A high accuracy open phase detection system for power transformers that uses a combination of logic controllers and current transformers to recognize an open phase condition experienced by the power transformers under no load, light load, and full load conditions. A current to voltage and current to current transformer on each phase are employed to detect the excitation current and load current conditions. During an open phase condition, a microprocessor detecting device, connected to the current to voltage and current to current transformers, monitors the appropriate power system quantities to determine the existence of one or more open phase(s) with or without a ground or an open phase with line charging capacitance. Through the microprocessor monitor, the microprocessor detecting device can alert operators to the loss of phase or abnormal conditions in the power source. The data used by the microprocessors can be used to calculate the magnitude and phase angle of the current in the power source and detect abnormal system conditions. This invention also employs a unique circuitry configuration to reduce the effects of ambient noise on the open phase detection system.