Bar primary ring type curent transformers are secondary coil wounded on a torroidal core if high permeability silicon steel. The conductor through which the current is to be measured is passed through the hole of the current transformer. This conductor acts as primary winding and generates proportionate low current in the winding wound on the ring of the current transformer. The windings of the current transformer provides lower proportional current which can be easily measured by the measurng instruments. Current Transformers also isolate measuring instruments and protective devices from mains and higher voltages.

It is easier to measure high current without breaking the bus bars or cables by means of bar primary current transformer, which can be supplied over the bus bar, or cable. These CT's are study, compact and economical.
The secondary of CT's should not be open circuited when primary is energized as it may harm the operator to high voltage.