PROTECTIVE relays are intended to protect expensive electrical equipment. With proper care they will perform this duty, but when neglected they may become inoperative and could become a hazard in themselves. Since the reliability is the most important quality of protective relays it follows that their maintenance must be first class.
It is generally accepted that protective relays and their trip circuits should be periodically checked in order to ensure that they will always be ready to operate with certainty. The recommended practice is to carry out three types oftest: (a) Acceptance tests at the installation or commissioning of the relays. (b) Periodic tests to check the calibration and condition of the relay. (c) More frequent tests of a simple nature to cause movement of the parts, and to check the continuity of the trip circuit.
The secondary injection test simulates the current and voltage transformer secondaries, by injecting single-phase or three-phase currents and voltages into relays, devices o into the secondary circuits, performing all the corresponding checking and testing with normal and fault conditions
Secondary injection tests are always done prior to primary injection tests. The purpose of secondary injection testing is to prove the correct operation of the protection scheme that is downstream from the inputs to the protection relay(s).
