Relay protection

Relay protection

Relay and protection test equipment | Megger

As power grids get bigger and carry more power, the need for quick, reliable disconnection when faults occur becomes more urgent. The purpose of protective relay equipment is to sense fault states and trip circuit breakers. If a fault isn’t corrected early, personal injuries and damage to circuits and equipment can occur with significant direct and indirect costs. At the same time as these decisions sometimes need to be taken within thousands of a second, it is necessary to only isolate the faulty part of the network to minimize customer impact.


The relay development can be divided in three main stages; the first stage was the era of electromechanical relays, which started over 100 years ago. The next era was static or electronic relays (also known as solid-state relays), which were introduced in the 1960s. The present era with microprocessor-based relays started in the beginning of the 1980s, where microprocessor performed the logics, but the filtering was analogue and late 1980s first fully numerical relay was introduced.


The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detection of abnormal operating conditions such as over-current, overvoltage, reverse power flow, over-frequency, and under-frequency. With later technologies more advanced protection features like distance protection and synchro check was introduced. In modern relays several protection functions can be taken care of by one device so one numerical terminal can replace up to five panels with electromechanical relays or two panels with static relays.