Microprocessor-based relay protection devices enable efficient operation of the electrical infrastructure of high voltage power lines and substations under emergency conditions. This is achieved by using highspeed fault detection algorithms and advanced electronic components. The paper deals with the elaboration of a such algorithm which novelty lies in the fact that it is based on amplitude-phase measurements of asymmetrical components of current and voltage signals, which detects accidents with symmetrical and asymmetric overloads significantly faster. The algorithm is implemented in microprocessor protection in the LIRA device.