Indoor vs Outdoor Current Transformer Differences
12 August, 2026
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On the nameplate of a current transformer (CT), one often encounters “mysterious codes” such as 1.0, 0.5, 0.5S, 0.2, 0.2S, etc. These codes are not simply arranged in ascending or descending order, but serve as core indicators that determine the “lower limit of measurement range and upper limit of error” of the CT.
According to IEC 61869-2, the accuracy level consists of two parts:
Number – Maximum ratio error at 100% rated current (±%)
Letter S (Special) – It can extend the accurate measurement range from the conventional 5% (or 10%) lower limit to 1%, ensuring precision even under light loads
Remember that the smaller the number, the more accurate it is, and the light load with an ‘S’ is also accurate. This way, you can quickly select the most suitable accuracy level for the current transformer during project initial design, equipment updates, or energy efficiency retrofits.
