Aiming at the single phase breakoff fault happended in the 110kV Hua-Fu transmission line,some theoretical formula is deduced.It proves that the operation of zero-sequence current protection only depends on the load current and has nothing to do with the fault position,when the wire break fault happens on the 110kV transmission line in a defined network configuration.Theoretical calculation proves that the operation of relaying and automatic equipment are right in the fault.