Impedance between faulted line and ground is generally regarded as a resistance， but in some cases the equivalent impedance can be reactive. Fault on a transmission line connected to the bus behind the protection is equal to fault on the bus through a reactive“fault impedance”. This problem has rarely been noticed. The impedance loci with fault impedance seen by the protection in this condition is analyzed. The theoretical analysis and digital simulation results show the possibility that distance protection with quadrangular characteristic may mal-operate under this condition.