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Since field evaluation does not provide sufficient data for detailed performance analysis, the multiprocessor-based UHV line relay developed at IREQ was tested on a Transient Network Analyzer (TNA). Results obtained from approximately 80,000 fault simulations revealed unpredicted behavior that had passed unnoticed during previous digital simulation and laboratory investigations. Special hardware and software designed for relay testing on the TNA are presented with examples. The final test session applied to the corrected relay showed that all faults generated within the reach of 80% of the line length are cleared in less than 1.5 cycles by the impedance algorithm alone.