These resources explain the selection and application of current transformers that reduce high ac currents to levels easily measured by panel or portable instrumentation.
Introduction to CT Measurements describes how instrument-grade current transformers are used to measure AC current, with either an isolated AC current or AC voltage output.
Current Transformer Turns Ratio & Polarity discusses proper CT orientation and how to specify and adjust the turns ratio.
How to Size a Current Transformer discusses VA rating, burden and wire length limitations for accurate current measurements.
Guide to Sizing a Motor CT provides a chart to select the CT rating for various three-phase motor sizes.