National Physical Laboratory

Current transformers and selective error voltage transformers

Current transformers 

NPL carries out UKAS accredited measurements of current transformers for customers including the Electricity Supply Industry and transformer manufacturers. We provide measurement of the current error and phase angle error of instrument current transformers, in accordance with BS EN 60044-1:1999.

Range of Measurements

Routine current transformer calibrations can be carried out over a large range of primary to secondary current ratios, generally to rated secondary currents of 1 A and 5 A. Measurements are made at defined frequencies, burdens and power factors according to the customer’s requirements.

Ratios /A

Frequency /Hz

0.25 – 10 000 / 5


5 – 1 000 / 5

50 – 400

The table above gives a simplified range of the ratios available for calibration, although others outside of this range can be calibrated. NPL also carries out calibrations of manual and automated current transformer test sets at 50 Hz.


The approximate uncertainties associated with the measurement of the current and phase angle errors are given in the table below. Uncertainties will be increased if a particular transformer’s errors are high or if the transformer exhibits poor repeatability.



Current Error

Phase Error

0.01, 0.02, 0.03

10 ppm

10 µrads

0.1 and higher

30 ppm

30 µrads

The reported uncertainties are based on a standard uncertainty multiplied by a coverage factor of k = 2, which provides a level of confidence of approximately 95 %.

Current Transducer Measurements

Current transducers, which give a voltage output, can be calibrated over a wide range of current levels. Instruments calibrated include current transformers with resistive loads connected across their secondary winding and Rogowski coils.

NPL's UKAS accreditation covers the calibration of current transducers that have an output voltage greater than 0.25 V at 50 Hz and have a base measurement uncertainty of 0.05%. NPL certificates can be issued for the calibration of transducers that have smaller output voltages.


Customer Service tel: +44 20 8943 7166

Last Updated: 18 Aug 2015
Created: 8 May 2009


Please note that the information will not be divulged to third parties, or used without your permission