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Up to the present time, the only data generally available to protection engineers for determining the performance of current transformers have been the ratio and phase-angle curves of the current transformers in question. More detailed analysis of important high-speed differential protection problems has shown that these data are hopelessly inadequate for the purpose of determining the performance of current transformers during the starting period when the current wave may be fully offset. If the time constant for decay of the d-c component exceeds 0.05 second, large errors of transformation may be expected if standard current transformers are used. While it is possible to make special designs that will not saturate because of the d-c component in the current,1 such designs are usually considerably oversize and expensive. Furthermore, space requirements and economy sometimes dictate that current transformers already installed be used if at all possible, even though it is known that their performance will be far from perfect. The problem then becomes one of determining the actual performance of existing current transformers, so that a suitable relay scheme may be chosen. It is the purpose of this paper to discuss the solution of this problem, and to present approximate methods of calculation not hitherto available. Attention is also given to the proper interpretation of the calculations with respect to relay operations, together with supporting test results.