A Compact Secondary Substation including a fault protection system and a method for fault protection in Compact Secondary Substations (CSS). The CSS includes a Ring Main Unit (RMU), a transformer and a Low Voltage (LV) switchgear. The CSS includes a number of detectors, which detectors can be placed at least in one of the RMU, the transformer or in the LV switchgear, which detectors can be connected to a control unit, which control unit by fault detection by the detectors can activate a fast operating switch for grounding. By using the fast operating grounding switch and controlling this on the basis of the detectors placed inside the CSS, an active arc fault protection is achieved. The control unit also controls the fast operating grounding switch so that it works as a working grounding switch.