Abstract: The advantage of the grid anode system is that the protection potential is evenly distributed on the storage tank's bottom plate, with small interference on nearby pipeline. The test results from 11 storage tanks in two pump stations with instantaneous synchronized switch-off method showed that when control potential reached -1.1 V, the tank still may not be effectively protected because the potentiostat output current or protection current density was too small, however, the daily management were usually only concerned with energized potential, easily overlooking under-protection due to the IR drop. The standards on storage tank cathodic protection require that the current density should be 1~10 mA/m², and the recommended range is 5~10 mA/m², but test results showed that when the current density of two pump stations was 1~2 mA/m², the switch off potential reached -850 mV. As the current density requirements are influenced by tank level and seasonal factors, although the standards only require that current density requirement is determined based on potential eliminating IR drop when cathodic protection system is put into operation, it is still required to regularly determine the current density requirements in operation and maintenance process based on switch off potential to effectively avoid the risk of under-protection.