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Transformer-Like Devices for High-Accuracy Current Measurements.



T. M. Souders


A theoretical and practical framework is presented to aid in the design, fabrication and testing of transformer-like devices for use in high-accuracy ac current metering applications. Current transformers, two-stage current transformers, and current comparators are discussed, as well as related devices that use passive and active error correction techniques. Transformer theory is developed in terms of simple electromagnetic theory and practical equivalent circuits. Magnetic design equations are presented and the measurement of relevant magnetic properties is discussed. Sources of error and their mitigation are covered in detail, including errors caused by magnetizing currents, winding and core inhomogenieties (so-called magnetic errors), and circulating capacitive currents. Calibration methods and current transformer testing are also covered.
Technical Note (NIST TN) - 1473
Report Number


ac current measurement, current comparators, current transformer testing, current transformers, equivalent circuits, error sources, magnetic shielding, transformer design


Souders, T. (2008), Transformer-Like Devices for High-Accuracy Current Measurements., Technical Note (NIST TN), National Institute of Standards and Technology, Gaithersburg, MD, [online], (Accessed April 19, 2024)
Created June 1, 2008, Updated June 2, 2021