electricity, the circulating water pump（3CRF001PO） sewage pump pit cannot start the automatic drainage. The power failed for more than 6 h, and the water entered in the lower radial bearing oil chamber of the water circulation pump. The reasons for this problem is analyzed, and the power supply for the sewage pump is rebuilt, to avoid the risk of equipment flooding caused by the single loss of power.