A test plug for use with protective relays has a circuit that when it detects an overvoltage on the secondary side of a current transformer limits the overvoltage amplitude and the occurrence of the overvoltage to a predetermined number of peaks that are less than all of the peaks that occur when the secondary side is open circuited and the test plug is connected to the relays. The test plug also ensures that there is a continuous flow of current in the relays. The test plug further that has an indicator visible external to the plug to indicate the occurrence of an open circuit transformer secondary and that indicator remains illuminated when the test plug is disconnected from the protective relays.