The switching apparatus and the method are for varying the impedance of a phase line of a segment of an electrical power line. The phase line includes n conductors electrically insulated from each other and short-circuited together at two ends of the segment. The apparatus comprises a controllable interrupter connected in series for each conductor; a parameter detector; a first controller for controlling the interrupters; and a disabling unit for disabling the interrupters. The disabling unit comprises n controllable switches associated with the interrupters, position detectors for detecting which of the interrupters is closed, and a second controller having a command output to command the controllable switches and ensure that, at all operating times, at least one of the interrupters is closed and disabled.