Abstract: Apparatus for electrically isolating a DC voltage source while the voltage f the source is being measured or otherwise monitored. An electronic switch is provided for selectively coupling the apparatus to the source, and a first optical device coupled to the switch receives a control signal which directs switch operation. The first optical device electrically isolates the switch from electrical equipment which generates the control signal. A device for generating an internal voltage data signal is also coupled to the switch, the frequency of such signal varying according to the voltage of the source. A second optical device receives the internal data signal, and couples an external data signal to voltage monitoring equipment, the external data signal having a frequency which varies according to the frequency of the internal data signal, the second optical device electrically isolating the switch from the voltage monitoring equipment.