Connect a test set to the relay current input terminals and inject current. How it is done depends on the type of relay, type of test set available, type of circuit connections and intent of the test.
If the relay is a motor overload relay, hard wired in a motor starter, the test techniques are different than if it is a modern digital relay mounted in a protection panel with test switches. If it is an older electro-mechanical relay in a draw-out case the way you connect the test set and the trip timing devices is different.
Different test sets use different methods. The basic MOF (Mess On the Floor) method uses resistor load banks, variable voltage transformers, discrete voltmeters and ammeters, separate timing devices and lots of test leads to get a calibrated current into the relay. A modern digital test set will have two current leads and a timing lead and a connection to a laptop. Older test sets will have variants in between. All basically push a test current of known value through the relay and measure the trip time.
There are several safety issues involved with secondary injection. An inexperienced technician might open circuit a CT under load trying to connect the test current or inject too much current and burn up the relay coil. Even experienced techs forget to disconnect the trip outputs to prevent dumping load during testing.
Bottom line - secondary injection testing is more complicated than it seems. Look up some testing guides on the internet form the major test set manufacturers - Omicron for one, or sign up for some training.
