Characteristic Impedance Ratio (C.I.R). : The maximum value of the system impedance ratio for which the relay performance remains within the prescribed limits of accuracy
Young & Company Electric Power Products and Related Engineering Services, Glossary of Electrical Terms, http://www.youngco.com/young2.asp?ID=4&Type=3, CA., USA 2001. (Electr Sfty - Amer) {514}