Megger Biddle AVO Transformer Turn Ratio(TTR) Test Set The Biddle 550503 Three-Phase Automatic TTR is designed to measure the turn ratio of power, instrument, and distribution transformers in a substation or manufacturing environment. A rugged and robust design makes this TTR well-suited for use in a variety of harsh environments. The 550503 TTR is also particularly suited for testing in power transformer manufacturing environments where testing of complex, three-phase substation power transformers can be performed quickly while minimizing the possibility of errors.