As we can see from the History articles in the magazine, different forms of protection have been used in electric power systems for more than a century. With the increase in the importance of protection, it has been necessary to ensure that the relays are able to perform their functions as expected, when necessary, and do not operate when not required. Throughout the years different testing tools have been used successfully with electromechanical, solid-state and microprocessor based relays. Protection engineers and testing specialists from around the world have developed confidence in traditional methods such as "constant current" or "constant voltage" that have served them well for over a century. These methods were more or less the things you could do with the test devices available through most of the twentieth century More