Instrument transformer

Current transformers are often constructed by passing a single primary turn (either an insulated cable or an uninsulated bus bar) through a well-insulated toroidal core wrapped with many turns of wire.

This affords easy implementation on high voltage bushings of grid transformers and other devices by installing the secondary turn core inside high-voltage bushing insulators and using the pass-through conductor as a single turn primary.

One type provides an IR voltage output that is proportional to the measured current; another, called a Rogowski coil, requires an external integrator in order to provide a proportional output.

Burden and accuracy are usually stated as a combined parameter due to being dependent on each other.

Care must be taken that the secondary winding of a CT is not disconnected from its low-impedance load while current flows in the primary, as this may produce a dangerously high voltage across the open secondary (especially in a relaying type CT) and could permanently affect the accuracy of the transformer.