Abstract: A ground isolation monitoring apparatus having a protective circuit is provided for use between an AC power source and a utilization device, such as an electronic instrument. The protective circuit permits operation of the utilization device with earth ground disconnected, automatically reconnecting earth ground if a hazardous voltage or current appears on device chassis or exposed metal parts. The protective circuit also monitors neutral-to-earth ground continuity and shuts off AC power to the utilization device if the ground path opens or if the impedance thereof increases. Various other features may be provided to ensure safe operation of the device.