An aircraft electrical system comprising: at least one circuit to be provided with an indication of a threshold being crossed on a DC power bus; a DC power bus including a high voltage positive line and a high voltage return line; a voltage detection circuit (20, 80) comprising: a tap connected to the positive line and a return tap to the return line on the DC power bus; a scaling resistor network for scaling down an input line voltage to be indicative of a voltage on the positive line, a comparator, having positive and negative inputs and an output, said scaling resistor network being connected to the negative input of the comparator; a reference voltage being connected to the positive input of the comparator; first and second resistors, wherein the first resistor is connected between the positive input of the comparator and the output of the comparator, wherein the second resistor is connected between the positive input and the anode of a Zener diode whose cathode is connected to the tap, wherein the reference voltage is defined in part by the resistances of the first and second resistors (36,38) and by the breakdown voltage of the Zener diode; wherein the anode of the Zener diode is further connected with the anode of a diode series, whereby the cathode of the diode series is connected to the return tap via a resistor or a current sink provided to regulate the current needed to power the detection circuit; and an optical isolation device, wherein the output of said comparator communicates a signal to the optical isolation device when said input line voltage exceeds a threshold voltage to turn said optical isolation device off and send a signal to an output of said optical isolation device, said optical isolation device being turned on if said input line voltage falls below said threshold voltage; and wherein the comparator comprises a comparator transistor, and said comparator transistor being off when said threshold voltage is exceeded by said input line voltage, and not powering said optical isolation device, and said comparator transistor being on when said threshold voltage exceeds said input line voltage such that said optical isolation device is powered; and further comprising a transistor which communicates the signal to the optical isolation device, wherein the transistor is off when said comparator transistor is on, and said transistor is on when said comparator transistor is off, and when said transistor is on, the power to said optical isolation device is blocked. at least one circuit to be provided with an indication of a threshold being crossed on a DC power bus; a DC power bus including a high voltage positive line and a high voltage return line; a voltage detection circuit (20, 80) comprising: a tap connected to the positive line and a return tap to the return line on the DC power bus; a scaling resistor network for scaling down an input line voltage to be indicative of a voltage on the positive line, a comparator, having positive and negative inputs and an output, said scaling resistor network being connected to the negative input of the comparator; a reference voltage being connected to the positive input of the comparator; a tap connected to the positive line and a return tap to the return line on the DC power bus; a scaling resistor network for scaling down an input line voltage to be indicative of a voltage on the positive line, a comparator, having positive and negative inputs and an output, said scaling resistor network being connected to the negative input of the comparator; a reference voltage being connected to the positive input of the comparator; first and second resistors, wherein the first resistor is connected between the positive input of the comparator and the output of the comparator, wherein the second resistor is connected between the positive input and the anode of a Zener diode whose cathode is connected to the tap, wherein the reference voltage is defined in part by the resistances of the first and second resistors (36,38) and by the breakdown voltage of the Zener diode; wherein the anode of the Zener diode is further connected with the anode of a diode series, whereby the cathode of the diode series is connected to the return tap via a resistor or a current sink provided to regulate the current needed to power the detection circuit; and an optical isolation device, wherein the output of said comparator communicates a signal to the optical isolation device when said input line voltage exceeds a threshold voltage to turn said optical isolation device off and send a signal to an output of said optical isolation device, said optical isolation device being turned on if said input line voltage falls below said threshold voltage; and wherein the comparator comprises a comparator transistor, and said comparator transistor being off when said threshold voltage is exceeded by said input line voltage, and not powering said optical isolation device, and said comparator transistor being on when said threshold voltage exceeds said input line voltage such that said optical isolation device is powered; and further comprising a transistor which communicates the signal to the optical isolation device, wherein the transistor is off when said comparator transistor is on, and said transistor is on when said comparator transistor is off, and when said transistor is on, the power to said optical isolation device is blocked.