This disclosure relates to a method of and a system for fault detection and localization of a fault in a DC-system comprising multiple serially connected DC-sources. The method may comprise an act of connecting a test apparatus to at least one terminal of the DC-system and to DC-system ground. The method encompasses at least one repetition of an act of applying a test AC-signal to one terminal of the DC-system, an act of detecting the response AC-signal to the test AC-signal on either one terminal alone and/or on DC-system ground; and an act of comparing the test AC signal with the response AC-signal to detect a fault and the location of the fault in the DC-system. The system comprises a fault detection and localization apparatus configured to perform the method disclosed.