Abstract: This device for protecting a line (10, 12) of a network of supply lines comprises two protective relays (18, 20; 22, 24) furnished with means for detecting faults and a circuit breaker adapted so as to disconnect the line from the network of lines in the presence of a fault, each relay being adapted so as to monitor a first zone (Z1) of the line corresponding to a zone of certain detection of a fault (10, 12) and at least one second zone (Z2) juxtaposed with the first zone and corresponding to a zone of uncertain detection of a fault. At least one of the relays comprises means for detecting the opening of the circuit breaker associated with the other relay in response to a detection, by the latter, of a fault in the first zone in such a way as to cause the opening of the circuit breaker associated with the said at least one relay (18, 20; 22, 24).