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
- 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