An electrically isolated high speed optical switching arrangement for use as a computer over-voltage sensing circuit utilizing first and second optoisolator devices, the first having an input, a first signal from the output of a bus sensing circuitry which provides two complementary outputs, with the output voltage of the optoisolator controlling power-down circuitry for the computer in response to an over-voltage condition. A second control optoisolator has its output in parallel with a resistor in the base to emitter path of the first optoisolator. The complementary signal of the bus-sensing circuitry provides the input to the second optoisolator. During normal power conditions, the first optoisolator is conductive and the second is non-conductive. As an over-voltage condition occurs, the second optoisolator is driven into conduction, thereby effectively providing a shunt across the resistor of the first optoisolator while the first optoisolator is shut off to instantaneously terminate conduction of the first optoisolator and provide an output signal to the power-down circuitry for shutting down the power in response to the over-voltage condition.