The practice of specifying multi-ratio CTs and extended accuracy classes has evolved over many years and, in many organizations, has become a standard specification rather than a consciously evaluated engineering decision. This paper has attempted to revisit that practice from the standpoint of actual system operation.

The examples presented demonstrate that a single, appropriately selected CT ratio is adequate for most transmission lines, cables and transformers. The unused primary taps provided in multi-ratio CTs rarely contribute any operational advantage throughout the service life of the equipment.

Similarly, the analysis shows that Class 0.2 accuracy is sufficient for the majority of EHV metering applications, while Class 1 is adequate for motor feeders. The Extended Accuracy Class (0.2S) should be reserved only for applications where improved low-current accuracy has genuine commercial or operational significance, such as generator metering, power evacuation lines from generating stations and selected revenue metering installations.

The paper also highlights an important engineering reality: the overall accuracy of an energy measurement system depends not only on the CT, but also on several other factors, including burden variations, voltage transformer errors, current summation, communication systems and data processing. Pursuing ever-higher CT accuracy without considering these factors offers little practical benefit.

The same reasoning applies to ABT metering, where the accuracy of demand forecasting is governed far more by load prediction algorithms, weather forecasts and data quality than by marginal improvements in CT accuracy.

Ultimately, good engineering is not about specifying the highest possible performance for every component. It is about selecting specifications that are fit for purpose, technically justified and economically sound.

Replacing the routine use of multi-ratio CTs with well-chosen single-ratio CTs can reduce cost, simplify design, facilitate testing and maintenance, and improve standardization—without sacrificing the accuracy required for reliable metering and commercial accounting.

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