In the field of current differential protection relays there is a need for improvements in or relating to the protection relays to allow them to accommodate current transformer saturation while maintaining a desired degree of reliability. A method of operating a current differential protection relay following a fault, the protection relay being electrically coupled to a protection zone of an electrical power system, comprises the steps of establishing a fault condition by discriminating between an internal fault inside the protection zone and an external fault outside the protection zone; and modifying the sensitivity of the protection relay according to the nature of the fault condition.