Otherwise, dangerously high voltages would be present at the open circuited current transformer. Also note that
currents can be injected into the protection relay from a secondary injection test set.
With the switch between terminals 7 and 21 open and the switch between terminals 8 and 22 closed as shown
below, a 515 test plug can be inserted between terminals 7 and 21 to monitor the CT current. See the diagrams
below for details. Note that the 515 test plug is made up of two conductors separated by an insulator