A method for active detection of current overload and leakage by an overload and active fault detection circuit of a transport refrigeration unit, the method characterised by: measuring, by a microcontroller of the active fault detection circuit, a line current for each of one or more phases output by a generator of the transport refrigeration unit supplying power to a load of a refrigerant vapor compression system of the transport refrigeration unit; measuring, by the microcontroller, a voltage received by the active fault detection circuit from a battery of the transport refrigeration unit and transmitted via a conductor of the active fault detection circuit to a microprocessor of the refrigerant vapor compression system; controlling, by the microcontroller, a switch along the conductor of the active fault detection circuit to control transmission of the voltage, via the conductor to the microprocessor, wherein controlling the switch comprises (i) closing the switch upon a determination that the transport refrigeration unit is operating in a run mode of operation, in which the transport refrigeration unit is powered on and running, and the voltage level is greater than or equal to an operational threshold voltage level, (ii) closing the switch upon a determination that the transport refrigeration unit is operating in a start mode of operation, in which the transport refrigeration unit is being powered on, and the voltage level is less than the operational threshold voltage level, (iii) opening (510; 516) the switch upon a determination that any of the one or more line currents exceeds a threshold, and (iv) determining, by the microcontroller, whether the switch is operating correctly, and opening another switch positioned along the conductor and upstream of the switch upon a determination that the switch is not operating correctly, wherein determining whether the switch is operating correctly comprises comparing a present state of the switch to an expected state of the switch, wherein the expected state of the switch is based on the voltage, and wherein the present state of the switch is one of an open state or a closed state. measuring, by a microcontroller of the active fault detection circuit, a line current for each of one or more phases output by a generator of the transport refrigeration unit supplying power to a load of a refrigerant vapor compression system of the transport refrigeration unit; measuring, by the microcontroller, a voltage received by the active fault detection circuit from a battery of the transport refrigeration unit and transmitted via a conductor of the active fault detection circuit to a microprocessor of the refrigerant vapor compression system; controlling, by the microcontroller, a switch along the conductor of the active fault detection circuit to control transmission of the voltage, via the conductor to the microprocessor, wherein controlling the switch comprises (i) closing the switch upon a determination that the transport refrigeration unit is operating in a run mode of operation, in which the transport refrigeration unit is powered on and running, and the voltage level is greater than or equal to an operational threshold voltage level, (ii) closing the switch upon a determination that the transport refrigeration unit is operating in a start mode of operation, in which the transport refrigeration unit is being powered on, and the voltage level is less than the operational threshold voltage level, (iii) opening (510; 516) the switch upon a determination that any of the one or more line currents exceeds a threshold, and (iv) determining, by the microcontroller, whether the switch is operating correctly, and opening another switch positioned along the conductor and upstream of the switch upon a determination that the switch is not operating correctly, wherein determining whether the switch is operating correctly comprises comparing a present state of the switch to an expected state of the switch, wherein the expected state of the switch is based on the voltage, and wherein the present state of the switch is one of an open state or a closed state.