A modern power system consisting of multi-bus EHV/UHV transmission links to increase the capacity and reliability of the system, requires faster and reliable operation of relays and circuit breakers for its protection. The present day available relays for protection of transmission lines against open circuit fault are not sufficient for application in EHV/UHV circuits for their larger operating time. In the present paper, a microprocessor based ultra high speed relay has been developed for protection of a transmission line against open circuit fault which operates on the principle of continuous tracing of the line current waveforms thus eliminating sequence filters, and it requires less than one cycle to operate even in the worst possible fault condition. Application of a microprocessor enhances the accuracy, reliability and speed of operation of the device eliminating complicated hardware and dead band in operation. The use of zero crossing detectors in signal processing keeps the signals free from any transient effects. © 1994 Taylor & Francis Group, LLC.