Bar primary ring type current transformers are secondary coil wound on a torroidal core of 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 a primary winding and generates proportionate low current in the winding wound on the ring of the current transformer. The winding of current transformer provides lower proportional current which can be easily measured by measuring instruments. Current transformers also isolate measuring instruments and protective devices from mains and higher voltages.

It is easier to measure high currents without breaking the bus bars or cables by means of bar primary current transformer which can be slipped over the bus bar or cable. These CT's are sturdy, compact and economical.

The Secondary of CT's should not be open circuited when primary is energized as it may harm the operator due to high voltage.