The core type three phase voltage transformer with neutral grounding in primary side consist three induced voltage in each phase even one of primary three phase voltage is disconnected; because the symmetrical magnetizing path of this type of transformer leads residual magnetic flux of two active limbs to other dead limb, therefore residual induced voltage due to vector summation of two voltage with 120 degree difference phase can cause third secondary induced voltage without existing corresponded phase in primary side. This condition can be cause non operation of under voltage relay in secondary side of same voltage transformer. In this case removing of primary grounding can help to prevention of relay error operation.