Guidance is offered on the application and use of optical voltage and current sensor systems for protective relaying, covering the selection, installation, testing, and operation of sensors. Information is provided on optical voltage and current-sensing technologies, and a number of performance parameters including: accuracy, bandwidth, noise, stability, and impact of temperature and vibration. Also covered are interface requirements of optical sensor systems, a number of their applications, relevant installation and commissioning, testing, reliability, and redundancy matters, as well as a comprehensive treatment of the relevant industry standards.