Based on the ratio between the fault component voltage phasor of the busbar and the sum of all branch current phasor of the busbar， which is defined as the fault component integrated impedance， a novel busbar protection principle is proposed. When an external fault occurs， the fault component integrated impedance reflects the capacitance impedance of the bus， whose value is very large. When an internal fault occurs， the fault component integrated impedance reflects the paralleling result of all the lines' impedances， which are connected to the bus. Its value is relatively small. According to such a characteristic， whether there is a fault on the busbar can be determined. The criterion is insensitive to transition resistance， and can be applied in breaker-and-a-half busbar when an internal fault occurs and when there is current flowing out of the bus. In addition， it can be easily set. EMTP simulation results show that the proposed principle is highly effective and reliable.