Current Transformer (CT)

(1) a transformer that is employed to provide a secondary current proportional to primary current flowing. The primary “winding” is often created by passing the system conductor or bus bar through an opening in the device and the secondary is typically rated at a standard value to match common meters and display units. Current transformers are used in current measurement, protective relays, and power metering applications. The load (meter) on a CT should never be removed without first shorting the secondary of the CT, otherwise dangerous voltage levels may result when the load is removed.

(2) a device which measures the instantaneous current through a conductor of an electric power line and transmits a signal proportional to this current to the power system’s instrumentation.