The differential- and common-modes currents flowing in a branch line of a power line are analyzed for each of the two connection types. It is found that the parallel connection does not cause mutual conversion between the two modes. On the other hand, the series connection of a branch line makes a connected backbone line imbalanced. This is one of the mechanisms for generation of the common-mode current in the backbone line. As an example of the series connection, theoretical analysis is carried out on a branch line connected for switching an electric appliance. It is found that a common-mode current is generated in this switching branch by the series connection to the backbone line. In addition, a common-mode current is induced in the backbone line, if the switching branch is imbalanced.%本報告では，分岐線を接続することによって幹線および分岐線に発生するコモンモード電流について，並列接続及び直列接続に分けて考察した．その結果，並列接続では，その接続自体によるモード変換は生じないが，直列接続では，分岐線の接続によって幹線が不平衡になり，モード変換を生じて，幹線にコモンモード電流が発生し，また分岐線にもコモンモード電流が発生する場合があることが分かった．
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