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The characteristics of line boundary are further studied. Then this paper analyzes the feature differences of backward traveling-waves between internal and external faults. The main difference is that the ratio of high frequency to low frequency components in case of external faults is less than that in case of internal faults. In terms of the different features, the paper proposes a novel non-unit transient-based protection using backward traveling-wave, named as boundary unit (BU). Then presents the practical algorithm using wavelet transform. A number of ATP simulation tests show not only that the novel boundary unit is feasible and ultra-high-speed, but that its performance is superior to that of the former non-unit transient-based protection using current transients. The paper first indicates that the non-unit transient-based protection is difficult to reliably protect the entire line, but it may be used as an ultra-high-speed unit of the entire-line protection.