Methods for Testing Current Transformers

In the sale of electric power especially in large amounts at high voltages, current transformers are universally used to operate the meters from the readings of which the bills are made out. While the secondary currents of such transformers reproduce fairly faithifully on a smaller scale the conditions of current strength and phase existing in their primary circuits, meter engineers are realizing more and more the importance of checking up the accuracy of this link in the chain of measurements. Many possible methods have been suggested for testing the accuracy of current transformers. Some are delightfully simple and inaccurate while others involve relatively intricate connections and sensitive apparatus, but give correspondingly more exact results. The present paper gives a critical discussion of the various possible methods with data as to the advantages and disadvantages of each, in order to serve as a guide to the meter engineer in selecting the method best suited to the particular working conditions of accuracy, speed, volume of testing, and intelligence of labor, existing in his plant. In all eleven distinct methods are described, and supplementary suggestions are made concerning various types of detecting instruments, etc.