A voltage detection circuit comprises a tap to be connected to a positive line and a return tap to be connected to a return line on a power supply. The potential difference between the positive line and the return line is the input line voltage. A scaling resistor network scales down the input line voltage to be indicative of a voltage on the input line. The scaled voltage is connected to an input for a comparator. A reference voltage is supplied to the comparator. The reference voltage supplied to the comparator is indicative of a threshold voltage at which an output of the circuit changes states. An output of the comparator communicates a signal to an optical isolation device when the input line voltage is below the threshold voltage to turn the optical isolation device on and send a signal to an output of the optical isolation device. The optical isolation device is not turned on if the input line voltage exceeds the threshold voltage. A DC power bus incorporating the voltage detection circuit, along with an aircraft electrical system incorporating the DC power bus are also disclosed and claimed.