A method of performing fault analysis, performed by an arrangement in a Passive Optical Network, PON, comprising at least one Optical Network Terminal, ONT, by using Optical Time Domain Reflectometry, OTDR, said method comprising: - triggering a new OTDR measurement, wherein a previous reference measurement has been made indicating an original state of said PON, the new OTDR measurement being triggered by the reception of a fault alarm or an Optical Transceiver Monitoring, OTM, measurement report from at least one ONT connected to said PON; - inserting an OTDR measurement signal into a multistage splitter (113, 133) before a last splitter stage of said multistage splitter (113, 133), and wherein said last splitter stage is of ratio 2: N root, so that the last splitter stage is connected to a plurality of drop links, and wherein at least one of the plurality of drop links being connected to said multistage splitter (113, 133) comprises at least one sub-splitter (134, 611, 612), wherein said at least one sub-splitter (134, 611, 612) is of a ratio 1: N branch so that the at least one sub-splitter (134, 611, 612) has one, two or more branches and one input port connected to the multistage splitter (113, 133); and characterized by - determining at least one new event location based on the OTDR measurement signal by analyzing OTDR measurement data, in a predefined sequence, relating to said at least one sub-splitter (134, 611, 612) connected to said last splitter stage, said predefined sequence, being individual for each branch being connected to the last splitter stage, starting based on the drop links for which an upstream optical signal needs to travel through the largest number of sub-splitters in order to reach the last splitter stage and then gradually through each level of serially connected sub-splitters in the branch until the last splitter stage is reached; and - calculating a fault magnitude for at least one determined new event location by subtracting an event magnitude obtained from the new OTDR measurement from said reference OTDR measurement and taking into account the number of drop links connected to the last splitter stage and to the at least one sub-splitter (134, 611, 612) in the reference measurement and the new OTDR measurement, thereby enabling determination of position and severity of said fault locations. - triggering a new OTDR measurement, wherein a previous reference measurement has been made indicating an original state of said PON, the new OTDR measurement being triggered by the reception of a fault alarm or an Optical Transceiver Monitoring, OTM, measurement report from at least one ONT connected to said PON; - inserting an OTDR measurement signal into a multistage splitter (113, 133) before a last splitter stage of said multistage splitter (113, 133), and wherein said last splitter stage is of ratio 2: N root, so that the last splitter stage is connected to a plurality of drop links, and wherein at least one of the plurality of drop links being connected to said multistage splitter (113, 133) comprises at least one sub-splitter (134, 611, 612), wherein said at least one sub-splitter (134, 611, 612) is of a ratio 1: N branch so that the at least one sub-splitter (134, 611, 612) has one, two or more branches and one input port connected to the multistage splitter (113, 133); and characterized by - determining at least one new event location based on the OTDR measurement signal by analyzing OTDR measurement data, in a predefined sequence, relating to said at least one sub-splitter (134, 611, 612) connected to said last splitter stage, said predefined sequence, being individual for each branch being connected to the last splitter stage, starting based on the drop links for which an upstream optical signal needs to travel through the largest number of sub-splitters in order to reach the last splitter stage and then gradually through each level of serially connected sub-splitters in the branch until the last splitter stage is reached; and - calculating a fault magnitude for at least one determined new event location by subtracting an event magnitude obtained from the new OTDR measurement from said reference OTDR measurement and taking into account the number of drop links connected to the last splitter stage and to the at least one sub-splitter (134, 611, 612) in the reference measurement and the new OTDR measurement, thereby enabling determination of position and severity of said fault locations.