A combiner box (SCB1, SCB2, SCB3) for combining n strings (S1, S2, S3, S4) of a DC part of a PV plant, wherein n are integers greater 2, the combiner box comprising: - a plurality of n DC inputs (21, 22, 23, 24) for electrical connection to the n strings, - a DC output for electrical connection to at least one of an inverter or an array combiner box of the DC part, - a power line busbar which is electrically connected to power line terminals of the n DC inputs by means of n first power line connections (252;) and to a power line terminal of the DC output by means of a second power line connection, - a return line busbar (26; 36) which is electrically connected to return line terminals (261; 361) of the n DC inputs by means of n first return line connections and to a return line terminal of the DC output by means of a second return line connection, - a plurality of n pairs of first switching units (255, 265), one member of each pair being arranged in one of the n first power line connections and another member of each pair in a corresponding one of the n first return line connections, - a plurality of at least n DC direction sensors (256, 266) for detecting the direction of direct currents (I S1,.., I S4) conducted in the n first power line connections or in the n first return line connections, wherein each sensor is associated to at least one member of a pair of the first switching units, and - a combiner control unit (SCCU) for tripping the first switching units, in which the DC direction sensors communicate their output signals to the combiner control unit (SCCU), characterized in that the combiner box comprises a pair of second switching units (258, 268), one member of which being arranged in the second power line connection and another member of which being arranged in the second return line connection, and in that the combiner control unit (SCCU) is configured to communicate a first tripping signal to the first switching units for opening n-1 of the n pairs of first switching units in order to interrupt the direct currents (I S1, I S2, I S4) between each of n-1 DC inputs (21, 22, 24) and the two busbars (25, 26) when in a remaining one of the DC inputs the direct current changes from a forward (I S3) to a first reverse current (I R), and to communicate the first tripping signal without any time delay to the pair of second switching units (258, 268) in order to open these second switching units and to break a second reverse current (I Ro) fed through the DC output, and that the combiner control unit (SCCU) is configured to communicate a second tripping signal for opening the pair of first switching units (255, 265) associated to the remaining DC input with a first time delay, wherein the first time delay is at least the time required for breaking the second reverse current (I Ro) or wherein the first time delay is configured to match the discharge of the filter capacitors of the inverter. - a plurality of n DC inputs (21, 22, 23, 24) for electrical connection to the n strings, - a DC output for electrical connection to at least one of an inverter or an array combiner box of the DC part, - a power line busbar which is electrically connected to power line terminals of the n DC inputs by means of n first power line connections (252;) and to a power line terminal of the DC output by means of a second power line connection, - a return line busbar (26; 36) which is electrically connected to return line terminals (261; 361) of the n DC inputs by means of n first return line connections and to a return line terminal of the DC output by means of a second return line connection, - a plurality of n pairs of first switching units (255, 265), one member of each pair being arranged in one of the n first power line connections and another member of each pair in a corresponding one of the n first return line connections, - a plurality of at least n DC direction sensors (256, 266) for detecting the direction of direct currents (I S1,.., I S4) conducted in the n first power line connections or in the n first return line connections, wherein each sensor is associated to at least one member of a pair of the first switching units, and - a combiner control unit (SCCU) for tripping the first switching units,