A method is disclosed for diagnosing faults on a communication bus used to transmit zone selective interlocking signals in a power distribution network. The method includes configuring interfaces of all protection devices connected to the same communication bus to be input ports within a first diagnosis time period; detecting, at each protection device connected to the communication bus, an input signal received by the input port thereof; and if abnormality is detected in the input signal, the protection device issuing a first alert associated with the corresponding interface.