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The identification of shielding failure and back striking in transmission lines has always been a difficult subject. This paper analyzed the process of back striking and shielding failure in plusmn800kV UHVDC transmission lines. Because the mechanisms of produced transient zero module voltage and its propagation paths is different between back striking and shielding failure, their wavelet energy of different frequency bands of transient voltages is different. Thereby, from the perspective of protective relaying, this paper proposed an identification method to shielding failure and back striking in plusmn800kV UHVDC transmission lines based on wavelet energy. The data can be acquired from high speed protection device. The EMTDC simulation shows that the proposed method can effectively identify shielding failure and back striking in the whole UHVDC transmission lines, and is not affected by various wave shapes of lightning current.