The problem that TA wire breakage closedown logic failed to work is researched

and it is caused by loop circuit between TA under 3/2 connection pattern.Two methods of merging TA circuit under 3/2 connection pattern and the lock rule about TA wire breakage in the protection systems of line

transformer and bus are also introduced.And the phenomenon of loop circuit between TA is described

and the character of loop circuit and defects of lock logic under TA disconnection in some protection equipments are analyzed.The improvements of lock logic of protection system under TA wire breakage are put forward.