Traceable Technique to Calibrate Clamp Meters in AC Current From 100 to 1500 A

A measurement technique to calibrate current clamp meters in ac current in the measurement range from 100 to 1500 A at 50 Hz has been developed at the National Institute of Metrological Research (INRIM) in addition to the primary reference measurement systems for ac current calibrations at INRIM. This technique is traceable to the national standards of dc resistance, dc voltage, and ac current ratio. It is based on a standard current transformer (CT), on a 50-mΩ standard shunt, and on a 61/2-digit digital voltmeter (DVM) that measures the voltage on the standard shunt inserted in a conductor, to which also the clamp meter is applied, whose current is supplied by a CT that can generate currents up to 1500 A. The CT is controlled by a variable transformer. The clamp meter is applied to a copper wire and placed on a tilting support simulating the actual operating mode of the meter. This measurement technique is applicable in industrial metrological laboratories and is an alternative to measurement practices of accredited secondary laboratories involving digital calibrators and turn current coils. This measurement technique was also tested utilizing an 81/2-digit high-accuracy DVM. The results of the evaluation are presented and discussed. The expanded (k = 2) uncertainties of the technique span from 1.0 × 10^-2 at 100 A to 2.0 ×10^-3 at 1500 A.