Current Transformers Design Considerations

The Precision Factor

CT's provide an isolated, accurate measurement of the current in the primary circuit. Usually CT's have a one turn primary - a conductor that is part of the circuit being measured. CT's have very little effect on the circuit being measured, as we measure AC currents.

Variables for Consideration

Current Range
Overload current rating
Primary conductor opening
Sensitivity - volts per amp
Burden VA and resistance
Internal / external termination
Frequency response
Accuracy and linearity
Ambient temperature / allowable temperature rise
Dielectric / insulation resistance
Cost

Miscellaneous Comments

Low frequency response is limited, primarily, by magnetizing inductance.
High frequency response is limited, primarily, by stray winding capacitance and magnetic coupling between primary and secondary.
Manufacturability and cost targets have major design influence.

Current Transformer Mechanical Drawing